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2014-06-24 | The Origin of the X-ray Emission from the High-velocity Cloud MS30.7-81.4-118 | A soft X-ray enhancement has recently been reported toward the high-velocity
cloud MS30.7-81.4-118 (MS30.7), a constituent of the Magellanic Stream. In
order to investigate the origin of this enhancement, we have analyzed two
overlapping XMM-Newton observations of this cloud. We find that the X-ray
enhancement is $\sim$6' or $\sim$100 pc across, and is concentrated to the
north and west of the densest part of the cloud. We modeled the X-ray
enhancement with a variety of spectral models. A single-temperature equilibrium
plasma model yields a temperature of $(3.69^{+0.47}_{-0.44}) \times 10^6$ K and
a 0.4-2.0 keV luminosity of $7.9 \times 10^{33}$ erg s$^{-1}$. However, this
model underpredicts the on-enhancement emission around 1 keV, which may
indicate the additional presence of hotter plasma ($T \gtrsim 10^7$ K), or that
recombination emission is important. We examined several different physical
models for the origin of the X-ray enhancement. We find that turbulent mixing
of cold cloud material with hot ambient material, compression or shock heating
of a hot ambient medium, and charge exchange reactions between cloud atoms and
ions in a hot ambient medium all lead to emission that is too faint. In
addition, shock heating in a cool or warm medium leads to emission that is too
soft (for reasonable cloud speeds). We find that magnetic reconnection could
plausibly power the observed X-ray emission, but resistive
magnetohydrodynamical simulations are needed to test this hypothesis. If
magnetic reconnection is responsible for the X-ray enhancement, the observed
spectral properties could potentially constrain the magnetic field in the
vicinity of the Magellanic Stream. | 1406.6363v1 |
2015-03-21 | Graphene as a p-type metal for ultimate miniaturization | We report macroscopic sheets of highly conductive bilayer graphene with
exceptionally high hole concentrations of ~ $10^{15}$ $cm^{-2}$ and
unprecedented sheet resistances of 20-25 {\Omega} per square over macroscopic
scales, and obtained in-situ over a thin cushion of molecular oxygen on a
silicon substrate. The electric and electronic properties of this specific
configuration remain stable upon thermal anneals and months of exposure to air.
We further report a complementary ab-initio study, predicting an enhancement of
graphene adhesion energy of up to a factor 20, also supported by experimental
fracture tests. Our results show that the remarkable properties of graphene can
be realized in a reliable fashion using a high-throughput process. In addition
to providing exceptional material properties, the growth process we employed is
scalable to large areas so that the outstanding conduction properties of
graphene can be harnessed in devices fabricated via conventional semiconductor
manufacturing processes. We anticipate that the approach will provide the
necessary scalability and reliability for future developments in the graphene
nanoscience and technology fields, especially in areas where further
miniaturization is hampered by size effects and electrical reliability of
classical conductors. | 1503.06253v2 |
2018-04-06 | Antisite pairs suppress the thermal conductivity of BAs | BAs was predicted to have an unusually high thermal conductivity at room
temperature of 2000$\,$Wm$^{-1}$$\,$K$^{-1}$, comparable to that of diamond.
However, the experimentally measured thermal conductivity of BAs single
crystals is an order of magnitude lower. To identify the origin of this large
inconsistency, we investigated the lattice structure and potential defects in
BAs single crystals at atomic scale using aberration-corrected scanning
transmission electron microscopy (STEM). Rather than finding a large
concentration As vacancies ($V_\mathrm{As}$), as widely thought to dominate the
thermal resistance in BAs crystals, our STEM results showed enhanced intensity
of some B columns and reduced intensity of some As columns, suggesting the
presence of antisite defects with As$_\mathrm{B}$ (As-atom on B site) and
B$_\mathrm{As}$ (B-atom on As site) with significant concentrations. Further
calculations show that the antisite pair with As$_\mathrm{B}$ next to
B$_\mathrm{As}$ is preferred energetically among the different types of point
defects investigated, and confirm that such defects lower the thermal
conductivity for BAs. Using a concentration of
6.6$\pm$3$\times$10$^{20}$$\,$cm$^{-3}$ for the antisite pairs estimated from
STEM images, thermal conductivity is estimated to be
65-100$\,$Wm$^{-1}$$\,$K$^{-1}$, in reasonable agreement with our measured
value. Our study suggests that As$_\mathrm{B}$-B$_\mathrm{As}$ antisite pairs
are the primary lattice defects suppressing thermal conductivity of BAs.
Possible approaches are proposed for growth of high quality crystals or films
with high thermal conductivity. | 1804.02381v1 |
2019-04-08 | Characterizations of thermal stability and electrical performance of Au-Ni coating on CuCrZr substrate for high vacuum radio-frequency contact application | Radio-frequency (RF) contacts-which are an example of electrical contacts-are
commonly employed on accelerators and nuclear fusion experimental devices. RF
contacts with a current load of 2 kA for steady-state operation were designed
for application to the International Thermonuclear Experimental Reactor (ITER)
device. In contrast to the typical working conditions of general commercial
electrical contacts, those of RF contacts employed on fusion devices include
high vacuum, high temperature, and neutron radiation. CuCrZr is currently of
interest as a base material for the manufacture of louvers of RF contacts,
which has excellent thermal and electrical properties and has low creep rate at
250 {\textdegree}C. In this study, a hard Au coating (Au-Ni) was electroplated
on CuCrZr samples and the samples were then subjected to thermal aging
treatment at 250 {\textdegree}C for 500 h in order to simulate the
vacuum-commissioning process of the ITER. The effects of thermal aging on the
hardness, elastic modulus, crystallite size, and compositions of the coating
were investigated via microstructural and mechanical characterizations of the
coating material. Metal atom migration in different coating layers during
thermal aging was characterized and evaluated via scanning electron
microscopy/energy dispersive X-ray spectroscopy observations of the
cross-sectional surfaces, and the obtained results could be used to directly
select the coating thickness for the final RF contact component. The contact
resistance-an important parameter of the RF contact-was measured in a dedicated
testbed built to simulate fusion reactor conditions between CuCrZr pins and
stainless steel plates coated with Au-Ni and Rh, respectively. | 1907.07236v1 |
2020-07-03 | Enhanced creep performance in a polycrystalline superalloy driven by atomic-scale phase transformation along planar faults | Predicting the mechanical failure of parts in service requires understanding
their deformation behavior, and associated dynamic microstructural evolution up
to the near-atomic scale. Solutes are known to interact with defects generated
by plastic deformation, thereby affecting their displacement throughout the
microstructure and hence the material mechanical response to solicitation. This
effect is studied here in a polycrystalline Ni-based superalloy with two
different Nb contents that lead to a significant change in their creep
lifetime. Creep testing at 750C and 600 MPa shows that the high-Nb alloy
performs better in terms of creep strain rate. Considering the similar initial
microstructures, the difference in mechanical behavior is attributed to a phase
transformation that occurs along planar faults, controlled by the different
types of stacking faults and alloy composition. Electron channeling contrast
imaging reveals the presence of stacking faults in both alloys. Microtwinning
is observed only in the low-Nb alloy, rationalizing in part the higher creep
strain rate. In the high-Nb alloy, atom probe tomography evidences two
different types of stacking faults based on their partitioning behavior.
Superlattice intrinsic stacking faults (SISF) were found enriched in Nb, Co, Cr
and Mo while only Nb and Co was segregated at superlattice extrinsic stacking
faults. Based on their composition, a local phase transformation occurring
along the faults is suggested, resulting in slower creep strain rate in the
high-Nb alloy. In comparison, mainly SISF enriched in Co, Cr, Nb and Mo were
found in the low-Nb alloy. Following the results presented here, and those
available in the literature, an atomic-scale driven alloy design approach that
controls and promotes local phase transformation along planar faults at 750C is
proposed, aiming to design superalloys with enhanced creep resistance. | 2007.01676v2 |
2022-03-15 | Flexible terahertz photonic light-cage modules for in-core sensing and high temperature applications | Terahertz (THz) technology is a growing and multi-disciplinary research
field, particularly for sensing and telecommunications. A number of THz
waveguides have emerged over the past years, which are set to complement the
capabilities of existing and bulky free space setups. In most designs however,
the guiding region is physically separated from the surroundings, making
interactions between light and the environment inefficient. We present photonic
THz light cages (THzLCs) operating at THz frequencies, consisting of
free-standing dielectric strands, which guide light within a hollow core with
immediate access to the environment. We show the versatility and design
flexibility of this concept, by 3D-printing several cm-length-scale modules
using a single design and four different polymer- and ceramic- materials, which
are either rigid, flexible, or resistant to high temperatures. We characterize
propagation- and bend-losses for straight- and curved- waveguides, which are of
order ~1 dB/cm in the former, and ~2-8 dB/cm in the latter for bend radii below
10 cm, and largely independent of the material. Our transmission experiments
are complemented by near-field measurements at the waveguide output, which
reveal antiresonant guidance for straight THzLCs, and a deformed fundamental
mode in the bent waveguides, in agreement with numerical conformal mapping
simulations. We show that these THzLCs can be used either as: (i) flexible,
reconfigurable, and bendable modular assemblies; (ii) in-core sensors of
structures contained directly inside the hollow core; (iii) high-temperature
sensors, with potential applications in industrial monitoring. These THzLCs are
a novel and useful addition to the growing library of THz waveguides, marrying
the waveguide-like advantages of reconfigurable, diffractionless propagation,
with the free-space-like immediacy of direct exposure to the surrounding
environment. | 2203.08316v1 |
2022-10-21 | Understanding The Reversible Electrodeposition of Al in Low-Cost Room Temperature Molten Salts | Aluminum is the most earth-abundant metal, is trivalent, is inert in ambient
humid air, and has a density approximately four-times that of lithium at room
temperature. These attributes make it an attractive material for
cost-effective, long-duration storage of electrical energy in batteries.
