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2018-09-09 | Efficient thermionic operation and phonon isolation by a semiconductor-superconductor junction | Control of heat flux at small length scales is crucial for numerous
solid-state devices and systems. In addition to the thermal management of
information and communication devices the mastering of heat transfer channels
down to the nanoscale also enable, e.g., new memory concepts, high sensitivity
detectors and sensors, energy harvesters and compact solid-state refrigerators.
Electronic coolers and thermal detectors for electromagnetic radiation,
especially, rely on the maximization of electro-thermal response and blockade
of phonon transport. In this work, we propose and demonstrate that efficient
electro-thermal operation and phonon transfer blocking can be achieved in a
single solid-state thermionic junction. Our experimental demonstration relies
on suspended semiconductor-superconductor junctions where the electro-thermal
response arises from the superconducting energy gap, and the phonon blocking
naturally results from the transmission bottleneck at the junction. We suspend
different size degenerately doped silicon chips (up to macroscopic scale)
directly from the junctions and cool these by biasing the junctions. The
electronic cooling operation characteristics are accompanied by measurement and
analysis of the thermal resistance components in the structures indicating the
operation principle of phonon blocking in the junctions. | 1809.02994v1 |
2018-11-02 | Conformal printing of graphene for single and multi-layered devices on to arbitrarily shaped 3D surfaces | Printing has drawn a lot of attention as a means of low per-unit cost and
high throughput patterning of graphene inks for scaled-up thin-form factor
device manufacturing. However, traditional printing processes require a flat
surface and are incapable of achieving patterning on to 3D objects. Here, we
present a conformal printing method to achieve functional graphene-based
patterns on to arbitrarily-shaped surfaces. Using experimental design, we
formulate a water-insoluble graphene ink with optimum conductivity. We then
print single and multi-layered electrically functional structures on to a
sacrificial layer using conventional screen printing. The print is then floated
on water, allowing the dissolution of the sacrificial layer, while retaining
the functional patterns. The single and multilayer patterns can then be
directly transferred on to arbitrarily-shaped 3D objects without requiring any
post deposition processing. Using this technique, we demonstrate conformal
printing of single and multilayer functional devices that include joule
heaters, resistive strain sensors and proximity sensors on hard, flexible and
soft substrates, such as glass, latex, thermoplastics, textiles, and even
candies and marshmallows. Our simple strategy offers great promises to add new
device and sensing functionalities to previously inert 3D surfaces. | 1811.01073v2 |
2018-11-12 | Homogeneous Large-area Quasi-freestanding Monolayer and Bilayer Graphene on SiC | In this study, we first show that the argon flow during epitaxial graphene
growth is an important parameter to control the quality of the buffer and the
graphene layer. Atomic force microscopy (AFM) and low-energy electron
diffraction (LEED) measurements reveal that the decomposition of the SiC
substrate strongly depends on the Ar mass flow rate while pressure and
temperature are kept constant. Our data are interpreted by a model based on the
competition of the SiC decomposition rate, controlled by the Ar flow, with a
uniform graphene buffer layer formation under the equilibrium process at the
SiC surface. The proper choice of a set of growth parameters allows the growth
of defect-free, ultra-smooth and coherent graphene-free buffer layer and
bilayer-free monolayer graphene sheets which can be transformed into large-area
high-quality quasi-freestanding monolayer and bilayer graphene (QFMLG and
QFBLG) by hydrogen intercalation. AFM, scanning tunneling microscopy (STM),
Raman spectroscopy and electronic transport measurements underline the
excellent homogeneity of the resulting quasi-freestanding layers. Electronic
transport measurements in four-point probe configuration reveal a homogeneous
low resistance anisotropy on both {\mu}m- and mm scales. | 1811.04998v1 |
2018-11-14 | Giant anisotropy in superconducting single crystals of CsCa$_2$Fe$_4$As$_4$F$_2$ | CsCa$_2$Fe$_4$As$_4$F$_2$ is a newly discovered iron-based superconductor
with $T_\mathrm{c}\sim$ 30 K containing double Fe$_2$As$_2$ layers that are
separated by insulating Ca$_2$F$_2$ spacer layers. Here we report the transport
and magnetization measurements on CsCa$_2$Fe$_4$As$_4$F$_2$ single crystals
grown for the first time using the self flux of CsAs. We observed a huge
resistivity anisotropy $\rho_c(T)/\rho_{ab}(T)$, which increases with
decreasing temperature, from 750 at 300 K to 3150 at 32 K. The $\rho_c(T)$ data
exhibit a non-metallic behavior above $\sim$140 K, suggesting an incoherent
electronic state at high temperatures due to the dimension crossover. The
superconducting onset transition temperature in $\rho_{ab}$ is 0.7 K higher
than that in $\rho_c$, suggesting two-dimensional (2D) superconducting
fluctuations. The lower and upper critical fields also show an exceptional
anisotropy among iron-based superconductors. The $H_{c1}^\bot(T)$ data are well
fitted using the model with two $s$-wave-like superconducting gaps,
$\Delta_1(0)=6.75$ meV and $\Delta_2(0)=2.32$ meV. The inter-plane coherence
length $\xi_c(0)$ is $3.6$ \AA, remarkably smaller than the distance between
conducting layers (8.6 \AA), consolidating the 2D nature in the title material. | 1811.05706v1 |
2019-02-01 | Light Enhanced Blue Energy Generation using MoS$_2$ Nanopores | Blue energy relies on the chemical potential difference generated between
solutions of high and low ionic strength and would provide a sun-and-wind
independent energy source at estuaries around the world. Converting this
osmotic energy through reverse-electrodialysis relies on ion-selective
membranes. A novel generation of these membranes is based on atomically thin
MoS$_2$ membranes to decrease the resistance to current flow to increase power
output. By modulating the surface charge by light we are able to raise the ion
selectivity of the membrane by a factor of 5 while staying at a neutral pH.
Furthermore, we find that the behavior of small nanopores is dominated by
surface conductance. We introduce a formalism based on the Dukhin number to
quantify these effects in the case of a concentration gradient system. As a
consequence, the charges created by light illumination provoke two important
changes. Increased surface charge at the pore rim enhances the ion selectivity
and therefore larger osmotic voltage (dominating in small pores), while the
increased surface charge of the overall membrane enhances the surface
conductance and therefore the osmotic current (dominating in larger pores). The
combination of these effects might be able to efficiently boost the energy
generation with arrays of nanopores with varying pore sizes. | 1902.00410v1 |
2019-02-25 | Synaptic Learning and Memory Functions Achieved in Self-rectifying BFO Memristor under Extreme Environmental Temperature | Memristors have been intensively studied in recent years as promising
building blocks for next-generation non-volatile memory, artificial neural
networks and brain-inspired computing systems. Even though the environment
adaptability of memristor has been required in many application fields, it has
been rarely reported due to the underlying mechanism could become invalid
especially at an elevated temperature. Here, we focus on achieving synaptic
learning and memory functions in BiFeO3 memristor in a wide range of
temperature. We have proved the ferroelectricity of BFO films at a record-high
temperature of 500 {\deg}C by piezoresponse force microscopy (PFM) measurement.
Due to the robust ferroelectricity of BFO thin film, an analog-like resistance
switching behavior has been clearly found in a wide range of temperature, which
is attributed to the reversal of ferroelectric polarization. Various synaptic
functions including long-term potentiation (LTP), depression (LTD), consecutive
potentiation/depression (P/D) and spike-timing dependent plasticity (STDP) have
been realized from -170 to 300 {\deg}C, illustrating their potential for
electronic applications even under extreme environmental temperature. | 1902.09081v1 |
2019-07-03 | Probing defect states in few-layer MoS$_{2}$ by conductance fluctuation spectroscopy | Despite the concerted effort of several research groups, a detailed
experimental account of defect dynamics in high-quality single- and few-layer
transition metal dichalcogenides remain elusive. In this paper we report an
experimental study of the temperature dependence of conductance and
conductance-fluctuations on several few-layer MoS$_{2}$ exfoliated on hexagonal
boron nitride and covered by a capping layer of high-$\kappa$ dielectric
HfO$_{2}$. The presence of the high-$\kappa$ dielectric made the device
extremely stable against environmental degradation as well as resistant to
changes in device characteristics upon repeated thermal cycling enabling us to
obtain reproducible data on the same device over a time-scale of more than one
year. Our device architecture helped bring down the conductance fluctuations of
the MoS$_2$ channel by orders of magnitude compared to previous reports. The
extremely low noise levels in our devices made in possible to detect the
generation-recombination noise arising from charge fluctuation between the
sulphur-vacancy levels in the band gap and energy-levels at the conductance
band-edge. Our work establishes conduction fluctuation spectroscopy as a viable
route to quantitatively probe in-gap defect levels in low-dimensional
semiconductors. | 1907.01830v1 |
2020-04-11 | High-field depinned phase and planar Hall effect in skyrmion-host Gd$_2$PdSi$_3$ | For the skyrmion-hosting intermetallic Gd$_2$PdSi$_3$ with centrosymmetric
hexagonal lattice and triangular net of rare earth sites, we report a thorough
investigation of the magnetic phase diagram. Our work reveals a new magnetic
phase with isotropic value of the critical field for all orientations, where
the magnetic ordering vector $\mathbf{q}$ is depinned from its preferred
directions in the basal plane. This is in contrast to the highly anisotropic
behavior of the low field phases, such as the skyrmion lattice (SkL), which are
easily destroyed by in-plane magnetic field. The bulk nature of the SkL and of
other magnetic phases was evidenced by specific-heat measurements. Resistivity
anisotropy, likely originating from partial gapping of the density of states
along $\mathbf{q}$ in this RKKY magnet, is picked up via the planar Hall effect
(PHE). The PHE confirms the single-$\mathbf{q}$ nature of the magnetic order
when the field is in the hexagonal plane, and allows to detect the preferred
directions of $\mathbf{q}$. For field aligned perpendicular to the basal plane,
several scenarios for the depinned phase (DP), such as tilted conical order,
are discussed on the basis of the data. | 2004.05385v1 |
2020-09-11 | Electronic nematic states tuned by isoelectronic substitution in bulk FeSe1-xSx | Isoelectronic substitution is an ideal tuning parameter to alter electronic
states and correlations in iron-based superconductors. As this substitution
takes place outside the conducting Fe planes, the electronic behaviour is less
affected by the impurity scattering experimentally and relevant key electronic
parameters can be accessed. In this short review, I present the experimental
progress made in understanding the electronic behaviour of the nematic
electronic superconductors, FeSe1-xSx. A direct signature of the nematic
electronic state is in-plane anisotropic distortion of the Fermi surface
triggered by orbital ordering effects and electronic interactions that result
in multi-band shifts detected by ARPES. Upon sulphur substitution, the
electronic correlations and the Fermi velocities decrease in the tetragonal
phase. Quantum oscillations are observed for the whole series in ultra-high
magnetic fields and show a complex spectra due to the presence of many small
orbits. Effective masses associated to the largest orbit display non-divergent
behaviour at the nematic end point (x~0.175(5)), as opposed to critical
spin-fluctuations in other iron pnictides. Magnetotransport behaviour has a
strong deviation from the Fermi liquid behaviour and linear T resistivity is
detected at low temperatures inside the nematic phase, where scattering from
low energy spin-fluctuations are likely to be present. The superconductivity is
not enhanced in FeSe1-xSx and there are no divergent electronic correlations at
the nematic end point. These manifestations indicate a strong coupling with the
lattice in FeSe1-xSx and a pairing mechanism likely promoted by spin
fluctuations. | 2009.05523v1 |
2020-09-23 | Superconducting fluctuations in overdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ | Fluctuating superconductivity - vestigial Cooper pairing in the resistive
state of a material - is usually associated with low dimensionality, strong
disorder or low carrier density. Here, we report single particle spectroscopic,
thermodynamic and magnetic evidence for persistent superconducting fluctuations
in heavily hole-doped cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$
($T_c$ = 66~K) despite the high carrier density. With a sign-problem free
quantum Monte Carlo calculation, we show how a partially flat band at ($\pi$,0)
can help enhance superconducting phase fluctuations. Finally, we discuss the
implications of an anisotropic band structure on the phase-coherence-limited
superconductivity in overdoped cuprates and other superconductors. | 2009.10932v2 |
2020-09-29 | Structural Phase Dependent Giant Interfacial Spin Transparency in W/CoFeB Thin Film Heterostructure | Pure spin current has transfigured the energy-efficient spintronic devices
and it has the salient characteristic of transport of the spin angular
momentum. Spin pumping is a potent method to generate pure spin current and for
its increased efficiency high effective spin-mixing conductance (Geff) and
interfacial spin transparency (T) are essential. Here, a giant T is reported in
Sub/W(t)/Co20Fe60B20(d)/SiO2(2 nm) heterostructures in \beta-tungsten (\beta-W)
phase by employing all-optical time-resolved magneto-optical Kerr effect
technique. From the variation of Gilbert damping with W and CoFeB thicknesses,
the spin diffusion length of W and spin-mixing conductances are extracted.
Subsequently, T is derived as 0.81 \pm 0.03 for the \beta-W/CoFeB interface. A
sharp variation of Geff and T with W thickness is observed in consonance with
the thickness-dependent structural phase transition and resistivity of W. The
spin memory loss and two-magnon scattering effects are found to have negligible
contributions to damping modulation as opposed to spin pumping effect which is
reconfirmed from the invariance of damping with Cu spacer layer thickness
inserted between W and CoFeB. The observation of giant interfacial spin
transparency and its strong dependence on crystal structures of W will be
important for pure spin current based spin-orbitronic devices. | 2009.14143v1 |
2017-04-20 | Accretion powered AGN feedback in the cores of galaxy clusters | Detection of the copious amount of X-ray emission from the dilute hot plasma
in galaxy clusters suggests that a substantial fraction of the central
intracluster medium (ICM) is cooling radiatively on a time scale much faster
than the Hubble time. Theoretical models predict the cooling rate as high as
about few hundred to few thousand solar mass per year, which would be then made
available for the formation of new stars in the core of these clusters.
However, systematic studies of the cores of such clusters failed to detect the
expected reservoirs of cooled gas. Thus, the gas in the cores of galaxy
clusters is losing substantial amount of energy in the form of X-rays but is
not cooling. This in turn point towards the famous cooling flow paradox and
hence demands some intermittent heating to balance the cooling over such a long
period. Several sources have been suggested to counteract on the cooling of the
ICM, however, the AGN feedback appeared to be the most promising and enough
energetic source to resist cooling of the ICM in the cores of such clusters. In
this presentation I will provide a brief overview on the feedback processes
that are involved in the cores of the galaxy clusters with an emphasis on the
AGN feedback and its observable signatures. | 1704.06047v1 |
2017-04-23 | Steep-slope Hysteresis-free Negative Capacitance MoS2 Transistors | The so-called Boltzmann Tyranny defines the fundamental thermionic limit of
the subthreshold slope (SS) of a metal-oxide-semiconductor field-effect
transistor (MOSFET) at 60 mV/dec at room temperature and, therefore, precludes
the lowering of the supply voltage and the overall power consumption. Adding a
ferroelectric negative capacitor to the gate stack of a MOSFET may offer a
promising solution to bypassing this fundamental barrier. Meanwhile,
two-dimensional (2D) semiconductors, such as atomically thin transition metal
dichalcogenides (TMDs) due to their low dielectric constant, and ease of
integration in a junctionless transistor topology, offer enhanced electrostatic
control of the channel. Here, we combine these two advantages and demonstrate
for the first time a molybdenum disulfide (MoS2) 2D steep slope transistor with
a ferroelectric hafnium zirconium oxide layer (HZO) in the gate dielectric
stack. This device exhibits excellent performance in both on- and off-states,
with maximum drain current of 510 {\mu}A/{\mu}m, sub-thermionic subthreshold
slope and is essentially hysteresis-free. Negative differential resistance
(NDR) was observed at room temperature in the MoS2 negative capacitance
field-effect-transistors (NC-FETs) as the result of negative capacitance due to
the negative drain-induced-barrier-lowering (DIBL). High on-current induced
self-heating effect was also observed and studied. | 1704.06865v2 |
2017-08-04 | The cuprate phase diagram and the influence of nanoscale inhomogeneities | The phase diagram associated with the high Tc superconductors is complicated
by an array of different ground states. The parent material represents an
antiferromagnetic insulator but with doping superconductivity becomes possible
with transition temperatures previously thought unattainable. The underdoped
region of the phase diagram is dominated by the so-called pseudogap phenomena
whereby in the normal state the system mimics superconductivity in its spectra
response but does not show the complete loss of resistivity associated with the
superconducting state. An understanding of this regime presents one of the
great challenges for the field. In the present study we revisit the structure
of the phase diagram as determined in photoemission studies. By careful
analysis of the role of nanoscale inhomogeneities in the overdoped region we
are able to more carefully separate out the gaps due to the pseudogap phenomena
from the gaps due to the superconducting transition. Within a mean field
description we are thus able to link the magnitude of the gap directly to the
Heisenberg exchange interaction term, $J\sum{s_i \cdot s_j}$, contained in the
$t-J$ model. This approach provides a clear indication that the pseudogap is
that associated with spin singlet formation. | 1708.01558v2 |
2019-03-12 | Precision Analysis of Evolved Stars | Evolved stars dominate galactic spectra, enrich the galactic medium, expand
to change their planetary systems, eject winds of a complex nature, produce
spectacular nebulae and illuminate them, and transfer material between binary
companions. While doing this, they fill the HR diagram with diagnostic loops
that write the story of late stellar evolution. Evolved stars sometimes release
unfathomable amounts of energy in neutrinos, light, kinetic flow, and
gravitational waves. During these late-life times, stars evolve complexly, with
expansion, convection, mixing, pulsation, mass loss. Some processes have
virtually no spatial symmetries, and are poorly addressed with low-resolution
measurements and analysis. Even a "simple" question as how to model mass loss
resists solution. However, new methods offer increasingly diagnostic tools.
