publicationDate stringlengths 10 10 | title stringlengths 17 233 | abstract stringlengths 20 3.22k | id stringlengths 9 12 |
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2024-03-20 | The influence of hydrogen on the electronic structure in transition metallic glasses | We investigate the influence of hydrogen on the electronic structure of a
binary transition metallic glass of V$_{80}$Zr$_{20}$. We examine the
hybridization between the hydrogen and metal atoms with the aid of hard x-ray
photoelectron spectroscopy. Combined with ab initio density functional theory,
we are able to show and predict the formation of $s$-$d$ hybridized energy
states. With optical transmission and resistivity measurements, we investigate
the emergent electronic properties formed out of those altered energy states,
and together with the theoretical calculations of the frequency-dependent
conductivity tensor, we qualitatively support the observed strong
wavelength-dependency of the hydrogen-induced changes on the optical absorption
and a positive parabolic change in resistivity with hydrogen concentration. | 2403.13371v1 |
2024-04-07 | Morphological stability of electrostrictive thin films | A large electric field is typically present in anodic or passive oxide films.
Stresses induced by such a large electric field are critical in understanding
the breakdown mechanism of thin oxide films and improving their corrosion
resistance. In this work, we consider electromechanical coupling through the
electrostrictive effect. A continuum model incorporating lattice misfit and
electric field-induced stresses is developed. We perform a linear stability
analysis of the full coupled model and show that, for typical oxides,
neglecting electrostriction underestimates the film's instability, especially
in systems with a large electric field. Moreover, a region where
electrostriction can potentially provide a stabilizing effect is identified,
allowing electrostriction to enhance corrosion resistance. We identified an
equilibrium electric field intrinsic to the system and the corresponding
equilibrium film thickness. The film's stability is very sensitive to the
electric field: a 40 percent deviation from the equilibrium electric field can
change the maximum growth rate by nearly an order of magnitude. Moreover, our
model reduces to classical morphological instability models in the limit of
misfit-only, electrostatic-only, and no-electrostriction cases. Finally, the
effect of various parameters on the film's stability is studied. | 2404.05093v1 |
2024-04-11 | Respective Roles of Electron-Phonon and Electron-Electron Interactions in the Transport and Quasiparticle Properties of SrVO$_3$ | The spectral and transport properties of strongly correlated metals, such as
SrVO$_3$ (SVO), are widely attributed to electron-electron ($e$-$e$)
interactions, with lattice vibrations (phonons) playing a secondary role. Here,
using first-principles electron-phonon ($e$-ph) and dynamical mean field theory
calculations, we show that $e$-ph interactions play an essential role in SVO:
they govern the electron scattering and resistivity in a wide temperature range
down to 30 K, and induce an experimentally observed kink in the spectral
function. In contrast, the $e$-$e$ interactions control quasiparticle
renormalizations and low temperature transport, and enhance the $e$-ph
coupling. We clarify the origin of the near $T^2$ temperature dependence of the
resistivity by analyzing the $e$-$e$ and $e$-ph limited transport regimes. Our
work disentangles the electronic and lattice degrees of freedom in a
prototypical correlated metal, revealing the dominant role of $e$-ph
interactions in SVO. | 2404.07772v1 |
2024-04-16 | Strain-dependent Insulating State and Kondo Effect in Epitaxial SrIrO$_{3}$ Films | The large spin-orbit coupling in iridium oxides plays a significant role in
driving novel physical behaviors, including emergent phenomena in the films and
heterostructures of perovskite and Ruddlesden-Popper iridates. In this work, we
study the role of epitaxial strain on the electronic behavior of thin SrIrO$_3$
films. We find that compressive epitaxial strain leads to metallic transport
behavior, but a slight tensile strain shows gapped behavior.
Temperature-dependent resistivity measurements are used to examine different
behaviors in films as a function of strain. We find Kondo contributions to the
resistivity, with stronger effects in films that are thinner and under less
compressive epitaxial strain. These results show the potential to tune
SrIrO$_3$ into Kondo insulating states and open possibilities for a quantum
critical point that can be controlled with strain in epitaxial films. | 2404.10909v1 |
2024-04-24 | Low thermal boundary resistance at bonded GaN/diamond interface by controlling ultrathin heterogeneous amorphous layer | Thermal boundary resistance (TBR) in semiconductor-on-diamond structure
bottlenecks efficient heat dissipation in electronic devices. In this study, to
reduce the TBR between GaN and diamond, surface-activated bonding with a hybrid
SiOx-Ar ion source was applied to achieve an ultrathin interfacial layer. The
simultaneous surface activation and slow deposition of the SiOx binder layer
enabled precise control over layer thickness (2.5-5.3 nm) and formation of an
amorphous heterogeneous nanostructure comprising a SiOx region between two
inter-diffusion regions. Crucially, the 2.5-nm-thick interfacial layer achieved
a TBR of 8.3 m2-W/GW, a record low for direct-bonded GaN/diamond interface. A
remarkable feature is that the TBR is extremely sensitive to the interfacial
thickness; rapidly increasing to 34 m2-K/GW on doubling the thickness to 5.3
nm. Theoretical analysis revealed the origin of this increase: a diamond/SiOx
interdiffusion layer extend the vibrational frequency, far-exceeding that of
crystalline diamond, which increases the lattice vibrational mismatch and
suppresses phonon transmission. | 2404.15738v1 |
2024-05-03 | Local insulator-to-superconductor transition in amorphous InO$_x$ films modulated by e-beam irradiation | We present a novel method enabling precise post-fabrication modulation of the
electrical resistance in micrometer-scale regions of amorphous indium oxide
(a-InO$_x$) films. By subjecting initially insulating films to an electron beam
at room temperature, we demonstrate that the exposed region of the films
becomes superconducting. The resultant superconducting transition temperature
($T_c$) is adjustable up to 2.8 K by changing the electron dose and
accelerating voltage. This technique offers a compelling alternative to
traditional a-InO$_x$ annealing methods for both fundamental investigations and
practical applications. Moreover, it empowers independent adjustment of
electrical properties across initially identical a-InO$_x$ samples on the same
substrate, facilitating the creation of superconducting microstructures with
precise $T_c$ control at the micrometer scale. The observed resistance
modifications likely stem from photoreduction induced by X-ray and/or UV
radiation emitted during electron beam interactions with the film and
substrate. | 2405.02276v1 |
2024-05-27 | Resistance Distribution of Decoherent Quantum Hall-Superconductor Edges | We study the probability distribution of the resistance, or equivalently the
charge transmission, of a decoherent quantum Hall-superconductor edge, with the
decoherence coming from metallic puddles along the edge. Such metallic puddles
may originate from magnetic vortex cores or other superconductivity suppressing
perturbations. In contrast to the distribution of a coherent edge which is
peaked away from zero charge transmission, we show analytically and numerically
that the distribution of a decoherent edge with metallic puddles is always
peaked at zero charge transmission, which serves as a probe of coherence of
superconducting chiral edge states. We further show that the distribution width
decays exponentially in magnetic field and temperature. Our theoretical
decoherent distribution agrees well with the recent experimental observation in
graphene with superconducting proximity. | 2405.17550v1 |
2017-12-13 | Magnetotransport in a model of a disordered strange metal | Despite much theoretical effort, there is no complete theory of the 'strange'
metal state of the high temperature superconductors, and its
linear-in-temperature, $T$, resistivity. Recent experiments showing an
unexpected linear-in-field, $B$, magnetoresistivity have deepened the puzzle.
We propose a simple model of itinerant electrons, interacting via random
couplings with electrons localized on a lattice of quantum 'dots' or 'islands'.
This model is solvable in a large-$N$ limit, and can reproduce observed
behavior. The key feature of our model is that the electrons in each quantum
dot are described by a Sachdev-Ye-Kitaev model describing electrons without
quasiparticle excitations. For a particular choice of the interaction between
the itinerant and localized electrons, this model realizes a controlled
description of a diffusive marginal-Fermi liquid (MFL) without momentum
conservation, which has a linear-in-$T$ resistivity and a $T \ln T$ specific
heat as $T\rightarrow 0$. By tuning the strength of this interaction relative
to the bandwidth of the itinerant electrons, we can additionally obtain a
finite-$T$ crossover to a fully incoherent regime that also has a linear-in-$T$
resistivity. We show that the MFL regime has conductivities which scale as a
function of $B/T$; however, its magnetoresistance saturates at large $B$. We
then consider a macroscopically disordered sample with domains of MFLs with
varying densities of electrons. Using an effective-medium approximation, we
obtain a macroscopic electrical resistance that scales linearly in the magnetic
field $B$ applied perpendicular to the plane of the sample, at large $B$. The
resistance also scales linearly in $T$ at small $B$, and as $T f(B/T)$ at
intermediate $B$. We consider implications for recent experiments reporting
linear transverse magnetoresistance in the strange metal phases of the
pnictides and cuprates. | 1712.05026v3 |
2018-10-05 | Static and Dynamic Signatures of Anisotropic Electronic Phase Separation in La2/3Ca1/3MnO3 Thin Films under Anisotropic Strain | The electronic phase separation (EPS) of optimally doped La2/3Ca1/3MnO3
(LCMO) thin films under various degrees of anisotropic strain is investigated
by static magnetotransport and dynamic relaxation measurements. Three LCMO
films were grown simultaneously on (001) NdGaO3 (NGO) substrates by pulsed
laser deposition, and then post-growth annealed at 780 oC in O2 for different
durations of time. With increasing annealing time, the films developed
significant strains of opposite signs along the two orthogonal in-plane
directions. The static temperature-dependent resistivity, R(T), was measured
simultaneously along the two orthogonal directions. With increasing annealing
time, both zero-field-cooled and field-cooled R(T) show significant increases,
suggesting strain-triggered EPS and appearance of antiferromagnetic insulating
(AFI) phases in a ferromagnetic metallic (FMM) ground state. Meanwhile, R(T)
along the tensile-strained [010] direction becomes progressively larger than
that along the compressive-strained [100]. The enhanced resistivity anisotropy
indicates that the EPS is characterized by phase-separated FMM entities with a
preferred orientation along [100], possibly due to the cooperative deformation
and rotation/tilting of the MnO6 octahedra under the enhanced anisotropic
strain. The anisotropic EPS can also be tuned by an external magnetic field.
During a field-cycle at several fixed temperatures, the AFI phases are melted
at high fields and recovered at low fields, resulting in sharp resistance
changes of the ratio as high as 104. Furthermore, the resistivity was found to
exhibit glass-like behavior, relaxing logarithmically in the phase-separated
states. Fitting the data to a phenomenological model, the resulting resistive
viscosity and characteristic relaxation time are found to evolve with
temperature, showing a close correlation with the static measurements in the
EPS states. | 1810.02516v1 |
2023-07-13 | Hydrodynamic magnetotransport in two-dimensional electron systems with macroscopic obstacles | In high-quality conductors, the hydrodynamic regime of electron transport has
been recently realized. In this work we theoretically investigate
magnetotransport of a viscous electron fluid in samples with
electron-impermeable obstacles. We use the two approaches to describe the fluid
flow. The first one is based on the equations of hydrodynamics of a charged
fluid, which assume that the kinetic equation takes into account the two
harmonics of the electron distribution function. The second approach is based
on the equations that are obtained by taking into account three harmonics of
the distribution function (''quasi-hydrodynamics''). Within the hydrodynamic
approach, we consider the cases of the rough and the smooth edges of the disks,
on which the electron scattering is diffusive or specular, respectively. The
longitudinal magnetoresistivity turns out to be strong and negative, the same
for both rough and smooth discs edges to within small corrections. For rough
discs, the Hall resistivity is equal to its standard value. For smooth discs
the Hall resistance acquire a small correction to the standard value,
proportional to the Hall viscosity. In the quasi-hydrodynamic approach, we
considered the case of smooth discs and small magnetic fields. In the regime
when the flow is substantially different from the hydrodynamic one, the
longitudinal resistivity does not depend on the shear stress relaxation time
(but depends on the relaxation time of the third angular harmonic), while the
correction to the standard Hall resistivity does not depend on both relaxation
times. We compare the results of the hydrodynamic calculation of the
longitudinal resistance with the experimental data on magnetotransport in
high-quality GaAs quantum wells with macroscopic defects. A good agreement of
theory and experiment evidences in favor of the realization of the hydrodynamic
transport regime in such systems. | 2307.06705v2 |
2023-11-13 | Pruning random resistive memory for optimizing analogue AI | The rapid advancement of artificial intelligence (AI) has been marked by the
large language models exhibiting human-like intelligence. However, these models
also present unprecedented challenges to energy consumption and environmental
sustainability. One promising solution is to revisit analogue computing, a
technique that predates digital computing and exploits emerging analogue
electronic devices, such as resistive memory, which features in-memory
computing, high scalability, and nonvolatility. However, analogue computing
still faces the same challenges as before: programming nonidealities and
expensive programming due to the underlying devices physics. Here, we report a
universal solution, software-hardware co-design using structural
plasticity-inspired edge pruning to optimize the topology of a randomly
weighted analogue resistive memory neural network. Software-wise, the topology
of a randomly weighted neural network is optimized by pruning connections
rather than precisely tuning resistive memory weights. Hardware-wise, we reveal
the physical origin of the programming stochasticity using transmission
electron microscopy, which is leveraged for large-scale and low-cost
implementation of an overparameterized random neural network containing
high-performance sub-networks. We implemented the co-design on a 40nm 256K
resistive memory macro, observing 17.3% and 19.9% accuracy improvements in
image and audio classification on FashionMNIST and Spoken digits datasets, as
well as 9.8% (2%) improvement in PR (ROC) in image segmentation on DRIVE
datasets, respectively. This is accompanied by 82.1%, 51.2%, and 99.8%
improvement in energy efficiency thanks to analogue in-memory computing. By
embracing the intrinsic stochasticity and in-memory computing, this work may
solve the biggest obstacle of analogue computing systems and thus unleash their
immense potential for next-generation AI hardware. | 2311.07164v1 |
2022-06-09 | Identification of High-Dielectric Constant Compounds from Statistical Design | The discovery of high-dielectric materials is crucial to increasing the
efficiency of electronic devices and batteries. Here, we report three
previously unexplored materials with very high dielectric constants (69 $<$
$\epsilon$ $<$ 101) and large band gaps (2.9$<$ $E_{\text{g}}$(eV) $<$ 5.5)
obtained by screening materials databases using statistical optimization
algorithms aided by artificial neural networks (ANN). Two of these new
dielectrics are mixed-anion compounds (Eu$_5$SiCl$_6$O$_4$ and HoClO), and are
shown to be thermodynamically stable against common semiconductors via
phase-diagram analysis. We also uncovered four other materials with relatively
large dielectric constants (20$<$$\epsilon$$<$40) and band gaps
(2.3$<$$E_{\text{g}}$(eV)$<$2.7). While the ANN training data is obtained from
Materials Project, the search-space consists of materials from Open Quantum
Materials Database (OQMD) - demonstrating a successful implementation of
cross-database materials design. Overall, we report dielectric properties of 17
materials calculated using ab-initio calculations, that were selected in our
design workflow. The dielectric materials with high dielectric properties
predicted in this work open up further experimental research opportunities. | 2206.04750v1 |
1997-10-23 | Stripes, Non-Fermi-Liquid Behavior, and Two-Component Transport in the High-Tc Cuprates | Non-Fermi-liquid features of the high-Tc cuprates, and specifically the
systematic behavior of the resistivity, Hall constant, and thermoelectric
power, are shown to result from an electronic structure based on "large-U" and
"small-U" orbitals, and the resulting striped structure. | 9710254v1 |
1998-12-03 | Anomalous Magnetothermal Resistance of High-Tc Superconductors: Anomalous Cyclotron Orbits at a Dirac Point | I derive equations of motion for quasiparticles near the nodes in the d-wave
gap of high Tc superconductors. Previous versions have not properly taken into
account the spatial dependence of the gap parameter phase. The results are
compatible with magnetothermal conductivity measurements in the superconducting
phase by Ong and Krishana. | 9812063v1 |
1999-05-18 | On Anisotropic Transport in High Landau Levels | We would like to point out that: (i) according to the edge-bulk transport
models for the quantum Hall regime, the direction of low bulk conductance is
actually perpendicular to that expected naively; and (ii) the values of
experimental "high resistance" peaks correspond to value of bulk conductance
$\approx 1 e^{2}/h$. | 9905237v1 |
2001-11-03 | Stability of antiferromagnetism at high magnetic fields in Mn3Si | We report low temperature measurements of the specific heat, resistivity and
magnetisation of the itinerant antiferromagnet Mn3Si. The unprecedented
stability of the magnetic state to high magnetic field up to 14 T inferred from
the invariance of these bulk properties is incompatible with itinerant
magnetism expected of a conventional Fermi liquid. | 0111048v1 |
2003-11-12 | The Driving Force of Superconducting Transition in High Temperature Superconductors | We show that both the kinetic energy and the exchange energy of the t-J model
can be read off from the optical data. We show that the optical data indicates
that the superconducting transition in high temperature superconductors is
kinetic energy driven and the exchange energy resist the transition. We also
show that kinetic energy may also be the driving force of the pseudogap
phenomenon. | 0311282v1 |
2009-04-10 | Linear in temperature resistivity of scalar fermions: application to high Tc cuprates | no longer applicable | 0904.1746v4 |
2024-01-06 | The 4-adic complexity of quaternary sequences with low autocorrelation and high linear complexity | Recently, Jiang et al. proposed several new classes of quaternary sequences
with low autocorrelation and high linear complexity by using the inverse Gray
mapping (JAMC, \textbf{69} (2023): 689--706). In this paper, we estimate the
4-adic complexity of these quaternary sequences. Our results show that these
sequences have large 4-adic complexity to resist the attack of the rational
approximation algorithm. | 2401.03204v1 |
2020-08-27 | Modeling cross-field demagnetization of superconducting stacks and bulks for up to 100 tapes and 2 million cycles | Superconducting stacks and bulks can act as very strong magnets (more than 17
T), but they lose their magnetization in the presence of alternating (or
ripple) transverse magnetic fields, due to the dynamic magneto-resistance. This
demagnetization is a major concern for applications requiring high run times,
such as motors and generators, where ripple fields are of high amplitude and
frequency. We have developed a numerical model based on dynamic
magneto-resistance that is much faster than the conventional
Power-Law-resistivity model, enabling us to simulate high number of cycles with
the same accuracy. We simulate demagnetization behavior of superconducting
stacks made of 10-100 tapes for up to 2 million cycles of applied ripple field.