Scientific discoveries in the past decade have established that secondary Al
batteries can be created by paring an Al anode with a graphite or transition
metal oxide cathode, in imidazolium-based, room-temperature
ionic-liquid-Aluminum chloride electrolytes. Here we report findings from a
systematic study that sheds light on the structural requirements,
physicochemical, and transport properties of the ionic liquid electrolytes
responsible for the high reversibility of Al battery anodes. We find that the
most important interfacial and transport properties of these electrolytes can
be achieved in other electrolytes, including ammonium-based molten salts that
are available at costs as much as twenty-times lower than the ionic
liquid-Aluminum chloride melt. High Al reversibility in ammonium- and
imidazolium-based electrolytes is specifically shown to require a critical
ratio of the solvation species, where Lewis acidity and beneficial interfacial
reactions continuously etch the alumina resistive interfacial layer and form
beneficial solid electrolyte interphase at the anode. We report further that
successful development of new electrolyte families that support high Al anode
reversibility also provides a good platform for detailed studies of the working
mechanisms of an intercalation graphite cathode using X-ray absorption
spectroscopy. Our findings therefore open new opportunities for developing
simple, cost-effective, room-temperature Al batteries that enable long-duration
electrical energy storage. | 2210.12131v1 |
2024-01-18 | Spreading of Low-viscosity Ink Filaments Driven by Bath Viscoelasticity in Embedded Printing | Inks deposited in conventional direct ink writing need to be able to support
their own weight and that of the upper layers with minimal deformation to
preserve the structural integrity of the three-dimensional (3D) printed parts.
This constraint limits the range of usable inks to high-viscosity materials.
Embedded printing enables the use of much softer inks by depositing the
materials in a bath of another fluid that provides external support, thus
diversifying the types of 3D printable structures. The interactions between the
ink and bath fluids, however, give rise to a unique type of defect: spreading
of the dispensed ink behind the moving nozzle. By printing horizontal threads
made of dyed water in baths of Carbopol suspensions, we demonstrate that the
spreading can be attributed to the pressure field generated in the viscous bath
by the relative motion of the nozzle. As the pressure gradient increases with
the viscosity of the bath fluid while the viscosity of the ink resists the
flow, a larger bath-to-ink viscosity ratio results in more spreading for
low-concentration Carbopol baths. For high-concentration, yield-stress-fluid
baths, we find that the steady-state viscosity alone cannot account for the
spreading, as the elastic stress becomes comparable to the viscous stress and
the bath fluid around the dispensed ink undergoes fluidization and
resolidification. By parameterizing the transient rheology of the
high-concentration Carbopol suspensions using a simple viscoelastic model, we
suggest that the ink spreading is exacerbated by the elasticity but is
mitigated by the yield stress as long as the yield stress is low enough to
allow steady injection of the ink. These results help illuminate the link
between the bath rheology and the printing quality in embedded 3D printing. | 2401.09684v1 |
2001-10-08 | Structural and electronic properties of the Nb$_4$-cluster compound Ga$_{1.33}$Nb$_4$X$_8$ (X = S, Se) | We report resistivity, thermopower and magnetic susceptibility measurements
on the Nb$_4$-cluster compounds Ga$_{1.33}$Nb$_4$X$_{8}$ (X = S, Se), derived
from vacancy ordered spinels A$_x$T$_4$X$_8$. The cubic selenide phase is
insulating and its resistivity crosses over from $\ln{\rho} \sim T^{-1}$ to
$\sim T^{-1/2}$ on cooling below 150 K, indicating variable range hopping (VRH)
conduction at low temperatures. This is similar to that previously reported for
V$_4$ and Mo$_4$ cluster compounds. The rhombohedrally distorted sulfide is
metallic and shows a minimum in resistance at $\sim $ 56 K below which the
resistivity varies as $\rho(T) \sim T^{-1/2}$. The thermopower of the selenide
becomes temperature independent below the crossover temperature. These Nb$_4$
compounds exhibit enhanced Pauli-paramagnetic magnetic susceptibility ($\chi$)
irrespective of their transport properties and both undergo a similar
transition in $\chi(T) \sim 30$ K. We discuss these properties in the model of
hopping conduction under long-range Coulomb repulsion effects and derive
consistency between some of the transport and magnetic parameters. | 0110155v1 |
2001-11-06 | Spin-dependent electrical transport in ion-beam sputter deposited Fe-Cr multilayers | The temperature dependence of the electrical resistivity and
magnetoresistance of Xe-ion beam sputtered Fe-Cr multilayers has been
investigated. The electrical resistivity between 5 and 300 K in the fully
ferromagnetic state, obtained by applying a field beyond the saturation field
(H_sat) necessary for the antiferromagnetic(AF)-ferromagnetic(FM) field-induced
transition, shows evidence of spin-disorder resistivity as in crystalline Fe
and an s-d scattering contribution (as in 3d metals and alloys). The sublattice
magnetization m(T) in these multilayers has been calculated in terms of the
planar and interlayer exchange energies. The additional spin-dependent
scattering \Delta \rho (T) = \rho(T,H=0)_AF - \rho(T,H=H_sat)_FM in the AF
state over a wide range of temperature is found to be proportional to the
sublattice magnetization, both \Delta \rho(T) and m(T) reducing along with the
antiferromagnetic fraction. At intermediate fields, the spin-dependent part of
the electrical resistivity (\rho_s (T)) fits well to the power law \rho_s (T) =
b - cT^\alpha where c is a constant and b and \alpha are functions of H. At low
fields \alpha \approx 2 and the intercept b decreases with H much the same way
as the decrease of \Delta \rho (T) with T. A phase diagram (T vs. H_sat) is
obtained for the field- induced AF to FM transition. Comparisons are made
between the present investigation and similar studies using dc magnetron
sputtered and molecular beam epitaxy (MBE) grown Fe-Cr multilayers. | 0111084v1 |
2002-10-29 | Quantum Resistive Behaviors in the Vortex Liquid Regimes at Finite Temperatures | Motivated by a {\it mean field-like} resistive behavior in magnetic fields
commonly seen in various superconducting (SC) cuprates and organics with strong
fluctuation, {\it quantum} SC fluctuation effects on resistive behaviors are
reexamined by putting emphasis on their roles in the so-called {\it thermal}
vortex liquid regime. By incorporating the quantum fluctuation and a vortex
pinning effect in the GL fluctuation theory, it is found that the resistivity
curve shows not a fan-shaped broadening but a sharp drop at a 3d vortex-glass
transition point far below an apparent upper critical field H_{c2}^*(T) as a
result of a quantm fluctuation enhanced by an adequately small condensation
energy or by a strong field. Fittings to data of cuprate and organic
superconductors are performed by including effects of SC pseudogap region. It
is argued on the cuprate materials that, irrespective of the presence of
fluctuations of competing non-SC orders, the in-plane coherence length of
hole-doped cuprates decreases on approaching the underdoped limit and that the
condensation energy density to be measured from the heat capacity data is
maximum near the optimal doping. The case of disordered quasi 2d s-wave films
showing the field-tuned superconductor-insulator transition (FSIT) behavior is
also studied for comparison, and an agreement with available data is found. | 0210626v5 |
2002-11-22 | Room Temperature Ballistic Conduction in Carbon Nanotubes | Multiwalled carbon nanotubes are shown to be ballistic conductors at room
temperature, with mean free paths of the order of tens of microns. These
experiments follow and extend the original experiments by Frank et al (Science,
280 1744 1998) including in-situ electron microscopy experiments and a detailed
analysis of the length dependence of the resistance. The per unit length
resistance r < 100 Ohm/micron, indicating free paths l > 65 microns,
unambiguously demonstrate ballistic conduction at room temperature up to
macroscopic distances. The nanotube-metal contact resistances are in the range
0.1-1 kOhm micron. Contact scattering can explain why the measured conductances
are about half the expected theoretical value of 2 G0 . For V>0.1V the
conductance rises linearly (dG/dV~0.3 G0 /V) reflecting the linear increase in
the density-of-states in a metallic nanotube above the energy gap. Increased
resistances (r =2- 10 k Ohm/micron) and anomalous I-V dependences result from
impurities and surfactants on the tubes.Evidence is presented that ballistic
transport occurs in undoped and undamaged tubed for which the top layer is
metallic and the next layer is semiconducting. The diffusive properties of
lithographically contacted multiwalled nanotubes most likely result from
purification and other processing steps that damage and dope the nanotubes
thereby making them structurally and electronically different than the pristine
nanotubes investigated here. | 0211515v1 |
2006-12-27 | Peculiarities of the transport properties of InMnAs layers, produced by the laser deposition, in strong magnetic fields | Magnetotransport properties of p-InMnAs layers are studied in pulsed magnetic
fields up to 30 T. Samples were prepared by the laser deposition and annealed
by ruby laser pulses. Well annealed samples show p-type conductivity while they
were n-type before the annealing. Surprisingly the anomalous Hall effect
resistance in paramagnetic state (T>40 K) and in strong magnetic fields (B > 20
T) appears to be greater than that in ferromagnetic state (T <= 40 K), while
the longitudinal resistance rises with the temperature decrease. The negative
magnetoresistance saturates in magnetic fields higher then 10T at T near 4 K
only, whereas the saturation fields of the anomalous Hall effect resistance are
much less (around 2 T at 30K). The total reduction of resistance exceeds 10
times in magnetic fields around of 10T. The obtained results are interpreted on
the base of the assumptions of the non-uniform distribution of Mn atoms acting
as acceptors, the local ferromagnetic transition and the percolation-like
character of the film conductivity, which prevailed under conditions of the
strong fluctuations of the exchange interaction. Characteristic scales of the
magneto-electric nonuniformity are estimated using analysis of the mesoscopic
fluctuations of the non-diagonal components of the magnetoresistivity tensor. | 0612641v1 |
2007-06-28 | Emergence of a flux tube through a partially ionised solar atmosphere | For a magnetic flux tube, or indeed any flux, to emerge into the Solar corona
from the convection zone it must pass through the partially ionised layers of
the lower atmosphere: the photosphere and the chromosphere. In such regions the
ion-neutral collisions lead to an increased resistivity for currents flowing
across magnetic field lines. This Cowling resistivity can exceed the Spitzer
resistivity by orders of magnitude and in 2.5D simulations has been shown to be
sufficient to remove all cross field current from emerging flux. Here we extend
this modelling into 3D. Once again it is found that the Cowling resistivity
removes perpendicular current. However the presence of 3D structure prevents
the simple comparison possible in 2.5D simulations. With a fully ionised
atmosphere the flux emergence leads to an unphysically low temperature region
in the overlying corona, lifting of chromospheric material and the subsequent
onset of the Rayleigh-Taylor instability. Including neutrals removes the low
temperature region, lifts less chromospheric matter and shows no signs of the
Rayleigh-Taylor instability. Simulations of flux emergence therefore should
include such a neutral layer in order to obtain the correct perpendicular
current, remove the Rayleigh-Taylor instability and get the correct temperature
profile. In situations when the temperature is not important, i.e. when no
simulated spectral emission is required, a simple model for the neutral layer
is demonstrated to adequately reproduce the results of fully consistent
simulations. | 0706.4223v1 |
2010-01-10 | Anomalies of conductivity behavior near the paramagnetic-antiferromagnetic transition in single-crystals La_{2}CuO_{4+δ} | The temperature dependences of resistance, R(T), of two single-crystals
La_{2}CuO_{4+\delta} samples have been studied with the aim to detect a
possible change in the R(T) behavior induced by paramagnetic-antiferromagnetic
(PM-AFM) transition. One of the samples with \delta \lesssim 0.01, was fairly
homogeneous in oxygen distribution (not phase-separated) with Neel temperature
T_{N}\approx 266 K. Conductivity of this sample has been determined by Mott's
variable-range hopping below T_N. The other, far less resistive, sample with
\delta \approx 0.05, was inhomogeneous (phase-separated) showing both PM-AFM
(T_N\approx 205 K) and superconducting (T_c\approx 25 K) transitions. It is
found that for the homogeneous sample the resistivity decreases above T_N far
faster with temperature than below it (for both directions of measuring
current, parallel and perpendicular to basal CuO_2 planes). A similar behavior
of conductivity near PM-AFM transition is also found for the phase-separated
and less resistive sample. In this case a clear kink in R(T) curve near
T_N\approx 205 K can be seen. Furthermore, a transition to metallic (dR/dT>0)
behavior occurs far enough above T_N. The observed behavior of the samples
studied is related to increased delocalization of charge carriers above T_N.