Astrometry reveals populations and groupings. Pulsations/oscillations support
study of stellar interiors. Optical/radio interferometry enable 2-3d imagery of
atmospheres and shells. Bright stars with rich molecular spectra and velocity
fields are a ripe opportunity for imaging with high spatial and spectral
resolution, giving insight into the physics and modeling of later stellar
evolution. | 1903.05109v1 |
2019-04-14 | Superconducting Praseodymium Superhydrides | Superhydrides have complex hydrogenic sublattices and are important
prototypes for studying metallic hydrogen and high-temperature superconductors.
Encouraged by the results on LaH10, in consideration of the differences between
La and Pr, Pr-H system is especially worth studying because of the magnetism
and valence-band f-electrons in element Pr. Here we successfully synthesized
praseodymium superhydrides (PrH9) in laser-heated diamond anvil cells.
Synchrotron X-ray diffraction (XRD) analysis demonstrated the presence of
previously predicted F43m-PrH9 and unexpected P63/mmc-PrH9 phases. Moreover,
Fm3m-PrH3, P4/nmm-PrH(3-{\delta}) and Fm3m-PrH(1+x) were found below 52 GPa.
F43m-PrH9 and P63/mmc-PrH9 were stable above 100 GPa in experiment.
Experimental studies of electrical resistance in the PrH9 sample showed the
emergence of superconducting transition (Tc) below 9K and a dependent Tc on
applied magnetic field. Theoretical calculations indicate that magnetic order
and electron-phonon interaction coexist in a very close range of pressures in
the PrH9 sample which may contribute to its low superconducting temperature Tc.
Our results highlight the intimate connections among hydrogenic sublattices,
density of states, magnetism and superconductivity in Pr-based superhydrides. | 1904.06643v2 |
2019-08-06 | Hump-like structure in Hall signal from SrRuO$_3$ ultra-thin films without inhomogeneous anomalous Hall effect | A controversy arose over the interpretation of the recently observed hump
features in Hall resistivity $\rho_{xy}$ from ultra-thin SrRuO$_3$ (SRO) film;
it was initially interpreted to be due to topological Hall effect but was later
proposed to be from existence of regions with different anomalous Hall effect
(AHE). In order to settle down the issue, we performed Hall effect as well as
magneto-optic Kerr-effect measurements on 4 unit cell SRO films grown on
SrTiO$_3$ (001) substrates. Clear hump features are observed in the measured
$\rho_{xy}$, whereas neither hump feature nor double hysteresis loop is seen in
the Kerr rotation which should be proportional to the magnetization. In
addition, magnetization measurement by superconducting quantum interference
device shows no sign of multiple coercive fields. These results show that
inhomogeneous AHE alone cannot explain the observed hump behavior in
$\rho_{xy}$ data from our SRO ultra-thin films. We found that emergence of the
hump structure in $\rho_{xy}$ is closely related to the growth condition, high
quality films having clear sign of humps. | 1908.02083v1 |
2019-08-22 | High-Pressure Synthesis of Magnetic Neodymium Polyhydrides | The current search for room-temperature superconductivity is inspired by the
unique properties of the electron-phonon interaction in metal superhydrides.
Encouraged by the recently found highest-$T_C$ superconductor fcc-$LaH_{10}$,
here we discover several superhydrides of another lanthanide - neodymium. We
identify three novel metallic Nd-H phases at pressure range from 85 to 135 GPa:
$I4/mmm$-$NdH_4$, $C2/c$-$NdH_7$, $P6_3/mmc$-$NdH_9$, synthesized by
laser-heating metal samples in NH3BH3 media for in situ generation of hydrogen.
A lower trihydride $Fm\bar{3}m$-$NdH_3$ is found at pressures from 2 to 52 GPa.
$I4/mmm$-$NdH_4$ and $C2/c$-$NdH_7$ are stable from 135 down to 85 GPa, and
$P6_3/mmc$-$NdH_9$ from 110 to 130 GPa. Measurements of the electrical
resistance of NdH9 demonstrate a possible superconducting transition at ~4.5 K
in $P6_3/mmc$-$NdH_9$. Our theoretical calculations predict that all the
neodymium hydrides have antiferromagnetic order at pressures below 150 GPa and
represent one of the first discovered examples of strongly correlated
superhydrides with large exchange spin-splitting in the electron band structure
(> 450 meV). The critical N$\'e$el temperatures for new neodymium hydrides are
estimated using the mean-field approximation as about 4 K ($NdH_4$), 251 K
($NdH_7$) and 136 K ($NdH_9$). | 1908.08304v3 |
2019-12-27 | Localized spin waves at low temperatures in a Cobalt Carbide nanocomposite | We study magnetic, transport and thermal properties of Cobalt carbide
nanocomposite with a mixture of Co2C and Co3C phases in 1:1 ratio, with an
average particle diameter of 40$\pm 15$ nm. We show that the behavior of the
nanocomposite is completely different from that of either Co3C or Co2C. We
observed that with decreasing temperature the saturation magnetization MS(T)
increases, however, below 100 K, there is a steep rise. A detail analysis shows
the increase in MS(T) down to 100 K is explained via the surface spin freezing
model. However, below 100 K the steep increase in MS(T) is explained by a
finite size effect related to a confinement of spin waves within the nano
particles. The measurement of heat capacity shows broad peak at 100 K along
with presence of another anomaly at a lower temperature 43 K(=Tex). Resistance
measurement in the nanocomposite shows metallic behavior at high T with an
unusual anomaly appearing at Tex, which is near the T regime where MS(T) begins
to increase steeply. A measurement of the temperature gradients across the
sample thickness indicates an abrupt change in thermal conductivity at Tex
which suggests a phase transition at Tex. Our results are explained in terms of
a transformation from a magnetically coupled state with a continuous spectrum
of spin waves into a magnetically decoupled state below 100 K with confined
spin waves. | 1912.12085v1 |
2012-06-06 | Irreversibility and time relaxation in electrostatic doping of oxide interfaces | Two-dimensional electron gas (2DEG) confined in quantum wells at insulating
oxide interfaces have attracted much attention as their electronic properties
display a rich physics with various electronics orders such as
superconductivity and magnetism. A particularly exciting features of these
hetero-structures lies in the possibility to control their electronic
properties by electrostatic gating, opening up new opportunities for the
development of oxide based electronics. However, unexplained gating hysteresis
and time relaxation of the 2DEG resistivity have been reported in some bias
range, raising the question of the precise role of the gate voltage. Here we
show that in LaTiO3/SrTiO3 and LaAlO3/SrTiO3 heterostructures, above a filling
threshold, electrons irreversibly escape out of the well. This mechanism, which
is directly responsible for the hysteresis and time relaxation, can be entirely
described by a simple analytical model derived in this letter. Our results
highlight the crucial role of the gate voltage both on the shape and the
filling of the quantum well. They also demonstrate that it is possible to
achieve a low-carrier density regime in a semiconductor limit, whereas the
high-carrier density regime is intrinsically limited. | 1206.1198v1 |
2018-07-04 | Doping effects of Cr on the physical properties of BaFe$_{1.9-x}$Ni$_{0.1}$Cr$_{x}$As$_{2}$ | We present a systematic study on the heavily Cr doped iron pnictides
BaFe$_{1.9-x}$Ni$_{0.1}$Cr$_{x}$As$_{2}$ by using elastic neutron scattering,
high-resolution synchrotron X-ray diffraction (XRD), resistivity and Hall
transport measurements. When the Cr concentration increases from $x=$ 0 to 0.8,
neutron diffraction experiments suggest that the collinear antiferromagnetism
persists in the whole doping range, where the N\'{e}el temperature $T_N$
coincides with the tetragonal-to-orthorhombic structural transition temperature
$T_s$, and both of them keeps around 35 K. The magnetic ordered moment, on the
other hand, increases within increasing $x$ until $x=$ 0.5, and then decreases
with further increasing $x$. Detailed refinement of the powder XRD patterns
reveals that the Cr substitutions actually stretch the FeAs$_4$ tetrahedron
along the $c-$axis and lift the arsenic height away Fe-Fe plane. Transport
results indicate that the charge carriers become more localized upon Cr doping,
then changes from electron-type to hole-type around $x=$ 0.5. Our results
suggest that the ordered moment and the ordered temperature of static magnetism
in iron pnictides can be decoupled and tuned separately by chemical doping. | 1807.01612v1 |
2018-07-26 | Excitation and coherent control of spin qudit modes with sub-MHz spectral resolution | Quantum bit or qubit is a two-level system, which builds the foundation for
quantum computation, simulation, communication and sensing. Quantum states of
higher dimension, i.e., qutrits (D = 3) and especially qudits (D = 4 or
higher), offer significant advantages. Particularly, they can provide
noise-resistant quantum cryptography, simplify quantum logic and improve
quantum metrology. Flying and solid-state qudits have been implemented on the
basis of photonic chips and superconducting circuits, respectively. However,
there is still a lack of room-temperature qudits with long coherence time and
high spectral resolution. The silicon vacancy centers in silicon carbide (SiC)
with spin S = 3/2 are quite promising in this respect, but until now they were
treated as a canonical qubit system. Here, we apply a two-frequency protocol to
excite and image multiple qudit modes in a SiC spin ensemble under ambient
conditions. Strikingly, their spectral width is about one order of magnitude
narrower than the inhomogeneous broadening of the corresponding spin resonance.
By applying Ramsey interferometry to these spin qudits, we achieve a spectral
selectivity of 600 kHz and a spectral resolution of 30 kHz. As a practical
consequence, we demonstrate absolute DC magnetometry insensitive to thermal
noise and strain fluctuations. | 1807.10383v1 |
2018-12-16 | Anti-fatigue-fracture hydrogels | The emerging applications of hydrogels in devices and machines require these
soft materials to maintain robustness under cyclic mechanical loads. Whereas
hydrogels have been made tough to resist fracture under a single cycle of
mechanical load, these toughened gels still suffer from fatigue fracture under
multiple cycles of loads. The reported fatigue threshold (i.e., the minimal
fracture energy at which crack propagation occurs under cyclic loads) for
synthetic hydrogels is on the order of 1-100 J/m2, which is primarily
associated with the energy required to fracture a single layer of polymer
chains per unit area. Here, we demonstrate that the controlled introduction of
crystallinity in hydrogels can significantly enhance their fatigue thresholds,
since the process of fracturing crystalline domains for fatigue-crack
propagation requires much higher energy than fracturing a single layer of
polymer chains. The fatigue threshold of polyvinyl alcohol (PVA) with a
crystallinity of 18.9 wt.% in the swollen state can exceed 1,000 J/m2. We
further develop a strategy to enhance the anti-fatigue-fracture properties of
PVA hydrogels, but still maintain their high water contents and low moduli by
patterning highly-crystalline regions in the hydrogels. The current work not
only reveals an anti-fatigue-fracture mechanism in hydrogels but also provides
a practical method to design anti-fatigue-fracture hydrogels for diverse
applications. | 1812.06403v1 |
2019-01-15 | Use of Cernox thermometers in AC specific heat measurements under pressure | We report on the resistance behavior of bare-chip Cernox thermometers under
pressures up to 2 GPa, generated in a piston-cylinder pressure cell. Our
results clearly show that Cernox thermometers, frequently used in
low-temperature experiments due to their high sensitivity, remain highly
sensitive even under applied pressure. We show that these thermometers are
therefore ideally suited for measurements of heat capacity under pressure
utilizing an ac oscillation technique up to at least 150 K. Our Cernox-based
system is very accurate in determining changes of the specific heat as a
function of pressure as demonstrated by measurements of the heat capacity on
three different test cases: (i) the superconducting transition in elemental Pb
(T_{c} = 7.2 K), (ii) the antiferromagnetic transition in the rare-earth
compound GdNiGe3 (T_{N} = 26 K) and (iii) the structural/magnetic transition in
the iron-pnictide BaFe2As2 (T_{s,N} = 130 K). The chosen examples demonstrate
the versatility of our technique for measuring the specific heat under pressure
of various condensed matter systems with very different transition temperatures
as well as amounts of removed entropy. | 1901.05089v2 |
2019-10-07 | High electrical conductivity of single metal-organic chains | Molecular wires are essential components for future nanoscale electronics.
However, the preparation of individual long conductive molecules is still a
challenge. MMX metal-organic polymers are quasi-one-dimensional sequences of
single halide atoms (X) bridging subunits with two metal ions (MM) connected by
organic ligands. They are excellent electrical conductors as bulk macroscopic
crystals and as nanoribbons. However, according to theoretical calculations,
the electrical conductance found in the experiments should be even higher. Here
we demonstrate a novel and simple drop-casting procedure to isolate bundles of
few to single MMX chains. Furthermore, we report an exponential dependence of
the electrical resistance of one or two MMX chains as a function of their
length that does not agree with predictions based on their theoretical band
structure. We attribute this dependence to strong Anderson localization
originated by structural defects. Theoretical modeling confirms that the
current is limited by structural defects, mainly vacancies of iodine atoms,
through which the current is constrained to flow. Nevertheless, measurable
electrical transport along distances beyond 250 nm surpasses that of all other
molecular wires reported so far. This work places in perspective the role of
defects in one-dimensional wires and their importance for molecular
electronics. | 1910.02834v1 |
2020-10-01 | Universal limiting transition temperature for the high $T_\mathrm{c}$ superconductors | Since their discovery three decades ago it has emerged that the physics of
high-$T_\mathrm{c}$ cuprate superconductors is characterised by multiple
temperature scales, and a phenomenology that deviates significantly from the
conventional paradigm of superconductivity. Below the "pseudogap" temperature,
$T^*$, a range of experiments indicate a reduction in the density of states.
Lower in temperature, $T_\mathrm{c}$ marks the onset of a bulk zero-resistance
state. Intermediate between these, numerous other temperature boundaries and
cross-overs have been identified, often postulated to be associated with
fluctuations of some kind. However, for the most part there is little consensus
either over their definitions or the physical mechanisms at play. One of these
temperature scales is the Nernst onset temperature, $T_\mathrm{onset}$, below
which thermoelectric phenomena characteristic of superconductivity are
observed. Depending on the material, the difference between $T_\mathrm{onset}$
and $T_\mathrm{c}$ ranges from almost nothing to over 100K. In this paper, we
identify $T_\mathrm{onset}$ from published experimental data according to a
consistent definition; this reveals a remarkable consistency of behaviour
across the whole high-$T_\mathrm{c}$ family, despite the appreciable variations
in other parameters. Our analysis suggests that in the cuprates there is an
inherent limit to $T_\mathrm{c} \sim 135$K. We compare this behaviour with
other strongly-correlated superconductors and propose a unified picture of
superconducting fluctuations close to the Mott state. | 2010.00572v1 |
2020-11-17 | Shallow Valence Band of Rutile GeO$_2$ and P-type Doping | GeO$_2$ has an $\alpha$-quartz-type crystal structure with a very wide
fundamental band gap of 6.6 eV and is a good insulator. Here we find that the
stable rutile-GeO$_2$ polymorph with a 4.6 eV band gap has a surprisingly low
$\sim$6.8 eV ionization potential, as predicted from the band alignment using
first-principles calculations. Because of the short O$-$O distances in the
rutile structure containing cations of small effective ionic radii such as
Ge$^{4+}$, the antibonding interaction between O 2p orbitals raises the valence
band maximum energy level to an extent that hole doping appears feasible.