We found that for high number of cycles, the trapped field reaches non-zero
stationary values for both superconducting bulks and stacks; as long as the
ripple field amplitudes are below the parallel penetration field, being
determined by the penetration field for a single tape in stacks. Bulks keep
substantial stationary values for much higher ripple field amplitudes than the
stacks, being relevant for high number of cycles. However, for low number of
cycles, stacks lose much less magnetization as compared to bulks. | 2008.12006v2 |
2021-03-07 | 130 mA/mm $β$-Ga$_2$O$_3$ MESFET with Low-Temperature MOVPE-Regrown Ohmic Contacts | We report on the demonstration of metalorganic vapor phase epitaxy-regrown
(MOVPE) ohmic contacts in an all MOVPE-grown $\beta$-Ga$_2$O$_3$
metal-semiconductor field effect transistor (MESFET). The low-temperature
(600$^{\circ}$C) heavy (n$^{+}$) Si-doped regrown layers exhibit extremely high
conductivity with sheet resistance of 73 $\Omega$/$\square$ and record low
metal/n$^{+}$-Ga$_2$O$_3$ contact resistance of 80 m$\Omega$.mm and specific
contact resistivity of 8.3$\times$10$^{-7}$ $\Omega$.cm$^{2}$ were achieved.
The fabricated MESFETs exhibit a maximum drain-to-source current of 130 mA/mm,
a high I$_{ON}$/I$_{OFF}$ of $>$10$^{10}$ with a high power FOM of 25
MW/cm$^{2}$ were achieved without any field plates. Nanoparticle-assisted Raman
thermometry, thermal modeling, and infrared thermography were performed to
assess the device self-heating under the high current and power conditions.
This demonstration shows the promise of MOVPE technique for the realization of
high-performance lateral $\beta$-Ga$_2$O$_3$ devices and also highlights the
need for device-level thermal management. | 2103.04275v2 |
2010-08-31 | Uniaxial linear resistivity of superconducting La(1.905)Ba(0.095)CuO(4) induced by an external magnetic field | We present an experimental study of the anisotropic resistivity of
superconducting La(2-x)Ba(x)CuO(4) with x=0.095 and transition temperature
Tc=32 K. In a magnetic field perpendicular to the CuO(2) layers, H(perp), we
observe that the resistivity perpendicular to the layers, \rho(perp), becomes
finite at a temperature consistent with previous studies on very similar
materials; however, the onset of finite parallel resistivity, \rho(par), occurs
at a much higher temperature. This behavior contradicts conventional theory,
which predicts that \rho(perp) and \rho(par) should become finite at the same
temperature. Voltage vs. current measurements near the threshold of voltage
detectability indicate linear behavior perpendicular to the layers, becoming
nonlinear at higher currents, while the behavior is nonlinear from the onset
parallel to the layers. These results, in the presence of moderate H(perp),
appear consistent with superconducting order parallel to the layers with
voltage fluctuations between the layers due to thermal noise. In search of
uncommon effects that might help to explain this behavior, we have performed
diffraction measurements that provide evidence for H(perp)-induced charge and
spin stripe order. The field-induced decoupling of superconducting layers is
similar to the decoupled phase observed previously in La(2-x)Ba(x)CuO(4) with
x=1/8 in zero field. | 1009.0031v4 |
2014-07-03 | Phonon-limited resistivity of graphene by first-principle calculations: electron-phonon interactions, strain-induced gauge field and Boltzmann equation | Electron-phonon coupling in graphene is extensively modeled and simulated
from first principles. We find that using an accurate model for the
polarizations of the acoustic phonon modes is crucial to obtain correct
numerical results. The interactions between electrons and acoustic phonon
modes, the gauge field and deformation potential, are calculated at the DFT
level in the framework of linear response. The zero-momentum limit of acoustic
phonons is interpreted as a strain pattern, allowing the calculation of the
acoustic gauge field parameter in the GW approximation. The role of electronic
screening on the electron-phonon matrix elements is investigated. We then solve
the Boltzmann equation semi-analytically in graphene, including both acoustic
and optical phonon scattering. We show that, in the Bloch-Gr\"uneisen and
equipartition regimes, the electronic transport is mainly ruled by the
unscreened acoustic gauge field, while the contribution due to the deformation
potential is negligible and strongly screened. By comparing with experimental
data, we show that the contribution of acoustic phonons to resistivity is
doping- and substrate-independent. The DFT+GW approach underestimates this
contribution to resistivity by about 30 %. Above 270K, the calculated
resistivity underestimates the experimental one more severely, the
underestimation being larger at lower doping. We show that, beside remote
phonon scattering, a possible explanation for this disagreement is the
electron-electron interaction that strongly renormalizes the coupling to
intrinsic optical-phonon modes. Finally, after discussing the validity of the
Matthiessen rule in graphene, we derive simplified analytical solutions of the
Boltzmann equation to extract the coupling to acoustic phonons, related to the
strain-induced gauge field, directly from experimental data. | 1407.0830v2 |
2015-08-10 | Fracture resistance of zigzag single walled carbon nanotubes | Brittle fracture is one of the important failure modes of Single-Walled
Carbon Nanotube (SWNT) due to mechanical loading. In this paper, the fracture
resistance of zigzag SWNTs with preexisting defects is calculated using
fracture mechanics concepts based on atomistic simulations. The problem of
unstable crack growth at finite temperature, presumably caused by lattice
trapping effect, is circumvented by computing the strain energy release rate
through a series of displacement-controlled tensile loading of SWNTs (applied
through moving the outermost layer of atoms at one end at constant strain rate
of 9.4x10-4/ps) with pre-existing crack-like defects of various lengths. The
strain energy release rate, G, is computed for (17,0), (28,0) and (35,0) SWNTs
(each with aspect ratio 4) with pre-existing cracks up to 29.5{\AA} long. The
fracture resistance, Gc, is determined as a function of crack length for each
tube at three different temperatures (1K, 300K and 500K). A significant
dependence of Gc on crack length is observed reminiscent of the rising R curve
behavior of metals at the macroscale: for the zigzag nanotubes Gc increases
with crack length at small length, and tends to reach a constant value if the
tube diameter is large enough. We suspect that the lattice trapping effect
plays the role of crack tip plasticity at the atomic scale. For example, at 300
Kelvin, Gc for the (35,0) tube with aspect ratio 4 converges to 6 Joule/m2 as
the crack length exceeds 20 Angstrom. This value is comparable with the
fracture toughness of graphite and Silicon. The fracture resistance of the
tubes is found to decrease significantly as the temperature increases. To study
the length effects, the computations are repeated for zigzag nanotubes with the
same three chiralities but with aspect ratio 8 at 1K. | 1508.02129v1 |
2015-11-23 | CPP Magnetoresistance of Magnetic Multilayers: A critical review | We present a comprehensive review of data and analysis of Giant (G)
Magnetoresistance (MR) with Current-flow Perpendicular-to-layer-Planes (CPP-MR)
of magnetic multilayers [F/N]n (n = number of repeats) with alternating
nanoscale layers of ferromagnetic (F) and non-magnetic (N) metals. GMR, a large
change in resistance when an applied magnetic field changes the moment ordering
of adjacent F-layers from anti-parallel (AP) to parallel (P), was discovered in
1988 in the Current-flow-in-layer-Planes (CIP) geometry. The CPP-MR has two
advantages over the CIP-MR: (1) it allows more direct access to the underlying
physics; and (2) it is usually larger, which should be advantageous for
devices. When the first CPP-MR data were published in 1991, it was not clear
whether electronic transport in GMR multilayers is fully diffusive or at least
partly ballistic. It was not known whether the properties of layers and
interfaces would vary with layer thickness or number. It was not known if the
CPP-MR would be dominated by scattering within the F-metals or at the F/N
interfaces. Nothing was known about: (1) spin-flipping within F-metals,
characterized by a spin-diffusion length, l(F)sf; (2) interface specific
resistances (AR = area A times resistance R) for N1/N2 interfaces; (3)
interface specific resistances and spin-dependent scattering asymmetry at F/N
and F1/F2 interfaces; and (4) spin-flipping at F/N, F1/F2 and N1/N2 interfaces.
Knowledge of spin-dependent scattering asymmetries in F-metals and F-alloys,
and of spin-flipping in N-metals and N-alloys was limited. We show how CPP-MR
measurements have quantified the scattering and spin-flipping parameters that
determine CPP-MR for a wide range of F- and N-metals and alloys and of F/N
pairs. We also review progress in finding techniques and F-alloys and F/N pairs
to enhance the CPP-MR to make it more competitive for devices. | 1511.07387v1 |
2016-07-01 | Size-dependence of nanosecond-scale spin-torque switching in perpendicularly magnetized tunnel junctions | We time-resolve the spin-transfer-torque-induced switching in perpendicularly
magnetized tunnel junctions of diameters from 50 to 250 nm in the thermally
activated regime. When the field and the spin-torque concur to favor the P to
AP transition, the reversal yields monotonic resistance ramps that can be
interpreted as a domain wall propagation through the device at velocities of 17
to 30 nm/ns; smaller cells switch hence faster. When the field hinders the P to
AP transition, the switching is preceded by repetitive switching attempts,
during which the resistance transiently increases until successful reversal
occurs. At 50 nm, the P to AP switching proceeds reproducibly in 3 ns, with a
monotonic increase of the device resistance. In the reverse transition (AP to
P), several reversal paths are possible even in the smallest junctions.
Besides, the non uniform nature of the response seems still present at
nanoscale, with sometimes electrical signatures of strong disorder during the
reversal. The AP to P transition is preceded by a strong instability of the AP
state in devices above 100 nm. The resistance becomes extremely agitated before
switching to P in a path yielding a slow (20-50 ns) irregular increase of the
conductance with variability. Unreversed bubbles of 60 nm can persist a few
microseconds in the largest junctions. The complexity of the AP to P switching
is reduced but not suppressed when the junctions are downsized below 60 nm. The
instability of the initial AP state is no longer detected but the other
features remain. In the smallest junctions (50 nm) we occasionally observe much
faster (sub-1 ns) switching events. We discuss the origin of the switching
asymmetry and its size dependence, with an emphasis on the role of the non
uniformities of the stray field emanating from the reference layers, which
affects the zones in which nucleation is favored. | 1607.00260v1 |
2018-03-08 | Helical magnetic structure and the anomalous and topological Hall effects in epitaxial B20 Fe$_{1-y}$Co$_y$Ge films | Epitaxial films of the B20-structure alloy Fe$_{1-y}$Co$_y$Ge were grown by
molecular beam epitaxy on Si (111) substrates. The magnetization varied
smoothly from the bulk-like values of one Bohr magneton per Fe atom for FeGe to
zero for non-magnetic CoGe. The chiral lattice structure leads to a
Dzyaloshinskii-Moriya interaction (DMI), and the films' helical magnetic ground
state was confirmed using polarized neutron reflectometry measurements. The
pitch of the spin helix, measured by this method, varies with Co content $y$
and diverges at $y \sim 0.45$. This indicates a zero-crossing of the DMI, which
we reproduced in calculations using first principle methods. We also measured
the longitudinal and Hall resistivity of our films as a function of magnetic
field, temperature, and Co content $y$. The Hall resistivity is expected to
contain contributions from the ordinary, anomalous, and topological Hall
effects. Both the anomalous and topological Hall resistivities show peaks
around $y \sim 0.5$. Our first principles calculations show a peak in the
topological Hall constant at this value of $y$, related to the strong
spin-polarisation predicted for intermediate values of $y$. Half-metallicity is
predicted for $y = 0.6$, consistent with the experimentally observed linear
magnetoresistance at this composition. Whilst it is possible to reconcile
theory with experiment for the various Hall effects for FeGe, the large
topological Hall resistivities for $y \sim 0.5$ are much larger then expected
when the very small emergent fields associated with the divergence in the DMI
are taken into account. | 1803.03281v2 |
2020-07-09 | Physical properties revealed by transport measurements on superconducting Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ thin films | The newly found superconductivity in infinite-layer nickelate superconducting
films has attracted much attention, because their crystalline and electronic
structures are similar to high-$T_c$ cuprate superconductors. The upper
critical field can provide much information on superconductivity, but detailed
experimental data are still lacking in these films. Here we present temperature
and angle dependence of resistivity measured under different magnetic fields
($H$) in Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ thin films. The onset superconducting
transition occurs at about 16.2 K at 0 T. Temperature dependent upper critical
fields determined by using a criterion very close to the onset transition show
a clear negative curvature near the critical transition temperature, which is
explained as the consequence of the paramagnetically limited effect on
superconductivity. The temperature dependent anisotropy of the upper critical
field is obtained from resistivity data, which yields a value decreasing from 3
to 1.2 with lowering temperature. This can be explained by a variable
contribution from the orbital limit effect on upper critical field. The angle
dependent resistivity at a fixed temperature and different magnetic fields
cannot be scaled to one curve, which deviates from the prediction of the
anisotropic Ginzburg-Landau theory. However, at low temperatures, the increased
resistivity by magnetic field can be scaled by the parameter $H^\beta
|\cos\theta|$ ($1<\beta<6$) with $\theta$ the angle enclosed between $c$-axis
and the applied magnetic field. As the first detailed study on the upper
critical field of the nickelate thin films, our results clearly indicate a
small anisotropy and paramagnetically limited effect of superconductivity in
nickelate superconductors. | 2007.04884v2 |
2020-11-11 | Interface controlled thermal properties of ultra-thin chalcogenide-based phase change memory devices | Phase change memory (PCM) is a rapidly growing technology that not only
offers advancements in storage-class memories but also enables in-memory data
storage and processing towards overcoming the von Neumann bottleneck. In PCMs,
the primary mechanism for data storage is thermal excitation. However, there is
a limited body of research regarding the thermal properties of PCMs at length
scales close to the memory cell dimension and, thus, the impact of interfaces
on PCM operation is unknown. Our work presents a new paradigm to manage thermal
transport in memory cells by manipulating the interfacial thermal resistance
between the phase change unit and the electrodes without incorporating
additional insulating layers. Experimental measurements show a substantial
change in thermal boundary resistance as GST transitions from one
crystallographic structure (cubic) to another (hexagonal) and as the thickness
of tungsten contacts is reduced from five to two nanometers. Simulations reveal
that interfacial resistance between the phase change unit and its adjacent
layer can reduce the reset current for 20 and 120 nm diameter devices by up to
~40% and ~50%, respectively. The resultant phase-dependent and geometric
effects on thermal boundary resistance dictate that the effective thermal
conductivity of the phase change unit can be reduced by a factor of four,
presenting a new opportunity to reduce operating currents in PCMs. | 2011.05492v1 |
2022-03-22 | Effect of ECAP and heat treatment on mechanical properties, stress relaxation behavior and corrosion resistance of a 321-type austenitic steel with increased delta-ferrite content | Hot rolled commercial metastable austenitic steel 0.8C-18Cr-10Ni-0.1Ti
(Russian industrial name 08X18H10T, analog 321L) with strongly elongated thin
delta-ferrite particles in its microstructure was the object of investigations.
The lengths of these delta-particles were up to 500 mkm, the thickness was 10
mkm. The formation of the strain-induced martensite as well as the grinding of
the austenite and of the delta-ferrite grains take place during ECAP. During
the annealing of the UFG steel, the formation of the delta-phase particles
takes place. These particles affect the grain boundary migration and the
strength of the steel. However, a reduction of the Hall-Petch coefficient as
compared to the coarse-grained (CG) steel due to the fragmentation of the
delta-ferrite particles was observed. The samples of the UFG steel were found
to have 2-3 times higher stress relaxation resistance as compared to the CG
steel (a higher macroelasticity stress and a lower stress relaxation
magnitude). The differences in the stress relaxation resistance of the UFG and
CG steels were investigated. ECAP was shown to result in an increase in the
corrosion rate and in an increased tendency to the intergranular corrosion
(IGC). The reduction of the corrosion resistance of the UFG steel was found to
originate from the increase in the fraction of the strain-induced martensite
during ECAP. | 2203.12102v1 |
2022-10-18 | Sensing Remote Bulk Defects Through Resistance Noise in a Large Area Graphene Field Effect Transistor | Substrate plays a crucial role in determining transport and low frequency
noise behavior of graphene field effect devices. Typically, heavily dope
Si/SiO$_2$ substrate is used to fabricate these devices for efficient gating.
Trapping-detrapping processes closed to the graphene/substrate interface are
the dominant sources of resistance fluctuations in the graphene channel, while
Coulomb fluctuations arising due to any remote charge fluctuations inside the
bulk of the substrate are effectively screened by the heavily doped substrate.