This is in accordance with decrease in the AFM correlation length and
corresponding enhancement of the hole mobility above T_N known for low-doped
lanthanum cuprates. | 1001.1518v1 |
2011-05-07 | Effect of 50 MeV Li3+ irradiation on structural and electrical properties of Mn doped ZnO | The present work aims to study the effect of ion irradiation on structural
and electrical properties and their correlation with the defects in Zn1-xMnxO
type system. Zn1-xMnxO (x = 0.02, 0.04) samples have been synthesized by
solid-state reaction method and have been irradiated with 50 MeV Li3+ ions. The
concomitant changes have been probed by x-ray diffraction (XRD), temperature
dependent electrical resistivity and positron annihilation lifetime (PAL)
spectroscopy. XRD result shows single phase wurtzite structure for
Zn0.98Mn0.02O, whereas for Zn0.96Mn0.04O sample an impurity phase has been
found apart from the usual peaks of ZnO. Ion irradiation dissolves this
impurity peak. Grain size of the samples found to be uniform. For
Zn0.98Mn0.02O, the observed sharp decrease in room temperature resistivity
(RhoRT) with irradiation is consistent with the lowering of FWHM of the XRD
peaks. However for Zn0.96Mn0.04O, RhoRT decreases for initial fluence but
increases for further increase of fluence. All the irradiated Zn0.98Mn0.02O
samples show metal-semiconductor transition in temperature dependent
resistivity measurement at low temperature. But all the irradiated
Zn0.96Mn0.04O samples show semiconducting nature in the whole range of
temperature. Results of room temperature resistivity, XRD and PAL measurements
are consistent with each other. | 1105.1414v1 |
2013-01-07 | Disorder-driven carrier transport in atomic layer deposited ZnO thin films | This paper addresses the effect of disorder on the carrier transport
mechanism of atomic layer deposited ZnO thin films as has been investigated by
temperature dependent electrical resistivity measurements in the temperature
range of 4.2K to 300K. Films were grown on (0001) sapphire substrate at
different substrate temperatures varying from 150 to 350 C. The defects and
structural disorder in the films were found to be strongly dependent on their
growth temperature. The films grown at 150, 300 and 350 C were found to be
semiconductor-like in the whole measurement temperature range of resistivity
due to the enhanced disorder in these films. However, a metal to semiconductor
transition (MST) at low temperature has been observed in the films grown at 200
and 250 C. It was also observed that the film grown at 250 C with higher
residual resistivity, the transition temperature shifted towards the higher
value due to the increased disorder in this film as compared to that grown at
200 C. The upturn in resistivity below the transition temperature has been well
explained by considering quantum corrections to the Boltzmanns conductivity
which includes the effect of weak localization and coulomb electron-electron
interactions related to the existence of disorder in these films. | 1301.1172v1 |
2014-11-27 | Temperature dependence of electrical and thermal conduction in single silver nanowire | Silver nanowires have great application potential in fields like flexible
electronic devices, solar cells and transparent electrodes. It is critical and
fundamental to study the thermal and electrical transport properties in a
single silver nanowire. In this work, the thermal and electrical transport in
an individual silver nanowire is characterized down to 35 K with the steady
state electro-thermal technique. The results indicate that, at room
temperature, the electrical resistivity increases by around 4 folds compared
from that of its bulk counterpart. After fitting the temperature dependent
electrical resistivity curves with the Bloch-Gr\"uneisen formula, the Debye
temperature (151 K) of the silver nanowire is found 36% lower than that (235 K)
of bulk silver, confirming strong phonon softening. The thermal conductivity is
reduced by 55% compared with that of its bulk counterpart at room temperature
and this reduction becomes larger as the temperature goes down. To explain the
opposite trends of thermal conductivity (\k{appa}) ~ temperature (T) of silver
nanowire and bulk silver, a unified thermal resistivity is used to elucidate
the electron scattering mechanism. A large residual unified thermal resistivity
for the silver nanowire is observed while that of the bulk silver is almost
zero. The same trend of variation against T indicates that the silver nanowire
and bulk silver share the same phonon-electron scattering mechanism.
Additionally, due to phonon-assisted electron energy transfer across the grain
boundaries, the Lorenz number of the silver nanowire is found much larger than
that of bulk silver and decreases with decreasing temperature. | 1411.7659v2 |
2015-09-18 | Evidence for hydrodynamic electron flow in PdCoO$_2$ | Electron transport is conventionally determined by the momentum-relaxing
scattering of electrons by the host solid and its excitations. The electrical
resistance is set by geometrical factors and the resistivity, which is a
microscopic property of the solid. Hydrodynamic fluid flow through channels, in
contrast, is determined by geometrical factors, boundary scattering and the
viscosity of the fluid, which is governed by momentum-conserving internal
collisions. A long-standing question in the physics of solids, brought into
focus by the advent of new calculational techniques, has been whether the
viscosity of the electron fluid plays an observable role in determining the
resistance. At first sight this seems unlikely, because in almost all known
materials the rate of momentum-relaxing collisions dominates that of the
momentum-conserving ones that give the viscous term. Here, we show this is not
always the case. We report experimental evidence that the resistance of
restricted channels of the ultra-pure two-dimensional metal PdCoO$_2$ has a
large viscous contribution. Comparison with theory allows an estimate of the
electronic viscosity in the range between $6\times 10^{-3}$~kg(ms)$^{-1}$ and
$3\times 10^{-4}$~kg(ms)$^{-1}$, which brackets that of water at room
temperature. | 1509.05691v2 |
2015-10-31 | The influence of system dynamics on the frictional resistance: insights from a discrete model | In order to examine the influence of system dynamics on sliding friction, we
introduce the so-called micro-walking machine. This model consists of a rigid
body with a number of elastic contact spots that is pulled by a constantly
moving base. The system slides with dry friction on a rigid substrate. The
kinematic coupling of the rotation and the translation of the rigid body
results in varying normal and tangential forces at the contact spots. For
certain parameter ranges this leads to self-excited oscillations in the
vertical direction. A particular dynamic mode occurs which is characterized by
a strong correlation between low or even zero normal forces and a fast forward
motion. This effect is referred to as micro-walking. In addition to an
experimental rig we use numerical integration and an extensive parameter study
for the analysis. In theory, the reduction of the frictional resistance reaches
up to 98%. These results are confirmed by the experiments where the maximal
reduction was 73%. Our model shows that micro-vibrations play an important role
for the dynamic influences on the frictional resistance of systems that exhibit
apparently smooth sliding. The identification of the critical parameter range
enables the systematic control of frictional resistance through the adjustment
of attributes such as geometry and stiffness. In addition, it is possible to
deduce guidelines for how tribological test rigs should be designed in order to
get reliable results. | 1511.00162v1 |
2016-05-02 | Interfilament Resistance at 77 K in Striated HTS Coated Conductors | Striating HTS coated conductor (CC) tapes into narrow filaments offers the
possibility of reducing the tapes' magnetization losses without unreasonably
decreasing their current-carrying capability. However, realizing well-separated
striations presents technological challenges, especially if the number of
filaments is large and/or if a thick layer of metallic stabilizer is present.