Experimentally, we report the flux growth of $1.5 \times 1.0 \times 0.8$ mm$^3$
large rutile GeO$_2$ single crystals and confirm the thermal stability for
temperatures up to $1021 \pm 10~^\circ$C. X-ray fluorescence spectroscopy shows
the inclusion of unintentional Mo impurities from the Li$_2$O$-$MoO$_3$ flux,
as well as the solubility of Ga in the r-GeO$_2$ lattice as a prospective
acceptor dopant. The resistance of the Ga- and Mo-codoped r-GeO$_2$ single
crystals is very high at room temperature, but it decreases by 2-3 orders of
magnitude upon heating to 300 $^\circ$C, which is attributed to
thermally-activated p-type conduction. | 2011.08928v1 |
2020-11-25 | Evidence for freezing of charge degrees of freedom across a critical point in CeCoIn$_5$ | The presence of a quantum critical point separating two distinct
zero-temperature phases is thought to underlie the `strange' metal state of
many high-temperature superconductors. The nature of this quantum critical
point, as well as a description of the resulting strange metal, are central
open problems in condensed matter physics. In large part, the controversy stems
from the lack of a clear broken symmetry to characterize the critical phase
transition, and this challenge is no clearer than in the example of the
unconventional superconductor CeCoIn$_5$. Through Hall effect and Fermi surface
measurements of CeCoIn$_5$, in comparison to ab initio calculations, we find
evidence for a critical point that connects two Fermi surfaces with different
volumes without apparent symmetry-breaking, indicating the presence of a
transition that involves an abrupt localization of one sector of the charge
degrees of freedom. We present a model for the anomalous electrical Hall
resistivity of this material based on the conductivity of valence charge
fluctuations. | 2011.12951v1 |
2020-12-03 | Effect of ageing on the properties of the W-containing IRIS-TiAl alloy | The effects of ageing at 800 $^\circ$C on the properties of the IRIS alloy
(Ti$_{49.9}$Al$_{48}$W$_2$B$_{0.1}$) are studied. The initial microstructure of
this alloy densified by Spark Plasma Sintering (SPS) is mainly composed of
lamellar colonies which are surrounded by $\gamma$ grains. The evolutions of
the alloy strength and creep resistance resulting from this ageing treatment
are measured by the related mechanical tests. The microstructural changes are
investigated by scanning and transmission electron microscopies and by X-ray
diffraction. The main structural evolutions consist in a shrinkage of the
lamellar areas and in a precipitation of $\beta_0$ phase, which is accompanied
by a moderate segregation of tungsten and a decrease of the $\alpha_2$ lamellar
width. However, these evolutions are relatively limited and the microstructural
stability is found to result mainly from the low diffusivity of tungsten.
Conversely, a moderate effect of this ageing treatment on mechanical
properties, at room and high temperatures, is measured. Such experimental
results are interpreted and discussed in terms of the microstructural
evolutions and of the deformation mechanisms which are activated at different
temperatures under various solicitations. | 2012.01760v1 |
2020-12-03 | First order transition in trigonal structure ${\textbf{Ca}}{\textbf{Mn}}_{2}{\textbf{P}}_{2}$ | We report structural and physical properties of the single crystalline
${\mathrm{Ca}}{\mathrm{Mn}}_{2}{\mathrm{P}}_{2}$. The X-ray diffraction(XRD)
results show that ${\mathrm{Ca}}{\mathrm{Mn}}_{2}{\mathrm{P}}_{2}$ adopts the
trigonal ${\mathrm{Ca}}{\mathrm{Al}}_{2}{\mathrm{Si}}_{2}$-type structure.
Temperature dependent electrical resistivity $\rho(T)$ measurements indicate an
insulating ground state for ${\mathrm{Ca}}{\mathrm{Mn}}_{2}{\mathrm{P}}_{2}$
with activation energies of 40 meV and 0.64 meV for two distinct regions,
respectively. Magnetization measurements show no apparent magnetic phase
transition under 400 K. Different from other
${\mathrm{A}}{\mathrm{Mn}}_{2}{\mathrm{Pn}}_{2}$ (A = Ca, Sr, and Ba, and Pn =
P, As, and Sb) compounds with the same structure, heat capacity
$C_{\mathrm{p}}(T)$ and $\rho(T)$ reveal that
${\mathrm{Ca}}{\mathrm{Mn}}_{2}{\mathrm{P}}_{2}$ has a first-order transition
at $T$ = 69.5 K and the transition temperature shifts to high temperature upon
increasing pressure. The emergence of plenty of new Raman modes below the
transition, clearly suggests a change in symmetry accompanying the transition.
The combination of the structural, transport, thermal and magnetic
measurements, points to an unusual origin of the transition. | 2012.01863v1 |
2021-02-10 | Electropolishing of single crystal and polycrystalline aluminum to achieve high optical and mechanical surfaces | Electropolishing has found wide application as the final surface treatment of
metal products in mechanical engineering and instrumentation, medicine and
reflective concentrators for PV cells. It was found that electropolishing (EP)
not only reduces the surface roughness and changes its appearance, but improves
many operational characteristics as well, such as corrosion resistance,
endurance, tensile strength, and many others, and also changes the
physicochemical properties, for example, reflectivity, electromagnetic
permeability and electronic emission of some ferro-magnetic metals. This fact
greatly expands the possibility of using this method in various fields of
science and technology. This work is part of a study, examining electro
polishing for reducing the crystalline defects adjacent to the surface, thus
improving its physical properties, contributing to higher optical efficiency,
when used in PV generation and storage devices. Five compositions were examined
using different temperature and current density parameters. The polished
samples were evaluated using reflectance spectrometry. The solution which was
composed of Phosphoric acid - 85%, Acetic acid - 10%, Nitric acid - 5% was
found to provide the best results. Another result obtained was that reflectance
increased as the current density increased up to 25 A/dm2. Further increasing
the current density resulted in deterioration of the surface and reduced
reflectance. It was shown that careful lapping and polishing followed by
electropolishing using the suggested solution may consist of an adequate
treatment for preparing reflective concentrators for PV cells. | 2102.05752v1 |
2021-04-21 | Electrically driven programmable phase-change meta-switch reaching 80% efficiency | Despite recent advances in active metaoptics, wide dynamic range combined
with high-speed reconfigurable solutions is still elusive. Phase-change
materials (PCMs) offer a compelling platform for metasurface optical elements,
owing to the large index contrast and fast yet stable phase transition
properties. Here, we experimentally demonstrate an in situ electrically-driven
reprogrammable metasurface by harnessing the unique properties of a
phase-change chalcogenide alloy, Ge$_{2}$Sb$_{2}$Te$_{5}$ (GST), in order to
realize fast, non-volatile, reversible, multilevel, and pronounced optical
modulation in the near-infrared spectral range. Co-optimized through a
multiphysics analysis, we integrate an efficient heterostructure resistive
microheater that indirectly heats and transforms the embedded GST film without
compromising the optical performance of the metasurface even after several
reversible phase transitions. A hybrid plasmonic-PCM meta-switch with a record
electrical modulation of the reflectance over eleven-fold (an absolute
reflectance contrast reaching 80%), unprecedented quasi-continuous spectral
tuning over 250 nm, and switching speed that can potentially reach a few kHz is
presented. Our work represents a significant step towards the development of
fully integrable dynamic metasurfaces and their potential for beamforming
applications. | 2104.10381v2 |
2021-07-01 | Solid Source Metal-Organic Molecular Beam Epitaxy of Epitaxial RuO2 | A seemingly simple oxide with a rutile structure, RuO2 has been shown to
possess several intriguing properties ranging from strain-stabilized
superconductivity to a strong catalytic activity. Much interest has arisen
surrounding the controlled synthesis of RuO2 films but, unfortunately,
utilizing atomically-controlled deposition techniques like molecular beam
epitaxy (MBE) has been difficult due to the ultra-low vapor pressure and low
oxidation potential of Ru. Here, we demonstrate the growth of epitaxial,
single-crystalline RuO2 films on different substrate orientations using the
novel solid-source metal-organic (MO) MBE. This approach circumvents these
issues by supplying Ru using a pre-oxidized solid metal-organic precursor
containing Ru. High-quality epitaxial RuO2 films with bulk-like
room-temperature resistivity of 55 micro-ohm-cm were obtained at a substrate
temperature as low as 300 C. By combining X-ray diffraction, transmission
electron microscopy, and electrical measurements, we discuss the effect of
substrate temperature, orientation, film thickness, and strain on the structure
and electrical properties of these films. Our results illustrating the use of
novel solid-source MOMBE approach paves the way to the atomic-layer controlled
synthesis of complex oxides of stubborn metals, which are not only difficult to
evaporate but also hard to oxidize. | 2107.00193v1 |
2021-08-02 | PID-like active control strategy for electroacoustic resonators to design tunable single-degree-of-freedom sound absorbers | Sound absorption at low frequencies still remains a challenge in both
scientific research and engineering practice. Natural porous materials are
ineffective in this frequency range, as well as acoustic resonators which
present too narrow bandwidth of absorption, thus requiring alternative
solutions based on active absorption techniques. In the present work, we
propose an active control framework applied on a closed-box loudspeaker to
enable the adjustment of the acoustic impedance at the loudspeaker diaphragm.
More specifically, based on the proportionality between the pressure inside the
enclosure and the axial displacement of the loudspeaker diaphragm at low
frequencies, we demonstrate both analytically and experimentally that a
PID-like feedback control approach allows tuning independently the compliance,
the resistance and the moving mass of the closed-box loudspeaker to implement a
prescribed impedance of a single-degree-of-freedom resonator. By considering
different control combinations to tailor the resonator characteristics, a
perfect absorption (with absorption coefficient equal to 1) is achievable at
the target resonance frequency, while enlarging the effective absorption
bandwidth. Moreover, the proposed feedback control strategy shows an excellent
control accuracy, especially compared to the feedforward-based control formerly
reported in the literature. The mismatches between the performance of
experimental prototype and the model, likely to result from the control time
delay and the inaccuracy in estimating the loudspeaker parameters, can be
compensated directly by tuning the control parameters in the control platform.
The active resonators implemented through the reported control scheme can be
used to build more complex acoustic devices/structures to enable
high-efficiency broadband sound absorption or other types of acoustic phenomena
such as wavefront shaping. | 2108.00765v1 |
2021-10-29 | Supercurrent diode effect and magnetochiral anisotropy in few-layer NbSe$_2$ | Nonreciprocal transport refers to charge transfer processes that are
sensitive to the bias polarity. Until recently, nonreciprocal transport was
studied only in dissipative systems, where the nonreciprocal quantity is the
resistance. Recent experiments have, however, demonstrated nonreciprocal
supercurrent leading to the observation of a supercurrent diode effect in
Rashba superconductors, opening the vision of dissipationless electronics. Here
we report on a supercurrent diode effect in NbSe$_2$ constrictions obtained by
patterning NbSe$_2$ flakes with both even and odd layer number. The observed
rectification is driven by valley-Zeeman spin-orbit interaction. We demonstrate
a rectification efficiency as large as 60%, considerably larger than the
efficiency of devices based on Rashba superconductors. In agreement with recent
theory for superconducting transition metal dichalcogenides, we show that the
effect is driven by an out-of-plane magnetic field component. Remarkably, we
find that the effect becomes field-asymmetric in the presence of an additional
in-plane field component transverse to the current direction. Supercurrent
diodes offer a further degree of freedom in designing superconducting quantum
electronics with the high degree of integrability offered by van der Waals
materials. | 2110.15752v2 |
2021-11-28 | CoFeVSb: A Promising Candidate for Spin Valve and Thermoelectric Applications | We report a combined theoretical and experimental study of a novel quaternary
Heusler system CoFeVSb from the view point of room temperature spintronics and
thermoelectric applications. It crystallizes in cubic structure with small
DO$_3$-type disorder. The presence of disorder is confirmed by room temperature
synchrotron X-ray diffraction(XRD) and extended X-ray absorption fine structure
(EXAFS) measurements. Magnetization data reveal high ordering temperature with
a saturation magnetization of 2.2 $\mu_B$/f.u. Resistivity measurements reflect
half-metallic nature. Double hysteresis loop along with asymmetry in the
magnetoresistance(MR) data reveals room temperature spin-valve feature, which
remains stable even at 300 K. Hall measurements show anomalous behavior with
significant contribution from intrinsic Berry phase. This compound also large
room temperature power factor ($\sim0.62$ mWatt/m/K$^{2}$) and ultra low
lattice thermal conductivity ($\sim0.4$ W/m/K), making it a promising candidate
for thermoelectric application. Ab-initio calculations suggest weak
half-metallic behavior and reduced magnetization (in agreement with experiment)
in presence of DO$_3$ disorder. We have also found an energetically competing
ferromagnetic FM)/antiferromagnetic (AFM) interface structure within an
otherwise FM matrix: one of the prerequisites for spin valve behavior.
Coexistence of so many promising features in a single system is rare, and hence
CoFeVSb gives a fertile platform to explore numerous applications in future. | 2111.14081v1 |
2021-12-16 | Constrained multi-objective optimization of process design parameters in settings with scarce data: an application to adhesive bonding | Adhesive joints are increasingly used in industry for a wide variety of
applications because of their favorable characteristics such as high
strength-to-weight ratio, design flexibility, limited stress concentrations,
planar force transfer, good damage tolerance, and fatigue resistance. Finding
the optimal process parameters for an adhesive bonding process is challenging:
the optimization is inherently multi-objective (aiming to maximize break
strength while minimizing cost), constrained (the process should not result in
any visual damage to the materials, and stress tests should not result in
failures that are adhesion-related), and uncertain (testing the same process
parameters several times may lead to different break strengths). Real-life
physical experiments in the lab are expensive to perform. Traditional
evolutionary approaches (such as genetic algorithms) are then ill-suited to
solve the problem, due to the prohibitive amount of experiments required for
evaluation. Although Bayesian optimization-based algorithms are preferred to
solve such expensive problems, few methods consider the optimization of more
than one (noisy) objective and several constraints at the same time. In this
research, we successfully applied specific machine learning techniques
(Gaussian Process Regression) to emulate the objective and constraint functions
based on a limited amount of experimental data. The techniques are embedded in
a Bayesian optimization algorithm, which succeeds in detecting Pareto-optimal
process settings in a highly efficient way (i.e., requiring a limited number of
physical experiments). | 2112.08760v3 |
2022-01-08 | Are Heavy Fermion Strange Metals Planckian? | Strange metal behavior refers to a linear temperature dependence of the
electrical resistivity at temperatures below the Mott-Ioffe-Regel limit. It is
seen in numerous strongly correlated electron systems, from the heavy fermion
compounds, via transition metal oxides and iron pnictides, to magic angle
twisted bi-layer graphene, frequently in connection with unconventional or
"high temperature" superconductivity. To achieve a unified understanding of
these phenomena across the different materials classes is a central open
problem in condensed matter physics. Tests whether the linear-in-temperature
law might be dictated by Planckian dissipation - scattering with the rate $\sim
k_{\rm B}T/\hbar$, are receiving considerable attention. Here we assess the
situation for strange metal heavy fermion compounds. They allow to probe the
regime of extreme correlation strength, with effective mass or Fermi velocity
renormalizations in excess of three orders of magnitude. Adopting the same
procedure as done in previous studies, i.e., assuming a simple Drude
conductivity with the above scattering rate, we find that for these strongly
renormalized quasiparticles, scattering is much weaker than Planckian, implying
that the linear temperature dependence should be due to other effects. We
discuss implications of this finding and point to directions for further work. | 2201.02820v1 |
2022-03-04 | Character of the "normal state" of the nickelate superconductors | The occurrence of superconductivity in proximity to various strongly
correlated phases of matter has drawn extensive focus on their normal state
properties, to develop an understanding of the state from which
superconductivity emerges. The recent finding of superconductivity in layered
nickelates raises similar interests. However, transport measurements of doped
infinite-layer nickelate thin films have been hampered by materials limitations
of these metastable compounds - in particular, a relatively high density of
extended defects. Here, by moving to a substrate
(LaAlO$_{3}$)$_{0.3}$(Sr$_{2}$TaAlO$_{6}$)$_{0.7}$ which better stabilizes the
growth and reduction conditions, we can synthesize the doping series of
Nd$_{1-x}$Sr$_{x}$NiO$_{2}$ essentially free from extended defects. This
enables the first examination of the 'intrinsic' temperature and doping
dependent evolution of the transport properties. The normal state resistivity
exhibits a low-temperature upturn in the underdoped regime, linear behavior
near optimal doping, and quadratic temperature dependence for overdoping. This
is strikingly similar to the copper oxides, despite key distinctions - namely
the absence of an insulating parent compound, multiband electronic structure,
and a Mott-Hubbard orbital alignment rather than the charge-transfer insulator
of the copper oxides. These results suggest an underlying universality in the
emergent electronic properties of both superconducting families. | 2203.02580v1 |
2022-03-22 | Prediction of resistance induced by surface complexity in lubricating layers: Application to super-hydrophobic surfaces | Super Hydrophobic (SH) coatings are widely used to mitigate drag in various
applications. Numerous studies have demonstrated that the beneficial wall-slip
effect produced by these materials disappears in laminar flow regimes. The main
mechanisms considered to be behind the decrease in performance are
Marangoni-induced stresses and air/liquid interface deformation. In the present
study, a new mechanism is proposed to explain the loss of performances of
SH-surfaces in laminar flow regimes. Here we consider the flow of air inside
the plastron and the associated momentum loses induced by roughness elements
with different geometric characteristics. The effects of air motion within the
plastron is coupled to the outer fluid with a homogenised boundary condition
approach. To this end, numerical simulations at the scale of the roughness
element were conducted as a function of the porosity and the tortuosity of the
domain to determine the slip velocity at the air-liquid interface.