Here, we present electronic transport and low frequency noise characteristics
of large area CVD graphene field effect transistor (FET) prepared on a lightly
doped Si/SiO$_2$ substrate (N$_A$ $\sim$ 10$^{15}$cm$^{-3}$). Through a
systematic characterization of transport, noise and capacitance at various
temperature, we reveal that remote Si/SiO$_2$ interface can affect the charge
transport in graphene severely and any charge fluctuations inside bulk of the
silicon substrate can be sensed by the graphene channel. The resistance (R) vs.
back gate voltage (V$_{bg}$) characteristics of the device shows a hump around
the depletion region formed at the SiO$_2$/Si interface, confirmed by the
capacitance (C) - Voltage (V) measurement. Low frequency noise measurement on
these fabricated devices shows a peak in the noise amplitude close to the
depletion region. This indicates that due to the absence of any charge layer at
Si/SiO$_2$ interface, screening ability decreases and as a consequence, any
fluctuations in the deep level coulomb impurities inside the silicon substrate
can be observed as a noise in resistance in graphene channel via mobility
fluctuations. Noise behavior on ionic liquid gated graphene on the same
substrate exhibits no such peak in noise and can be explained by the
interfacial trapping - detrapping processes closed to the graphene channel. | 2210.09851v1 |
2023-11-24 | Multi-tap Resistive Sensing and FEM Modeling enables Shape and Force Estimation in Soft Robots | We address the challenge of reliable and accurate proprioception in soft
robots, specifically those with tight packaging constraints and relying only on
internally embedded sensors. While various sensing approaches with single
sensors have been tried, often with a constant curvature assumption, we look
into sensing local deformations at multiple locations of the sensor. In our
approach, we multi-tap an off-the-shelf resistive sensor by creating multiple
electrical connections onto the resistive layer of the sensor and we insert the
sensor into a soft body. This modification allows us to measure changes in
resistance at multiple segments throughout the length of the sensor, providing
improved resolution of local deformations in the soft body. These measurements
inform a model based on a finite element method (FEM) that estimates the shape
of the soft body and the magnitude of an external force acting at a known
arbitrary location. Our model-based approach estimates soft body deformation
with approximately 3% average relative error while taking into account internal
fluidic actuation. Our estimate of external force disturbance has an 11%
relative error within a range of 0 to 5 N. The combined sensing and modeling
approach can be integrated, for instance, into soft manipulation platforms to
enable features such as identifying the shape and material properties of an
object being grasped. Such manipulators can benefit from the inherent softness
and compliance while being fully proprioceptive, relying only on embedded
sensing and not on external systems such as motion capture. Such proprioception
is essential for the deployment of soft robots in real-world scenarios. | 2311.14566v1 |
2024-04-05 | Direct Electrical Detection of Spin Chemical Potential Due to Spin Hall Effect in $β$-Tungsten and Platinum Using a Pair of Ferromagnetic and Normal Metal Voltage probes | The phenomenon of Spin Hall Effect (SHE) generates a pure spin current
transverse to an applied current in materials with strong spin-orbit coupling,
although not detectable through conventional electrical measurement. An
intuitive Hall effect like measurement configuration is implemented to directly
measure pure spin chemical potential of the accumulated spins at the edges of
heavy metal (HM) channels that generates large SHE. A pair of transverse
linearly aligned voltage probes in placed in ohmic contact with the top surface
of HM , one being a ferromagnetic metal (FM) with non-zero spin polarization
and other is the reference metal (RM) with zero polarization of carriers. This
combination of FM/RM electrodes is shown to induce an additional voltage
proportional to a spin accumulation potential, which is anti symmetric with
respect to opposite orientations of FM controlled by a 2D vector magnet. Proof
of concept of the measurement scheme is verified by comparing the signs of
voltages for HM channels of Tungsten (W) and Platinum (Pt) which are known to
generate opposite spin accumulation under similar conditions of applied
current. The same devices are also able to detect the reciprocal effect,
inverse spin Hall effect (ISHE) by swapping the current and voltage leads and
the results are consistent with reciprocity principle. Further, exploiting a
characteristic feature of W thin film deposition, a series of devices were
fabricated with W resistivity varying over a wide range of 10 - 750 $\mu
\Omega$-cm and the calculated spin Hall resistivity exhibits a pronounced power
law dependence on resistivity. Our measurement scheme combined with almost two
decades of HM resistivity variation provides the ideal platform required to
test the underlying microscopic mechanism responsible for SHE/ISHE. | 2404.03934v1 |
2013-02-11 | Feedback from Winds and Supernovae in Massive Stellar Clusters. I: Hydrodynamics | We use 3D hydrodynamical models to investigate the effects of massive star
feedback from winds and supernovae on inhomogeneous molecular material left
over from the formation of a massive stellar cluster. We simulate the
interaction of the mechanical energy input from a cluster with 3 O-stars into a
giant molecular cloud (GMC) clump containing 3240 solar masses of molecular
material within a 4 pc radius. The cluster wind blows out of the molecular
clump along low-density channels, into which denser clump material is
entrained. We find that the densest molecular regions are surprisingly
resistant to ablation by the cluster wind, in part due to shielding by other
dense regions closer to the cluster. Nonetheless, molecular material is
gradually removed by the cluster wind during which mass-loading factors in
excess of several 100 are obtained. Because the clump is very porous, 60-75 per
cent of the injected wind energy escapes the simulation domain, with the
difference being radiated. After 4.4 Myr, the massive stars in our simulation
begin to explode as supernovae. The highly structured environment into which
the SN energy is released allows even weaker coupling to the remaining dense
material and practically all of the SN energy reaches the wider environment.
The molecular material is almost completely dispersed and destroyed after 6
Myr. The escape fraction of ionizing radiation is estimated to be about 50 per
cent during the first 4 Myr of the cluster's life. A similar model with a
larger and more massive GMC clump reveals the same general picture, though more
time is needed for it to be destroyed. | 1302.2443v1 |
2016-12-22 | Insight into the Role of Oxygen in Phase-Change Material GeTe | Oxygen is widely used to tune the performance of chalcogenide phase-change
materials in the usage of phase-Change random access memory (PCRAM) which is
considered as the most promising next-generation non-volatile memory. However,
the microscopic role of oxygen in the write-erase process, i.e., the reversible
phase transition between crystalline and amorphous state of phase-change
materials is not clear yet. Using oxygen doped GeTe as an example, this work
unravels the role of oxygen at the atomic scale by means of ab initio total
energy calculations and ab initio molecular dynamics simulations. Our main
finding is that after the amorphization and the subsequent re-crystallization
process simulated by ab initio molecular dynamics, oxygen will drag one Ge atom
out of its lattice site and both atoms stay in the interstitial region near the
Te vacancy that was originally occupied by the oxygen, forming a
"dumbbell-like" defect (O-VTe-Ge), which is in sharp contrast to the results of
ab initio total energy calculations at 0 K showing that the oxygen prefers to
substitute Te in crystalline GeTe. This specific defect configuration is found
to be responsible for the slower crystallization speed and hence the improved
data retention of oxygen doped GeTe as reported in recent experimental work.
Moreover, we find that the oxygen will increase the effective mass of the
carrier and thus increases the resistivity of GeTe. Our results unravel the
microscopic mechanism of the oxygen-doping optimization of phase-change
material GeTe, and the present reported mechanism can be applied to other
oxygen doped ternary chalcogenide phase-change materials. | 1612.07467v1 |
2013-12-20 | Lessons from a Large-Scale Assessment: Results from Conceptual Inventories | We report the conceptual inventory results of a large-scale assessment
project at a large university. We studied an attempt at introducing materials
and instructional methods informed by physics education research (PER-informed
materials) into a department where most instruction has been traditional and a
significant number of faculty are hesitant, ambivalent or even resistant about
the introduction of such reforms. The changes were made in the laboratories and
recitation sections of the introductory classes, both calculus-based and
algebra-based, introducing PER-informed materials and training the teaching
assistants in student-centered instructional methods. In addition to the
results found in the large lecture classes, we present the results of a small
PER-informed, inquiry-based, laboratory-based class that has been taught as a
special section of the algebra-based course for about 10 years. The assessment
reported in this paper was done using available PER-developed assessment
instruments. The results of other assessment instruments used in the project,
such as free-response pre- and post-tests, are reported in subsequent papers.
The results in this paper inform researchers in PER of the use of PER-informed
materials and instructional methods in a department not unified in the
introduction and implementation of these materials and the results of the
implementation as assessed by PER-based conceptual inventories. We found that
our conceptual inventory scores were lower than many results reported elsewhere
in the literature. However, we did see a statistically significant increase in
the conceptual inventory scores with the implementation of PER-informed
laboratories and the use of student-centered pedagogy in the labs and
recitations. The increase was much greater, if the lecture instructor also used
PER-informed materials. | 1312.6000v1 |
2017-11-19 | Elastomeric focusing enables application of hydraulic principles to solid materials in order to create micromechanical actuators with giant displacements | A continuing challenge in material science is how to create active materials
in which shape changes or displacements can be generated electrically or
thermally. Here we borrow principles from hydraulics, in particular that
confined geometries can be used to focus expansion into large displacements, to
create solid materials with amplified shape changes. Specifically, we confined
an elastomeric poly(dimethylsiloxane) sheet between two more rigid layers and
caused focused expansion into embossed channels by local resistive heating,
resulting in a 10x greater relative displacement than the unconfined geometry.
We used this effect to create electrically controlled microfluidic valves that
open and close in less than 100 ms, can cycle >10,000 times, and operate with
as little as 20 mW of power. We investigate this mechanism and establish design
rules by varying dimensions, configurations, and materials. We show the
generality of elastomeric focusing by creating additional devices where local
heating and expansion are generated either wirelessly through inductive
coupling or optically with a laser, allowing arbitrary and dynamic positioning
of a microfluidic valve along the channels. | 1711.07102v1 |
2020-04-29 | Superconducting Materials for Microwave Kinetic Inductance Detectors | The superconducting materials that make up an MKID have a significant effect
on its performance. The $T_\textrm{c}$ and normal state resistivity
$\rho_\textrm{N}$ of the film determine the penetration depth $\lambda$ and
therefore how much kinetic inductance it has. The ratio of kinetic inductance
to total inductance ($\alpha$), the volume of the inductor, and $Q_\textrm{m}$
determines the magnitude of the response to incoming energy. The quasiparticle
lifetime $\tau_\textrm{qp}$ is the characteristic time during which the MKID's
surface impedance is modified by the incoming energy. Many materials have been
explored for use in superconducting resonators and MKIDs, but that information
is often not published or scattered around the literature. This chapter
contains information and references on the work that has been done with thin
film lithographed circuits for MKIDs over the last two decades. Note that
measured material properties such as the internal loss quality factor
$Q_\textrm{i}$ and quasiparticle lifetime $\tau_\textrm{qp}$ vary significantly
depending on how the MKID superconducting thin film is made and the system they
are measured in, so it is best to interpret all stated values as typical but
not definitive. Values are omitted in cases when there aren't enough
measurements or there is too much disagreement in the literature to estimate a
typical value. In order to be as complete as possible some unpublished results
from the author's lab are included and can be identified by the lack of a
reference. Unless noted all films are polycrystalline or amorphous. | 2004.14576v1 |
2018-12-20 | Dynamics of Fluid Driven Autonomous Materials: Interconnected Fluid Filled Cavities to Realize Autonomous Materials | The study of elastic structures embedded with fluid-filled cavities received
considerable attention in fields such as autonomous materials, sensors,
actuators, and smart systems. This work studies an elastic beam embedded with a
set of fluid-filled bladders, similar to a honeycomb structure, which are
interconnected via an array of slender tubes. The configuration of the
connecting tubes is arbitrary, and each tube may connect any two bladders. Beam
deformation both creates, and is induced by, the internal viscous flow- and
pressure-fields which deform the bladders and thus the surrounding solid.
Applying concepts from poroelasticity, and leveraging Cosserat beam
large-deformation models, we obtain a set of three coupled equations relating
the fluidic pressure within the bladders to the large transverse and
longitudinal displacements of the beam. We show that by changing the viscous
resistance of the connecting tubes we are able to modify the amplitude of
oscillatory deformation modes created due to external excitations on the
structure. In addition, rearranging tube configuration in a given bladder
system is shown to add an additional degree of control, and generate varying
mode shapes for the same external excitation. The presented modified Cosserat
model is applied to analyze a previously suggested energy harvester
configuration and estimate the efficiency of such a device. The results of this
work are validated by a transient three-dimensional numerical study of the full
fluid-structure-interaction problem. The presented model allows for the
analysis and design of soft smart-metamaterials with unique mechanical
properties.
Keywords: autonomous materials, adaptive materials, programmed materials,
smart systems, autonomous systems, soft matter, soft robotics, energy
harvesting, fluid dynamics, fluid-structure interaction, large deformation. | 1812.08717v2 |
2019-01-14 | Anomalous relation between in-plane and out-of-plane stiffnesses in 2D networked materials | For thin networked materials, which are spatial discrete structures
constructed by continuum components, a paradox on the effective thickness
defined by the in-plane and out-of-plane stiffnesses is found, i.e. the
effective thickness is not a constant but varies with loading modes. To reveal
the mechanism underneath the paradox, we have established a micromechanical
framework to investigate the deformation mechanism and predict the stiffness
matrix of the networked materials. It is revealed that the networked materials
can carry in-plane loads by axial stretching/compression of the components in
the networks and resist out-of-plane loading by bending and torsion of the
components. The bending deformation of components has a corresponding relation
to the axial stretching/compression through the effective thickness, as the
continuum plates do, while the torsion deformation has no relation to the axial
stretching/compression. The isolated torsion deformation breaks the classical
stiffness relation between the in-plane stiffness and the out-of-plane
stiffness, which can even be further distorted by the stiffness threshold
effect in randomly networked materials. Accordingly, a new formula is
summarized to describe the anomalous stiffness relation. This network model can
also apply in atomic scale 2D nanomaterials when combining with the molecular
structural mechanics model. This work gives an insight into the understanding
of the mechanical properties of discrete materials/structures ranging from
atomic scale to macro scale. | 1901.04203v1 |
2019-10-16 | Stumbling Through the Research Wilderness, Standard Methods to Shine Light on Electrically Conductive Nanocomposites for Future Health-Care Monitoring | Electrically conductive nanocomposites are an exciting ever expanding area of
research that has yielded many new technologies for wearable health devices.
Acting as strain sensing materials, they have paved the way towards real time
medical diagnostic tools that may very well lead to a golden age of healthcare.
Currently, the goal in research is to create a material that simultaneously has
both a large gauge factor G and sensing range. However, a weakness in the area
of electromechanical research is the lack of standardisation in the reporting
of the figure of merit, i.e. G, and the need for new metrics to give
researchers a more complete view of the research landscape of resistive type
sensors. A paradigm shift in the way in which data is reported is required, to
push research in the right direction and to facilitate achieving research
goals. Here, we report a standardised method for reporting strain sensing
performance and the introduction of the working factor W and the Young's
modulus Y of a material as two new material criteria. Using this new method, we
can now for the first time define the benchmarks for an optimum sensing
material, G > 7, W > 1, Y < 300 kPa, using limits set by standard commercial
materials and the human body. Using extrapolated data from 200 publications
normalised to this standard method, we can review what composite types meet
these benchmark limits, what governs composite performances, the literary
trends in composites and individual nanomaterial performance and the future
prospects of research. | 1910.07249v1 |
2022-04-11 | Electron transport in the single-layer semiconductor | Two-dimensional (2D) materials are a new class of materials with interesting
physical properties and applications ranging from nanoelectronics to sensing
and photonics. In addition to graphene, the most studied 2D material,
monolayers of other layered materials such as semiconducting dichalcogenides
MoS2 or WSe2 are gaining in importance as promising channel materials for
field-effect transistors (FETs) and phototransistors. However, it is unclear
that how the specific process of electron transport is affected by temperature.
So, nowadays the electron dynamics of single-layer semiconductor cannot be
understood fundamentally. Here, we develop an analytical theory distinguishing
from traditional energy band theory, backed up by Monte-Carlo simulations, that
predicts the process of electron transport and the effect of temperature on the
electron transport in the single-layer semiconductor. In this paper, A new
model is built to deal with electron transporting in the sing-layer
semiconductor. The resistance is decided by the barrier rather than the
electron scattering in the single-layer semiconductor, which is macroscopic
quantum effect. Electron transport in FETs with different dielectric
configurations are investigated at different temperatures and a new control
factor that is decided by top-gate voltage or bottom-gate voltage is introduced
to describe the effect of gate voltage on the electron transport in 2D
semiconductor. The results of simulation show the drain current is mainly
determined by some elements, such as temperature, top-gate voltage, bottom-gate
voltage and source-drain voltage. | 2204.04850v1 |
2022-12-15 | Physics-Informed Neural Networks for Material Model Calibration from Full-Field Displacement Data | The identification of material parameters occurring in constitutive models
has a wide range of applications in practice. One of these applications is the
monitoring and assessment of the actual condition of infrastructure buildings,
as the material parameters directly reflect the resistance of the structures to
external impacts. Physics-informed neural networks (PINNs) have recently
emerged as a suitable method for solving inverse problems. The advantages of
this method are a straightforward inclusion of observation data. Unlike
grid-based methods, such as the least square finite element method (LS-FEM)
approach, no computational grid and no interpolation of the data is required.