In these situations, the filaments can be easily coupled and their
effectiveness to reduce magnetization losses is strongly diminished or
eliminated. While the onset of coupling is well visible from magnetization loss
measurements, the actual path of the coupling current is unknown. In this
contribution we present a systematic study of the transverse resistance in HTS
CC samples in order to get a deeper understanding of those paths. The measured
samples differ in terms of manufacturer (SuperPower and SuperOx), and presence
and thickness of stabilizer material. In addition, oxidation is used as a means
to increase the resistance between the filaments in non-stabilized samples. The
results are interpreted with a chain network model. This work provides useful
insights on the factors determining the transverse resistance in striated HTS
CCs, thus indicating ways to improve the effectiveness of the striation process
for AC loss reduction. | 1605.00401v2 |
2016-08-09 | Disorder-sensitive pump-probe measurements on Nd$_{1.83}$Ce$_{0.17}$CuO$_{4\pmδ}$ films | We find an unambiguous relationship between disorder-driven features in the
temperature dependence of the resistance and the behavior, as functions of the
temperature, of the parameters necessary to describe some of the relaxation
processes in the photoinduced differential time-resolved reflectivity of three
samples of Nd$_{1.83}$Ce$_{0.17}$CuO$_{4\pm\delta}$. The latter, sharing the
same Ce content, have been fabricated and annealed ad-hoc in order to differ
only for the degree of disorder, mainly related to oxygen content and location,
and, consequently, for the temperature dependence of the resistance: two of
them present a minimum in the resistance and behave as a superconductor and as
a metal, respectively, the third behaves as an insulator. The systematic
coherence between the resistance and the relaxation parameters behaviors in
temperature for all three samples is absolutely remarkable and shows that
pump-probe measurements can be extremely sensitive to disorder as it drives the
emergence of new excitations and of the related relaxation channels as in this
paradigmatic case. | 1608.03006v1 |
2018-05-10 | Activation Energy of Metastable Amorphous Ge2Sb2Te5 from Room Temperature to Melt | Resistivity of metastable amorphous Ge2Sb2Te5 (GST) measured at device level
show an exponential decline with temperature matching with the steady-state
thin-film resistivity measured at 858 K (melting temperature). This suggests
that the free carrier activation mechanisms form a continuum in a large
temperature scale (300 K - 858 K) and the metastable amorphous phase can be
treated as a super-cooled liquid. The effective activation energy calculated
using the resistivity versus temperature data follow a parabolic behavior, with
a room temperature value of 333 meV, peaking to ~377 meV at ~465 K and reaching
zero at ~930 K, using a reference activation energy of 111 meV (3kBT/2) at
melt. Amorphous GST is expected to behave as a p-type semiconductor at Tmelt ~
858 K and transitions from the semiconducting-liquid phase to the
metallic-liquid phase at ~ 930 K at equilibrium. The simultaneous Seebeck (S)
and resistivity versus temperature measurements of amorphous-fcc mixed-phase
GST thin-films show linear S-T trends that meet S = 0 at 0 K, consistent with
degenerate semiconductors, and the dS/dT and room temperature activation energy
show a linear correlation. The single-crystal fcc is calculated to have dS/dT =
0.153 {\mu}V/K for an activation energy of zero and a Fermi level 0.16 eV below
the valance band edge. | 1805.04054v1 |
2019-09-17 | Algorithm for Training Neural Networks on Resistive Device Arrays | Hardware architectures composed of resistive cross-point device arrays can
provide significant power and speed benefits for deep neural network training
workloads using stochastic gradient descent (SGD) and backpropagation (BP)
algorithm. The training accuracy on this imminent analog hardware however
strongly depends on the switching characteristics of the cross-point elements.
One of the key requirements is that these resistive devices must change
conductance in a symmetrical fashion when subjected to positive or negative
pulse stimuli. Here, we present a new training algorithm, so-called the
"Tiki-Taka" algorithm, that eliminates this stringent symmetry requirement. We
show that device asymmetry introduces an unintentional implicit cost term into
the SGD algorithm, whereas in the "Tiki-Taka" algorithm a coupled dynamical
system simultaneously minimizes the original objective function of the neural
network and the unintentional cost term due to device asymmetry in a
self-consistent fashion. We tested the validity of this new algorithm on a
range of network architectures such as fully connected, convolutional and LSTM
networks. Simulation results on these various networks show that whatever
accuracy is achieved using the conventional SGD algorithm with symmetric
(ideal) device switching characteristics the same accuracy is also achieved
using the "Tiki-Taka" algorithm with non-symmetric (non-ideal) device switching
characteristics. Moreover, all the operations performed on the arrays are still
parallel and therefore the implementation cost of this new algorithm on array
architectures is minimal; and it maintains the aforementioned power and speed
benefits. These algorithmic improvements are crucial to relax the material
specification and to realize technologically viable resistive crossbar arrays
that outperform digital accelerators for similar training tasks. | 1909.07908v1 |
2017-04-13 | Light Induced Electron-Phonon Scattering Mediated Resistive Switching in Nanostructured Nb Thin Film Superconductor | The elemental Nb is mainly investigated for its eminent superconducting
properties. In contrary, we report of a relatively unexplored property, namely,
its superior optoelectronic property in reduced dimension. We demonstrate here
that nanostructured Nb thin films (NNFs), under optical illumination, behave as
room temperature photo-switches and exhibit bolometric features below its
superconducting critical temperature. Both photo-switch and superconducting
bolometric behavior are monitored by its resistance change with light in
visible and near infrared (NIR) wavelength range. Unlike the conventional
photodetectors, the NNF devices switch to higher resistive states with light
and the corresponding resistivity change is studied with thickness and grain
size variations. At low temperature in its superconducting state, the light
exposure shifts the superconducting transition towards lower temperature. The
room temperature photon sensing nature of the NNF is explained by the photon
assisted electron-phonon scattering mechanism while the low temperature light
response is mainly related to the heat generation which essentially changes the
effective temperature for the device and the device is capable of sensing a
temperature difference of few tens of milli-kelvins. The observed
photo-response on the transport properties of NNFs can be very important for
future superconducting photon detectors, bolometers and phase slip based device
applications. | 1704.04092v1 |
2017-07-05 | Anomalous Hall effect in two-dimensional non-collinear antiferromagnetic semiconductor Cr0.68Se | Cr0.68Se single crystals with two-dimensional (2D) character have been grown,
and the detailed magnetization M(T), electrical transport properties (including
longitudinal resistivity and Hall resistivity and thermal transport ones
(including heat capacity Cp(T) and thermoelectric power (TEP) S(T)) have been
measured. There are some interesting phenomena: (i) Cr0.68Se presents a
non-collinear antiferromagnetic (AFM) semiconducting behavior with the Neel
temperature TN = 42 K and the activated energy Eg=3.9 meV; (ii) It exhibits the
anomalous Hall effect (AHE) below TN and large negative magnetoresistance (MR)
about 83.7% (2 K, 8.5 T). The AHE coefficient RS is 0.385 cm-3/C at T=2 K and
the AHE conductivity {\sigma}H is about 1 ohm-1cm-1 at T=40 K, respectively;
(iii) The scaling behavior between the anomalous Hall resistivity and the
longitudinal resistivity is linear and further analysis implies that the origin
of the AHE in Cr0.68Se is dominated by the skew-scattering mechanism. Our
results may be helpful for exploring the potential application of these kind of
2D AFM semiconductors. | 1707.01252v1 |
2017-07-20 | Stochastic Dynamics of Resistive Switching: Fluctuations Lead to Optimal Particle Number | Resistive switching is one of the foremost candidates for building novel
types of non-volatile random access memories. Any practical implementation of
such a memory cell calls for a strong miniaturization, at which point
fluctuations start playing a role that cannot be neglected. A detailed
understanding of switching mechanisms and reliability is essential. For this
reason, we formulate a particle model based on the stochastic motion of oxygen
vacancies. It allows us to investigate fluctuations in the resistance states of
a switch with two active zones. The vacancies' dynamics is governed by a master
equation. Upon the application of a voltage pulse, the vacancies travel
collectively through the switch. By deriving a generalized Burgers equation we
can interpret this collective motion as nonlinear traveling waves, and
numerically verify this result. Further, we define binary logical states by
means of the underlying vacancy distributions, and establish a framework of
writing and reading such memory element with voltage pulses. Considerations
about the dis- criminability of these operations under fluctuations together
with the markedness of the resistive switching effect itself lead to the
conclusion, that an intermediate vacancy number is optimal for performance. | 1707.06648v1 |
2018-08-02 | Non-Classical Longitudinal Magneto-Resistance in Anisotropic Black Phosphorus | Resistivity measurements of a few-layer black phosphorus (bP) crystal in
parallel magnetic fields up to 45 T are reported as a function of the angle
between the in-plane field and the source-drain (S-D) axis of the device. The
crystallographic directions of the bP crystal were determined by Raman
spectroscopy, with the zigzag axis found within 5{\deg} of the S-D axis, and
the armchair axis in the orthogonal planar direction. A transverse
magneto-resistance (TMR) as well as a classically-forbidden longitudinal
magneto-resistance (LMR) are observed. Both are found to be strongly
anisotropic and non-monotonic with increasing in-plane field. Surprisingly, the
relative magnitude (in %) of the positive LMR is larger than the TMR above
$\sim$32 T. Considering the known anisotropy of bP whose zigzag and armchair
effective masses differ by a factor of approximately seven, our experiment
strongly suggests this LMR to be a consequence of the anisotropic Fermi surface
of bP. | 1808.00858v2 |
2019-08-12 | The role of graphitic filaments in resistive switching behaviour of amorphous silicon carbide thin films | Resistive switching in amorphous silicon carbide (a-SiC) films deposited by a
single composite target magnetron sputtering process is reported. Switching
performance as a function of thickness of the films (50, 100 and 300 nm) as
well as different top metal electrodes (Cu, Pt and Ag) with the bottom
electrode fixed as Au, is investigated. The switching parameters (Forming
Voltage, Set and Reset voltages and corresponding currents) are found to be
dependent on thickness of SiC films and it is observed that 100 nm is the
optimal thickness for best endurance. The interface between metal electrode and
a-SiC films plays a more significant role in achieving switching performance.