The homogenised boundary condition is then implemented in a theoretical model
for the outer flow to compute drag on SH-spheres at low $Re$ numbers.
Experiments of laminar SH falling spheres indicate that high values of the
tortuosity and low values of porosity lead to a loss of performances when
considering drag reduction. As anticipated, a 3D printed sphere with low
tortuosity and similar porosity demonstrated near-optimal drag reductions. A
comparative study between the predicted values and experiments shows that the
homogenised model is able to accurately predict the drag on SH surfaces for
values of the porosity and tortuosity estimated from microscopy images of the
SH textured surface. | 2203.12039v1 |
2022-04-06 | Thermal activation of low-density Ga implanted in Ge | The nuclear spins of low-density implanted Ga atoms in Ge are interesting
candidates for solid state-based qubits. To date, activation studies of
implanted Ga in Ge have focused on high densities. Here we extend activation
studies into the low-density regime. We use spreading resistance profiling and
secondary ion mass spectrometry to derive electrical activation of Ga ions
implanted into Ge as a function of rapid thermal anneal temperature and implant
density. We show that for our implant conditions the activation is best for
anneal temperatures between 400 and 650 $^\circ$C, with a maximum activation of
64% at the highest fluence. Below 400 $^\circ$C, remaining implant damage
results in defects that act as superfluous carriers, and above 650 $^\circ$C,
surface roughening and loss of Ga ions are observed. The activation increased
monotonically from 10% to 64% as the implant fluence increased from
$6\times10^{10}$ to $6\times10^{12}$ cm$^{-2}$. The results provide thermal
anneal conditions to be used for initial studies of using low-density Ga atoms
in Ge as nuclear spin qubits. | 2204.02878v1 |
2022-05-05 | Intrinsic spin Hall torque in a moire Chern magnet | In spin torque magnetic memories, electrically actuated spin currents are
used to switch a magnetic bit. Typically, these require a multilayer geometry
including both a free ferromagnetic layer and a second layer providing spin
injection. For example, spin may be injected by a nonmagnetic layer exhibiting
a large spin Hall effect, a phenomenon known as spin-orbit torque. Here, we
demonstrate a spin-orbit torque magnetic bit in a single two-dimensional system
with intrinsic magnetism and strong Berry curvature. We study AB-stacked
MoTe2/WSe2, which hosts a magnetic Chern insulator at a carrier density of one
hole per moire superlattice site. We observe hysteretic switching of the
resistivity as a function of applied current. Magnetic imaging using a
superconducting quantum interference device reveals that current switches
correspond to reversals of individual magnetic domains. The real space pattern
of domain reversals aligns precisely with spin accumulation measured near the
high-Berry curvature Hubbard band edges. This suggests that intrinsic spin- or
valley-Hall torques drive the observed current-driven magnetic switching in
both MoTe2/WSe2 and other moire materials. The switching current density of
10^3 Amps per square centimeter is significantly less than reported in other
platforms paving the way for efficient control of magnetic order. | 2205.02823v1 |
2022-06-02 | A multi-fidelity approach coupling parameter space reduction and non-intrusive POD with application to structural optimization of passenger ship hulls | Nowadays, the shipbuilding industry is facing a radical change towards
solutions with a smaller environmental impact. This can be achieved with low
emissions engines, optimized shape designs with lower wave resistance and noise
generation, and by reducing the metal raw materials used during the
manufacturing. This work focuses on the last aspect by presenting a complete
structural optimization pipeline for modern passenger ship hulls which exploits
advanced model order reduction techniques to reduce the dimensionality of both
input parameters and outputs of interest. We introduce a novel approach which
incorporates parameter space reduction through active subspaces into the proper
orthogonal decomposition with interpolation method. This is done in a
multi-fidelity setting. We test the whole framework on a simplified model of a
midship section and on the full model of a passenger ship, controlled by 20 and
16 parameters, respectively. We present a comprehensive error analysis and show
the capabilities and usefulness of the methods especially during the
preliminary design phase, finding new unconsidered designs while handling high
dimensional parameterizations. | 2206.01243v3 |
2022-06-08 | Dielectric properties and impedance spectroscopy of NASICON type Na$_3$Zr$_2$Si$_2$PO$_{12}$ | We report the temperature dependent dielectric properties and impedance
spectroscopy investigation of Na$_3$Zr$_2$Si$_2$PO$_{12}$ in the frequency
range of 20 Hz--2 MHz. The Rietveld refinement of x-ray diffraction pattern
confirms the monoclinic phase with C2/c space group. The {\it d.c.} resistivity
behavior shows its strong insulating nature at low temperatures, and follows
Arrhenius law of thermal conduction with an activation energy of 0.68 eV. The
decrease in electric permittivity ($\epsilon_r$) with frequency is explained
based on the space polarization mechanism and its increment with temperature by
thermal activation of charge carriers. The dielectric loss (D=tan$\delta$) peak
follows the Arrhenius law of thermal activation with an energy of 0.25 eV. We
observe an enhancement in {\it a.c.} conductivity with frequency and
temperature due to the decrease in the activation energy, which results in
enhancing the conduction between defect states. Further, we observe an abrupt
increase in the {\it a.c.} conductivity at high frequencies, which is explained
using the universal Jonschers power law. The analysis of {\it a.c.}
conductivity shows two types of conduction mechanisms namely correlated barrier
hopping and non-overlapping small polaron tunnelling in the measured
temperature range. The imaginary part of the electric modulus confirms the
non-Debye type relaxation in the sample. The shifting of the relaxation peak
towards higher frequency side with an increase in temperature ensures its
thermally activated nature. The scaling behavior of the electric modulus shows
similar type of relaxation over the measured temperature range. The combined
analysis of electric modulus and impedance with frequency shows the short-range
mobility of charge carriers. | 2206.03668v1 |
2022-11-18 | Synthesis of infinite-layer nickelates and influence of the capping-layer on magnetotransport | The recent discovery of a zero-resistance state in nickel-based compounds has
generated a re-excitement about the long-standing problem in condensed matter
of high-critical-temperature superconductivity, in light of the analogies
between infinite-layer nickelates and cuprates. However, despite some formal
valence and crystal symmetry analogies, the electronic properties of
infinite-layer nickelates are remarkably original accounting, among other
properties, of a unique Nd5d-Ni3d hybridization. This designates infinite-layer
nickelates as a new class of oxide superconductors which should be considered
on their own. Here we report about Nd1-xSrxNiO2 (x = 0, 0.05 and 0.2) thin
films synthesized with and without a SrTiO3 capping-layer, showing very smooth
and step-terraced surface morphologies. Angle-dependent anisotropic
magnetoresistance measurements performed with a magnetic field rotating
in-plane or out-of-plane with respect to the sample surface, rendered important
information about the magnetic properties of undoped SrTiO3-capped and uncapped
samples. The results point at a key role of the capping-layer in controlling
the magnitude and the anisotropy of the anisotropic magnetoresistance
properties. We discuss this control in terms of a combined effect between the
Nd-Ni hybridization and an intra-atomic exchange coupling between the Nd-4f and
Nd-5d states, the latter essentially contributing to the (magneto)transport.
Further studies foresee the influence of the capping layer on infinite-layer
nickelates with no magnetic rare-earth. | 2211.10251v1 |
2022-12-12 | Unconventional localization of electrons inside of a nematic electronic phase | The magnetotransport behaviour inside the nematic phase of bulk FeSe reveals
unusual multiband effects that cannot be reconciled with a simple two-band
approximation proposed by surface-sensitive spectroscopic probes. In order to
understand the role played by the multiband electronic structure and the degree
of two-dimensionality we have investigated the electronic properties of
exfoliated flakes of FeSe by reducing their thickness. Based on
magnetotransport and Hall resistivity measurements, we assess the mobility
spectrum that suggests an unusual asymmetry between the mobilities of the
electrons and holes with the electron carriers becoming localized inside the
nematic phase. Quantum oscillations in magnetic fields up to 38 T indicate the
presence of a hole-like quasiparticle with a lighter effective mass and a
quantum scattering time three times shorter, as compared with bulk FeSe. The
observed localization of negative charge carriers by reducing dimensionality
can be driven by orbitally-dependent correlation effects, enhanced interband
spin-fluctuations or a Lifshitz-like transition which affect mainly the
electron bands. The electronic localization leads to a fragile two-dimensional
superconductivity in thin flakes of FeSe, in contrast to the two-dimensional
high-Tc induced with electron doping via dosing or using a suitable interface. | 2212.06196v1 |
2023-01-19 | Ab initio comparison of spin-transport properties in MgO-spaced ferrimagnetic tunnel junctions based on Mn$_3$Ga and Mn$_3$Al | We report on first-principles spin-polarised quantum transport calculations
(from NEGF+DFT) in MgO-spaced magnetic tunnel junctions (MTJs) based on two
different Mn-based Heusler ferrimagnetic metals, namely Mn$_3$Al and Mn$_3$Ga
in their tetragonal DO$_{22}$ phase. The former is a fully compensated
half-metallic ferrimagnet, while the latter is a low-moment
high-spin-polarisation ferrimagnet, both with a small lattice mismatch from
MgO. In identical symmetric and asymmetric interface reconstructions across a
3-monolayer thick MgO barrier for both ferrimagets, the linear response
(low-voltage) spin-transfer torque (STT) and tunneling magneto-resistance (TMR)
effects are evaluated. A larger staggered in-plane STT is found in the Mn$_3$Ga
case, while the STT in Mn$_3$Al vanishes quickly away from the interface
(similarly to STT in ferromagnetic MTJs). The roles are reversed for the TMR,
which is practically 100\% in the half-metallic Mn$_3$Al-based MTJs (using the
conservative definition) as opposed to 60\% in the Mn$_3$Ga case. The weak
dependence on the exact interface reconstruction would suggest
Mn$_3$Ga-Mn$_3$Al solid solutions as a possible route towards optimal trade-off
of STT and TMR in the low-bias, low-temperature transport regime. | 2301.08300v1 |
2023-05-09 | Influence of Shape on Heteroaggregation of Model Microplastics: A Simulation Study | Nano- and microplastics are a growing threat for the environment, especially
in aqueous habitats. For assessing the influence on the ecosystem and possible
solution strategies, it is necessary to investigate the fate of microplastics
(MP) in the environment. MPs are typically surrounded by natural organic
matter, which can cause them to aggregate. However, the effect of MP shape and
flow conditions on this heteroaggregation is not well understood. To address
this gap, we perform simulations of heteroaggregation of different MP shapes
with smaller spherical organic matter. We demonstrate that the shape had a
strong impact on the aggregate structure. MPs with mostly smooth surfaces
formed compact structures with a large number of neighbors with weak connection
strength and a higher fractal dimension. MPs with edges and corners aggregated
into more fractal structures with fewer neighbors, but with stronger
connections. Using MPCD, we investigated aggregates under shear flow. The
critical shear rate at which the aggregates break up is much larger for
spherical and rounded cube MPs, i.e, the compact aggregate structure of spheres
outweighs their weaker connection strength. Most notably, the rounded cube
exhibited unexpectedly high resistance against breakup under shear. We
attribute this to being fairly compact due to weaker, flexible neighbor
connections, which are still strong enough to prevent particles to break off
during shear flow. Irrespective of the stronger connections between
neighbouring MPs, the fractal aggregates of cubes break up at lower shear
rates. We find that cube aggregates reduced their radius of gyration
significantly, indicating restructuring, while most neighbor connections were
kept intact. Aggregates of spheres, however, kept their overall size while
undergoing local rearrangements, that broke a significant portion of their
neighbor interactions. | 2305.05453v1 |
2023-06-08 | Epitaxial thin films of binary Eu-compounds close to a valence transition | Intermetallic binary compounds of europium reveal a variety of interesting
phenomena due to the interconnection between two different magnetic and 4f
electronic (valence) states, which are particularly close in energy. The
valence states or magnetic properties are thus particularly sensitive to
strain-tuning in these materials. Consequently, we grew epitaxial EuPd$_2$
(magnetic Eu$^{2+}$) and EuPd$_3$ (nonmagnetic Eu$^{3+}$) thin films on
MgO(001) substrates using molecular beam epitaxy. Ambient X-ray diffraction
confirms an epitaxial relationship of cubic Laves-type (C15) EuPd$_2$ with an
(111)-out-of-plane orientation, whereby eight distinct in-plane
crystallographic domains develop. For simple cubic EuPd$_3$ two different
out-of-plane orientations can be obtained by changing the substrate annealing
temperature under ultra-high vacuum conditions from 600 {\deg}C to 1000 {\deg}C
for one hour. A small resistance minimum evolves for EuPd$_3$ thin films grown
with low temperature substrate annealing, which was previously found even in
single crystals of EuPd$_3$ and might be attributed to a Kondo or weak
localization effect. Absence of influence of an applied magnetic fields and
magnetotransport measurements suggest a nonmagnetic ground state for EuPd$_3$
thin films, i. e., a purely trivalent Eu valence, as found in EuPd$_3$ single
crystals. For EuPd$_2$ magnetic ordering below ~72 K is observed, quite similar
to single crystal behavior. Field dependent measurements of the
magnetoresistance and the Hall effect show hysteresis effects below ~0.4 T and
an anomalous Hall effect below ~70 K, which saturates around 1.4 T, thus
proving a ferromagnetic ground state of the divalent Eu. | 2306.05355v2 |
2023-06-26 | Phase purity and surface morphology of high-Jc superconducting Bi2Sr2Ca1Cu2O8+δ thin films | Bi2Sr2Ca1Cu2O8+d (Bi-2212) thin films with thicknesses less than 50 nm (<20
unit cells) are grown by pulsed laser deposition (PLD) onto (001) LaAlO3 (LAO)
single crystal substrates. Phase-pure and smooth c-axis oriented Bi-2212 films
with optimal oxygen doping, critical temperature Tc0 up to 86 K, and critical
current density Jc(60 K) above 1 MA/cm2 are obtained for samples that are
annealed in situ at temperatures below 700 {\deg}C. At higher temperature
Bi-2212 films on LAO substrates partially decompose to non-superconducting
impurity phases, while films on MgO and SrTiO3 substrates are stable. The
broadening of Tc of the metal-to-superconductor resistive phase transition in
magnetic fields is much larger for thin films of Bi-2212 as compared to
YBa2Cu3O7. The magnetic field-induced suppression of Tc0 is stronger for
Bi-2212 films containing impurity phases as compared to the phase-pure Bi-2212
films. The degradation of LAO substrate crystals after several steps of
deposition and chemical removal of the Bi-2212 layer is investigated. New,
commercially prepared substrates provide Bi-2212 films with smallest surface
roughness (3 nm) and strong out-of-plane texture. However, thin films of almost
the same quality are obtained on re-used LAO substrates that are mechanically
polished after the chemical etching. | 2306.14481v1 |
2023-07-12 | Coexistence of Competing Microbial Strains under Twofold Environmental Variability and Demographic Fluctuations | Microbial populations generally evolve in volatile environments, under
conditions fluctuating between harsh and mild, e.g. as the result of sudden
changes in toxin concentration or nutrient abundance. Environmental variability
thus shapes the long-time population dynamics, notably by influencing the
ability of different strains of microorganisms to coexist. Inspired by the
evolution of antimicrobial resistance, we study the dynamics of a community
consisting of two competing strains subject to twofold environmental
variability. The level of toxin varies in time, favouring the growth of one
strain under low drug concentration and the other strain when the toxin level
is high. We also model time-changing resource abundance by a randomly switching
carrying capacity that drives the fluctuating size of the community. While one
strain dominates in a static environment, we show that species coexistence is
possible in the presence of environmental variability. By computational and
analytical means, we determine the environmental conditions under which
long-lived coexistence is possible and when it is almost certain. Notably, we
study the circumstances under which environmental and demographic fluctuations
promote, or hinder, the strains coexistence. We also determine how the make-up
of the coexistence phase and the average abundance of each strain depend on the
environmental variability. | 2307.06314v3 |
2023-07-31 | Synthesis of possible room temperature superconductor LK-99:Pb$_9$Cu(PO$_4$)$_6$O | The quest for room-temperature superconductors has been teasing scientists
and physicists, since its inception in 1911 itself. Several assertions have
already been made about room temperature superconductivity but were never
verified or reproduced across the labs. The cuprates were the earliest high
transition temperature superconductors, and it seems that copper has done the
magic once again. Last week, a Korean group synthesized a Lead Apatite-based
compound LK-99, showing a T$_c$ of above 400$^\circ$K. The signatures of
superconductivity in the compound are very promising, in terms of resistivity
(R = 0) and diamagnetism at T$_c$. Although, the heat capacity (C$_p$) did not
show the obvious transition at T$_c$. Inspired by the interesting claims of
above room temperature superconductivity in LK-99, in this article, we report
the synthesis of polycrystalline samples of LK-99, by following the same heat
treatment as reported in [1,2] by the two-step precursor method. The phase is
confirmed through X-ray diffraction (XRD) measurements, performed after each
heat treatment. The room temperature diamagnetism is not evidenced by the
levitation of a permanent magnet over the sample or vice versa. Further
measurements for the confirmation of bulk superconductivity on variously
synthesized samples are underway. Our results on the present LK-99 sample,
being synthesized at 925$^\circ$C, as of now do not approve the appearance of
bulk superconductivity at room temperature. Further studies with different heat
treatments are though, yet underway. | 2307.16402v2 |
2023-10-11 | Temperature-Dependent Collective Excitations in a Three-Dimensional Dirac System ZrTe$_{5}$ | Zirconium pentatelluride (ZrTe$_{5}$), a system with a Dirac linear band
across the Fermi level and anomalous transport features, has attracted
considerable research interest for it is predicted to be located at the
boundary between strong and weak topological insulators separated by a
topological semimetal phase. However, the experimental verification of the
topological phase transition and the topological ground state in ZrTe$_{5}$ is
full of controversies, mostly due to the difficulty of precisely capturing the
small gap evolution with single-particle band structure measurements.