In the current work, we propose PINNs for the calibration of constitutive
models from full-field displacement and global force data in a realistic regime
on the example of linear elasticity. We show that conditioning and
reformulation of the optimization problem play a crucial role in real-world
applications. Therefore, among others, we identify the material parameters from
initial estimates and balance the individual terms in the loss function. In
order to reduce the dependency of the identified material parameters on local
errors in the displacement approximation, we base the identification not on the
stress boundary conditions but instead on the global balance of internal and
external work. We demonstrate that the enhanced PINNs are capable of
identifying material parameters from both experimental one-dimensional data and
synthetic full-field displacement data in a realistic regime. Since
displacement data measured by, e.g., a digital image correlation (DIC) system
is noisy, we additionally investigate the robustness of the method to different
levels of noise. | 2212.07723v2 |
2022-07-15 | Screening 0D materials for 2D nanoelectronics applications | As nanoelectronic devices based on two-dimensional (2D) materials are moving
towards maturity, optimization of the properties of the active 2D material must
be accompanied by equal attention to optimizing the properties of and the
interfaces to the other materials around it, such as electrodes, gate
dielectrics, and the substrate. While these are usually either 2D or 3D
materials, recently K. Liu et al. [Nat. Electron. 4, 906 (2021)] reported on
the use of zero-dimensional (0D) material, consisting of vdW-bonded Sb$_2$O$_3$
clusters, as a highly promising insulating substrate and gate dielectric. Here,
we report on computational screening study to find promising 0D materials for
use in nanoelectronics applications, in conjunction with 2D materials in
particular. By combining a database and literature searches, we found 16
materials belonging to 6 structural prototypes with high melting points and
high band gaps, and a range of static dielectric constants. We carried out
additional first-principles calculations to evaluate selected technologically
relevant material properties, and confirmed that all these materials are van
der Waals-bonded, thus allowing for facile separation of 0D clusters from the
3D host and also weakly perturbing the electronic properties of the 2D material
after deposition. | 2207.07364v1 |
2015-08-07 | Pressure Effects on Superconducting Properties of the BiS2-Based Superconductor Bi2(O,F)S2 | Pressure effects on a recently discovered BiS2-based superconductor
Bi2(O,F)S2 (Tc = 5.1 K) were examined via two different methods; high pressure
resistivity measurement and high pressure annealing. The effects of these two
methods on the superconducting properties of Bi2(O,F)S2 were significantly
different although in both methods hydrostatic pressure is applied to the
sample by the cubic-anvil-type apparatus. In high pressure resistivity
measurement, Tc linearly decreased at the rate of -1.2 K GPa-1. In contrast,
the Tc of 5.1 K is maintained after high pressure annealing under 2 GPa and
470{\deg}C of optimally doped sample despite significant change of lattice
parameters. In addition, superconductivity was observed in fluorine-free Bi2OS2
after high pressure annealing. These results suggest that high pressure
annealing would cause a unique effect on physical properties of layered
compounds. | 1508.01656v1 |
2018-09-11 | Atomic positions independent descriptor for machine learning of material properties | The high-throughput screening of periodic inorganic solids using machine
learning methods requires atomic positions to encode structural and
compositional details into appropriate material descriptors. These atomic
positions are not available {\it a priori} for new materials which severely
limits exploration of novel materials. We overcome this limitation by using
only crystallographic symmetry information in the structural description of
materials. We show that for materials with identical structural symmetry,
machine learning is trivial and accuracies similar to that of density
functional theory calculations can be achieved by using only atomic numbers in
the material description. For machine learning of formation energies of bulk
crystalline solids, this simple material descriptor is able to achieve
prediction mean absolute errors of only 0.07 eV/atom on a test dataset
consisting of more than 85,000 diverse materials. This atomic-position
independent material descriptor presents a new route of materials discovery
wherein millions of materials can be screened by training a machine learning
model over a drastically reduced subspace of materials. | 1809.03960v2 |
2019-01-14 | Materials discovery and properties prediction in thermal transport via materials informatics: a mini-review | There has been an increasing demand for materials with special thermal
properties, whereas experimental discovery is high-cost and time-consuming. The
emerging discipline `Materials Informatics' is an effective approach that can
accelerate materials development by combining material science and big data
technique. Recently materials informatics has been applied to the design of
novel materials such as thermal interface materials for heat-dissipation, and
thermoelectric materials for power generation. This mini-review summarized the
research progress on the applications of materials informatics for the thermal
transport properties prediction and discovery of materials with special thermal
properties, including optimal thermal conductivity, interfacial thermal
conductance and thermoelectricity efficiency. In addition, some perspectives
are given for the outlook of materials informatics in the field of thermal
transport. | 1901.04133v1 |
2024-02-07 | Are LLMs Ready for Real-World Materials Discovery? | Large Language Models (LLMs) create exciting possibilities for powerful
language processing tools to accelerate research in materials science. While
LLMs have great potential to accelerate materials understanding and discovery,
they currently fall short in being practical materials science tools. In this
position paper, we show relevant failure cases of LLMs in materials science
that reveal current limitations of LLMs related to comprehending and reasoning
over complex, interconnected materials science knowledge. Given those
shortcomings, we outline a framework for developing Materials Science LLMs
(MatSci-LLMs) that are grounded in materials science knowledge and hypothesis
generation followed by hypothesis testing. The path to attaining performant
MatSci-LLMs rests in large part on building high-quality, multi-modal datasets
sourced from scientific literature where various information extraction
challenges persist. As such, we describe key materials science information
extraction challenges which need to be overcome in order to build large-scale,
multi-modal datasets that capture valuable materials science knowledge.
Finally, we outline a roadmap for applying future MatSci-LLMs for real-world
materials discovery via: 1. Automated Knowledge Base Generation; 2. Automated
In-Silico Material Design; and 3. MatSci-LLM Integrated Self-Driving Materials
Laboratories. | 2402.05200v1 |
1997-11-19 | Inter-Band Pairing Theory of Superconductivity | A model for high temperature superconductors based on the idea of Cooper
pairs comprised of electrons from different bands is studied. We propose that
the two bands relevant for the cuprates are comprised of Cu dx2-y2, dz2, planar
O psigma, and apical O pz orbitals. Along the diagonal, kx=ky in the Brillouin
zone, the two band Fermi surfaces may cross. We associate the optimal doping
for the highest Tc with this point because only in the vicinity of this
touching point are inter-band Cooper pairs energetically possible. Due to the
lack of time reversal invariance of an inter-band Cooper pair with itself, the
standard interpretation of Josephson tunneling is altered such that the
detailed nature of the single particle tunneling matrix elements contributes to
the supercurrent. The dx2-y2 gap observations from Josephson tunneling are
shown to arise from our model with pairing due to phonons. A Hubbard model is
written down for the two bands at the Fermi energy with realistic parameters
for LaSrCuO. The anomalous normal state features in the nmr are calculated and
qualitatively explained as due to the character of the two bands in the
vicinity of the crossing point. The Hall effect is calculated using standard
Bloch-Boltzmann transport theory. The observed strong temperature dependence of
the Hall coefficient is reproduced and is due to the strong reshaping of the
current carrying band Fermi surface due to band repulsion with the other band
for dopings very close to the Fermi surface touching point. Reasonable
quantitative agreement is also obtained for the nmr and Hall effect. A linear
resistivity at optimal doping is expected due to the proximity of the second
band in k space which can strongly relax the current and the "smallness" of the
current carrying Fermi surface. | 9711170v2 |
2001-02-01 | The electronic state of vortices in YBa2Cu3Oy investigated by complex surface impedance measurement | The electromagnetic response to microwaves in the mixed state of
YBa2Cu3Oy(YBCO) was measured in order to investigate the electronic state
inside and outside the vortex core. The magnetic-field dependence of the
complex surface impedance at low temperatures was in good agreement with a
general vortex dynamics description assuming that the field-independent viscous
damping force and the linear restoring force were acting on the vortices. In
other words, both real and imaginary parts of the complex resistivity, \rho_1,
and \rho_2, were linear in B. This is explained by theories for d-wave
superconductors. Using analysis based on the Coffey-Clem description of the
complex penetration depth, we estimated that the vortex viscosity \eta at 10 K
was (4 \sim 5) \times 10^{-7} Ns/m^2. This value corresponds to \omega_0 \tau
\sim 0.3 - 0.5, where \omega_0 and \tau are the minimal gap frequency and the
quasiparticle lifetime in the vortex core, respectively. These results suggest
that the vortex core in YBCO is in the moderately clean regime. Investigation
of the moderately clean vortex core in high-temperature superconductors is
significant because physically new effects may be expected due to d-wave
characteristics and to the quantum nature of cuprate superconductors. The
behavior of Z_s as a function of B across the first order transition (FOT) of
the vortex lattice was also investigated. Unlike Bi2Sr2CaCu2Oy (BSCCO), no
distinct anomaly was observed around the FOT in YBCO. Our results suggest that
the rapid increase of X_s due to the change of superfluid density at the FOT
would be observed only in highly anisotropic two-dimensional vortex systems
like BSCCO. We discuss these results in terms of the difference of the
interlayer coupling and the energy scale between the two materials. | 0102021v2 |
2010-06-23 | Physical properties of FeSe$_{0.5}$Te$_{0.5}$ single crystals grown under different conditions | We report on structural, magnetic, conductivity, and thermodynamic studies of
FeSe$_{0.5}$Te$_{0.5}$ single crystals grown by self-flux and Bridgman methods.
The samples were prepared from starting materials of different purity at
various temperatures and cooling rates. The lowest values of the susceptibility
in the normal state, the highest transition temperature $T_c$ of 14.5 K, and
the largest heat-capacity anomaly at $T_c$ were obtained for pure (oxygen-free)
samples. The critical current density $j_c$ of $8 \times 10^4$ A/cm$^2$ (at 2
K) achieved in pure samples is attributed to intrinsic inhomogeneity due to
disorder at the cation and anion sites. The impure samples show increased $j_c$
up to $2.3 \times 10^5$ A/cm$^2$ due to additional pinning centers of
Fe$_3$O$_4$. The upper critical field $H_{c2}$ of $\sim 500$ kOe is estimated
from the resistivity study in magnetic fields parallel to the \emph{c}-axis.
The anisotropy of the upper critical field $\gamma_{H_{c2}} =
H_{_{c2}}^{ab}/H_{_{c2}}^{c}$ reaches a value $\sim 6$ at $T\longrightarrow
T_c$. Extremely low values of the residual Sommerfeld coefficient for pure
samples indicate a high volume fraction of the superconducting phase (up to
97%). The electronic contribution to the specific heat in the superconducting
state is well described within a single-band BCS model with a temperature
dependent gap $\Delta_0 = 27(1)$ K. A broad cusp-like anomaly in the electronic
specific heat of samples with suppressed bulk superconductivity is ascribed to
a splitting of the ground state of the interstitial Fe$^{2+}$ ions. This
contribution is fully suppressed in the ordered state in samples with bulk
superconductivity. | 1006.4453v2 |
2011-01-25 | Extreme 54Cr-rich nano-oxides in the CI chondrite Orgueil -Implication for a late supernova injection into the Solar System | Systematic variations in 54Cr/52Cr ratios between meteorite classes (Qin et
al., 2010a; Trinquier et al., 2007) point to large scale spatial and/or
temporal isotopic heterogeneity in the solar protoplanetary disk. Two
explanations for these variations have been proposed, with important
implications for the formation of the Solar System: heterogeneous seeding of
the disk with dust from a supernova, or energetic-particle irradiation of dust
in the disk. The key to differentiating between them is identification of the
carrier(s) of the 54Cr anomalies. Here we report the results of our recent
NanoSIMS imaging search for the 54Cr-rich carrier in the acid-resistant residue
of the CI chondrite Orgueil. A total of 10 regions with extreme 54Cr-excesses
({\delta}54Cr values up to 1500 %) were found. Comparison between SEM, Auger
and NanoSIMS analyses showed that these 54Cr-rich regions are associated with
one or more sub-micron (typically less than 200 nm) Cr oxide grains, most
likely spinels. Because the size of the NanoSIMS primary O- ion beam is larger
than the typical grain size on the sample mount, the measured anomalies are
lower limits, and we estimate that the actual 54Cr enrichments in three grains
are at least 11 times Solar and in one of these may be as high as 50 times
Solar. Such compositions strongly favor a Type II supernova origin. The
variability in bulk 54Cr/52Cr between meteorite classes argues for a
heterogeneous distribution of the 54Cr carrier in the solar protoplanetary disk
following a late supernova injection event. Such a scenario is also supported
by the O-isotopic distribution and variable abundances in different planetary
materials of other presolar oxide and silicate grains from supernovae. | 1101.4949v1 |
2011-12-02 | Surviving the hole I: Spatially resolved chemistry around Sgr A* | The interstellar region within the few central parsecs around the
super-massive black hole, Sgr A* at the very Galactic center is composed by a
number of overlapping molecular structures which are subject to one of the most
hostile physical environments in the Galaxy. We present high resolution
(4"x3"~0.16x0.11 pc) interferometric observations of CN, 13CN, H2CO, SiO,
c-C3H2 and HC3N emission at 1.3 mm towards the central ~4 pc of the Galactic
center region. Strong differences are observed in the distribution of the
different molecules. The UV resistant species CN, the only species tracing all
previously identified circumnuclear disk (CND) structures, is mostly
concentrated in optically thick clumps in the rotating filaments around Sgr A*.
H2CO emission traces a shell-like structure that we interpret as the expansion
of Sgr A East against the 50 km/s and 20 km/s giant molecular clouds (GMCs). We
derive isotopic ratios 12C/13C~15-45 across most of the CND region. The densest
molecular material, traced by SiO and HC3N, is located in the southern CND. The
observed c-C3H2/HC3N ratio observed in the region is more than an order of
magnitude lower than in Galactic PDRs. Toward the central region only CN was
detected in absorption. Apart from the known narrow line-of-sight absorptions,
a 90 km/s wide optically thick spectral feature is observed. We find evidences
of an even wider (>100 km/s) absorption feature. Around 70-75% of the gas mass,
concentrated in just the 27% densest molecular clumps, is associated with
rotating structures and show evidences of association with each of the arcs of
ionized gas in the mini-spiral structure. Chemical differentiation has been
proven to be a powerful tool to disentangle the many overlapping molecular
components in this crowded and heavily obscured region. | 1112.0566v1 |
2012-09-17 | Transport, Thermal, and Magnetic Properties of the Narrow-Gap Semiconductor CrSb2 | Resistivity, Hall effect, Seebeck coefficient, thermal conductivity, heat
capacity, and magnetic susceptibility data are reported for CrSb2 single
crystals. In spite of some unusual features in electrical transport and Hall
measurements below 100 K, only one phase transition is found in the temperature
range from 2 to 750 K corresponding to long-range antiferromagnetic order below
T_N ~ 273 K. Many of the low temperature properties can be explained by the
thermal depopulation of carriers from the conduction band into a low mobility
band located approximately 16 meV below the conduction band edge, as deduced
from the Hall effect data. In analogy with what occurs in Ge, the low mobility
band is likely an impurity band. The Seebeck coefficient, S, is large and
negative for temperatures from 2 to 300 K ranging from ~ -70\muV/K at 300 K to
-4500\muV/K at 18 K. A large maximum in |S| at 18 K is likely due to phonon
drag with the abrupt drop in |S| below 18 K due to the thermal depopulation of
the high mobility conduction band. The large thermal conductivity between 10
and 20 K (~350 W/m/K) is consistent with this interpretation, as are detailed
calculations of the Seebeck coefficient made using the complete calculated
electronic structure. These data are compared to data reported for FeSb2, which
crystallizes in the same marcasite structure, and FeSi, another unusual
narrow-gap semiconductor. | 1209.3676v2 |
2013-11-13 | Thermodynamic and transport properties of single crystalline RCo$_{2}$Ge$_{2}$ (R = Y, La-Nd, Sm-Tm) | Single crystals of RCo$_{2}$Ge$_{2}$ (R = Y, La-Nd, Sm-Tm) were grown using a
self-flux method and were characterized by room-temperature powder x-ray
diffraction; anisotropic, temperature and field dependent magnetization;
temperature and field dependent, in-plane resistivity; and specific heat
measurements. In this series, the majority of the moment-bearing members order
antiferromagnetically; YCo$_{2}$Ge$_{2}$ and LaCo$_{2}$Ge$_{2}$ are
non-moment-bearing. Ce is trivalent in CeCo$_{2}$Ge$_{2}$ at high temperatures,
and exhibits an enhanced electronic specific heat coefficient due to Kondo
effect at low temperatures. In addition, CeCo$_{2}$Ge$_{2}$ shows two
low-temperature anomalies in temperature-dependent magnetization and specific
heat measurements. Three members (R = Tb-Ho) have multiple phase transitions
above 1.8 K. Eu appears to be divalent with total angular momentum L = 0. Both
EuCo$_{2}$Ge$_{2}$ and GdCo$_{2}$Ge$_{2}$ manifest essentially isotropic
paramagnetic properties consistent with J = S = 7/2. Clear magnetic anisotropy
for rare-earth members with finite L was observed, with ErCo$_{2}$Ge$_{2}$ and
TmCo$_{2}$Ge$_{2}$ manifesting planar anisotropy and the rest members
manifesting axial anisotropy. The experimentally estimated crystal electric
field (CEF) parameters B$_{2}^{0}$ were calculated from the anisotropic
paramagnetic $\theta_{ab}$ and $\theta_{c}$ values and follow a trend that
agrees well with theoretical predictions. The ordering temperatures, T$_{N}$,
as well as the polycrystalline averaged paramagnetic Curie-Weiss temperature,
$\Theta_{avg}$, for the heavy rare-earth members deviate from the de Gennes
scaling, as the magnitude of both are the highest for Tb, which is sometimes
seen for extremely axial systems. Except for SmCo$_{2}$Ge$_{2}$, metamagnetic
transitions were observed at 1.8 K for all members that ordered
antiferromagnetically. | 1311.3321v1 |
2014-07-19 | Ferromagnetic cluster spin-glass behavior in PrRhSn3 | We report the synthesis, structure, and magnetic and transport properties of
a new ternary intermetallic compound PrRhSn3 which crystallizes in LaRuSn3-type
cubic structure (space group Pm-3n). At low applied fields the dc magnetic
susceptibility exhibits a sharp anomaly below 6~K with an irreversible behavior
in zero field cooled (ZFC) and field cooled (FC) susceptibility below 5.5 K.