Resistance Off/On ratios of 108, retention times >104 s and endurance of 50
cycles are achieved in the best devices. Cross-sectional scanning electron
microscopy provides evidence that the mechanism of switching involves the
formation of carbonaceous filaments and Raman spectroscopy indicates that these
filaments are nanocrystalline graphite in nature. The current work clearly
establishes that there is dissociation of SiC during the switching cycles
leading to formation of nanocrystalline graphitic filaments. These contribute
to switching, in addition to the metallic filaments, in the a-SiC based
resistive memory device. | 1908.04079v1 |
2018-07-13 | Tuning vortex fluctuations and the resistive transition in superconducting films with a thin overlayer | It is shown that the temperature of the resistive transition $T_r$ of a
superconducting film can be increased by a thin superconducting or normal
overlayer. For instance, deposition of a highly conductive thin overlayer onto
a dirty superconducting film can give rise to an "anti-proximity effect" which
manifests itself in an initial increase of $T_r(d_2)$ with the overlayer
thickness $d_2$ followed by a decrease of $T_r(d_2)$ at larger $d_2$. Such a
nonmonotonic thickness dependence of $T_r(d_2)$ results from the interplay of
the increase of a net superfluid density mitigating phase fluctuations and the
suppression of the critical temperature $T_c$ due to the conventional proximity
effect. This behavior of $T_r(d_2)$ is obtained by solving the Usadel equations
to calculate the temperature of the Berezinskii-Kosterletz-Thouless transition,
and the temperature of the resistive transition due to thermally-activated
hopping of single vortices in dirty bilayers. The theory incorporates relevant
materials parameters such as thicknesses and conductivities of the layers,
interface contact resistance between them and the subgap quasiparticle states
which affect both phase fluctuations and the proximity effect suppression of
$T_c$. The transition temperature $T_r$ can be optimized by tuning the
overlayer parameters, which can significantly weaken vortex fluctuations and
nearly restore the mean-field critical temperature. The calculated behavior of
$T_r(d_2)$ may explain the nonmonotonic dependence of $T_r(d_2)$ observed on
(Ag, Au, Mg, Zn)-coated Bi films, Ag-coated Ga and Pb films or NbN and NbTiN
films on AlN buffer layers. These results suggest that bilayers can be used as
model systems for systematic investigations of optimization of fluctuations in
superconductors. | 1807.04891v1 |
2020-01-02 | Numerical investigation into fracture resistance of bone following adaptation | Bone adapts in response to its mechanical environment. This evolution of bone
density is one of the most important mechanisms for developing fracture
resistance. A finite element framework for simulating bone adaptation, commonly
called bone remodelling, is presented. This is followed by a novel method to
both quantify fracture resistance and to simulate fracture propagation. The
authors' previous work on the application of configurational mechanics for
modelling fracture is extended to include the influence of heterogeneous bone
density distribution. The main advantage of this approach is that
configurational forces, and fracture energy release rate, are expressed
exclusively in terms of nodal quantities. This approach avoids the need for
post-processing and enables a fully implicit formulation for modelling the
evolving crack front. In this paper density fields are generated from both (a)
bone adaptation analysis and (b) subject-specific geometry and material
properties obtained from CT scans. It is shown that, in order to correctly
evaluate the configurational forces at the crack front, it is necessary to have
a spatially smooth density field with higher regularity than if the field is
directly approximated on the finite element mesh. Therefore, discrete density
data is approximated as a smooth density field using a Moving Weighted Least
Squares method. Performance of the framework is demonstrated using numerical
simulations for bone adaptation and subsequent crack propagation, including
consideration of an equine 3rd metacarpal bone. The degree of bone adaption is
shown to influence both fracture resistance and the resulting crack path. | 2001.00647v1 |
2020-06-19 | Contrasting physical properties of the trilayer nickelates Nd$_4$Ni$_3$O$_{10}$ and Nd$_4$Ni$_3$O$_8$ | We report the crystal structures and physical properties of trilayer
nickelates Nd$_4$Ni$_3$O$_{10}$ and Nd$_4$Ni$_3$O$_8$. Measurements of
magnetization and electrical resistivity display contrasting behaviors in the
two compounds. Nd$_4$Ni$_3$O$_{10}$ shows a paramagnetic metallic behavior with
a metal to metal phase transition($T^{\ast}$) at about 162 K, as revealed by
both magnetic susceptibility and resistivity. Further magnetoresistance and
Hall coefficient results show a negative magnetoresistance at low temperatures
and the carrier type of Nd$_4$Ni$_3$O$_{10}$ is dominated by hole-type charge
carriers. The significant enhancement of Hall coefficient and resistivity below
$T^{\ast}$ suggest that effective charge carrier density decreases when cooling
through the transition temperature. In contrast, Nd$_4$Ni$_3$O$_8$ shows an
insulating behavior despite small value of resistivity at room temperature. The
compound shows paramagnetic behavior, with the similar magnetic moments as in
Nd$_4$Ni$_3$O$_{10}$ derived from the Curie-Weiss fitting. This may suggest
that the magnetic moments in both systems are contributed by the Nd ions. By
applying pressures up to about 49 GPa, the insulating behavior is still present
and becomes even stronger. Our results suggest that the different Ni
configurations ($Ni^{1+/2+}$ or $Ni^{2+/3+}$) and competition between localized
and itinerant electrons may account for the contrasting behaviors in trilayer
nickelates Nd$_4$Ni$_3$O$_{10}$ and Nd$_4$Ni$_3$O$_8$. | 2006.10988v1 |
2020-10-13 | How to solve problems in micro- and nanofabrication caused by the emission of electrons and charged metal atoms during e-beam evaporation | We discuss how the emission of electrons and ions during
electron-beam-induced physical vapor deposition can cause problems in micro-
and nanofabrication processes. After giving a short overview of different types
of radiation emitted from an electron-beam (e-beam) evaporator and how the
amount of radiation depends on different deposition parameters and conditions,
we highlight two phenomena in more detail: First, we discuss an unintentional
shadow evaporation beneath the undercut of a resist layer caused by the one
part of the metal vapor which got ionized by electron-impact ionization. These
ions first lead to an unintentional build-up of charges on the sample, which in
turn results in an electrostatic deflection of subsequently incoming ionized
metal atoms towards the undercut of the resist. Second, we show how low-energy
secondary electrons during the metallization process can cause cross-linking,
blisters, and bubbles in the respective resist layer used for defining micro-
and nanostructures in an e-beam lithography process. After the metal
deposition, the cross-linked resist may lead to significant problems in the
lift-off process and causes leftover residues on the device. We provide a
troubleshooting guide on how to minimize these effects, which e.g. includes the
correct alignment of the e-beam, the avoidance of contaminations in the
crucible and, most importantly, the installation of deflector electrodes within
the evaporation chamber. | 2010.06459v2 |
2021-02-17 | Optimization of epitaxial graphene growth for quantum metrology | (See the complete abstract within the thesis in both English and German
versions)
In this thesis, the process conditions of the epitaxial graphene growth
through a socalled polymer-assisted sublimation growth method are minutely
investigated. Atomic force microscopy (AFM) is used to show that the previously
neglected flow-rate of the argon process gas has a significant influence on the
morphology of the SiC substrate and atop carbon layers. The results can be well
explained using a simple model for the thermodynamic conditions at the layer
adjacent to the surface. The resulting control option of step-bunching on the
sub-nanometer scales is used to produce the ultra-flat, monolayer graphene
layers without the bilayer inclusions that exhibit the vanishing of the
resistance anisotropy. The comparison of four-point and scanning tunneling
potentiometry measurements shows that the remaining small anisotropy represents
the ultimate limit, which is given solely by the remaining resistances at the
SiC terrace steps. ... The precise control of step-bunching using the Ar flow
also enables the preparation of periodic non-identical SiC surfaces under the
graphene layer. Based on the work function measurements by Kelvin-Probe force
microscopy and X-ray photoemission electron microscopy, it is shown for the
first time that there is a doping variation in graphene, induced by a proximity
effect of the different near-surface SiC stacks. The comparison of the AFM and
low-energy electron microscopy measurements have enabled the exact assignment
of the SiC stacks, and the examinations have led to an improved understanding
of the surface restructuring in the framework of a step-flow mode. ... | 2102.08691v1 |
2021-05-30 | Enhanced electron-phonon coupling in doubly aligned hexagonal boron nitride bilayer graphene heterostructure | The relative twist angle in heterostructures of two-dimensional (2D)
materials with similar lattice constants result in a dramatic alteration of the
electronic properties. Here, we investigate the electrical and magnetotransport
properties in bilayer graphene (BLG) encapsulated between two hexagonal boron
nitride (hBN) crystals, where the top and bottom hBN are rotationally aligned
with bilayer graphene with a twist angle $\theta_t\sim 0^{\circ} \text{and}~
\theta_b < 1^{\circ}$, respectively. This results in the formation of two
moir\'e superlattices, with the appearance of satellite resistivity peaks at
carrier densities $n_{s1}$ and $n_{s2}$, in both hole and electron doped
regions, together with the resistivity peak at zero carrier density.
Furthermore, we measure the temperature(T) dependence of the resistivity
($\rho$). The resistivity shows a linear increment with temperature within the
range 10K to 50K for the density regime $n_{s1} <n<n_{s2}$ with a large slope
d$\rho$/dT $\sim$ 8.5~$\Omega$/K. The large slope of d$\rho$/dT is attributed
to the enhanced electron-phonon coupling arising due to the suppression of
Fermi velocity in the reconstructed minibands, which was theoretically
predicted, recently in doubly aligned graphene with top and bottom hBN. Our
result establishes the uniqueness of doubly aligned moire system to tune the
strength of electron-phonon coupling and to modify the electronic properties of
multilayered heterostructures. | 2105.14589v1 |
2021-09-01 | Thickness dependent transition from the 1T$^\prime$ to Weyl semimetal phase in ultrathin MoTe$ _{2} $: Electrical transport, Noise and Raman studies | Bulk 1T$^\prime$-MoTe$_2$ shows a structural phase transition from
1T$^\prime$ to Weyl semimetallic (WSM) $ T_{d} $ phase at $\sim$ 240 K. This
phase transition and transport properties in the two phases have not been
investigated on ultra-thin crystals. Here we report electrical transport, $1/f$
noise and Raman studies in ultra-thin 1T$^\prime$-MoTe$_2$ ($\sim$ 5 to 16 nm
thick) field-effect transistors (FETs) devices as a function of temperature.
The electrical resistivities for thickness 16 nm and 11 nm show maxima at
temperatures 208 K and 178 K, respectively, making a transition from
semiconducting to semi-metallic phase, hitherto not observed in bulk samples.
Raman frequencies and linewidths for 11nm thick crystal show change around 178
K, attributed to additional contribution to the phonon self-energy due to
enhanced electron-phonon interaction in the WSM phase. Further, the resistivity
at low-temperature shows an upturn below 20 K along with the maximum in the
power spectral density of the low frequency $1/f$ noise. The latter rules out
the metal-insulator transition (MIT) being responsible for the upturn of
resistivity below 20 K. The low temperature resistivity follows $\rho \propto
1/T$, changing to $\rho \propto T$ with increasing temperature supports
electron-electron interaction physics at electron-hole symmetric Weyl nodes
below 20 K. These observations will pave the way to unravel the properties of
WSM state in layered ultra-thin van der Waals materials. | 2109.00504v2 |
2021-10-05 | Carrier transport and performance limit of semi-transparent photovoltaics: CuIn$_{1-x}$Ga$_x$Se$_2$ as a case study | Semi-transparent photovoltaic devices for building integrated applications
have the potential to provide simultaneous power generation and natural light
penetration. CuIn$_{1-x}$Ga$_x$Se$_2$ (CIGS) has been established as a mature
technology for thin-film photovoltaics, however, its potential for
Semi-Transparent Photovoltaics (STPV) is yet to be explored. In this paper, we
present its carrier transport physics explaining the trend seen in recently
published experiments. STPV requires deposition of films of only a few hundred
nanometers to make them transparent and manifests several unique properties
compared to a conventional thin-film solar cell. Our analysis shows that the
short-circuit current, Jsc is dominated by carriers generated in the depletion
region, making it nearly independent of bulk and back-surface recombination.