Alternatively, the collective excitations of electric charges, known as
plasmons, in Dirac systems exhibiting unique behavior, can well reflect the
topological nature of the band structure. Here, using reflective
high-resolution electron energy loss spectroscopy (HREELS), we investigate the
temperature-dependent collective excitations of ZrTe$_{5}$, and discover that
the plasmon energy in ZrTe$_{5}$ is proportional to the $1/3$ power of the
carrier density $n$, which is a unique feature of plasmons in three-dimensional
Dirac systems. Based on this conclusion, the origin of the resistivity anomaly
of ZrTe$_{5}$ can be attributed to the temperature-dependent chemical potential
shift in extrinsic Dirac semimetals. | 2310.07232v2 |
2023-10-19 | Comparison of the Resistivities of Nanostructured Films Made from Silver, Copper-Silver and Copper Nanoparticle and Nanowire Suspensions | Spray deposition and inkjet printing of various nanostructures are emerging
complementary methods for creating conductive coatings on different substrates.
In comparison to established deposition techniques like vacuum metal coating
and lithography-based metallization processes, spray deposition and inkjet
printing benefit from significantly simplified equipment. However, there are
number of challenges related to peculiar properties and behaviour of
nanostructures that require additional studies. In present work, we investigate
electroconductive properties and sintering behaviour of thin films produced
from nanostructures of different metals (Ag, Cu and Cu-Ag) and different shapes
(nanowires and spherical nanoparticles), and compare them to the reference Ag
and Cu magnetron deposited films. Synthesized nanostructures were studied with
transmission electron microscopy. Morphology and crystallinity of produced
metal films were studied with scanning electron microscopy and X-ray
diffraction. The electrical parameters were measured by the van der Pauw
method. All nanowires-based films provided high conductivity and required only
modest thermal treatment (200 C). To achieve sufficient sintering and
conductivity of nanoparticles-based films, higher temperatures are required
(300 C for Ag nanoparticles and 350 C for Cu and Cu-Ag nanoparticles).
Additionally, stability of nanowires was studied by annealing the samples in
vacuum conditions inside a scanning electron microscope at 500 C. | 2310.12506v1 |
2023-11-01 | Active Noise Control Portable Device Design | While our world is filled with its own natural sounds that we can't resist
enjoying, it is also chock-full of other sounds that can be irritating, this is
noise. Noise not only influences the working efficiency but also the human's
health. The problem of reducing noise is one of great importance and great
difficulty. The problem has been addressed in many ways over the years. The
current methods for noise reducing mostly rely on the materials and
transmission medium, which are only effective to some extent for the high
frequency noise. However, the effective reduction noise method especially for
low frequency noise is very limited.
Here we come up with a noise reduction system consist of a sensor to detect
the noise in the environment. Then the noise will be sent to an electronic
control system to process the noise, which will generate a reverse phase
frequency signal to counteract the disturbance. Finally, the processed smaller
noise will be broadcasted by the speaker. Through this smart noise reduction
system, even the noise with low-frequency can be eliminated.
The system is also integrated with sleep tracking and music player
applications. It can also remember and store settings for the same environment,
sense temperature, and smart control of home furniture, fire alarm, etc. This
smart system can transfer data easily by Wi-Fi or Bluetooth and controlled by
its APP.
In this project, we will present a model of the above technology which can be
used in various environments to prevent noise pollution and provide a solution
to the people who have difficulties finding a peaceful and quiet environment
for sleep, work or study. | 2311.00535v1 |
2023-11-13 | Superconductivity in trilayer nickelate La4Ni3O10 under pressure | Nickelate superconductors have attracted a great deal of attention over the
past few decades due to their similar crystal and electronic structures with
high-temperature cuprate superconductors. Here, we report the superconductivity
in a pressurized Ruddlesden-Popper phase single crystal, La4Ni3O10 (n = 3), and
its interplay with the density wave order in the phase diagram. With increasing
pressure, the density wave order as indicated by the anomaly in the resistivity
is progressively suppressed, followed by the emergence of the superconductivity
around 25 K. Our angle-resolved photoemission spectroscopy measurements reveal
that the electronic structure of La4Ni3O10 is very similar to that of La3Ni2O7,
suggesting unified electronic properties of nickelates in Ruddlesden-Popper
phases. Moreover, theoretical analysis unveils that antiferromagnetic (AFM)
super-exchange interactions can serve as the effective pairing interaction for
the emergence of superconductivity (SC) in pressurized La4Ni3O10. Our research
provides a new platform for the investigation of the unconventional
superconductivity mechanism in Ruddlesden-Popper trilayer perovskite
nickelates. | 2311.07423v2 |
2023-11-15 | Quantification of cell contractile behavior based on non-destructive macroscopic measurement of tension forces on bioprinted hydrogel | Contraction assay based on surface measurement have been widely used to
evaluate cell contractility in 3D models. This method is straightforward and
requires no specific equipment, but it does not provide quantitative data about
contraction forces generated by cells. We expanded this method with a new
biomechanical model, based on the work-energy theorem, to provide
non-destructive longitudinal monitoring of contraction forces generated by
cells in 3D.We applied this method on hydrogels seeded with either fibroblasts
or osteoblasts. Hydrogel mechanical characteristics were modulated to enhance
(condition HCA$_{High}$: hydrogel contraction assay high contraction) or limit
(condition HCA$_{Low}$: hydrogel contraction assay low contraction) cell
contractile behaviors. Macroscopic measures were further correlated with cell
contractile behavior and descriptive analysis of their physiology in response
to different mechanical environments. Fibroblasts and osteoblasts contracted
their matrix up to 47% and 77% respectively. Contraction stress peaked at day 5
with 1.1 10$^{-14}$Pa for fibroblasts and 3.5 10$^{-14}$Pa for osteoblasts,
which correlated with cell attachment and spreading. Negligible contraction was
seen in HCA$_{Low}$. Both fibroblasts and osteoblasts expressed $\alpha$-SMA
contractile fibers in HCA$_{High}$ and HCA$_{Low}$. Failure to contract
HCA$_{Low}$ was attributed to increased cross-linking and resistance to
proteolytic degradation of the hydrogel. | 2311.08773v1 |
2023-12-15 | Plasma-enhanced atomic layer deposition of titanium nitride for superconducting devices | This study presents a comprehensive investigation into the exceptional
superconducting attributes of titanium nitride (TiN) achieved through
plasma-enhanced atomic layer deposition (PEALD) on both planar and intricate
three-dimensional (3D) structures. We introduced an additional substrate
biasing cycle to densify the film and remove ligand residues, augmenting the
properties while minimizing impurities. While reactive-sputtered TiN films
exhibit high quality, our technique ensures superior uniformity by consistently
maintaining a desired sheet resistance greater than 95 percent across a 6inch
wafer, a critical aspect for fabricating extensive arrays of superconducting
devices and optimizing wafer yield. Moreover, our films demonstrate exceptional
similarity to conventional reactive-sputtered films, consistently reaching a
critical temperature (Tc) of 4.35 K with a thickness of around 40 nm. This
marks a notable achievement compared to previously reported ALD-based
superconducting TiN. Using the same process as for planar films, we obtained Tc
for aspect ratios (ARs) ranging from 2 to 40, observing a Tc of approximately 2
K for ARs between 2 and 10.5. We elucidate the mechanisms contributing to the
limitations and degradation of superconducting properties over these aggressive
3D structures. Our results seamlessly align with both current and
next-generation superconducting technologies, meeting stringent criteria for
thin-film constraints, large-scale deposition, conformality, 3D integration
schemes, and yield optimization. | 2312.09984v1 |
2023-12-19 | On the role of Grain Boundary Character in the Stress Corrosion Cracking of Nanoporous Gold Thin Films | For its potential as a catalyst, nanoporous gold (NPG) prepared through
dealloying of bulk Ag-Au alloys has been extensively investigated. NPG thin
films can offer ease of handling, better tunability of the chemistry and
microstructure of the nanoporous structure, and represent a more sustainable
usage of scarce resources. These films are however prone to intergranular
cracking during dealloying, limiting their stability and potential
applications. Here, we set out to systematically investigate the grain
boundaries in Au28Ag72 thin films. We observe that a sample synthesized at 400
{\deg}C is at least 2.5 times less prone to cracking compared to a sample
synthesized at room temperature. This correlates with a higher density of
coincident site lattice grain boundaries, especially the density of coherent
sigma 3, increased, which appear resistant against cracking. Nanoscale
compositional analysis of random high-angle grain boundaries reveals prominent
Ag enrichment up to 77 at.%, whereas sigma 3 coherent twin boundaries show Au
enrichment of up to 30 at.%. The misorientation and the chemistry of grain
boundaries hence affect their dealloying behavior, which in turn controls the
cracking, and the possible longevity of NPG thin films for application in
electrocatalysis. | 2312.12235v1 |
2024-02-26 | Electronic phase transitions and superconductivity in ferroelectric Sn$_2$P$_2$Se$_6$ under pressure | Since there is both strong electron-phonon coupling during a ferroelectric/FE
transition and superconducting/SC transition, it has been an important topic to
explore superconductivity from the FE instability. Sn$_2$P$_2$Se$_6$ arouses
broad attention due to its unique FE properties. Here, we reported the
electronic phase transitions and superconductivity in this compound based on
high-pressure electrical transport measurement, optical absorption spectroscopy
and Raman based structural analysis. Upon compression, the conductivity of
Sn$_2$P$_2$Se$_6$ was elevated monotonously, an electronic phase transition
occurred near 5.4 GPa, revealed by optical absorption spectroscopy, and the
insulating state is estimated to be fully suppressed near 15 GPa. Then, it
started to show the signature of superconductivity near 15.3 GPa. The
zero-resistance state was presented from 19.4 GPa, and the superconductivity
was enhanced with pressure continuously. The magnetic field effect further
confirmed the SC behavior and this compound had a $T_c$ of 5.4 K at 41.8 GPa
with a zero temperature upper critical field of 6.55 T. The Raman spectra
confirmed the structural origin of the electronic transition near 5.4 GPa,
which should due to the transition from the paraelectric phase to the
incommensurate phase, and suggested a possible first-order phase transition
when the sample underwent the semiconductor-metal transition near 15 GPa. This
work demonstrates the versatile physical properties in ferroelectrics and
inspires the further investigation on the correlation between FE instability
and SC in M$_2$P$_2$X$_6$ family. | 2402.16427v1 |
2024-02-29 | Anomalous frequency and temperature dependent scattering in the dilute metallic phase in lightly doped-SrTiO$_3$ | The mechanism of superconductivity in materials with aborted ferroelectricity
and the emergence of a dilute metallic phase in systems like doped-SrTiO$_3$
are outstanding issues in condensed matter physics. This dilute metal has
features both similar and different to those found in the normal state of other
unconventional superconductors. We have investigated the optical properties of
the dilute metallic phase in doped-SrTiO$_3$ using THz time-domain
spectroscopy. At low frequencies the THz response exhibits a Drude-like form as
expected for typical metal-like conductivity. We observed the frequency and
temperature dependencies to the low energy scattering rate $\Gamma(\omega, T)
\propto (\hbar\omega)^2 + (p \pi k_BT)^2 $ expected in a conventional Fermi
liquid, despite the fact that densities are too small to allow current decay
through electron-electron scattering. However we find the lowest known $p$
values of 0.39-0.72. As $p$ is 2 in a canonical Fermi liquid and existing
models based on energy dependent elastic scattering bound $p$ from below to 1,
our observation lies outside current explanation. Our data also gives insight
into the high temperature regime and shows that the temperature dependence of
the resistivity derives in part from strong T dependent mass renormalizations. | 2402.18767v1 |
2024-03-06 | Crystal, ferromagnetism, and magnetoresistance with sign reversal in a EuAgP semiconductor | We synthesized the ferromagnetic EuAgP semiconductor and conducted a
comprehensive study of its crystalline, magnetic, heat capacity, band gap, and
magnetoresistance properties. Our investigation utilized a combination of X-ray
diffraction, optical, and PPMS DynaCool measurements. EuAgP adopts a hexagonal
structure with the $P6_3/mmc$ space group. As the temperature decreases, it
undergoes a magnetic phase transition from high-temperature paramagnetism to
low-temperature ferromagnetism. We determined the ferromagnetic transition
temperature to be $T_{\textrm{C}} =$ 16.45(1) K by fitting the measured
magnetic susceptibility using a Curie-Weiss law. Heat capacity analysis of
EuAgP considered contributions from electrons, phonons, and magnons, revealing
$\eta$ = 0.03 J/mol/$\textrm{K}^\textrm{2}$, indicative of semiconducting
behavior. Additionally, we calculated a band gap of $\sim$ 1.324(4) eV based on
absorption spectrum measurements. The resistivity versus temperature of EuAgP
measured in the absence of an applied magnetic field shows a pronounced peak
around $T_{\textrm{C}}$, which diminishes rapidly with increasing applied
magnetic fields, ranging from 1 to 14 T. An intriguing phenomenon emerges in
the form of a distinct magnetoresistance transition, shifting from positive
(e.g., 1.95\% at 300 K and 14 T) to negative (e.g., -30.73\% at 14.25 K and 14
T) as the temperature decreases. This behavior could be attributed to
spin-disordered scattering. | 2403.03650v1 |
2024-03-20 | Optimizing Transparent Electrodes: Interplay of High Purity SWCNTs network and a Polymer | The discovery of transparent electrodes led to the development of
optoelectronic devices such as OLEDs, LCDs, touchscreens, IR sensors, etc.