The ac susceptibility exhibits a frequency dependent anomaly revealing a
spin-glass behavior with a freezing temperature, T_f = 4.3 K. The observation
of spin-glass behavior is further supported by a very slow decay of
thermo-remnant magnetization (mean relaxation time tau = 2149 s). However, a
small jump at very low field in the isothermal magnetization at 2 K and a weak
anomaly in the specific heat near 5.5 K reveal the presence of ferromagnetic
clusters. The frequency dependence of the transition temperature T_f in the ac
susceptibility obeys the Vogel-Fulcher law, nu = nu_0exp[-E_a/k_B(T_f-T_0)]
with activation energy E_a/k_B = 19.1 K. This together with an intermediate
value of the parameter delta T_f = Delta T_f/T_f Delta(log nu) = 0.086 provide
an evidence for the formation of a cluster-glass state in PrRhSn3. The magnetic
contribution of the specific heat reveals a broad Schottky-type anomaly
centered around 10 K and the analysis based on the crystal electric field model
indicates a singlet ground state. Further, below T_f the magnetic part of the
specific heat exhibits a T^{3/2} temperature dependence. The strong influence
of the crystal electric field and a T^{3/2} temperature dependence are also
seen in the electrical resistivity which reveals a metallic character and a
high magnetoresistance. We also obtain a surprisingly large value of
Sommerfeld-Wilson ratio R_W ~ 247$. | 1407.5201v1 |
2015-10-01 | Are quantum spin Hall edge modes more resilient to disorder, sample geometry and inelastic scattering than quantum Hall edge modes? | On the surface of 2D Topological insulators occur 1D quantum spin Hall(QSH)
edge modes with Dirac like dispersion. Unlike quantum Hall(QH) edge modes which
occur at high magnetic fields in 2DEGs, the occurrence of QSH edge modes is
because of spin-orbit scattering in the bulk of the material. These QSH edge
modes are spin dependent and chiral- opposite spins move in opposing
directions. Electronic spin has larger decoherence and relaxation time than
charge- in view of this its expected that QSH edge modes will be more robust to
disorder and inelastic scattering than QH edge modes which are charge dependent
and spin unpolarized. However, we notice no such advantage accrues to QSH edge
modes when subjected to same degree of contact disorder and/or inelastic
scattering in similar setups as QH edge modes. In fact we observe that QSH edge
modes are more susceptible to inelastic scattering and contact disorder than QH
edge modes. Further, while a single disordered contact has no effect on QH edge
modes it leads to a finite charge Hall current in case of quantum spin Hall
edge modes and thus vanishing of pure quantum spin Hall effect. For more than a
single disordered contact while quantum Hall states continue to remain immune
to disorder, quantum spin Hall edge modes become more susceptible- the Hall
resistance for quantum spin Hall effect changes sign with increasing disorder.
In case of many disordered contacts with inelastic scattering included while
quantization of Hall edge modes holds, for quantum spin Hall edge modes- a
finite charge Hall current still flows. For quantum spin Hall edge modes in the
inelastic scattering regime we distinguish between two cases: with spin-flip
and without spin-flip scattering. Finally, while asymmetry in sample geometry
can have a deleterious effect on quantum spin Hall case it has no impact in
quantum Hall case. | 1510.00105v2 |
2015-10-06 | Conservative regularization of compressible flow and ideal magnetohydrodynamics | Ideal systems like MHD and Euler flow may develop singularities in vorticity
(w = curl v). Viscosity and resistivity are dissipative regularizations. We
propose a minimal, local, conservative, nonlinear, dispersive regularization of
compressible flow and ideal MHD, in analogy with the KdV regularization of the
1D Hopf equation. This work significantly extends earlier work on
incompressible Euler and ideal MHD. It involves a cut-off lambda inversely
proportional to square-root of density rho, which is like a position-dependent
mean free path. In MHD, lambda can be taken of order the electron collisionless
skin depth. The regularizing `twirl' term is - lambda w x curl w. Such a term
could be important in high speed flows with vorticity and arise in an expansion
of kinetic equations in Knudsen number. A magnetic analogue of the twirl term -
(B x curl B)/(rho mu_0), arises as the Lorentz force in ideal MHD. Our
regularization preserves symmetries of the ideal systems, and with appropriate
boundary conditions, implies associated conservation laws. Energy and enstrophy
are subject to a priori bounds determined by initial data. A Hamiltonian and
Poisson bracket formulation is developed and used to generalize the
constitutive relation to bound higher moments of w and curl w. A `swirl'
velocity field is shown to transport w/rho and B/rho, generalizing the
Kelvin-Helmholtz and Alfv\'en theorems. The steady regularized equations are
used to model a rotating vortex, MHD pinch, plane vortex sheet, channel flow,
plane flow and propagating spherical and cylindrical vortices; solutions are
more regular than corresponding Eulerian ones. The proposed regularization
could facilitate simulations of fluid/MHD equations and provide a consistent
statistical mechanics of vortices/current filaments in 3D, without blowup of
enstrophy. Implications for detailed analyses of fluid and plasma systems are
discussed. | 1510.01606v2 |
2015-10-20 | Photoconversion in the HIT solar cells: Theory vs experiment | We obtain theoretical expressions for the photocurrent in the Heterojunction
solar cells with Intrinsic Thin layer (HIT cells). Our calculations take into
account tunneling of electrons and holes through wide-bandgap layers of
$\alpha$-Si:H or $\alpha$-SiC:H. We introduce the criteria, under which
tunneling does not lead to the deterioration of solar cell characteristics, in
particular, to the reduction of the short-circuit current and open-circuit
voltage. We propose an algorithm to compute the photoconversion efficiency of
HIT elements, taking into account the peculiarities of the open-circuit voltage
generation, in particular, its rather high values. We test our theoretical
predictions against the experimental results. For this, we fabricate HIT
elements with the efficiency of about $20\,\%$. We measured the temperature
dependence of the short-circuit current, open-circuit voltage, photoconversion
power, and fill factor of the current-voltage curve of these elements in a wide
temperature range from 80 to 420\,K. In the low-temperature range, the
open-circuit voltage and the photoconversion power decrease on cooling. At $T
\ge 200$\,K, the theoretical expressions and the experimental curves agree
rather well. The behavior of the fill factor and output power at low
temperatures is explained by the increase of the series resistance on cooling.
We discuss the reasons behind the reduction of the power temperature
coefficient in HIT elements. We show that they are related to the low value of
the combined surface and volume recombination rate. Finally, we derive a
theoretical expression for the HIT element's operation temperature under
natural working conditions. | 1510.06007v2 |
2016-01-16 | Signatures of the Adler-Bell-Jackiw chiral anomaly in a Weyl Fermion semimetal | Weyl semimetals provide the realization of Weyl fermions in solid-state
physics. Among all the physical phenomena that are enabled by Weyl semimetals,
the chiral anomaly is the most unusual one. Here, we report signatures of the
chiral anomaly in the magneto-transport measurements on the first Weyl
semimetal TaAs. We show negative magnetoresistance under parallel electric and
magnetic fields, that is, unlike most metals whose resistivity increases under
an external magnetic field, we observe that our high mobility TaAs samples
become more conductive as a magnetic field is applied along the direction of
the current for certain ranges of the field strength. We present systematically
detailed data and careful analyses, which allow us to exclude other possible
origins of the observed negative magnetoresistance. Our transport data,
corroborated by photoemission measurements, first-principles calculations and
theoretical analyses, collectively demonstrate signatures of the Weyl fermion
chiral anomaly in the magneto-transport of TaAs. | 1601.04208v2 |
2016-08-15 | Physical properties of single crystalline $R$Mg$_{2}$Cu$_{9}$ ($R$ = Y, Ce-Nd, Gd-Dy, Yb) and the search for in-plane magnetic anisotropy in hexagonal systems | Single crystals of $R$Mg$_{2}$Cu$_{9}$ ($R$=Y, Ce-Nd, Gd-Dy, Yb) were grown
using a high-temperature solution growth technique and were characterized by
measurements of room-temperature x-ray diffraction, temperature-dependent
specific heat and temperature-, field-dependent resistivity and anisotropic
magnetization. YMg$_{2}$Cu$_{9}$ is a non-local-moment-bearing metal with an
electronic specific heat coefficient, $\gamma \sim$ 15 mJ/mol K$^2$. Yb is
divalent and basically non-moment bearing in YbMg$_{2}$Cu$_{9}$. Ce is
trivalent in CeMg$_{2}$Cu$_{9}$ with two magnetic transitions being observed at
2.1 K and 1.5 K. PrMg$_{2}$Cu$_{9}$ does not exhibit any magnetic phase
transition down to 0.5 K. The other members being studied ($R$=Nd, Gd-Dy) all
exhibits antiferromagnetic transitions at low-temperatures ranging from 3.2 K
for NdMg$_{2}$Cu$_{9}$ to 11.9 K for TbMg$_{2}$Cu$_{9}$. Whereas
GdMg$_{2}$Cu$_{9}$ is isotropic in its paramagnetic state due to zero angular
momentum ($L$=0), all the other local-moment-bearing members manifest an
anisotropic, planar magnetization in their paramagnetic states. To further
study this planar anisotropy, detailed angular-dependent magnetization was
carried out on magnetically diluted (Y$_{0.99}$Tb$_{0.01}$)Mg$_{2}$Cu$_{9}$ and
(Y$_{0.99}$Dy$_{0.01}$)Mg$_{2}$Cu$_{9}$. Despite the strong, planar
magnetization anisotropy, the in-plane magnetic anisotropy is weak and
field-dependent. A set of crystal electric field parameters are proposed to
explain the observed magnetic anisotropy. | 1608.04310v1 |
2017-06-19 | Chronic neural probe for simultaneous recording of single-unit, multi-unit, and local field potential activity from multiple brain sites | Drug resistant focal epilepsy can be treated by resecting the epileptic focus
requiring a precise focus localization using stereoelectroencephalography
(SEEG) probes. As commercial SEEG probes offer only a limited spatial
resolution, probes of higher channel count and design freedom enabling the
incorporation of macro and microelectrodes would help increasing spatial
resolution and thus open new perspectives for investigating mechanisms
underlying focal epilepsy and its treatment. This work describes a new
fabrication process for SEEG probes with materials and dimensions similar to
clinical probes enabling recording single neuron activity at high spatial
resolution. Polyimide is used as a biocompatible flexible substrate into which
platinum electrodes and leads are...
The resulting probe features match those of clinically approved devices.
Tests in saline solution confirmed the probe stability and functionality.
Probes were implanted into the brain of one monkey (Macaca mulatta), trained to
perform different motor tasks. Suitable configurations including up to 128
electrode sites allow the recording of task-related neuronal signals. Probes
with 32 and 64 electrode sites were implanted in the posterior parietal cortex.
Local field potentials and multi-unit activity were recorded as early as one
hour after implantation. Stable single-unit activity was achieved for up to 26
days after implantation of a 64-channel probe. All recorded signals showed
modulation during task execution. With the novel probes it is possible to
record stable biologically relevant data over a time span exceeding the usual
time needed for epileptic focus localization in human patients. This is the
first time that single units are recorded along cylindrical polyimide probes
chronically implanted 22 mm deep into the brain of a monkey, which suggests the
potential usefulness of this probe for human applications. | 1706.05899v1 |
2017-06-29 | Effects of Incomplete Ionization on Beta - Ga2O3 Power Devices: Unintentional Donor with Energy 110 meV | Understanding the origin of unintentional doping in Ga2O3 is key to
increasing breakdown voltages of Ga2O3 based power devices. Therefore,
transport and capacitance spectroscopy studies have been performed to better
understand the origin of unintentional doping in Ga2O3. Previously unobserved
unintentional donors in commercially available (-201) Ga2O3 substrates have
been electrically characterized via temperature dependent Hall effect
measurements up to 1000 K and found to have a donor energy of 110 meV. The
existence of the unintentional donor is confirmed by temperature dependent
admittance spectroscopy, with an activation energy of 131 meV determined via
that technique, in agreement with Hall effect measurements. With the
concentration of this donor determined to be in the mid to high 10^16 cm^-3
range, elimination of this donor from the drift layer of Ga2O3 power
electronics devices will be key to pushing the limits of device performance.
Indeed, analytical assessment of the specific on-resistance (Ronsp) and
breakdown voltage of Schottky diodes containing the 110 meV donor indicates
that incomplete ionization increases Ronsp and decreases breakdown voltage as
compared to Ga2O3 Schottky diodes containing only the shallow donor. The
reduced performance due to incomplete ionization occurs in addition to the
usual tradeoff between Ronsp and breakdown voltage. To achieve 10 kV operation
in Ga2O3 Schottky diode devices, analysis indicates that the concentration of
110 meV donors must be reduced below 5x10^14 cm^-3 to limit the increase in
Ronsp to one percent. | 1706.09960v2 |
2018-01-24 | Superconductivity in the vicinity of a ferroelectric quantum phase transition | Superconductivity has been observed in doped SrTiO$_3$ at charge-carrier
densities below 10$^{18}$ cm$^{-3}$, where the density of states at the Fermi
level of the itinerant electrons is several orders of magnitude lower than that
of conventional metals. In terms of the Bardeen-Cooper-Schrieffer description,
this implies the existence of an extraordinarily strong interaction driving the
formation of Cooper pairs, potentially comparable in order of magnitude to that
in some high Tc superconductors. Under suitable conditions the interaction
might remain effective at densities approaching metallic densities, leading to
the possibility of pair formation at elevated temperatures. Here we investigate
the pressure dependence of the resistivity and superconducting transition
temperature, Tc, of SrTiO$_3$ at a carrier density near to optimal doping. Our
experiments show that Tc collapses rapidly with pressure and hence with
increasing frequency of the soft transverse-optical phonon mode connected to
the ferroelectric quantum critical point. We show that the superconductivity
phase diagram can be understood in terms of the coupling of electrons via two
hybrid longitudinal polar modes, based on a model of dipolar fluctuations of
the charge carrier-ion system. In particular, we predict that for carrier
densities above the order of 10$^{18}$ cm$^{-3}$, Tc can be strongly enhanced
on approaching the ferroelectric quantum critical point, as seen in our
measurements of SrTiO$_3$ and as found in many electrically conducting magnetic
analogues. However below this density we predict the reverse behaviour, namely
that Tc is suppressed on approaching the ferroelectric quantum critical point.
Our model is also relevant to superconductivity found in gated ferroelectric
quantum critical systems such as KTaO$_3$ and can guide searches for new
superconductors in a diversity of materials. | 1801.08121v2 |
2018-03-01 | Conductance relaxation in GeBiTe - slow thermalization in an open quantum system | This work describes the microstructure and transport properties of GeBiTe
films with emphasis on their out-of-equilibrium behavior.
Persistent-photoconductivity (PPC), previously studied in the phase-change
compound GeSbTe is also quite prominent in this system. Much weaker PPC
response is observed in the pure GeTe compound and when alloying GeTe with
either In or Mn. Films made from these compounds share the same
crystallographic structure, the same p-type conductivity, a similar
compositional disorder extending over mesoscopic scales, and similar mosaic
morphology. The enhanced PPC response exhibited by the Sb and Bi alloys may
therefore be related to their common chemistry. PPC is observable in GeBiTe
films at the entire range of sheet resistances studied in this work. The excess
conductance produced by a brief exposure to infrared illumination decays with
time as a stretched-exponential (Kohlrausch law). Intrinsic electron-glass
effects on the other hand, are observable in thin films of GeBiTe only for
samples that are strongly-localized just like it was noted with the seven
electron-glasses previously studied. These include a memory-dip which is the
defining attribute of the phenomenon. The memory-dip in GeBiTe is the widest
among the germanium-telluride alloys studied to date consistent with the high
carrier-concentration of this compound. The thermalization process exhibited in
either, the PPC-state or in the electron-glass regime is sluggish but the
temporal law of the relaxation from the out-of-equilibrium state is distinctly
different. Coexistence of the two phenomena give rise to some non-trivial
effects, in particular, the visibility of the memory-dip is enhanced in the
PPC-state. The relation between this effect and the dependence of the
memory-effect magnitude on the ratio between the interparticle-interaction and
quench-disorder is discussed. | 1803.00564v1 |
2018-04-12 | Bulk and surface characterization of In$_2$O$_3$(001) single crystals | A comprehensive bulk and surface investigation of high-quality
In$_2$O$_3$(001) single crystals is reported. The transparent-yellow,
cube-shaped single crystals were grown using the flux method. Inductively
coupled plasma mass spectrometry (ICP-MS) reveals small residues of Pb, Mg, and
Pt in the crystals. Four-point-probe measurements show a resistivity of 2.0
$\pm$ 0.5 $\times$ 10$^5$ {\Omega} cm, which translates into a carrier
concentration of $\approx$10$^{12}$ cm$^{-3}$. The results from x-ray
diffraction (XRD) measurements revise the lattice constant to 10.1150(5) {\AA}
from the previously accepted value of 10.117 {\AA}. Scanning tunneling
microscopy (STM) images of a reduced (sputtered/annealed) and oxidized
(exposure to atomic oxygen at 300 {\deg}C) surface show a step height of 5
{\AA}, which indicates a preference for one type of surface termination. The
surfaces stay flat without any evidence for macroscopic faceting under any of
these preparation conditions. A combination of low-energy ion scattering (LEIS)
and atomically resolved STM indicates an indium-terminated surface with small
islands of 2.5 {\AA} height under reducing conditions, with a surface structure
corresponding to a strongly distorted indium lattice. Scanning tunneling
spectroscopy (STS) reveals a pronounced surface state at the Fermi level
($E_F$). Photoelectron spectroscopy (PES) shows additional, deep-lying band gap
states, which can be removed by exposure of the surface to atomic oxygen.