The bulk recombination, which limits the open-circuit voltage Voc, appears to
be higher than usual attributable to numerous grain boundaries. When the
absorber layer is reduced below 500 nm, grain size reduces resulting in more
grain boundaries and higher resistance. This produces an inverse relationship
between series resistance and absorber thickness. We also present a
thickness-dependent model of shunt resistance showing its impact in these
ultra-thin devices. For various scenarios of bulk and interface recombinations,
shunt and series resistances, AVT and composition of CuIn$_{1-x}$Ga$_x$Se$_2$,
we project the efficiency limit which - for most practical cases - is found to
be $\leq$10% for AVT $\geq$25%. | 2110.02146v1 |
2022-01-11 | Spectral Thermal Spreading Resistance of Wide Bandgap Semiconductors in Ballistic-Diffusive Regime | To develop efficient thermal management strategies for wide bandgap (WBG)
semiconductor devices, it is essential to have a clear understanding of the
heat transport process within the device and accurately predict the junction
temperature. In this paper, we used the phonon Monte Carlo (MC) method with the
phonon dispersion of various typical WBG semiconductors, including GaN, SiC,
AlN, and \ce{\beta-Ga_2O_3}, to investigate the thermal spreading resistance in
a ballistic-diffusive regime. It was found that when compared with Fourier's
law-based predictions, the increase in the thermal resistance caused by
ballistic effects was strongly related to different phonon dispersions. Based
on the model deduced under the gray-medium approximation and the results of
dispersion MC, we obtained a thermal resistance model that can well address the
issues of thermal spreading and ballistic effects, and the influences of phonon
dispersion. The model can be easily coupled with FEM based thermal analysis and
applied to different materials. This paper can provide a clearer understanding
of the influences of phonon dispersion on the thermal transport process, and it
can be useful for the prediction of junction temperatures and the development
of thermal management strategies for WBG semiconductor devices. | 2201.03788v1 |
2022-03-23 | Improved Gradual Resistive Switching Range and 1000x On/Off Ratio in HfOx RRAM Achieved with a $Ge_2Sb_2Te_5$ Thermal Barrier | Gradual switching between multiple resistance levels is desirable for analog
in-memory computing using resistive random-access memory (RRAM). However, the
filamentary switching of $HfO_x$-based conventional RRAM often yields only two
stable memory states instead of gradual switching between multiple resistance
states. Here, we demonstrate that a thermal barrier of $Ge_2Sb_2Te_5$ (GST)
between $HfO_x$ and the bottom electrode (TiN) enables wider and weaker
filaments, by promoting heat spreading laterally inside the $HfO_x$. Scanning
thermal microscopy suggests that $HfO_x+GST$ devices have a wider heating
region than control devices with only $HfO_x$, indicating the formation of a
wider filament. Such wider filaments can have multiple stable conduction paths,
resulting in a memory device with more gradual and linear switching. The
thermally-enhanced $HfO_x+GST$ devices also have higher on/off ratio ($>10^3$)
than control devices ($<10^2$), and a median set voltage lower by approximately
1 V (~35%), with a corresponding reduction of the switching power. Our
$HfO_x+GST$ RRAM shows 2x gradual switching range using fast (~ns) identical
pulse trains with amplitude less than 2 V. | 2203.12190v1 |
2022-05-09 | Reconciling scaling of the optical conductivity of cuprate superconductors with Planckian resistivity and specific heat | Materials tuned to a quantum critical point display universal scaling
properties as a function of temperature $T$ and frequency $\omega$. A
long-standing puzzle regarding cuprate superconductors has been the observed
power-law dependence of optical conductivity with an exponent smaller than one,
in contrast to $T$-linear dependence of the resistivity and $\omega$-linear
dependence of the optical scattering rate. Here, we present and analyze
resistivity and optical conductivity of La$_{2-x}$Sr$_x$CuO$_4$ with $x=0.24$.
We demonstrate $\hbar\omega/k_{\mathrm{B}} T$ scaling of the optical data over
a wide range of frequency and temperature, $T$-linear resistivity, and optical
effective mass proportional to $\sim \ln T$ corroborating previous specific
heat experiments. We show that a $T,\omega$-linear scaling Ansatz for the
inelastic scattering rate leads to a unified theoretical description of the
experimental data, including the power-law of the optical conductivity. This
theoretical framework provides new opportunities for describing the unique
properties of quantum critical matter. | 2205.04030v2 |
2022-05-23 | Electrical Resistivity of Polycrystalline Graphene: Effect of Grain-Boundary-Induced Strain Fields | We have revealed the decisive role of grain-boundary-induced strain fields in
electron scattering in polycrystalline graphene. To this end, we have
formulated the model based on Boltzmann transport theory which properly takes
into account the microscopic structure of grain boundaries (GB) as a repeated
sequence of heptagon-pentagon pairs. The effect of strain field is described
within the deformation potential theory. For comparison, we consider the
scattering due to electrostatic potential of charged grain boundary. We show
that at naturally low GB charges the deformation potential scattering dominates
and leads to physically reasonable and, what is important, experimentally
observable values of the electrical resistivity. It ranges from 0.1 to 10
k$\Omega $$\mu $m for different types of GBs with a size of 1 $\mu$m and has a
strong dependence on misorientation angle. For low-angle highly charged GBs,
two scattering mechanisms may compete. The resistivity increases markedly with
decreasing GB size and reaches values of 60 k$\Omega $$\mu $m and more. It is
also very sensitive to the presence of irregularities modeled by embedding of
partial disclination dipoles. With significant distortion, we found an increase
in resistance by more than an order of magnitude, which is directly related to
the destruction of diffraction on the GB. Our findings may be of interest both
in the interpretation of experimental data and in the design of electronic
devices based on poly- and nanocrystalline graphene. | 2205.11270v1 |
2022-05-31 | Anisotropic signatures of the electronic correlations in the electrical resistivity of UTe$_2$ | Multiple unconventional superconducting phases are suspected to be driven by
magnetic fluctuations in the heavy-fermion paramagnet UTe$_2$, and a challenge
is to identify the signatures of the electronic correlations, including the
magnetic fluctuations, in the bulk physical quantities. Here, we investigate
thoroughly the anisotropy of the electrical resistivity of UTe$_2$ under
intense magnetic fields up to 70~T, for different electrical-current and
magnetic-field configurations. Two characteristic temperatures and an
anisotropic low-temperature Fermi-liquid-like coefficient $A$, controlled by
the electronic correlations, are extracted. Their critical behavior near the
metamagnetic transition induced at $\mu_0H_m\simeq35$~T for
$\mathbf{H}\parallel\mathbf{b}$ is characterized. Anisotropic scattering
processes are evidenced and magnetic fluctuations are proposed to contribute,
via a Kondo hybridization, to the electrical resistivity. Our work appeals for
a microscopic modeling of the anisotropic contributions to the electrical
resistivity as a milestone for understanding magnetically-mediated
superconductivity in UTe$_2$. | 2205.15789v2 |
2022-06-11 | On the Fermi gas, the Sommerfeld fine structure constant, and the electron-electron scattering in conductors | Electrical energy is considered as a fundamental parameter for inclusion in
Fermi gas theory, in addition to thermal energy. It is argued that electrical
energy can move some electrons to above the Fermi Level, providing free charges
to carry the electrical current, even at absolute zero temperature. The Drude
model, Ohm's law, quantum resistance, and the electrical resistivity due to
electron-electron scattering appear naturally as a consequence of the
theoretical description, which is based on the quantization of the angular
momentum and the Fermi-Dirac distribution, considering total energy as
${\epsilon}$ = k$_B$$T$ + ${\Phi}_0$$I$. The electrical and magnetic forces
acting on an electron are related to the ratio between the Fermi velocity and
the speed of light and show that the electron motion is due to helical paths.