Since ITO has many drawbacks in respect of its production cost and limited
transparency in IR, carbon nanotubes (CNTs) have been a potential replacement
for ITO due to their exceptional electrical and optical properties, especially
in the IR region. In this work, we present the development of a CNT-polymer
composite thin film that exhibits outstanding transparency across both visible
and IR spectra prepared by layer-by-layer (LbL) technique. This approach not
only ensures uniform integration and crosslinking of CNTs into lightweight
matrices, but also represents a cost-effective method for producing transparent
electrodes with remarkable optical and electrical properties. The produced
films achieved a transparency above 80% in the UV-VIS range and approximately
70% in the mid-IR range. The sheet resistance of the fabricated thin films was
measured at about 4 kOhm/sq, showing a tendency to decrease with the number of
bilayers. Furthermore, in this work we have investigated electrical properties
and transport mechanisms in more detail with computational analysis.
Computational analysis was performed to better understand the electrical
behavior of nanotube-polymer junctions in the interbundle structure. Based on
all results, we propose that the transparent electrodes with 4 and 6 bilayers
are the most optimal structures in terms of optical and electrical properties. | 2403.13594v1 |
1996-10-01 | Itinerant Antiferromagnetism in FeGe_2 | FeGe_2, and lightly doped compounds based on it, have a Fermi surface driven
instability which drive them into an incommensurate spin density wave state.
Studies of the temperature and magnetic field dependence of the resistivity
have been used to determine the magnetic phase diagram of the pure material
which displays an incommensurate phase at high temperatures and a commensurate
structure below 263 K in zero field. Application of a magnetic field in the
tetragonal basal plane decreases the range of temperatures over which the
incommensurate phase is stable. We have used inelastic neutron scattering to
measure the spin dynamics of FeGe_2. Despite the relatively isotropic transport
the magnetic dynamics is quasi-one dimensional in nature. Measurements carried
out on HET at ISIS have been used to map out the spin wave dispersion along the
c-axis up the 400 meV, more than an order of magnitude higher than the zone
boundary magnon for wavevectors in the basal plane. | 9610009v1 |
1998-05-25 | Metal-insulator transition induced by 16O -18O oxygen isotope exchange in colossal negative magnetoresistance manganites | The effect of 16O-18O isotope exchange on the electric resistivity was
studied for (La(1-y)Pr(y))0.7Ca0.3MnO3 ceramic samples. Depending on y, this
mixed perovskite exhibited different types of low-temperature behavior ranging
from ferromagnetic metal (FM) to charge ordered (CO) antiferromagnetic
insulator. It was found that at y=0.75, the substitution of 16O by 18O results
in the reversible transition from a FM to a CO insulator at zero magnetic
field. The applied magnetic field (H >= 2 T) transformed the sample with 18O
again to the metallic state and caused the increase in the FM transition
temperature Tc of the 16O sample. As a result, the isotope shift of Tc at H = 2
T was as high as 63 K. Such unique sensitivity of the system to oxygen isotope
exchange, giving rise even to the metal-insulator transition, is discussed in
terms of the isotope dependence of the effective electron bandwidth which
shifts the balance between the CO and FM phases. | 9805315v1 |
2001-04-13 | Metal-insulator transition in 2D: a role of the upper Habbard band | To explain the main features of the metal-insulator transition (MIT) in 2D we
suggest a simple model taking into account strongly localized states in the
band tail of 2D conductivity band with a specific emphasize of a role of
doubly-occupied states (upper Hubbard band). The metallic behavior of
resistance is explained as result of activation of localized electrons to
conductance band leading to a suppression of non-linear screening of the
disorder potential. The magnetoresistance (MR) in the critical region is
related to depopulation of double occupied localized states also leading to
partial suppression of the nonlinear screening. The most informative data are
related to nearly activated temperature dependence of MR in strongly insulating
limit (which can be in particular reached from the metallic state in high
enough fields). According to our model this behavior originates due to a
lowering of a position of chemical potential in the upper Hubbard band due to
Zeeman splitting. We compare the theoretical predictions to the existing
experimental data and demonstrate that the model explains such features of the
2D MIT as scaling behavior in the critical region, saturation of MR and H/T
scaling of MR in the insulating limit. The quantitative analysis of MR in
strongly insulating limit based on the model suggested leads to the values of
g-factors being in good agreement with known values for localized states in
corresponding materials. | 0104254v1 |
2001-05-31 | Anomalous Nonlinear Microwave Response of Epitaxial YBa2Cu3O7-x Films on MgO | We have investigated the anomalous nonlinear microwave response of epitaxial
electron-beam coevaporated YBa2Cu3O7-x films on MgO. The power and temperature
dependent surface impedance and two-tone intermodulation distortion were
measured in stripline resonators at several frequencies between 2.3 and 11.2
GHz and at temperatures between 1.7 K and Tc. All of the eight films measured
to date show a decrease of the surface resistance Rs of up to one order of
magnitude as the microwave current is increased up to approximately 1 mA for T
< 20 K. The surface reactance Xs showed only a weak increase in the same
region. The usual nonlinear increase of Rs and Xs was observed at high currents
in the 100-mA range. The minimum of Rs correlates with a pronounced plateau in
the third-order intermodulation signal. We have developed a phenomenological
two-fluid model, incorporating a microwave current-dependent quasiparticle
scattering rate g and normal fraction fn. The model contains only three
adjustable parameters. We find good agreement with the measured data for
constant fn and a current-dependent scattering rate g whose magnitude increases
almost linearly with temperature and frequency. Obvious mechanisms leading to
nonlinear microwave response like Josephson coupling across grain boundaries,
nonlocal or nonequilibrium effects cannot explain the data. The anomalies could
rather reflect the specific charge and spin ordering and transport phenomena in
the low-dimensional cuprate superconductors. | 0105613v1 |
2002-01-25 | The influence of microstructures and crystalline defects on the superconductivity of MgB2 | This work studies the influence of microstructures and crystalline defects on
the superconductivity of MgB2, with the objective to improve its flux pinning.
A MgB2 sample pellet that was hot isostatic pressed (HIPed) was found to have
significantly increased critical current density (Jc) at high fields than its
un-HIPed counterpart. The HIPed sample had a Jc of 10000 A/cm2 in 50000 Oe (5
T) at 5K. This was 20 times higher than that of the un-HIPed sample, and the
same as the best Jc reported by other research groups. Microstructures observed
in scanning and transmission electron microscopy indicate that the HIP process
eliminated porosity present in the MgB2 pellet resulting in an improved
intergrain connectivity. Such improvement in intergrain connectivity was
believed to prevent the steep Jc drop with magnetic field H that occurred in
the un-HIPed MgB2 pellet at H > 45000 Oe (4.5 T) and T = 5 K. The HIP process
was also found to disperse the MgO that existed at the grain boundaries of the
un-HIPed MgB2 pellet and to generate more dislocations in the HIPed the
pellets. These dispersed MgO particles and dislocations improved flux pinning
also at H<45000 Oe. The HIPing process was also found to lower the resistivity
at room temperature. | 0201486v2 |
2004-04-19 | Low temperature transport in granular metals | We investigate transport in a granular metallic system at large tunneling
conductance between the grains. We show that at low temperatures, $T\leq
g_T\delta $, where $\delta$ is the single mean energy level spacing in a grain,
the coherent electron motion at large distances dominates the physics, contrary
to the high temperature ($T > g_T \delta $) behavior where conductivity is
controlled by the scales of the order of the grain size. The conductivity of
one and two dimensional granular metals, in the low temperature regime, decays
with decreasing temperature in the same manner as that in homogeneous
disordered metals, indicating thus an insulating behavior. However, even in
this temperature regime the granular structure remains important and there is
an additional contribution to conductivity coming from short distances. Due to
this contribution the metal-insulator transition in three dimensions occurs at
the value of tunnel conductance $g_T^C=(1/6\pi)\ln (E_C/\delta)$, where $E_C$
is the charging energy of an isolated grain, and not at the generally expected
$g_T^C \propto 1$. Corrections to the density of states of granular metals due
to the electron-electron interaction are calculated. Our results compare
favorably with the logarithmic dependence of resistivity in the high-$T_c$
cuprate superconductors indicating that these materials may have a granular
structure. | 0404443v2 |
2004-10-21 | Novel magnetic behavior of single crystalline Er2PdSi3 | We report the results of ac and dc magnetic susceptibility (chi) and
electrical resistivity (rho) measurements on the single crystals of Er2PdSi3,
crystallizing in an AlB2-derived hexagonal structure, for two orientations
H//[0001] and H//[2 -1 -1 0]. For H//[0001], there are apparently two magnetic
transitions as revealed by the ac chi data, one close to 7 K attributable to
antiferromagnetic ordering and the other around 2 K. However, for H // [2 -1 -1
0], we observe additional features above 7 K (near 11 and 23 K) in the plot of
low-field chi(T); also, there is no corresponding anomaly in the rho(T) plot.
In this respect, the magnetic behavior of this compound is novel, particularly
while compared with other members of this series. The features in ac chi
respond differently to the application of a small dc magnetic field for the two
directions. As far as low temperature (T= 1.8 and 5 K) isothermal magnetization
(M) behaviour is concerned, it exhibits meta-magnetic-like features around 2
kOe saturating at high fields for the former orientation, whereas for the
latter, there is no saturation even at 120 kOe. The sign of paramagnetic Curie
temperature is different for these two directions. Thus, there is a strong
anisotropy in the magnetic behavior. However, interestingly, the rho(T) plots
are found to be essentially isotropic, with the data revealing possible
formation of magnetic superzone formation below 7 K. | 0410532v1 |
2006-06-22 | Metal-insulator transition in Nd$_{1-x}$Eu$_{x}$NiO$_{3}$ compounds | Polycrystalline Nd$_{1-x}$Eu$_{x}$NiO$_3$ ($0 \leq x \leq 0.5$) compounds
were synthesized in order to investigate the character of the metal-insulator
(MI) phase transition in this series. Samples were prepared through the sol-gel
route and subjected to heat treatments at $\sim$1000 $^\circ$C under oxygen
pressures as high as 80 bar. X-ray Diffraction (XRD) and Neutron Powder
Diffraction (NPD), electrical resistivity $\rho(T)$, and Magnetization $M(T)$
measurements were performed on these compounds. The results of NPD and XRD
indicated that the samples crystallize in an orthorhombic distorted perovskite
structure, space group $Pbnm$. The analysis of the structural parameters
revealed a sudden and small expansion of $\sim$0.2% of the unit cell volume
when electronic localization occurs. This expansion was attributed to a small
increase of $\sim$0.003 \AA{} of the average Ni-O distance and a simultaneous
decrease of $\sim$$- 0.5^\circ$ of the Ni-O-Ni superexchange angle. The
$\rho(T)$ measurements revealed a MI transition occurring at temperatures
ranging from $T_{\rm MI}\sim 193$ to 336 K for samples with $x = 0$ and 0.50,
respectively. These measurements also show a large thermal hysteresis in
NdNiO$_{3}$ during heating and cooling processes suggesting a first-order
character of the phase transition at $T_{\rm MI}$. The width of this thermal
hysteresis was found to decrease appreciably for the sample
Nd$_{0.7}$Eu$_{0.3}$NiO$_{3}$. The results indicate that cation disorder
associated with increasing substitution of Nd by Eu is responsible for changing
the first order character of the transition in NdNiO$_{3}$. | 0606573v1 |
2007-03-21 | Dynamics of the Magnetic Flux Trapped in Fractal Clusters of a Normal Phase in Percolative Superconductors | The effect of the fractal clusters of a normal phase, which act as pinning
centers, on the dynamics of magnetic flux in percolative type-II superconductor
is considered. The main features of these clusters are studied in detail: the
cluster statistics is analyzed; the fractal dimension of their boundary is
estimated; the distribution of critical currents is obtained, and its
peculiarities are explored. It is found that there is the range of fractal
dimension where this distribution has anomalous statistical properties,
specifically, its dispersion becomes infinite. It is examined how the finite
resolution capacity of the cluster geometric size measurement affects the
estimated value of fractal dimension. The effect of fractal properties of the
normal phase clusters on the electric field arisen from magnetic flux motion is
investigated for the cluster area distribution of different kinds. The
voltage-current characteristics of fractal superconducting structures in the
resistive state are obtained for an arbitrary fractal dimension. It is revealed
that the fractality of the boundaries of the normal phase clusters intensifies
the magnetic flux trapping and thereby raises the critical current of a
superconductor. | 0703541v1 |
2007-03-27 | Diffusion and activation of n-type dopants in germanium | The diffusion and activation of $n$-type impurities (P and As) implanted into
$p$-type Ge(100) substrates were examined under various dose and annealing
conditions. The secondary ion mass spectrometry profiles of chemical
concentrations indicated the existence of a sufficiently high number of
impurities with increasing implanted doses. However, spreading resistance probe
profiles of electrical concentrations showed electrical concentration
saturation in spite of increasing doses and indicated poor activation of As
relative to P in Ge. The relationships between the chemical and electrical
concentrations of P in Ge and Si were calculated, taking into account the
effect of incomplete ionization. The results indicated that the activation of P
was almost the same in Ge and Si. The activation ratios obtained experimentally
were similar to the calculated values, implying insufficient degeneration of
Ge. The profiles of P in Ge substrates with and without damage generated by Ge
ion implantation were compared, and it was clarified that the damage that may
compensate the activated $n$-type dopants has no relationship with the
activation of P in Ge. | 0703708v3 |
2008-10-14 | Superconductivity in Fluorine-Arsenide Sr_{1-x}La_xFeAsF | Since the discovery of superconductivity\cite{1} at 26 K in oxy-pnictide
$LaFeAsO_{1-x}F_x$, enormous interests have been stimulated in the fields of
condensed matter physics and material sciences. Among the five different
structures in this broad type of superconductors\cite{2,3,4,5,6}, the ZrCuSiAs
structure has received special attention since the $T_c$ has been quickly
promoted to 55-56 K\cite{7,8,9,10,11} in fluorine doped oxy-pnictides REFeAsO
(RE = rare earth elements). The superconductivity can also be induced by
applying a high pressure to the undoped samples\cite{12,13}. The mechanism of
superconductivity in the FeAs-based system remains unclear yet, but it turns
out to be clear that any change to the structure or the building blocks will
lead to a change of the superconducting transition temperatures. In this
Letter, we report the fabrication of the new family of compounds, namely
fluorine-arsenides DvFeAsF (Dv = divalent metals) with the ZrCuSiAs structure
and with the new building block DvF instead of the REO (both the layers DvF and
REO have the combined cation state of "+1"). The undoped parent phase has a
Spin-Density-Wave like transition at about 173 K for SrFeAsF, 118 K for CaFeAsF
and 153 K for EuFeAsF. By doping electrons into the system the resistivity
anomaly associated with this SDW transition is suppressed and superconductivity
appears at 32 K in the fluorine-arsenide Sr$_{1-x}$La$_x$FeAsF (x = 0.4). Our
discovery here initiates a new method to obtain superconductors in the
FeAs-based system. | 0810.2531v3 |
2009-07-09 | Annular Spin-Transfer Memory Element | An annular magnetic memory that uses a spin-polarized current to switch the
magnetization direction or helicity of a magnetic region is proposed. The
device has magnetic materials in the shape of a ring (1 to 5 nm in thickness,
20 to 250 nm in mean radius and 8 to 100 nm in width), comprising a reference
magnetic layer with a fixed magnetic helicity and a free magnetic layer with a
changeable magnetic helicity. These are separated by a thin non-magnetic layer.