Oxidation also results in a shoulder at the O 1s core level at a higher binding
energy, possibly indicative of a surface peroxide species. A downward band
bending of 0.4 eV is observed for the reduced surface, while the band bending
of the oxidized surface is of the order of 0.1 eV or less. | 1804.04478v1 |
2018-04-19 | Modelling massive-star feedback with Monte Carlo radiation hydrodynamics: photoionization and radiation pressure in a turbulent cloud | We simulate a self-gravitating, turbulent cloud of 1000 Msol with
photoionization and radiation pressure feedback from a 34 Msol star. We use a
detailed Monte Carlo radiative transfer scheme alongside the hydrodynamics to
compute photoionization and thermal equilibrium with dust grains and multiple
atomic species. Using these gas temperatures, dust temperatures, and ionization
fractions, we produce self-consistent synthetic observations of line and
continuum emission. We find that all material is dispersed from the (15.5
pc)$^3$ grid within 1.6 Myr or 0.74 free-fall times. Mass exits with a peak
flux of $2 \times 10^{-3}$ Msol/yr, showing efficient gas dispersal. The model
without radiation pressure has a slight delay in the breakthrough of
ionization, but overall its effects are negligible. 85 per cent of the volume,
and 40 per cent of the mass, become ionized -- dense filaments resist
ionization and are swept up into spherical cores with pillars that point
radially away from the ionizing star. We use free-free emission at 20 cm to
estimate the production rate of ionizing photons. This is almost always
underestimated: by a factor of a few at early stages, then by orders of
magnitude as mass leaves the volume. We also test the ratio of dust continuum
surface brightnesses at 450 and 850 micron to probe dust temperatures. This
underestimates the actual temperature by more than a factor of 2 in areas of
low column density or high line-of-sight temperature dispersion; the HII region
cavity is particularly prone to this discrepancy. However, the probe is
accurate in dense locations such as filaments. | 1804.07309v1 |
2016-06-06 | Origin and magnitude of 'designer' spin-orbit interaction in graphene on semiconducting transition metal dichalcogenides | We use a combination of experimental techniques to demonstrate a general
occurrence of spin-orbit interaction (SOI) in graphene on transition metal
dichalcogenide (TMD) substrates. Our measurements indicate that SOI is
ultra-strong and extremely robust, despite it being merely
interfacially-induced, with neither graphene nor the TMD substrates changing
their structure. This is found to be the case irrespective of the TMD material
used, of the transport regime, of the carrier type in the graphene band, and of
the thickness of the graphene multilayer. Specifically, we perform weak
antilocalization measurements as the simplest and most general diagnostic of
SOI, and show that the spin relaxation time is very short in all cases
regardless of the elastic scattering time. Such a short spin-relaxation time
strongly suggests that the SOI originates from a modification of graphene band
structure. We confirmed this expectation by measuring a gate-dependent beating,
and a corresponding frequency splitting, in the low-field Shubnikov-de Haas
magneto-resistance oscillations in high quality bilayer graphene on WSe$_2$.
These measurements provide an unambiguous diagnostic of a SOI-induced splitting
in the electronic band structure, and their analysis allows us to determine the
SOI coupling constants for the Rashba term and the so-called spin-valley
coupling term, i.e., the terms that were recently predicted theoretically for
interface-induced SOI in graphene. The magnitude of the SOI splitting is found
to be on the order of 10 meV, more than 100 times greater than the SOI
intrinsic to graphene. Both the band character of the interfacially induced
SOI, as well as its robustness and large magnitude make graphene-on-TMD a
promising system to realize and explore a variety of spin-dependent transport
phenomena, such as, in particular, spin-Hall and valley-Hall topological
insulating states. | 1606.01789v1 |
2019-02-14 | Quantum magnetic imaging of iron biomineralisation in teeth of the chiton Acanthopleura hirtosa | Iron biomineralisation is critical for life. Nature capitalises on the
physical attributes of iron biominerals for a variety of functional, structural
and sensory applications. Although magnetism is an integral property of iron
biominerals, the role it plays in their nano-assembly remains a fundamental,
unanswered question. This is well exemplified by the magnetite-bearing radula
of chitons. Chitons, a class of marine mollusc, create the hardest biomineral
of any animal in their abrasion-resistant, self-sharpening teeth4. Despite this
system being subjected to a range of high resolution imaging studies, the
mechanisms that drive mineral assembly remain unresolved. However, the advent
of quantum imaging technology provides a new avenue to probe magnetic
structures directly. Here we use quantum magnetic microscopy, based on
nitrogen-vacancy centres in diamond, to attain the first subcellular magnetic
profiling of a eukaryotic system. Using complementary magnetic imaging
protocols, we spatially map the principal mineral phases (ferrihydrite and
magnetite) in the developing teeth of Acanthopleura hirtosa with submicron
resolution. The images reveal previously undiscovered long-range magnetic
order, established at the onset of magnetite mineralisation. This is in
contrast to electron microscopy studies that show no strong common
crystallographic orientation. The quantum-based magnetic profiling techniques
presented in this work have broad application in biology, earth science,
chemistry and materials engineering and can be applied across the range of
systems for which iron is vital. | 1902.09637v2 |
2019-05-24 | Interference measurements of non-Abelian e/4 & Abelian e/2 quasiparticle braiding | The quantum Hall states at filling factors $\nu=5/2$ and $7/2$ are expected
to have Abelian charge $e/2$ quasiparticles and non-Abelian charge $e/4$
quasiparticles. The non-Abelian statistics of the latter has been predicted to
display a striking interferometric signature, the even-odd effect. By measuring
resistance oscillations as a function of magnetic field in Fabry-P\'erot
interferometers using new high purity heterostructures, we for the first time
report experimental evidence for the non-Abelian nature of excitations at
$\nu=7/2$. At both $\nu=5/2$ and $7/2$ we also examine, for the first time, the
fermion parity, a topological quantum number of an even number of non-Abelian
quasiparticles. The phase of observed $e/4$ oscillations is reproducible and
stable over long times (hours) near both filling factors, indicating stability
of the fermion parity. At both fractions, when phase fluctuations are observed,
they are predominantly $\pi$ phase flips, consistent with either fermion parity
change or change in the number of the enclosed $e/4$ quasiparticles. We also
examine lower-frequency oscillations attributable to Abelian interference
processes in both states. Taken together, these results constitute new evidence
for the non-Abelian nature of $e/4$ quasiparticles; the observed life-time of
their combined fermion parity further strengthens the case for their utility
for topological quantum computation. | 1905.10248v5 |
2020-03-21 | Fluctuation dynamo in a weakly collisional plasma | The turbulent amplification of cosmic magnetic fields depends upon the
material properties of the host plasma. In many hot, dilute astrophysical
systems, such as the intracluster medium (ICM) of galaxy clusters, the rarity
of particle--particle collisions allows departures from local thermodynamic
equilibrium. These departures exert anisotropic viscous stresses on the plasma
motions that inhibit their ability to stretch magnetic-field lines. We present
a numerical study of the fluctuation dynamo in a weakly collisional plasma
using magnetohydrodynamic (MHD) equations endowed with a field-parallel viscous
(Braginskii) stress. When the stress is limited to values consistent with a
pressure anisotropy regulated by firehose and mirror instabilities, the
Braginskii-MHD dynamo largely resembles its MHD counterpart. If instead the
parallel viscous stress is left unabated -- a situation relevant to recent
kinetic simulations of the fluctuation dynamo and to the early stages of the
dynamo in a magnetized ICM -- the dynamo changes its character, amplifying the
magnetic field while exhibiting many characteristics of the saturated state of
the large-Prandtl-number (${\rm Pm}\gtrsim{1}$) MHD dynamo. We construct an
analytic model for the Braginskii-MHD dynamo in this regime, which successfully
matches magnetic-energy spectra. A prediction of this model, confirmed by our
simulations, is that a Braginskii-MHD plasma without pressure-anisotropy
limiters will not support a dynamo if the ratio of perpendicular and parallel
viscosities is too small. This ratio reflects the relative allowed rates of
field-line stretching and mixing, the latter of which promotes resistive
dissipation of the magnetic field. In all cases that do exhibit a dynamo, the
generated magnetic field is organized into folds that persist into the
saturated state and bias the chaotic flow to acquire a scale-dependent spectral
anisotropy. | 2003.09760v2 |
2020-09-30 | OMC-1 dust polarisation in ALMA Band 7: Diagnosing grain alignment mechanisms in the vicinity of Orion Source I | We present ALMA Band 7 polarisation observations of the OMC-1 region of the
Orion molecular cloud. We find that the polarisation pattern observed in the
region is likely to have been significantly altered by the radiation field of
the $>10^{4}$ L$_{\odot}$ high-mass protostar Orion Source I. In the
protostar's optically thick disc, polarisation is likely to arise from dust
self-scattering. In material to the south of Source I - previously identified
as a region of 'anomalous' polarisation emission - we observe a polarisation
geometry concentric around Source I. We demonstrate that Source I's extreme
luminosity may be sufficient to make the radiative precession timescale shorter
than the Larmor timescale for moderately large grains ($> 0.005-0.1\,\mu$m),
causing them to precess around the radiation anisotropy vector (k-RATs) rather
than the magnetic field direction (B-RATs). This requires relatively unobscured
emission from Source I, supporting the hypothesis that emission in this region
arises from the cavity wall of the Source I outflow. This is one of the first
times that evidence for k-RAT alignment has been found outside of a
protostellar disc or AGB star envelope. Alternatively, the grains may remain
aligned by B-RATs and trace gas infall onto the Main Ridge. Elsewhere, we
largely find the magnetic field geometry to be radial around the BN/KL
explosion centre, consistent with previous observations. However, in the Main
Ridge, the magnetic field geometry appears to remain consistent with the
larger-scale magnetic field, perhaps indicative of the ability of the dense
Ridge to resist disruption by the BN/KL explosion. | 2009.14758v2 |
2016-03-22 | Strongly Anisotropic Thermal and Electrical Conductivities of Self-assembled Silver Nanowire Network | Heat dissipation issues are the emerging challenges in the field of flexible
electronics. Thermal management of flexible electronics creates a demand for
flexible materials with highly anisotropic thermal conductivity, which work as
heat spreaders to remove excess heat in the in-plane direction and as heat
shields to protect human skin or device components under them from heating.
This study proposes a self-assembled silver nanowire network with high thermal
and electrical anisotropy with the potential to solve these challenges. The
in-plane thermal conductivity of the network along the axial direction of
silver nanowires is measured as 37 W/m-K while the cross-plane thermal
conductivity is only 0.36 W/m-K. The results of measurements of electrical and
thermal conductivities suggest that abundant wire-wire contacts strongly impede
thermal transport. The excellent alignment of nanowires results in the same
anisotropy ratio of 3 for both thermal and electrical conduction in the two
in-plane directions. The ratio remains unchanged as the temperature decrease to
50 K, which indicates that wire-wire contacts lower the thermal and electrical
conduction in the two directions to the same extent and their effect is
independent of temperature. In addition, phonon softening markedly reduces the
Debye temperatures of the network, which are fitted from the electrical
resistivity data. As a result of phonon thermal conduction, the Lorenz numbers
of the film in the two directions, which are approximately the same, are larger
than the Sommerfeld value at room temperature and decrease as temperature
decreases because of small angle scattering and the reduced phonon
contribution. This nanowire network provides a solution to the emerging
challenges of thermal management of flexible electronics. | 1603.06845v1 |
2017-05-18 | Superconductivity and charge carrier localization in ultrathin $\mathbf{{La_{1.85}Sr_{0.15}CuO_4}/{La_2CuO_4}}$ bilayers | $\mathrm{La_{1.85}Sr_{0.15}CuO_4}$/$\mathrm{La_2CuO_4}$ (LSCO15/LCO) bilayers
with a precisely controlled thickness of N unit cells (UCs) of the former and M
UCs of the latter ([LSCO15\_N/LCO\_M]) were grown on (001)-oriented {\slao}
(SLAO) substrates with pulsed laser deposition (PLD). X-ray diffraction and
reciprocal space map (RSM) studies confirmed the epitaxial growth of the
bilayers and showed that a [LSCO15\_2/LCO\_2] bilayer is fully strained,
whereas a [LSCO15\_2/LCO\_7] bilayer is already partially relaxed. The
\textit{in situ} monitoring of the growth with reflection high energy electron
diffraction (RHEED) revealed that the gas environment during deposition has a
surprisingly strong effect on the growth mode and thus on the amount of
disorder in the first UC of LSCO15 (or the first two monolayers of LSCO15
containing one $\mathrm{CuO_2}$ plane each). For samples grown in pure
$\mathrm{N_2O}$ gas (growth type-B), the first LSCO15 UC next to the SLAO
substrate is strongly disordered. This disorder is strongly reduced if the
growth is performed in a mixture of $\mathrm{N_2O}$ and $\mathrm{O_2}$ gas
(growth type-A). Electric transport measurements confirmed that the first UC of
LSCO15 next to the SLAO substrate is highly resistive and shows no sign of
superconductivity for growth type-B, whereas it is superconducting for growth
type-A. Furthermore, we found, rather surprisingly, that the conductivity of
the LSCO15 UC next to the LCO capping layer strongly depends on the thickness
of the latter. A LCO capping layer with 7~UCs leads to a strong localization of
the charge carriers in the adjacent LSCO15 UC and suppresses superconductivity.
The magneto-transport data suggest a similarity with the case of weakly hole
doped LSCO single crystals that are in a so-called {"{cluster-spin-glass
state}"} | 1705.06587v1 |
2017-07-18 | Extremely large magnetoresistance and Kohler's rule in PdSn4: a complete study of thermodynamic, transport and band structure properties | The recently discovered material PtSn$_4$ is known to exhibit extremely large
magnetoresistance (XMR) that also manifests Dirac arc nodes on the surface.
PdSn$_4$ is isostructure to PtSn$_4$ with same electron count. We report on the
physical properties of high quality single crystals of PdSn$_4$ including
specific heat, temperature and magnetic field dependent resistivity and
magnetization, and electronic band structure properties obtained from angle
resolved photoemission spectroscopy (ARPES). We observe that PdSn$_4$ has
physical properties that are qualitatively similar to those of PtSn$_4$, but
find also pronounced differences. Importantly, the Dirac arc node surface state
of PtSn$_4$ is gapped out for PdSn$_4$. By comparing these similar compounds,
we address the origin of the extremely large magnetoresistance in PdSn$_4$ and
PtSn$_4$; based on detailed analysis of the magnetoresistivity, $\rho(H,T)$, we
conclude that neither carrier compensation nor the Dirac arc node surface state
are primary reason for the extremely large magnetoresistance. On the other
hand, we find that surprisingly Kohler's rule scaling of the
mangnetoresistance, which describes a self-similarity of the field induced
orbital electronic motion across different length scales and is derived for a
simple electronic response of metals to applied in a magnetic field is obeyed
over the full range of temperatures and field strengths that we explore. | 1707.05706v2 |
2019-03-19 | Pressure-induced superconductivity in layered pnictogen diselenide NdO$_{0.8}$F$_{0.2}$Sb$_{1-x}$Bi$_x$Se$_2$ (x = 0.3 and 0.7) | Polycrystalline samples of layered pnictogen diselenide NdO0.8F0.2Sb1-xBixSe2
(x = 0 to 0.8) were successfully synthesized by solid-state reactions.
Electrical resistivity in the synthesized samples was systematically decreased
with an increase in Bi content x. Crystal structure analysis using synchrotron
X-ray diffraction suggests that insulator to metal transition upon Bi doping
correlates with anomalous change in c-axis length and/or corrugation in
conducting layer. The emergence of superconductivity under high pressure is
demonstrated using diamond anvil cell (DAC) with boron-doped diamond
electrodes, for x = 0.3 and 0.7 as the representative samples. For Sb-rich one
(x = 0.3), we observed a superconducting transition with Tconset = 5.3 K at 50
GPa, which is the first-ever report of the superconductivity in layered
SbCh2-based (Ch: chalcogen) compounds. The Tconset of x = 0.3 increased with
increasing pressure and reached 7.9 K at 70.8 GPa, followed by the gradual
decrease in Tc up to 90 GPa. For Bi-rich one (x = 0.7), a superconducting
transition with Tconset = 5.9 K was observed at 43.5 GPa, which is the almost
comparable to that of x = 0.3; besides, upper critical field (Hc2) is evaluated
to be ~10 T for x = 0.7, which is higher than that of x = 0.3 (Hc2 = 6.7 T at
50 GPa). | 1903.07791v3 |
2015-06-18 | Evolution of electronic states in n-type copper oxide superconductor via electric double layer gating | Since the discovery of n-type copper oxide superconductors, the evolution of
electron- and hole-bands and its relation to the superconductivity have been
seen as a key factor in unveiling the mechanism of high-Tc superconductors. So
far, the occurrence of electrons and holes in n-type copper oxides has been
achieved by chemical doping, pressure, and/or deoxygenation. However, the
observed electronic properties are blurred by the concomitant effects such as
change of lattice structure, disorder, etc. Here, we report on successful
tuning the electronic band structure of n-type Pr2-xCexCuO4 (x = 0.15)
ultrathin films, via the electric double layer transistor technique. Abnormal
transport properties, such as multiple sign reversals of Hall resistivity in
normal and mixed states, have been revealed within an electrostatic field in
range of -2 V to +2 V, as well as varying the temperature and magnetic field.
In the mixed state, the intrinsic anomalous Hall conductivity invokes the
contribution of both electron and hole-bands as well as the energy dependent
density of states near the Fermi level. The two-band model can also describe
the normal state transport properties well, whereas the carrier concentrations
of electrons and holes are always enhanced or depressed simultaneously in
electric fields. This is in contrast to the scenario of Fermi surface
reconstruction by antiferromagnetism, where an anti-correlation between
electrons and holes is commonly expected. Our findings paint the picture where
Coulomb repulsion plays an important role in the evolution of the electronic
states in n-type cuprate superconductors. | 1506.05727v1 |
2019-01-08 | Thermal bridging of graphene nanosheets via covalent molecular junctions. a Non-Equilibrium Green Functions Density Functional Tight-Binding study | Despite the uniquely high thermal conductivity of graphene is well known, the
exploitation of graphene into thermally conductive nanomaterials and devices is
limited by the inefficiency of thermal contacts between the individual
nanosheets. A fascinating yet experimentally challenging route to enhance
thermal conductance at contacts between graphene nanosheets is through
molecular junctions, allowing covalently connecting nanosheets otherwise
interacting only via weak Van der Waals forces. Beside the bare existence of
covalent connections, the choice of molecular structures to be used as thermal
junctions should be guided by their vibrational properties, in terms of phonon
transfer through the molecular junction. In this paper, density functional
tight-binding combined with Green functions formalism was applied for the
calculation of thermal conductance and phonon spectra of several different
aliphatic and aromatic molecular junctions between graphene nanosheets. Effect
of molecular junction length, conformation, and aromaticity were studied in
detail and correlated with phonon tunnelling spectra. The theoretical insight
provided by this work can guide future experimental studies to select suitable
molecular junctions in order to enhance the thermal transport by suppressing
the interfacial thermal resistances, particularly attractive for various
systems, including graphene nanopapers and graphene polymer nanocomposites, as
well as related devices. In a broader view, the possibility to design molecular
junctions to control phonon transport currently appears as an efficient way to
produce phononic devices and controlling heat management in nanostructures. | 1901.02396v2 |
2019-01-09 | Printing surface charge as a new paradigm to program droplet transport | Directed, long-range and self-propelled transport of droplets on solid
surfaces, especially on water repellent surfaces, is crucial for many
applications from water harvesting to bio-analytical devices. One appealing
strategy to achieve the preferential transport is to passively control the
surface wetting gradients, topological or chemical, to break the asymmetric
contact line and overcome the resistance force. Despite extensive progress, the
directional droplet transport is limited to small transport velocity and short
transport distance due to the fundamental trade-off: rapid transport of droplet
demands a large wetting gradient, whereas long-range transport necessitates a
relatively small wetting gradient. Here, we report a radically new strategy
that resolves the bottleneck through the creation of an unexplored gradient in
surface charge density (SCD). By leveraging on a facile droplet printing on
superamphiphobic surfaces as well as the fundamental understanding of the
mechanisms underpinning the creation of the preferential SCD, we demonstrate
the self-propulsion of droplets with a record-high velocity over an ultra-long
distance without the need for additional energy input. Such a Leidenfrost-like
droplet transport, manifested at ambient condition, is also genetic, which can
occur on a variety of substrates such as flexible and vertically placed
surfaces. Moreover, distinct from conventional physical and chemical gradients,
the new dimension of gradient in SCD can be programmed in a rewritable fashion.