Considering the center of mass description for the Bohr atom, it was possible
to show that the magnetic force is related to the electrical force as $F_M$ =
(${\alpha}$/${\pi}$) $F_E$, which demonstrates that the electrons move in
helical paths along the orbit. The helical motion naturally provides for
quantization of the magnetic flux, the spin of the electron, and the first
correction term of the anomalous magnetic moment. Applying the model to
describe the electron-electron scattering allows prediction of the behavior of
the electrical resistivity of many metals at low temperatures, which is in
excellent agreement with empirical observations. | 2206.05393v2 |
2022-06-30 | Antipolar transitions in GaNb$_4$Se$_8$ and GaTa$_4$Se$_8$ | We present dielectric, polarization, resistivity, specific heat, and magnetic
susceptibility data on single crystals of the lacunar spinels GaNb4Se8 and
GaTa4Se8, tetrahedral cluster-based materials with substantial spin-orbit
coupling. We concentrate on the possible occurrence of antipolar order in these
compounds, as previously reported for the isoelectronic GaNb4S8, where
spin-orbit coupling plays a less important role. Our broadband
dielectric-spectroscopy investigations reveal clear anomalies of the intrinsic
dielectric constant at the magneto-structural transitions in both systems that
are in accord with the expectations for antipolar transitions. A similar
anomaly is also observed at the cubic-cubic transition of the Nb compound
leading to an intermediate phase. Similar to other polar and antipolar lacunar
spinels, we find indications for dipolar relaxation dynamics at low
temperatures. Polarization measurements on GaNb4Se8 reveal weak ferroelectric
ordering below the magneto-structural transition, either superimposed to
antipolar order or emerging at structural domain walls. The
temperature-dependent dc resistivity evidences essentially thermally-activated
charge transport with different activation energies in the different phases. A
huge step-like increase of the resistivity at the magneto-structural transition
of the Ta compound points to a fundamental change in the electronic structure
or the mechanism of the charge transport. At low temperatures, charge transport
is governed by in-gap impurity states, as also invoked to explain the resistive
switching in these compounds. | 2206.15200v2 |
2022-11-16 | Decoupling the Roles of Defects/Impurities and Wrinkles in Thermal Conductivity of Wafer-scale hBN Films | We demonstrate a non-monotonic evolution of thermal conductivity of
large-area hexagonal boron nitride films with thickness. Wrinkles and
defects/impurities are present in these films. Raman spectroscopy, an
optothermal non-contact technique, is employed to probe the temperature and
laser power dependence property of the Raman active E2ghigh phonon mode, which
in turn is used to estimate the rise in the temperature of the films under
different laser powers. As the conventional Fourier law of heat diffusion
cannot be directly employed analytically to evaluate the thermal conductivity
of these films with defects and wrinkles, finite element modeling is used
instead. In the model, average heat resistance is used to incorporate an
overall defect structure, and Voronoi cells with contact resistance at the cell
boundaries are constructed to mimic the wrinkled domains. The effective thermal
conductivity is estimated to be 87, 55, and 117 W/m.K for the 2, 10, and 30
nm-thick films, respectively. We also present a quantitative estimation of the
thermal resistance by defects and wrinkles individually to the heat flow. Our
study reveals that the defects/impurities render a much higher resistance to
heat transfer in the films than wrinkles. | 2211.08683v2 |
2022-11-18 | Magnetotransport induced by anomalous Hall effect | In a magnetic metal, the Hall resistance is generally taken to be the sum of
the ordinary Hall resistance and the anomalous Hall resistance. Here it is
shown that this empirical relation is no longer valid when either the ordinary
Hall angle or the anomalous Hall angle is not small. Using the proper
conductivity relation, we reveal an unexpected magnetoresistance (MR) induced
by the anomalous Hall effect (AHE). A $B$-linear MR arises and the sign of the
slope depends on the sign of the anomalous Hall angle, giving rise to a
characteristic bowtie shape. The Hall resistance in a single-band system can
exhibit a nonlinearity which is usually considered as a characteristic of a
two-band system. A $B$-symmetric component appears in the Hall. These effects
reflect the fundamental difference between the ordinary Hall effect and the
AHE. Furthermore, we experimentally reproduce the unusual MR and Hall reported
before in Co$_3$Sn$_2$S$_2$ and show that these observations can be well
explained by the proposed mechanism. MR often observed in quantum anomalous
Hall insulators provides further confirmation of the picture. The effect may
also account for the large MR observed in non-magnetic three-dimensional
topological Dirac semimetals. | 2211.10222v2 |
2022-12-08 | Theory of the Little-Parks effect in spin-triplet superconductors | The celebrated Little-Parks effect in mesoscopic superconducting rings has
recently gained great attention due to its potential to probe half-quantum
vortices in spin-triplet superconductors. However, despite the large number of
works reporting anomalous Little-Parks measurements attributed to
unconventional superconductivity, the general signatures of spin-triplet
pairing in the Little-Parks effect have not yet been systematically
investigated. Here we use Ginzburg-Landau theory to study the Little-Parks
effect in a spin-triplet superconducting ring that supports half-quantum
vortices; we calculate the field-induced Little-Parks oscillations of both the
critical temperature itself and the residual resistance resulting from thermal
vortex tunneling below the critical temperature. We observe two separate
critical temperatures with a single-spin superconducting state in between and
find that, due to the existence of half-quantum vortices, each minimum in the
upper critical temperature splits into two minima for the lower critical
temperature. From a rigorous calculation of the residual resistance, we confirm
that these two minima in the lower critical temperature translate into two
maxima in the residual resistance below and establish the general conditions
under which the two maxima can be practically resolved. In particular, we
identify a fundamental trade-off between sharpening each maximum and keeping
the overall magnitude of the resistance large. Our results will guide
experimental efforts in designing mesoscopic ring geometries for probing
half-quantum vortices in spin-triplet candidate materials on the device scale. | 2212.04591v1 |
2023-04-16 | Anomalous and Topological Hall Resistivity in Ta/CoFeB/MgO Magnetic Systems for Neuromorphic Computing Applications | Topologically protected spin textures, such as magnetic skyrmions, have the
potential for dense data storage as well as energy-efficient computing due to
their small size and a low driving current. The evaluation of the writing and
reading of the skyrmion's magnetic and electrical characteristics is a key step
toward the implementation of these devices. In this paper, we present the
magnetic heterostructure Hall bar device and study the anomalous Hall and
topological Hall signals in the device. Using the combination of different
measurements like magnetometry at different temperatures, Hall effect
measurement from 2K to 300K, and magnetic force microscopy imaging, we
investigate the magnetic and electrical characteristics of the magnetic
structure. We measure the skyrmion topological resistivity at different
temperatures as a function of the magnetic field. The topological resistivity
is maximum around the zero magnetic field and it decreases to zero at the
saturating field. This is further supported by MFM imaging. Interestingly the
resistivity decreases linearly with the field, matching the behavior observed
in the corresponding micromagnetic simulations. We combine the experimental
results with micromagnetic simulations, thus propose a skyrmion-based synaptic
device and show spin-orbit torque-controlled potentiation/depression in the
device. The device performance as the synapse for neuromorphic computing is
further evaluated in a convolutional neural network CNN. The neural network is
trained and tested on the MNIST data set we show devices acting as synapses
achieving a recognition accuracy close to 90%, on par with the ideal
software-based weights which offer an accuracy of 92%. | 2304.07742v1 |
2023-09-01 | Multilayer Ferromagnetic Spintronic Devices for Neuromorphic Computing Applications | Spintronics has gone through substantial progress due to its applications in
energy-efficient memory, logic and unconventional computing paradigms.
Multilayer ferromagnetic thin films are extensively studied for understanding
the domain wall and skyrmion dynamics. However, most of these studies are
confined to the materials and domain wall/skyrmion physics. In this paper, we
present the experimental and micromagnetic realization of a multilayer
ferromagnetic spintronic device for neuromorphic computing applications. The
device exhibits multilevel resistance states and the number of resistance
states increases with lowering temperature. This is supported by the multilevel
magnetization behavior observed in the micromagnetic simulations. Furthermore,
the evolution of resistance states with spin-orbit torque is also explored in
experiments and simulations. Using the multi-level resistance states of the
device, we propose its applications as a synaptic device in hardware neural
networks and study the linearity performance of the synaptic devices. The
neural network based on these devices is trained and tested on the MNIST
dataset using a supervised learning algorithm. The devices at the chip level
achieve 90\% accuracy. Thus, proving its applications in neuromorphic
computing. Furthermore, we lastly discuss the possible application of the
device in cryogenic memory electronics for quantum computers. | 2309.00476v1 |
2024-01-30 | Effect of the Sc/Zr ratio on the corrosion resistance of Al-Mg cast alloys | The results of investigations of the corrosion resistance of Al-Mg-Sc-Zr
alloys with varying Mg content and different Sc/Zr ratios are presented. The
objects of investigations were the Al-Mg-Sc-Zr alloys with total Sc + Zr
content of 0.32 wt%. The concentration of Sc and Zr in the alloys varied with
the increments of 0.02 wt%. The alloys were produced by induction casting. The
effect of annealing temperature on the microhardness and electrical resistivity
of the Al-Mg-Sc-Zr alloys was investigated. Corrosion tests were carried out in
a medium simulating intergranular corrosion in aluminum alloys. Electrochemical
studies and mass loss tests were performed. An increase in the Sc concentration
and a decrease in the Zr one were shown to result in an increase in the
corrosion rate. The primary Al3(ScxZr1-x) particles were found to have the main
effect on the corrosion resistance of Al-Mg-Sc-Zr alloys. The dependence of the
corrosion current on the annealing temperature of the Al-Mg-Sc-Zr alloy was
found to have a non-monotonous character (with a maximum). | 2401.17429v1 |
1997-02-27 | High temperature anomaly of the conductance of a tunnel junction | The linear conductance of a tunnel junction in series with an ohmic resistor
is determined in the high temperature limit. The tunneling current is treated
nonperturbatively by means of path integral techniques. Due to quantum effects
the conductance is smaller than the classical series conductance. The reduction
factor is found to be nonanalytic in the environmental resistance for vanishing
resistance. This behavior is a high temperature manifestation of the Coulomb
blockade effect. | 9702242v3 |
2002-10-11 | High-field side of Superconductor-Insulator Transition | We report the experimental observation of a magnetic-field-tuned
superconductor-insulator transition (SIT) in ultrathin TiN films. The low
temperature transport properties of these films show scaling behavior
consistent with a transition driven by quantum phase fluctuations in
two-dimensional superconductor. The magnetoresistance reveals peak and a
subsequent decrease in fields higher than the critical field. The temperature
dependences of the isomagnetic resistance data on the high-field side of the
SIT have been analyzed and the transition from insulating to metallic phase is
found, with at high fields the zero-temperature asymptotic value of the
resistance being equal to h/e^2. | 0210250v1 |
2003-11-11 | Nonlocal effects in high energy charged particle beams | Within the framework of the thermal wave model, an investigation is made of
the longitudinal dynamics of high energy charged particle beams. The model
includes the self-consistent interaction between the beam and its surroundings