Information is written using a current flowing perpendicular to the layers,
inducing a spin-transfer torque that alters the magnetic state of the free
layer. The resistance, which depends on the magnetic state of the device, is
used to read out the stored information. This device offers several important
advantages compared to conventional spin-transfer magnetic random access memory
(MRAM) devices. First, the ring geometry offers stable magnetization states,
which are, nonetheless, easily altered with short current pulses. Second, the
ring geometry naturally solves a major challenge of spin-transfer devices:
writing requires relatively high currents and a low impedance circuit, whereas
readout demands a larger impedance and magnetoresistance. The annular device
accommodates these conflicting requirements by performing reading and writing
operations at separate read and write contacts placed at different locations on
the ring. | 0907.1600v1 |
2009-09-16 | Band structure engineering of epitaxial graphene on SiC by molecular doping | Epitaxial graphene on SiC(0001) suffers from strong intrinsic n-type doping.
We demonstrate that the excess negative charge can be fully compensated by
non-covalently functionalizing graphene with the strong electron acceptor
tetrafluorotetracyanoquinodimethane (F4-TCNQ). Charge neutrality can be reached
in monolayer graphene as shown in electron dispersion spectra from angular
resolved photoemission spectroscopy (ARPES). In bilayer graphene the band gap
that originates from the SiC/graphene interface dipole increases with
increasing F4-TCNQ deposition and, as a consequence of the molecular doping,
the Fermi level is shifted into the band gap. The reduction of the charge
carrier density upon molecular deposition is quantified using electronic Fermi
surfaces and Raman spectroscopy. The structural and electronic characteristics
of the graphene/F4-TCNQ charge transfer complex are investigated by X-ray
photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy
(UPS). The doping effect on graphene is preserved in air and is temperature
resistant up to 200\degree C. Furthermore, graphene non-covalent
functionalization with F4-TCNQ can be implemented not only via evaporation in
ultra-high vacuum but also by wet chemistry. | 0909.2966v2 |
2010-01-07 | Anisotropic Connectivity and its Influence on Critical Current Densities, Irreversibility Fields, and Flux Creep in In-Situ-Processed MgB2 Strands | The anisotropy of the critical current density (Jc) and its influence on
measurement of irreversibility field (Birr) has been investigated for high
quality, in-situ MgB2 strands. Comparison of transport and magnetization
measurements has revealed the onset of a regime where large differences exist
between transport and magnetically measured values of the critical current
density and Birr. These effects, initially unexpected due to the lack of
crystalline texture in these in-situ processed strands, appear to be due to a
fibrous microstructure, connected with the details of the wire fabrication and
MgB2 formation reactions. Scanning electron micrographs of in-situ-processed
MgB2 monocore strands have revealed a fibrous microstructure. Grains (~100 nm)
are randomly oriented, and there is no apparent local texture of the grains.
However, this randomly oriented polycrystalline material has a fibrous texture
at a larger length scale, with stringers of MgB2 (~ 60 {\mu}m long and ~5
{\mu}m in diameter) partially separated by elongated pores -- the spaces
previously occupied by stringers of elemental Mg. This leads to an
interpretation of the differences observed in transport and magnetically
determined critical currents, in particular a large deviation between the two
at higher fields, in terms of different transverse and longitudinal
connectivities within the strand. The different values of connectivity also
lead to different resistive transition widths, and thus irreversibility field
values, as measured by transport and magnetic techniques. Finally, these
considerations are seen to influence estimated pinning potentials for the
strands. | 1001.1030v1 |
2010-04-21 | Determination of the intrinsic ferroelectric polarization in orthorhombic HoMnO3 | Whether large ferroelectric polarization P exists in the orthorhombic HoMnO3
with the E-type antiferromagnetic spin ordering or not remains as one of
unresolved, challenging issues in the physics of multiferroics. The issue is
closely linked to an intriguing experimental difficulty for determining P of
polycrystalline specimens that conventional pyroelectric current measurements
performed after a poling procedure under high dc electric fields are subject to
large errors due to the problems caused by leakage currents or space charges.
To overcome the difficulty, we employed the PUND method, which uses
successively the two positive and two negative electrical pulses, to directly
measure electrical hysteresis loops in several polycrystalline HoMnO3 specimens
below their N\'eel temperatures. We found that all the HoMnO3 samples had
similar remnant polarization Pr values at each temperature, regardless of their
variations in resistivity, dielectric constant, and pyroelectric current
levels. Moreover, Pr of ~0.07 \mu\C/cm2 at 6 K is consistent with the P value
obtained from the pyroelectric current measurement performed after a short
pulse poling. Our findings suggest that intrinsic P of polycrystalline HoMnO3
can be determined through the PUND method and P at 0 K may reach ~0.24
\mu\C/cm2 in a single crystalline specimen. | 1004.3643v1 |
2010-07-26 | Python Regius (Ball Python) shed skin: Biomimetic analogue for function-targeted design of tribo-surfaces | A major concern in designing tribo-systems is to minimize friction, save
energy, and to reduce wear. Satisfying these requirements depends on the
integrity of the rubbing surface and its suitability to sliding conditions. As
such, designers currently focus on constructing surfaces that are an integral
part of the function of the tribo-system. Inspirations for such constructs come
from studying natural systems and from implementing natural design rules. One
species that may serve as an analogue for design is the Ball python. This is
because such a creature while depending on legless locomotion when sliding
against various surfaces, many of which are deemed tribologically hostile,
doesn't sustain much damage. Resistance to damage in this case originates from
surface design features. As such, studying these features and how do they
contribute to the control of friction and wear is very attractive for design
purposes. In this work we apply a multi scale surface characterization approach
to study surface design features of the Python regius that are beneficial to
design high quality lubricating surfaces (such as those obtained through
plateau honing). To this end, we studied topographical features by SEM and
through White Light Interferrometery (WLI). We further probe the roughness of
the surface on multi scale and as a function of location within the body. The
results are used to draw a comparison to metrological features of commercial
cylinder liners obtained by plateau honing. | 1007.4419v1 |
2010-11-09 | Phase diagram of iron-arsenide superconductors Ca(Fe1-xCox)2As2 (0 <= x <= 0.2) | Platelet-like single crystals of the Ca(Fe1-xCox)2As2 series having lateral
dimensions up to 15 mm and thickness up to 0.5 mm were obtained from the high
temperature solution growth technique using Sn flux. Upon Co doping, the c-axis
of the tetragonal unit cell decreases, while the a-axis shows a less
significant variation. Pristine CaFe2As2 shows a combined spin-density-wave and
structural transition near T = 166 K which gradually shifts to lower
temperatures and splits with increasing Co-doping. Both transitions terminate
abruptly at a critical Co-concentration of xc = 0.075. For x \geq 0.05,
superconductivity appears at low temperatures with a maximum transition
temperature TC of around 20 K. The superconducting volume fraction increases
with Co concentration up to x = 0.09 followed by a gradual decrease with
further increase of the doping level. The electronic phase diagram of
Ca(Fe1-xCox)2As2 (0 \leq x \leq 0.2) series is constructed from the
magnetization and electric resistivity data. We show that the low-temperature
superconducting properties of Co-doped CaFe2As2 differ considerably from those
of BaFe2As2 reported previously. These differences seem to be related to the
extreme pressure sensitivity of CaFe2As2 relative to its Ba counterpart. | 1011.2085v1 |
2011-08-17 | Impact of Protostellar Outflow on Star Formation: Effects of Initial Cloud Mass | Star formation efficiency controlled by the protostellar outflow in a single
cloud core is investigated by three-dimensional resistive MHD simulations.
Starting from the prestellar cloud core, the star formation process is
calculated until the end of the main accretion phase. In the calculations, the
mass of the prestellar cloud is parameterized. During the star formation, the
protostellar outflow is driven by the circumstellar disk. The outflow extends
also in the transverse direction until its width becomes comparable to the
initial cloud scale, and thus, the outflow has a wide opening angle of >40
degrees. As a result, the protostellar outflow sweeps up a large fraction of
the infalling material and ejects it into the interstellar space. The outflow
can eject at most over half of the host cloud mass, significantly decreasing
star formation efficiency. The outflow power is stronger in clouds with a
greater initial mass. Thus, the protostellar outflow effectively suppresses
star formation efficiency in a massive cloud. The outflow weakens significantly
and disappears in several free-fall timescales of the initial cloud after the
cloud begins to collapse. The natal prestellar core influences the lifetime and
size of the outflow. At the end of the main accretion phase, a massive
circumstellar disk comparable in mass to the protostar remains. Calculations
show that typically, ~30% of the initial cloud mass is converted into the
protostar and ~20% remains in the circumstellar disk, while ~40% is ejected
into the interstellar space by the protostellar outflow. Therefore, a single
cloud core typically has a star formation efficiency of 30-50%. | 1108.3564v1 |
2012-05-02 | Negative thermal expansion and antiferromagnetism in the actinide oxypnictide NpFeAsO | A neptunium analogue of the LaFeAsO tetragonal layered compound has been
synthesized and characterized by a variety of experimental techniques. The
occurrence of long-range magnetic order below a critical temperature T_N = 57 K
is suggested by anomalies in the temperature-dependent magnetic susceptibility,
electrical resistivity, Hall coefficient, and specific heat curves. Below T_N,
powder neutron diffraction measurements reveal an antiferromagnetic structure
of the Np sublattice, with an ordered magnetic moment of 1.70(0.07) \mu_B
aligned along the crystallographic c-axis. No magnetic order has been observed
on the Fe sublattice, setting an upper limit of about 0.3 \mu_B for the ordered
magnetic moment on the iron. High resolution x-ray powder diffraction
measurements exclude the occurrence of lattice transformations down to 5 K, in
sharp contrast to the observation of a tetragonal-to-orthorhombic distortion in
the rare-earth analogues, which has been associated with the stabilization of a
spin density wave on the iron sublattice. Instead, a significant expansion of
the NpFeAsO lattice parameters is observed with decreasing temperature below
T_N, corresponding to a relative volume change of about 0.2% and to an invar
behavior between 5 and 20 K. First-principle electronic structure calculations
based on the local-spin density plus Coulomb interaction and the local density
plus Hubbard-I approximations provide results in good agreement with the
experimental findings. | 1205.0438v1 |
2012-09-13 | Magnetic phase transitions in single crystals of the chiral helimagnet Cr1/3NbS2 | The chiral helimagnet Cr1/3NbS2 has been investigated by magnetic, transport
and thermal properties measurements on single crystals and by first principles
electronic structure calculations. From the measured field and temperature
dependence of the magnetization for fields applied perpendicular to the c axis,
the magnetic phase diagram has been constructed in the vicinity of the phase
transitions. A transition from a paramagnetic to a magnetically ordered phase
occurs near 120 K. With increasing magnetic field and at temperatures below 120
K, this material undergoes transitions from a helimagnetic to a soliton-lattice
phase near 900 Oe, and then to a ferromagnetic phase near 1300 Oe. The
transitions are found to strongly affect the electrical transport. The
resistivity decreases sharply upon cooling near 120 K, and the spin
reorientation from the helimagnetic ground state to the commensurate
ferromagnetic state is evident in the magnetoresistance. At high fields a large
magnetoresistance (55 % at 140 kOe) is observed near the magnetic transition
temperature. Heat capacity and electronic structure calculations show the
density of states at the Fermi level is low in the magnetically ordered state.
Effects of spin fluctuations are likely important in understanding the behavior
of Cr1/3NbS2 near and above the magnetic ordering transitions. | 1209.2783v1 |
2013-04-15 | Non-Fermi liquids and the Wiedemann-Franz law | A general discussion of the ratio of thermal and electrical conductivities in
non-Fermi liquid metals is given. In metals with sharp Drude peaks, the
relevant physics is correctly organized around the slow relaxation of
almost-conserved momenta. While in Fermi liquids both currents and momenta
relax slowly, due to the weakness of interactions among low energy excitations,
in strongly interacting non-Fermi liquids typically only momenta relax slowly.