We envision that our work enriches and extends our capability in the
manipulation of droplet transport and would find numerous potential
applications otherwise impossible. | 1901.02612v1 |
2019-11-06 | Absence of superconductivity in bulk Nd$_{1-x}$Sr$_x$NiO$_2$ | Recently superconductivity at 9 - 15 K was discovered in an infinite-layer
nickelate (Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ films), which has received enormous
attention. Since the $Ni^{1+}$ ionic state in NdNiO$_2$ may have the $3d^9$
outer-shell electronic orbit which resembles that of the cuprates, it is very
curious to know whether superconductivity discovered here has similar mechanism
as that in the cuprates. By using a three-step method, we successfully
synthesize the bulk samples of Nd$_{1-x}$Sr$_x$NiO$_2$ (x=0, 0.2, 0.4). The
X-ray diffractions reveal that all the samples contain mainly the infinite
layer phase of 112 with some amount of segregated Ni. This has also been well
proved by the SEM image and the EDS composition analysis. The resistive
measurements on the Sr doped samples show insulating behavior without the
presence of superconductivity. Temperature dependence of the magnetic moment
under high magnetic fields exhibits a Curie-Weiss law feature with the
paramagnetic moment of about 2$\mu_B$/f.u.. By applying pressure on
Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ up to about 50.2 GPa, we find that the strong
insulating behavior at ambient pressure is significantly suppressed, but
superconductivity has not been observed either. Since the lattice constants
derived from our XRD data are very close to those of the reported
superconducting films, we argue that the superconductivity in the reported film
may not originate from the expected Nd$_{0.8}$Sr$_{0.2}$NiO$_2$, but arise from
the interface or the stress effect. | 1911.02420v3 |
2019-11-12 | Absence of Superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_x$ Thin Films without Chemical Reduction | The recently reported superconductivity 9-15 K in Nd0.8Sr0.2NiO2/SrTiO3
heterostructures that were fabricated by a soft-chemical topotactic reduction
approach based on precursor Nd0.8Sr0.2NiO3 thin films deposited on SrTiO3
substrates, has excited an immediate surge of research interest. To explore an
alternative physical path instead of chemical reduction for realizing
superconductivity in this compound, using pulsed laser deposition, we
systematically fabricated 63 Nd0.8Sr0.2NiOx (NSNO) thin films at a wide range
of oxygen partial pressures on various different oxide substrates. Transport
measurements did not find any signature of superconductivity in all the 63
thin-film samples. With reducing the oxygen content in the NSNO films by
lowering the deposition oxygen pressure, the NSNO films are getting more
resistive and finally become insulating. Furthermore, we tried to cap a
20-nm-thick amorphous LaAlO3 layer on a Nd0.8Sr0.2NiO3 thin film deposited at a
high oxygen pressure of 150 mTorr to create oxygen vacancies on its surface and
did not succeed in higher conductivity either. Our experimental results
together with the recent report on the absence of superconductivity in
synthesized bulk Nd0.8Sr0.2NiO2 crystals suggest that the chemical reduction
approach could be unique for yielding superconductivity in NSNO/SrTiO3
heterostructures. However, SrTiO3 substrates could be reduced to generate
oxygen vacancies during the chemical reduction process as well, which may thus
partially contribute to conductivity. | 1911.04662v2 |
2019-11-19 | Multi-GMR sensors controlled by additive dipolar coupling | Vertical packaging of multiple Giant Magnetoresistance (multi-GMR) stacks is
a very interesting noise reduction strategy for local magnetic sensor
measurements, which has not been reported experimentally so far. Here, we have
fabricated multi-GMR sensors (up to 12 repetitions) keeping good GMR ratio,
linearity and low roughness. From magnetotransport measurements, two different
resistance responses have been observed with a crossover around 5 GMR
repetitions: step-like (N<5) and linear (N>5) behavior, respectively. With the
help of micromagnetic simulations, we have analyzed in detail the two main
magnetic mechanisms: the Neel coupling distribution induced by the roughness
propagation and the additive dipolar coupling between the N free layers.
Furthermore we have correlated the dipolar coupling mechanism, controlled by
the number of GMRs (N) and lateral dimensions (width), to the sensor
performance (sensitivity, noise and detectivity) in good agreement with
analytical theory. The noise roughly decreases in multi-GMRs as 1/\sqrt{N} in
both regimes (low frequency 1/f and thermal noise). The sensitivity is even
stronger reduced, scaling as 1/N, in the strong dipolar regime (narrow devices)
while converges to a constant value in the weak dipolar regime (wide devices).
Very interestingly, they are more robust against undesirable RTN noise than
single GMRs at high voltages and the linearity can be extended towards much
larger magnetic field range without dealing with the size and the reduction of
GMR ratio. Finally, we have identified the optimal conditions for which
multi-GMRs exhibit lower magnetic field detectivity than single GMRs: wide
devices operating in the thermal regime where much higher voltage can be
applied without generating remarkable magnetic noise. | 1911.08592v1 |
2020-07-30 | The effect of structural and magnetic disorder on the 3$d$-5$d$ exchange interactions of La$_{2-x}$Ca$_{x}$CoIrO$_{6}$ | The delicate balance between spin-orbit coupling, Coulomb repulsion and
crystalline electric field interactions observed in Ir-based oxides is usually
manifested as exotic magnetic behavior. Here we investigate the evolution of
the exchange coupling between Co and Ir for partial La substitution by Ca in
La$_{2}$CoIrO$_6$. A great advantage of the use of Ca$^{2+}$ as replacement for
La$^{3+}$ is the similarity of its ionic radii. Thus, the observed magnetic
changes can more easily be associated to electronic variations. A thorough
investigation of the structural, electronic and magnetic properties of the
La$_{2-x}$Ca$_{x}$CoIrO$_6$ system was carried out by means of synchrotron
x-ray powder diffraction, muon spin rotation and relaxation ($\mu$SR), AC and
DC magnetization, XAS, XMCD, Raman spectroscopy, electrical resistivity and
dielectric permittivity. Our XAS results show that up to 25% of Ca substitution
at the La site results in the emergence of Co$^{3+}$, possibly in high spin
state, while the introduction of larger amount of Ca leads to the increase of
Ir valence. The competing magnetic interactions resulting from the mixed
valences lead to a coexistence of a magnetically ordered and an emerging spin
glass (SG) state for the doped samples. Our $\mu$SR results indicate that for
La$_{2}$CoIrO$_6$ a nearly constant fraction of a paramagnetic (PM) phase
persists down to low temperature, possibly related to the presence of a small
amount of Ir$^{3+}$ and to the anti-site disorder at Co/Ir sites. For the doped
compounds the PM phase freezes below 30 K, but there is still some dynamics
associated with the SG. The dielectric data obtained for the parent compound
and the one with 25% of Ca-doping indicate a possible magnetodielectric effect,
which is discussed in terms of the electron hopping between the TM ions, the
anti-site disorder and the distorted crystalline structure. | 2007.15318v1 |
2020-09-30 | A model for atomic precision p-type doping with diborane on Si(100)-2$\times$1 | Diborane (B$_2$H$_6$) is a promising molecular precursor for atomic precision
p-type doping of silicon that has recently been experimentally demonstrated [T.
{\v{S}}kere{\v{n}}, \textit{et al.,} Nature Electronics (2020)]. We use density
functional theory (DFT) calculations to determine the reaction pathway for
diborane dissociating into a species that will incorporate as electrically
active substitutional boron after adsorbing onto the Si(100)-2$\times$1
surface. Our calculations indicate that diborane must overcome an energy
barrier to adsorb, explaining the experimentally observed low sticking
coefficient ($< 10^{-4}$ at room temperature) and suggesting that heating can
be used to increase the adsorption rate. Upon sticking, diborane has an $\sim
50\%$ chance of splitting into two BH$_3$ fragments versus merely losing
hydrogen to form a dimer such as B$_2$H$_4$. As boron dimers are likely
electrically inactive, whether this latter reaction occurs is shown to be
predictive of the incorporation rate. The dissociation process proceeds with
significant energy barriers, necessitating the use of high temperatures for
incorporation. Using the barriers calculated from DFT, we parameterize a
Kinetic Monte Carlo model that predicts the incorporation statistics of boron
as a function of the initial depassivation geometry, dose, and anneal
temperature. Our results suggest that the dimer nature of diborane inherently
limits its doping density as an acceptor precursor, and furthermore that
heating the boron dimers to split before exposure to silicon can lead to poor
selectivity on hydrogen and halogen resists. This suggests that while diborane
works as an atomic precision acceptor precursor, other non-dimerized acceptor
precursors may lead to higher incorporation rates at lower temperatures. | 2010.00129v1 |
2020-10-26 | Solubility limit of Ge Dopants in AlGaN: a Chemical and Microstructural Investigation down to the Nanoscale | Attaining low resistivity AlGaN layers is the keystone to improve the
efficiency of light emitting devices in the ultraviolet spectral range. Here,
we present a microstructural analysis of Ge-doped AlGaN samples with Al mole
fraction from x=0 to 1, and nominal doping level in the range of 1E20 cm-3,
together with the measurement of Ge concentration and its spatial distribution
down to the nm scale. AlGaN:Ge samples with x smaller or equal to 0.2 do not
present any sign of inhomogeneity. However, samples with x > 0.4 display
micrometer-size Ge crystallites at the surface. Ge segregation is not
restricted to the surface: Ge-rich regions with a size of tens of nanometers
are observed inside the AlGaN:Ge layers, generally associated with Ga-rich
regions around structural defects. With this local exceptions, the AlGaN:Ge
matrix present an homogenous Ge composition which can be significantly lower
than the nominal doping level. Precise measurements of Ge in the matrix provide
a view of the solubility diagram of Ge in AlGaN as a function of the Al mole
fraction. The solubility of Ge in AlN is extremely low. Between AlN and GaN,
the solubility increases linearly with the Ga mole fraction in the ternary
alloy, which suggests that the Ge incorporation takes place by substitution of
Ga atoms only. The maximum percentage of Ga sites occupied by Ge saturates
around 1%. The solubility issues and Ge segregation phenomena at different
length scales likely play a role in the efficiency of Ge as n-type AlGaN
dopant, even at Al concentrations where Ge DX centers are not expected to
manifest. Therefore, this information can have direct impact in the performance
of Ge-doped AlGaN light emitting diodes, particularly in the spectral range for
disinfection (around 260 nm), which requires heavily-doped alloys with high Al
mole fraction. | 2010.13577v1 |
2020-10-26 | Domain Wall-Magnetic Tunnel Junction Spin Orbit Torque Devices and Circuits for In-Memory Computing | There are pressing problems with traditional computing, especially for
accomplishing data-intensive and real-time tasks, that motivate the development
of in-memory computing devices to both store information and perform
computation. Magnetic tunnel junction (MTJ) memory elements can be used for
computation by manipulating a domain wall (DW), a transition region between
magnetic domains. But, these devices have suffered from challenges: spin
transfer torque (STT) switching of a DW requires high current, and the multiple
etch steps needed to create an MTJ pillar on top of a DW track has led to
reduced tunnel magnetoresistance (TMR). These issues have limited experimental
study of devices and circuits. Here, we study prototypes of three-terminal
domain wall-magnetic tunnel junction (DW-MTJ) in-memory computing devices that
can address data processing bottlenecks and resolve these challenges by using
perpendicular magnetic anisotropy (PMA), spin-orbit torque (SOT) switching, and
an optimized lithography process to produce average device tunnel
magnetoresistance TMR = 164%, resistance-area product RA = 31
{\Omega}-{\mu}m^2, close to the RA of the unpatterned film, and lower switching
current density compared to using spin transfer torque. A two-device circuit
shows bit propagation between devices. Device initialization variation in
switching voltage is shown to be curtailed to 7% by controlling the DW initial
position, which we show corresponds to 96% accuracy in a DW-MTJ full adder
simulation. These results make strides in using MTJs and DWs for in-memory and
neuromorphic computing applications. | 2010.13879v1 |
2020-12-30 | Cooperation in a fluid swarm of fuel-free micro-swimmers | Cooperation is vital for the survival of a swarm$^1$. Large scale cooperation
allows murmuring starlings to outmaneuver preying falcons$^2$, shoaling
sardines to outsmart sea lions$^3$, and homo sapiens to outlive their
Pleistocene peers$^4$. On the micron-scale, bacterial colonies show excellent
resilience thanks to the individuals' ability to cooperate even when densely
packed, mitigating their internal flow pattern to mix nutrients, fence the
immune system, and resist antibiotics$^{5-14}$. Production of an artificial
swarm on the micro-scale faces a serious challenge $\frac{\;\;}{\;\;}$ while an
individual bacterium has an evolutionary-forged internal machinery to produce
propulsion, until now, artificial micro-swimmers relied on the precise chemical
composition of their environment to directly fuel their drive$^{14-23}$. When
crowded, artificial micro-swimmers compete locally for a finite fuel supply,
quenching each other's activity at their greatest propensity for cooperation.
Here we introduce an artificial micro-swimmer that consumes no chemical fuel
and is driven solely by light. We couple a light absorbing particle to a fluid
droplet, forming a colloidal chimera that transforms light energy into
propulsive thermo-capillary action. The swimmers' internal drive allows them to
operate and remain active for a long duration (days) and their effective
repulsive interaction allows for a high density fluid phase. We find that above
a critical concentration, swimmers form a long lived crowded state that
displays internal dynamics. When passive particles are introduced, the dense
swimmer phase can re-arrange and spontaneously corral the passive particles. We
derive a geometrical, depletion-like condition for corralling by identifying
the role the passive particles play in controlling the effective concentration
of the micro-swimmers. | 2012.15087v1 |
2021-02-09 | Evidence for multiband superconductivity and charge density waves in Ni-doped ZrTe$_2$ | We carried out a comprehensive study of the electronic, magnetic, and
thermodynamic properties of Ni-doped ZrTe$_2$. High quality
Ni$_{0.04}$ZrTe$_{1.89}$ single crystals show a possible coexistence of charge
density waves (CDW, T$_{CDW}\approx287$\,K) with superconductivity (T$_c\approx
4.1$\,K), which we report here for the first time. The temperature dependence
of the lower (H$_{c_1}$) and upper (H$_{c_2}$) critical magnetic fields both
deviate significantly from the behaviors expected in conventional single-gap
s-wave superconductors. However, the behaviors of the normalized superfluid
density $\rho_s(T)$ and H$_{c_2}(T)$ can be described well using a two-gap
model for the Fermi surface, in a manner consistent with conventional multiband
superconductivity. Electrical resistivity and specific heat measurements show
clear anomalies centered near 287\,K consistent with a CDW phase transition.