It follows that the conductivities of such non-Fermi liquids are obtained
within a fundamentally different kinematics to Fermi liquids. Among these
strongly interacting non-Fermi liquids we distinguish cases with only one
almost-conserved momentum, which we term hydrodynamic metals, and with many
patchwise almost-conserved momenta. For all these cases, we obtain universal
expressions for the ratio of conductivities that violate the Wiedemann-Franz
law. We further discuss the case in which long-lived `cold' quasiparticles, in
general with unconventional scattering rates, coexist with strongly interacting
hot spots, lines or bands. For these cases, we characterize circumstances under
which non-Fermi liquid transport, in particular a linear in temperature
resistivity, is and is not compatible with the Wiedemann-Franz law. We suggest
the likely outcome of future transport experiments on CeCoIn5, YbRh2Si2 and
Sr3Ru2O7 at their critical magnetic fields. | 1304.4249v3 |
2014-05-15 | Anomalous frequency and temperature dependent scattering and Hund's coupling in the almost quantum critical heavy fermion system CeFe$_2$Ge$_2$ | We present THz range optical conductivity data of a thin film of the near
quantum critical heavy fermion compound CeFe$_2$Ge$_2$. Our complex
conductivity measurements find a deviation from conventional Drude-like
transport in a temperature range previously reported to exhibit unconventional
behavior. We calculate the frequency dependent effective mass and scattering
rate using an extended Drude model analysis. We find the inelastic scattering
rate can be described by a temperature dependent power-law $\omega^{n(T)}$
where $n(T)$ approaches $\sim1.0 \pm 0.2$ at 1.5 K. This is compared to the
$\rho \sim T^{1.5}$ behavior claimed in dc resistivity data and the $\rho \sim
T^{2}$ expected from Fermi-liquid theory. In addition to a low temperature mass
renormalization, we find an anomalous mass renormalization that persists to
high temperature. We attribute this to a Hund's coupling in the Fe states in a
manner similar to that recently proposed in the ferro-pnictides. CeFe$_2$Ge$_2$
appears to be a very interesting system where one may study the interplay
between the usual $4f$ lattice Kondo effect and this Hund's enhanced Kondo
effect in the $3d$ states. | 1405.4007v3 |
2014-05-23 | Growth and characterization of heteroepitaxial La-substituted BaSnO$_3$ films on SrTiO$_3$ (001) and SmScO$_3$ (110) substrates | Heteroepitaxial growth of BaSnO$_3$ (BSO) and Ba$_{1-x}$La$_x$SnO$_3$ (x = 7
%) (LBSO) thin films on different perovskite single crystal (SrTiO$_3$ (001)
and SmScO$_3$ (110)) substrates has been achieved by Pulsed Laser Deposition
(PLD) under optimized deposition conditions. X-ray diffraction measurements
indicate that the films on either of these substrates are relaxed due to the
large mismatch and present a high degree of crystallinity with narrow rocking
curves and smooth surface morphology while analytical quantification by proton
induced x-ray emission (PIXE) confirms the stoichiometric La transfer from a
polyphasic target, producing films with La contents above the bulk solubility
limit. The films show degenerate semiconducting behavior on both substrates,
with the observed room temperature resistivities, Hall mobilities and carrier
concentrations of 4.4 $m \Omega cm$, 10.11 $cm^2 V^{-1} s^{-1}$, and 1.38
$\cdot 10^{20} cm^{-3}$ on SmScO$_3$ and 7.8 $m \Omega cm$, 5.8 $cm^2 V^{-1}
s^{-1}$, and 1.36 $\cdot 10^{20} cm^{-3}$ on SrTiO$_3$ ruling out any extrinsic
contribution from the substrate. The superior electrical properties observed on
the SmScO3 substrate are attributed to reduction in dislocation density from
the lower lattice mismatch. | 1405.6233v1 |
2014-05-31 | Scale-Invariant Dissipationless Chiral Transport in Magnetic Topological Insulators beyond the Two-Dimensional Limit | We investigate the quantum anomalous Hall Effect (QAHE) and related chiral
transport in the millimeter-size (Cr0.12Bi0.26Sb0.62)2Te3 films. With high
sample quality and robust magnetism at low temperatures, the quantized Hall
conductance of e2/h is found to persist even when the film thickness is beyond
the two-dimensional (2D) hybridization limit. Meanwhile, the Chern
insulator-featured chiral edge conduction is manifested by the non-local
transport measurements. In contrast to the 2D hybridized thin film, an
additional weakly field-dependent longitudinal resistance is observed in the 10
quintuple-layer film, suggesting the influence of the film thickness on the
dissipative edge channel in the QAHE regime. The extension of QAHE into the
three-dimensional thickness region addresses the universality of this quantum
transport phenomenon and motivates the exploration of new QAHE phases with
tunable Chern numbers. In addition, the observation of the scale-invariant
dissipationless chiral propagation on a macroscopic scale makes a major stride
towards ideal low-power interconnect applications. | 1406.0106v2 |
2014-09-11 | High performance solar cells based on graphene-GaAs heterostructures | The honeycomb connection of carbon atoms by covalent bonds in a macroscopic
two-dimensional scale leads to fascinating graphene and solar cell based on
graphene/silicon Schottky diode has been widely studied. For solar cell
applications, GaAs is superior to silicon as it has a direct band gap of 1.42
eV and its electron mobility is six times of that of silicon. However,
graphene/GaAs solar cell has been rarely explored. Herein, we report
graphene/GaAs solar cells with conversion efficiency (Eta) of 10.4% and 15.5%
without and with anti-reflection layer on graphene, respectively. The Eta of
15.5% is higher than the state of art efficiency for graphene/Si system
(14.5%). Furthermore, our calculation points out Eta of 25.8% can be reached by
reasonably optimizing the open circuit voltage, junction ideality factor,
resistance of graphene and metal/graphene contact. This research strongly
support graphene/GaAs hetero-structure solar cell have great potential for
practical applications. | 1409.3500v3 |
2014-10-13 | Breakdown of Three-dimensional Dirac Semimetal State in pressurized Cd3As2 | We report the first observation of a pressure-induced breakdown of the 3D-DSM
state in Cd3As2, evidenced by a series of in-situ high-pressure synchrotron
X-ray diffraction (XRD) and single crystal transport measurements. We find that
Cd3As2 undergoes a structural phase transition from a metallic tetragonal (T)
phase in space group I41/acd to a semiconducting monoclinic (M) phase in space
group P21/c at critical pressure 2.57 GPa, above this pressure, an activation
energy gap appears, accompanied by distinct switches in Hall resistivity slope
and electron mobility. These changes of crystal symmetry and corresponding
transport properties manifest the breakdown of the 3D-DSM state in pressurized
Cd3As2. | 1410.3213v4 |
2014-11-13 | Analysis of viscoelastic soft dielectric elastomer generators operating in an electrical circuit | A predicting model for soft Dielectric Elastomer Generators (DEGs) must
consider a realistic model of the electromechanical behaviour of the elastomer
filling, the variable capacitor and of the electrical circuit connecting all
elements of the device. In this paper such an objective is achieved by
proposing a complete framework for reliable simulations of soft energy
harvesters. In particular, a simple electrical circuit is realised by
connecting the capacitor, stretched periodically by a source of mechanical
work, in parallel with a battery through a diode and with an electrical load
consuming the energy produced. The electrical model comprises resistances
simulating the effect of the electrodes and of the conductivity current
invariably present through the dielectric film. As these devices undergo a high
number of electro-mechanical loading cycles at large deformation, the
time-dependent response of the material must be taken into account as it
strongly affects the generator outcome. To this end, the viscoelastic behaviour
of the polymer and the possible change of permittivity with strains are
analysed carefully by means of a proposed coupled electro-viscoelastic
constitutive model, calibrated on experimental data available in the literature
for an incompressible polyacrilate elastomer (3M VHB4910). Numerical results
showing the importance of time-dependent behaviour on the evaluation of
performance of DEGs for different loading conditions, namely equi-biaxial and
uniaxial, are reported in the final section. | 1411.3613v2 |
2014-11-28 | Growth control of the oxidation state in vanadium oxide thin films | Precise control of the chemical valence or oxidation state of vanadium in
vanadium oxide thin films is highly desirable for not only fundamental
research, but also technological applications that utilize the subtle change in
the physical properties originating from the metal- insulator transition (MIT)
near room temperature. However, due to the multivalent nature of vanadium and
the lack of a good understanding on growth control of the oxidation state,
stabilization of phase pure vanadium oxides with a single oxidation state is
extremely challenging. Here, we systematically varied the growth conditions to
clearly map out the growth window for preparing phase pure epitaxial vanadium
oxides by pulsed laser deposition for providing a guideline to grow high
quality thin films with well-defined oxidation states of V2(+3)O3, V(+4)O2, and
V2(+5)O5. A well pronounced MIT was only observed in VO2 films grown in a very
narrow range of oxygen partial pressure P(O2). The films grown either in lower
(< 10 mTorr) or higher P(O2) (> 25 mTorr) result in V2O3 and V2O5 phases,
respectively, thereby suppressing the MIT for both cases. We have also found
that the resistivity ratio before and after the MIT of VO2 thin films can be
further enhanced by one order of magnitude when the films are further oxidized
by post-annealing at a well-controlled oxidizing ambient. This result indicates
that stabilizing vanadium into a single valence state has to compromise with
insufficient oxidation of an as grown thin film and, thereby, a subsequent
oxidation is required for an improved MIT behavior. | 1411.7922v1 |
2014-12-05 | Effects of Oxygen Adsorption on the Surface State of Epitaxial Silicene on Ag(111) | Epitaxial silicene, which is one single layer of silicon atoms packed in a
honeycomb structure, demonstrates a strong interaction with the substrate that
dramatically affects its electronic structure. The role of electronic coupling
in the chemical reactivity between the silicene and the substrate is still
unclear so far, which is of great importance for functionalization of silicene
layers. Here, we report the reconstructions and hybridized electronic
structures of epitaxial 4x4 silicene on Ag(111), which are revealed by scanning
tunneling microscopy and angle-resolved photoemission spectroscopy. The
hybridization between Si and Ag results in a metallic surface state, which can
gradually decay due to oxygen adsorption. X-ray photoemission spectroscopy
confirms the decoupling of Si-Ag bonds after oxygen treatment as well as the
relatively oxygen resistance of Ag(111) surface, in contrast to 4x4 silicene
[with respect to Ag(111)]. First-principles calculations have confirmed the
evolution of the electronic structure of silicene during oxidation. It has been
verified experimentally and theoretically that the high chemical activity of
4x4 silicene is attributable to the Si pz state, while the Ag(111) substrate
exhibits relatively inert chemical behavior. | 1412.1887v1 |
2015-02-10 | Ultrafast dynamic conductivity and scattering rate saturation of photoexcited charge carriers in silicon investigated with a midinfrared continuum probe | We employ ultra-broadband terahertz-midinfrared probe pulses to characterize
the optical response of photoinduced charge-carrier plasmas in high-resistivity
silicon in a reflection geometry, over a wide range of excitation densities
(10^{15}-10^{19} cm^{-3}) at room temperature. In contrast to conventional
terahertz spectroscopy studies, this enables one to directly cover the
frequency range encompassing the resultant plasma frequencies. The intensity
reflection spectra of the thermalized plasma, measured using sum-frequency
(up-conversion) detection of the probe pulses, can be modeled well by a
standard Drude model with a density-dependent momentum scattering time of
approx. 200 fs at low densities, reaching approx. 20 fs for densities of
approx. 10^{19} cm^{-3}, where the increase of the scattering rate saturates.
This behavior can be reproduced well with theoretical results based on the
generalized Drude approach for the electron-hole scattering rate, where the
saturation occurs due to phase-space restrictions as the plasma becomes
degenerate. We also study the initial sub-picosecond temporal development of
the Drude response, and discuss the observed rise in the scattering time in
terms of initial charge-carrier relaxation, as well as the optical response of
the photoexcited sample as predicted by finite-difference time-domain
simulations. | 1502.02829v1 |
2015-03-03 | Construction of force measuring optical tweezers instrumentation and investigations of biophysical properties of bacterial adhesion organelles | Optical tweezers are a technique in which microscopic-sized particles,
including living cells and bacteria, can be non-intrusively trapped with high
accuracy solely using focused light. The technique has therefore become a
powerful tool in the field of biophysics. Optical tweezers thereby provide
outstanding manipulation possibilities of cells as well as semi-transparent
materials, both non-invasively and non-destructively, in biological systems. In
addition, optical tweezers can measure minute forces (< 10-12 N), probe
molecular interactions and their energy landscapes, and apply both static and
dynamic forces in biological systems in a controlled manner. The assessment of
intermolecular forces with force measuring optical tweezers, and thereby the
biomechanical structure of biological objects, has therefore considerably
facilitated our understanding of interactions and structures of biological
systems. Adhesive bacterial organelles, so called pili, mediate adhesion to
host cells and are therefore crucial for the initial bacterial-cell contact.
Thus, they serve as an important virulence factor. The investigation of pili,
both their biogenesis and their expected in vivo properties, brings information
that can be of importance for the design of new drugs to prevent bacterial
infections, which is crucial in the era of increased bacterial resistance
towards antibiotics. In this thesis, an experimental setup of a force measuring
optical tweezers system and the results of a number of biomechanical
investigations of adhesive bacterial organelles are presented. Force measuring
optical tweezers have been used to characterize three different types of
adhesive organelles under various conditions, P, type 1, and S pili, which all
are expressed by uropathogenic Escherichia coli. A quantitative biophysical
force-extension model, built upon the structure and force response, has been
developed. | 1503.00881v1 |
2015-03-30 | Microbranching in mode-I fracture using large scale simulations of amorphous and perturbed lattice models | We study the high-velocity regime mode-I fracture instability when small
microbranches start to appear near the main crack, using large scale
simulations. Some of the features of those microbranches have been reproduced
qualitatively in smaller scale studies (using ${\cal O}(10^4)$ atoms) on both a
model of an amorphous materials (via the continuous random network model) and
using perturbed lattice models. In this study, larger scale simulations (${\cal
O}(10^6)$ atoms) were performed using multi-threading computing on a GPU
device, in order to achieve more physically realistic results. First, we find
that the microbranching pattern appears to be converging with the lattice
width. Second, the simulations reproduce the growth of the size of a
microbranch as a function of the crack velocity, as well as the increase of the
amplitude of the derivative of the electrical resistance RMS with respect to
the time as a function of the crack velocity. In addition, the simulations
yield the correct branching angle of the microbranches, and the power law
governing the shape of the microbranches seems to be lower than one, so that
the side cracks turn over in the direction of propagation of the main crack as
seen in experiment. | 1503.08616v2 |
2016-04-12 | An invisible non-volatile solid-state memory | Information technologies require entangling data stability with encryption
for a next generation of secure data storage. Current magnetic memories,
ranging from low-density stripes up to high-density hard drives, can ultimately
be detected using routinely available probes or manipulated by external
magnetic perturbations. Antiferromagnetic resistors feature unrivalled
robustness but the stable resistive states reported scarcely differ by more
than a fraction of a percent at room temperature. Here we show that the
metamagnetic (ferromagnetic to antiferromagnetic) transition in intermetallic
Fe0.50Rh0.50 can be electrically controlled in a magnetoelectric
heterostructure to reveal or cloak a given ferromagnetic state. From an aligned
ferromagnetic phase, magnetic states are frozen into the antiferromagnetic
phase by the application of an electric field, thus eliminating the stray field
and likewise making it insensitive to external magnetic field. Application of a
reverse electric field reverts the antiferromagnetic state to the original
ferromagnetic state. Our work demonstrates the building blocks of a feasible,
extremely stable, non-volatile, electrically addressable, low-energy
dissipation, magnetoelectric multiferroic memory. | 1604.03383v2 |
2016-05-19 | Robust two-dimensional superconductivity and vortex system in Bi2Te3/FeTe heterostructures | The discovery of two-dimensional superconductivity in Bi2Te3/FeTe
heterostructure provides a new platform for the search of Majorana fermions in
condensed matter systems. Since Majorana fermions are expected to reside at the
core of the vortices, a close examination of the vortex dynamics in
superconducting interface is of paramount importance. Here, we report the
robustness of the interfacial superconductivity and 2D vortex dynamics in four
as-grown and aged Bi2Te3/FeTe heterostructure with different Bi2Te3 epilayer
thickness (3, 5, 7, 14 nm). After two years' air exposure, superconductivity
remains robust even when the thickness of Bi2Te3 epilayer is down to 3 nm.
Meanwhile, a new feature at ~13 K is induced in the aged samples, and the high
field studies reveal its relevance to superconductivity. The resistance of all
as-grown and aged heterostructures, just below the superconducting transition
temperature follows the Arrhenius relation, indicating the thermally activated
flux flow behavior at the interface of Bi2Te3 and FeTe. Moreover, the
activation energy exhibits a logarithmic dependence on the magnetic field,
providing a compelling evidence for the 2D vortex dynamics in this novel
system. The weak disorder associated with aging-induced Te vacancies is
possibly responsible for these observed phenomena. | 1605.05903v1 |
2016-07-06 | Chiral anomaly from strain-induced gauge fields in Dirac and Weyl semimetals | Dirac and Weyl semimetals form an ideal platform for testing ideas developed
in high energy physics to describe massless relativistic particles. One such
quintessentially field-theoretic idea of chiral anomaly already resulted in the
prediction and subsequent observation of the pronounced negative
magnetoresistance in these novel materials for parallel electric and magnetic
fields. Here we predict that the chiral anomaly occurs - and has experimentally
observable consequences - when real electromagnetic fields E and B are replaced
by strain-induced pseudo-electromagnetic fields e and b. For example, a uniform
pseudomagnetic field b is generated when a Weyl semimetal nanowire is put under
torsion. In accord with the chiral anomaly equation we predict a negative
contribution to the wire resistance proportional to the square of the torsion
strength. Remarkably, left and right moving chiral modes are then spatially
segregated to the bulk and surface of the wire forming a "topological coaxial
cable". This produces hydrodynamic flow with potentially very long relaxation
time. Another effect we predict is the ultrasonic attenuation and
electromagnetic emission due to a time periodic mechanical deformation causing
pseudoelectric field e. These novel manifestations of the chiral anomaly are
most striking in the semimetals with a single pair of Weyl nodes but also occur
in Dirac semimetals such as Cd3As2 and Na3Bi and Weyl semimetals with unbroken
time reversal symmetry. | 1607.01810v2 |
2016-11-17 | Interfacial thermal conductance in graphene/black phosphorus heterogeneous structures | Graphene, as a passivation layer, can be used to protect the black phosphorus
from the chemical reaction with surrounding oxygen and water. However, black
phosphorus and graphene heterostructures have low efficiency of heat
dissipation due to its intrinsic high thermal resistance at the interfaces. The
accumulated energy from Joule heat has to be removed efficiently to avoid the
malfunction of the devices. Therefore, it is of significance to investigate the
interfacial thermal dissipation properties and manipulate the properties by
interfacial engineering on demand. In this work, the interfacial thermal
conductance between few-layer black phosphorus and graphene is studied
extensively using molecular dynamics simulations. Two critical parameters, the
critical power Pcr to maintain thermal stability and the maximum heat power
density Pmax with which the system can be loaded, are identified. Our results
show that interfacial thermal conductance can be effectively tuned in a wide
range with external strains and interracial defects. The compressive strain can
enhance the interfacial thermal conductance by one order of magnitude, while
interface defects give a two-fold increase. These findings could provide
guidelines in heat dissipation and interfacial engineering for thermal
conductance manipulation of black phosphorus-graphene heterostructure-based
devices. | 1611.05598v1 |
2017-06-08 | Electronic and phonon excitations in α-RuCl$_3$ | We report on THz, infrared reflectivity and transmission experiments for wave
numbers from 10 to 8000 cm$^{-1}$ ($\sim$ 1 meV - 1 eV) and for temperatures
from 5 to 295 K on the Kitaev candidate material {\alpha}-RuCl$_3$. As reported
earlier, the compound under investigation passes through a first-order
structural phase transition, from a monoclinic high-temperature to a
rhombohedral low-temperature phase. The phase transition shows an extreme and
unusual hysteretic behavior, which extends from 60 to 166 K. In passing this
phase transition, in the complete frequency range investigated we found a
significant reflectance change, which amounts almost a factor of two. We
provide a broadband spectrum of dielectric constant, dielectric loss and
optical conductivity from the THz to the mid infrared regime and study in
detail the phonon response and the low-lying electronic density of states. We
provide evidence for the onset of an optical energy gap, which is of order 200
meV, in good agreement with the gap derived from measurements of the DC
electrical resistivity. Remarkably, the onset of the gap exhibits a strong blue
shift on increasing temperatures. | 1706.02724v2 |
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