Additionally, electronic-structure calculations support the coexistence of
electron-phonon multiband superconductivity and CDW order due to the
compensated disconnected nature of the electron- and hole-pockets at the Fermi
surface. Our electronic structure calculations also suggest that ZrTe$_2$ could
reach a non-trivial topological type-II Dirac semimetallic state. These
findings highlight that Ni-doped ZrTe2 can be uniquely important for probing
the coexistence of superconducting and CDW ground states in an electronic
system with non-trivial topology. | 2102.04812v3 |
2021-02-18 | $f$-electron hybridised metallic Fermi surface in magnetic field-induced metallic YbB$_{12}$ | The nature of the Fermi surface observed in the recently discovered family of
unconventional insulators starting with SmB$_6$ and subsequently YbB$_{12}$ is
a subject of intense inquiry. Here we shed light on this question by comparing
quantum oscillations between the high magnetic field-induced metallic regime in
YbB$_{12}$ and the unconventional insulating regime. In the field-induced
metallic regime beyond 47 T, we find prominent quantum oscillations in the
contactless resistivity characterised by multiple frequencies up to at least
3000 T and heavy effective masses up to at least 17 $m_\text{e}$,
characteristic of an $f$-electron hybridised metallic Fermi surface. The growth
of quantum oscillation amplitude at low temperatures in electrical transport
and magnetic torque in insulating YbB$_{12}$ is closely similar to the
Lifshitz-Kosevich low temperature growth of quantum oscillation amplitude in
field-induced metallic YbB$_{12}$, pointing to an origin of quantum
oscillations in insulating YbB$_{12}$ from in-gap neutral low energy
excitations. The field-induced metallic regime of YbB$_{12}$ is characterised
by more Fermi surface sheets of heavy quasiparticle effective mass that emerge
in addition to the heavy Fermi surface sheets yielding multiple quantum
oscillation frequencies below 1000 T observed in both insulating and metallic
regimes. We thus observe a heavy multi-component Fermi surface in which
$f$-electron hybridisation persists from the unconventional insulating to the
field-induced metallic regime of YbB$_{12}$, which is in distinct contrast to
the unhybridised conduction electron Fermi surface observed in the case of the
unconventional insulator SmB$_6$. Our findings require a different theoretical
model of neutral in-gap low energy excitations in which the $f$-electron
hybridisation is retained in the case of the unconventional insulator
YbB$_{12}$. | 2102.09545v2 |
2021-05-18 | Microwave response of a metallic superconductor subject to a high-voltage gate electrode | Processes that lead to the critical-current suppression and change of
impedance of a superconductor under the application of an external voltage is
an active area of research, especially due to various possible technological
applications. In particular, field-effect transistors and radiation detectors
have been developed in the recent years, showing the potential for precision
and sensitivity exceeding their normal-metal counterparts. In order to describe
the phenomenon that leads to the critical-current suppression in metallic
superconducting structures, a field-effect hypothesis has been formulated,
stating that an electric field can penetrate the metallic superconductor and
affect its characteristics. The existence of such an effect would imply the
incompleteness of the underlying theory, and hence indicate an important gap in
the general comprehension of superconductors. In addition to its theoretical
value, a complete understanding of the phenomenon underneath the electric-field
response of the superconductor is important in the light of the related
technological applications. In this paper, we study the change of the
characteristics of a superconductor implementing a coplanar-waveguide resonator
as a tank circuit, by relating our measurements to the reactance and resistance
of the material. Namely, we track the state of the superconductor at different
voltages and resulting leakage currents of a nearby gate electrode which is not
galvanically connected to the resonator. By comparing the effects of the
leakage current and of a change in the temperature of the system, we conclude
that the observed behaviour in the superconductor is [...] | 2105.08322v4 |
2021-05-24 | A new ideality factor for perovskite solar cells and an analytical theory for their impedance spectroscopy response | Impedance spectroscopy (IS) is a relatively straightforward experimental
technique that is commonly used to obtain information about the physical and
chemical characteristics of photovoltaic devices. However, the non-standard
physical behaviour of perovskite solar cells (PSC), which are heavily
influenced by the motion of mobile ion vacancies, has hindered efforts to
obtain a consistent theory to interpret PSC impedance data. This work rectifies
this omission by deriving a simple analytic model of the impedance response of
a PSC from the underlying drift-diffusion model of charge carrier dynamics and
ion vacancy motion. Extremely good agreement is shown between the analytic
model and the much more complex drift-diffusion model in regimes (including
maximum power point) where the applied voltage is close to the open circuit
voltage $V_{oc}$. Both models show good qualitative agreement to experimental
IS data in the literature and predict many of the observed anomalous features
found in impedance measurements on PSCs, such as `the giant low frequency
capacitance` and `inductive arcs' in the Nyquist plots. Where the physical
properties of the PSC are already known the analytic model can be used to
predict the recombination current $j_{rec}$ and the high and low frequency
resistances and capacitances of the cell, $R_{HF}$, $C_{HF}$, $R_{LF}$ and
$C_{LF}$. In scenarios where the physical properties of the cell are unknown
the analytic model can also used to extract physical parameters from
experimental PSC impedance data. {A novel physical parameter of particular
significance to PSC physics is identified. This is termed the electronic
ideality factor, $n_{el}$, and (as opposed to the standard ideality factor) can
be used to deduce the dominant source of recombination in a PSC, independent of
its ionic properties. | 2105.11226v1 |
2021-05-27 | Van der Waals interaction affects wrinkle formation in two-dimensional materials | Nonlinear mechanics of solids is an exciting field that encompasses both
beautiful mathematics, such as the emergence of instabilities and the formation
of complex patterns, as well as multiple applications. Two-dimensional crystals
and van der Waals (vdW) heterostructures allow revisiting this field on the
atomic level, allowing much finer control over the parameters and offering
atomistic interpretation of experimental observations. In this work, we
consider the formation of instabilities consisting of radially-oriented
wrinkles around mono- and few-layer "bubbles" in two-dimensional vdW
heterostructures. Interestingly, the shape and wavelength of the wrinkles
depend not only on the thickness of the two-dimensional crystal forming the
bubble, but also on the atomistic structure of the interface between the bubble
and the substrate, which can be controlled by their relative orientation. We
argue that the periodic nature of these patterns emanates from an energetic
balance between the resistance of the top membrane to bending, which favors
large wavelength of wrinkles, and the membrane-substrate vdW attraction, which
favors small wrinkle amplitude. Employing the classical "Winkler foundation"
model of elasticity theory, we show that the number of radial wrinkles conveys
a valuable relationship between the bending rigidity of the top membrane and
the strength of the vdW interaction. Armed with this relationship, we use our
data to demonstrate a nontrivial dependence of the bending rigidity on the
number of layers in the top membrane, which shows two different regimes driven
by slippage between the layers, and a high sensitivity of the vdW force to the
alignment between the substrate and the membrane. | 2105.13229v1 |
2021-08-04 | System Modelling of Very Low Earth Orbit Satellites for Earth Observation | The operation of satellites in very low Earth orbit (VLEO) has been linked to
a variety of benefits to both the spacecraft platform and mission design.
Critically, for Earth observation (EO) missions a reduction in altitude can
enable smaller and less powerful payloads to achieve the same performance as
larger instruments or sensors at higher altitude, with significant benefits to
the spacecraft design. As a result, renewed interest in the exploitation of
these orbits has spurred the development of new technologies that have the
potential to enable sustainable operations in this lower altitude range. In
this paper, system models are developed for (i) novel materials that improve
aerodynamic performance enabling reduced drag or increased lift production and
resistance to atomic oxygen erosion and (ii) atmosphere-breathing electric
propulsion (ABEP) for sustained drag compensation or mitigation in VLEO.
Attitude and orbit control methods that can take advantage of the aerodynamic
forces and torques in VLEO are also discussed. These system models are
integrated into a framework for concept-level satellite design and this
approach is used to explore the system-level trade-offs for future EO
spacecraft enabled by these new technologies. A case-study presented for an
optical very-high resolution spacecraft demonstrates the significant potential
of reducing orbital altitude using these technologies and indicates possible
savings of up to 75% in system mass and over 50% in development and
manufacturing costs in comparison to current state-of-the-art missions. For a
synthetic aperture radar (SAR) satellite, the reduction in mass and cost with
altitude were shown to be smaller, though it was noted that currently available
cost models do not capture recent commercial advancements in this segment... | 2108.01945v2 |
2021-09-10 | One-Dimensional Luttinger Liquids in a Two-Dimensional Moiré Lattice | The Luttinger liquid (LL) model of one-dimensional (1D) electronic systems
provides a powerful tool for understanding strongly correlated physics
including phenomena such as spin-charge separation. Substantial theoretical
efforts have attempted to extend the LL phenomenology to two dimensions (2D),
especially in models of closely packed arrays of 1D quantum wires, each being
described as a LL. Such coupled-wire models have been successfully used to
construct 2D anisotropic non-Fermi liquids, quantum Hall states, topological
phases, and quantum spin liquids. However, an experimental demonstration of
high-quality arrays of 1D LLs suitable for realizing these models remains
absent. Here we report the experimental realization of 2D arrays of 1D LLs with
crystalline quality in a moir\'e superlattice made of twisted bilayer tungsten
ditelluride (tWTe$_{2}$). Originating from the anisotropic lattice of the
monolayer, the moir\'e pattern of tWTe$_{2}$ hosts identical, parallel 1D
electronic channels, separated by a fixed nanoscale distance, which is tunable
by the interlayer twist angle. At a twist angle of ~ 5 degrees, we find that
hole-doped tWTe$_{2}$ exhibits exceptionally large transport anisotropy with a
resistance ratio of ~ 1000 between two orthogonal in-plane directions. The
across-wire conductance exhibits power-law scaling behaviors, consistent with
the formation of a 2D anisotropic phase that resembles an array of LLs. Our
results open the door for realizing a variety of correlated and topological
quantum phases based on coupled-wire models and LL physics. | 2109.04637v3 |
2021-09-16 | MEAM parameterization for cyclic and tensile deformations of Gold-Silver core-shell systems | Gold-Silver (Au-Ag) core-shell nanostructures are gaining importance in
stretchable electronics where high tensile and fatigue resistance is of
paramount importance. This work proposes the parameterization of a modified
embedded atomic model (MEAM) interatomic potential through density functional
theory (DFT) calculations for investigating the role of dislocations and defect
interaction governing the mechanical behavior of Au-Ag and Ag-Au Core-shell
nanostructures under tensile and fatigue loading using molecular dynamics (MD)
simulations. A comparative analysis between the Core-shell structures and their
pristine counterparts is also conducted. Throughout this work, pseudo-potential
and all-electron full potential DFT schemes are used for parameterizing MEAM by
calculating cohesive energy, lattice parameter, and bulk modulus of pure Au, Ag
and their alloy. Using the new force-field for MD simulations, the tensile
behavior of pristine and core-shell nanowires is explored for temperatures
between 300K to 600K. The fatigue properties of two pristine and two core-shell
nanowires in a strain range of -15% to 15% for 10 cycles is also conducted. Our
results suggest that Ag-Au Core-shell nanowire shows the best reversibility
under fatigue loading among the structures examined. Moreover, Ag-Au exhibit
the highest dislocation formation and complete annihilation of defects
consistently. While, Au-Ag present improved fatigue properties than its
pristine counterparts but have some residual defects leading to lower
reversibility when compared to Ag-Au. For tensile loading, all four structures
exhibited deterioration in strength with increasing temperature. Thermal
softening is seen to be more prominent in Au-Ag core-shell nanowires compared
to Ag-Au. | 2109.08196v2 |
2021-10-08 | Synthesis and study of ScN thin films | To contemplate an alternative approach for the minimization of diffusion at
high temperature depositions, present findings impart viability of
room-temperature deposited reactively sputtered ScN thin film samples. The
adopted room temperature route endows precise control over the $R_{N_2}$ flow
for a methodical structural phase evolution from Sc$\to$ScN and probe the
correlated physical aspects of the highly textured ScN samples. In the nitrided
regime i.e. at $R_{N_2}$ = 2.5-100% flow, incorporation of unintentional oxygen
defects were evidenced from surface sensitive soft x-ray absorption
spectroscopy study, though less compared to their metal ($R_{N_2} = 0\%$) and
interstitial ($R_{N_2} = 1.6\%$) counterparts, due to higher Gibb's free energy
for Sc-O-N formation with no trace of ligand field splitting around the O
K-edge spectra. To eradicate the sceptism of appearance of N K-edge (401.6 eV)
and Sc L-edge (402.2 eV) absorption spectra adjacent to each other, the nascent
Sc K-edge study has been adopted for the first time to validate complementary
insight on the metrical parameters of the Sc-N system taken into consideration.
Optical bandgaps of the polycrystalline ScN thin film samples were found to
vary between 2.25-2.62 eV as obtained from the UV-Vis spectroscopy, whereas,
the nano-indentation hardness and modulus of the as-deposited samples lie
between 15-34GPa and 152-476GPa, respectively following a linearly increasing
trend of resistance to plastic deformations. Besides, contrary to other early
3d transition metal nitrides (TiN, VN, CrN), a comprehensive comparison of
noticeably large homogeneity range in Sc-N has been outlined to apprehend the
minuscule lattice expansion over the large $R_{N_2}$ realm. | 2110.04008v1 |
2021-12-28 | Highly sensitive fire alarm system based on cellulose paper with low temperature response and wireless signal conversion | Highly sensitive smart sensors for early fire detection with remote warning
capabilities are urgently required to improve the fire safety of combustible
materials in diverse applications. The highly-sensitive fire alarm can detect
fire situation within a short time quickly when a fire disaster is about to
occur, which is conducive to achieve fire tuned. Herein, a novel fire alarm is
designed by using flame-retardant cellulose paper loaded with graphene oxide
(GO) and two-dimensional titanium carbide (Ti3C2, MXene). Owing to the
excellent temperature dependent electrical resistance switching effect of GO,
it acts as an electrical insulator at room temperature and becomes electrically
conductive at high temperature. During a fire incident, the partial
oxygen-containing groups on GO will undergo complete removal, which results in
the conductivity transformation.Besides the use of GO feature, this work also
introduces conductive MXene to enhance fire detection speed and warning at low
temperature, especially below 300 {\deg}C. The designed flame-retardant fire
alarm is sensitive enough to detect fire incident, showing a response time of 2
s at 250 {\deg}C, which is calculated by a novel and quantifiable technique.
More importantly, the designed fire alarm sensor is coupled to a wireless
communication interface to conveniently transmit fire signal remotely.
Therefore, when an abnormal temperature is detected, the signal is wirelessly
transmitted to a liquid crystal display (LCD) screen when displays a message
such as "FIRE DANGER". The designed smart fire alarm paper is promising for use
as a smart wallpaper for interior house decoration and other applications
requiring early fire detection and warning. | 2201.05442v1 |
2022-10-24 | Anomalous Hall effect and two-dimensional Fermi surfaces in the charge-density-wave state of kagome metal RbV$_3$Sb$_5$ | AV$_3$Sb$_5$ (A=Cs, K, Rb) are recently discovered superconducting systems
($T_{\rm c}\sim0.9-2.5$ K) in which the vanadium atoms adopt the kagome
structure. Intriguingly, these systems enter a charge-density-wave (CDW) phase
($T_{\rm CDW}\sim80-100$ K), and further evidence shows that the time-reversal
symmetry is broken in the CDW phase. Concurrently, the anomalous Hall effect
has been observed in KV$_3$Sb$_5$ and CsV$_3$Sb$_5$ inside the novel CDW phase.
Here, we report a comprehensive study of a high-quality RbV$_3$Sb$_5$ single
crystal with magnetotransport measurements. Our data demonstrate the emergence
of anomalous Hall effect in RbV$_3$Sb$_5$ when the charge-density-wave state
develops. The magnitude of anomalous Hall resistivity at the low temperature
limit is comparable to the reported values in KV$_3$Sb$_5$ and CsV$_3$Sb$_5$.
The magnetoresistance channel further reveals a rich spectrum of quantum
oscillation frequencies, many of which have not been reported before. In
particular, a large quantum oscillation frequency (2235 T), which occupies
$\sim$56% of the Brillouin zone area, has been recorded. For the quantum
oscillation frequencies with sufficient signal-to-noise ratio, we further
perform field-angle dependent measurements and our data indicate
two-dimensional Fermi surfaces in RbV$_3$Sb$_5$. Our results provide
indispensable information for understanding the anomalous Hall effect and band
structure in kagome metals AV$_3$Sb$_5$. | 2210.13250v2 |
2022-11-17 | An effective anisotropic visco-plastic model dedicated to high contrast ductile laminated microstructures: Application to lath martensite substructure | In particular types of layer- or lamellar-like microstructures such as
pearlite and lath martensite, plastic slip occurs favorably in directions
parallel to inter-lamellar boundaries. This may be due to the interplay between
morphology and crystallographic orientation or, more generally, due to
constraints imposed on the plastic slip due to the lamellar microstructural
geometry. This paper proposes a micromechanics based, computationally
efficient, scale independent model for particular type of lamellar
microstructures containing softer lamellae, which are sufficiently thin to be
considered as discrete slip planes embedded in a matrix representing the harder
lamellae. Accordingly, the model is constructed as an isotropic visco-plastic
model which is enriched with an additional orientation-dependent planar plastic
deformation mechanism. This additional mode is activated when the applied load,
projected on the direction of the soft films, induces a significant amount of
shear stress. Otherwise, the plastic deformation is governed solely by the
isotropic part of the model. The response of the proposed model is assessed via
a comparison to direct numerical simulations (DNS) of an infinite periodic
two-phase laminate. It is shown that the yielding behavior of the model follows
the same behavior as the reference model. It is observed that the proposed
model is highly anisotropic, and the degree of anisotropy depends on the
contrast between the slip resistance (or yield stress) of the planar mode
versus that of the isotropic part. The formulation is then applied to model the
substructure of lath martensite with inter-layer thin austenite films. It is
exploited in a mesoscale simulation of a dual-phase (DP) steel
microstructure.The results are compared with those of a standard isotropic
model and a full crystal plasticity model. | 2211.09754v1 |
2022-11-19 | Density-tuned effective metal-insulator transitions in 2D semiconductor layers: Anderson localization or Wigner crystallization | Electrons (or holes) confined in 2D semiconductor layers have served as model
systems for studying disorder and interaction effects for almost 50 years. In
particular, strong disorder drives the metallic 2D carriers into a strongly
localized Anderson insulator (AI) at low densities whereas pristine 2D
electrons in the presence of no (or little) disorder should solidify into a
Wigner crystal at low carrier densities. Since the disorder in 2D
semiconductors is mostly Coulomb disorder arising from random charged
impurities, the applicable physics is complex as the carriers interact with
each other as well as with the random charged impurities through the same
long-range Coulomb coupling. By critically theoretically analyzing the
experimental transport data in depth using a realistic transport theory to
calculate the low-temperature 2D resistivity as a function of carrier density
in 11 different experimental samples covering 9 different materials, we
establish, utilizing the Ioffe-Regel-Mott criterion for strong localization, a
direct connection between the critical localization density for the 2D
metal-insulator transition (MIT) and the sample mobility deep into the metallic
state, which for clean samples could lead to a localization density low enough
to make the transition appear to be a Wigner crystallization. We believe that
the insulating phase is always an effective Coulomb disorder-induced localized
AI, which may have short-range WC-like correlations at low carrier densities.
Our theoretically calculated disorder-driven critical MIT density agrees with
experimental findings in all 2D samples, even for the ultra-clean samples. In
particular, the extrapolated critical density for the 2D MIT seems to vanish
when the high-density mobility goes to infinity, indicating that transport
probes a disorder-localized insulating ground state independent of how low the
carrier density might be. | 2211.10673v2 |
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