in terms of a nonlinear coupling impedance, and when resistive as well as
reactive parts are included, the evolution equation becomes a generalised
nonlinear Schroedinger equation including a nonlocal nonlinear term. The
consequences of the resistive part on the propagation of particle bunches are
examined using analytical as well as numerical methods. | 0311139v1 |
2008-11-05 | High-pressure, transport, and thermodynamic properties of CeTe3 | We have performed high-pressure, electrical resistivity, and specific heat
measurements on CeTe3 single crystals. Two magnetic phases with nonparallel
magnetic easy axes were detected in electrical resistivity and specific heat at
low temperatures. We also observed the emergence of an additional phase at high
pressures and low temperatures and a possible structural phase transition
detected at room temperature and at 45 kbar, which can possibly be related with
the lowering of the charge-density wave transition temperature known for this
compound. | 0811.0792v2 |
2009-07-30 | High order fractional microwave induced resistance oscillations in 2D systems | We report on the observation of microwave-induced resistance oscillations
associated with the fractional ratio n/m of the microwave irradiation frequency
to the cyclotron frequency for m up to 8 in a two-dimensional electron system
with high electron density. The features are quenched at high microwave
frequencies independent of the fractional order m. We analyze temperature,
power, and frequency dependencies of the magnetoresistance oscillations and
discuss them in connection with existing theories. | 0907.5315v1 |
2010-11-24 | A spark-resistant bulk-micromegas chamber for high-rate applications | We report on the design and performance of a spark-resistant bulk-micromegas
chamber. The principle of this design lends itself to the construction of
large-area muon chambers for the upgrade of the detectors at the Large Hadron
Collider at CERN for luminosities in excess of 10**34/cm2/s or other high-rate
applications. | 1011.5370v1 |
2012-12-10 | Quantized escape and formation of edge channels at high Landau levels | We present nonlocal resistance measurements in an ultra high mobility two
dimensional electron gas. Our experiments show that even at weak magnetic
fields classical guiding along edges leads to a strong non local resistance on
macroscopic distances. In this high Landau level regime the transport along
edges is dissipative and can be controlled by the amplitude of the voltage drop
along the edge. We report resonances in the nonlocal transport as a function of
this voltage that are interpreted as escape and formation of edge channels. | 1212.2026v1 |
2019-06-26 | Studies on small charge packet transport in high-resistivity fully-depleted CCDs | In this work, we will present a physical model and measurements of the
transport of small charge packets in the bulk of thick high resistivity CCD
before being collected by the pixel potential wells. A new technique to measure
the lateral spread of the charge as a function of the ionization depth in the
bulk is presented. Results from measurements on CCD currently in use for
several scientific instruments are shown and validated with a new mathematical
algorithm to extend the current modeling based only on the diffusion of the
charge in silicon. | 1906.11379v1 |
2011-07-09 | Resistance to TB drugs in KwaZulu-Natal: causes and prospects for control | In 2005 there was an outbreak of XDR (extensively drug resistant) TB in
Tugela Ferry, which is served by the Church of Scotland Hospital (COSH), in the
uMzinyathi District, KwaZulu-Natal, South Africa. An investigation was carried
out to determine if XDR TB was occurring elsewhere in the province, and to
develop hypotheses for the rise in drug resistance with a view to developing a
strategy for the control of MDR (multi-drug resistant) and XDR TB in the
province and elsewhere. TB incidence and treatment success rates, for each of
the 11 districts in the province, were obtained from the provincial electronic
TB register for the years 2002-2007. The results of culture and drug
sensitivity tests for the years 2002 to 2007 in each of the districts were
compiled and culture taking practices were compared to the number of MDR TB
cases. Interviews were conducted with key personnel in affected sites. In 2007,
2799, or 2.3% of 119,218 notified TB cases in the province were multi-drug
resistant (MDR), and of these 270 (9.6%) were XDR. The two worst affected
districts were uMzinyathi where 226 (4.1%) of 5522 notified TB cases were MDR,
and of these 120 (53%) were extensively drug resistant (XDR), and uMkhanyakude
where 337 (4.8%) of 6991 notified TB cases were MDR, but of these only four or
(1.2%) were XDR. The worst affected medical centre was COSH where 164 or 9.8%
of notified TB cases were MDR and of these 99 (60%) were XDR. Very high rates
of XDR TB in the province are only found in uMzinyathi district even though MDR
TB is common in most other districts. XDR may arisen at COSH because of the
early and effective integration of the TB and HIV programmes in overcrowded and
poorly ventilated facilities particular to COS.H To control XDR TB better
management of both susceptible and resistant forms of TB is needed including
treatment supervision, infection control and HIV management. | 1107.1800v1 |
2012-02-06 | Performances of Anode-resistive Micromegas for HL-LHC | Micromegas technology is a promising candidate to replace Atlas forward muon
chambers -tracking and trigger- for future HL-LHC upgrade of the experiment.
The increase on background and pile-up event probability requires detector
performances which are currently under studies in intensive RD activities.
We studied performances of four different resistive Micromegas detectors with
different read-out strip pitches. These chambers were tested using \sim120 GeV
momentum pions, at H6 CERN-SPS beam line in autumn 2010. For a strip pitch 500
micrometers we measure a resolution of \sim90 micrometers and a efficiency of
~98%. The track angle effect on the efficiency was also studied. Our results
show that resistive techniques induce no degradation on the efficiency or
resolution, with respect to the standard Micromegas. In some configuration the
resistive coating is able to reduce the discharge currents at least by a factor
of 100.Micromegas technology is a promising candidate to replace Atlas forward
muon chambers -tracking and trigger- for future HL-LHC upgrade of the
experiment. The increase on background and pile-up event probability requires
detector performances which are currently under studies in intensive RD
activities. We studied performances of four different resistive Micromegas
detectors with different read-out strip pitches. These chambers were tested
using \sim120 GeV momentum pions, at H6 CERN-SPS beam line in autumn 2010. For
a strip pitch 500 micrometers we measure a resolution of \sim90 micrometers and
a efficiency of \sim98%. The track angle effect on the efficiency was also
studied. Our results show that resistive techniques induce no degradation on
the efficiency or resolution, with respect to the standard Micromegas. In some
configuration the resistive coating is able to reduce the discharge currents at
least by a factor of 100. | 1202.1074v1 |
2018-11-12 | Phonon-induced giant linear-in-$T$ resistivity in magic angle twisted bilayer graphene: Ordinary strangeness and exotic superconductivity | We study the effect of electron-acoustic phonon interactions in twisted
bilayer graphene on resistivity in the high-temperature transport and
superconductivity in the low-temperature phase diagram. We theoretically show
that twisted bilayer graphene should have an enhanced and strongly twist-angle
dependent linear-in-temperature resistivity in the metallic regime with the
resistivity magnitude increasing as the twist angle approaches the magic angle.
The slope of the resistivity versus temperature could approach one hundred ohms
per kelvin with a strong angle dependence, but with a rather weak dependence on
the carrier density. This higher-temperature density-independent linear-in-$T$
resistivity crosses over to a $T^4$ dependence at a low density-dependent
characteristic temperature, becoming unimportant at low temperatures. This
angle-tuned resistivity enhancement arises from the huge increase in the
effective electron-acoustic phonon coupling in the system due to the
suppression of graphene Fermi velocity induced by the flatband condition in the
moir\'e superlattice system. Our calculated temperature dependence is
reminiscent of the so-called `strange metal' transport behavior except that it
is arising from the ordinary electron-phonon coupling in a rather unusual
parameter space due to the generic moir\'e flatband structure of twisted
bilayer graphene. We also show that the same enhanced electron-acoustic phonon
coupling also mediates effective attractive interactions in $s$, $p$, $d$ and
$f$ pairing channels with a theoretical superconducting transition temperature
on the order of $\sim$5 K near magic angle. The fact that ordinary acoustic
phonons can produce exotic non-$s$-wave superconducting pairing arises from the
unusual symmetries of the system. | 1811.04920v4 |
2017-03-07 | A Strange Metal from Gutzwiller correlations in infinite dimensions | Recent progress in extremely correlated Fermi liquid theory (ECFL) and
dynamical mean field theory (DMFT) enables us to compute in the $d \to \infty$
limit the resistivity of the $t-J$ model after setting $J\to0$. This is also
the $U=\infty$ Hubbard model. We study three densities $n=.75,.8,.85$ that
correspond to a range between the overdoped and optimally doped Mott insulating
state. We delineate four distinct regimes characterized by different behaviors
of the resistivity $\rho$. We find at the lowest $T$ a Gutzwiller Correlated
Fermi Liquid regime with $\rho \propto T^2$ extending up to an effective Fermi
temperature that is dramatically suppressed from the non-interacting value.
This is followed by a Gutzwiller Correlated Strange Metal regime with $\rho
\propto (T-T_0)$, i.e. a linear resistivity extrapolating back to $\rho=0$ at a
positive $T_0$. At a higher $T$ scale, this crosses over into the Bad Metal
regime with $\rho \propto (T+T_1)$ extrapolating back to a finite resistivity
at $T=0$, and passing through the Ioffe-Regel-Mott value where the mean free
path is a few lattice constants. This regime finally gives way to the High $T$
Metal regime, where we find $\rho \propto T$. The present work emphasizes the
first two, where the availability of an analytical ECFL theory is of help in
identifying the changes in related variables entering the resistivity formula
that accompany the onset of linear resistivity, and the numerically exact DMFT
helps to validate the results. We also examine thermodynamic variables such as
the magnetic susceptibility, compressibility, heat capacity and entropy, and
correlate changes in these with the change in resistivity. This exercise casts
valuable light on the nature of charge and spin correlations in the strange
metal regime, which has features in common with the physically relevant strange
metal phase seen in strongly correlated matters. | 1703.02206v3 |
2019-07-16 | General relativistic resistive magnetohydrodynamics with robust primitive variable recovery for accretion disk simulations | Recent advances in black hole astrophysics, particularly the first visual
evidence of a supermassive black hole at the center of the galaxy M87 by the
Event Horizon Telescope (EHT), and the detection of an orbiting "hot spot"
nearby the event horizon of Sgr A* in the Galactic center by the Gravity
Collaboration, require the development of novel numerical methods to understand
the underlying plasma microphysics. Non-thermal emission related to such hot
spots is conjectured to originate from plasmoids that form due to magnetic
reconnection in thin current layers in the innermost accretion zone.
Resistivity plays a crucial role in current sheet formation, magnetic
reconnection, and plasmoid growth in black hole accretion disks and jets. We
included resistivity in the three-dimensional general-relativistic
magnetohydrodynamics (GRMHD) code BHAC and present the implementation of an
Implicit-Explicit scheme to treat the stiff resistive source terms of the GRMHD
equations. The algorithm is tested in combination with adaptive mesh refinement
to resolve the resistive scales and a constrained transport method to keep the
magnetic field solenoidal. Several novel methods for primitive variable
recovery, a key part in relativistic magnetohydrodynamics codes, are presented
and compared for accuracy, robustness, and efficiency. We propose a new
inversion strategy that allows for resistive-GRMHD simulations of low
gas-to-magnetic pressure ratio and highly magnetized regimes as applicable for
black hole accretion disks, jets, and neutron star magnetospheres. We apply the
new scheme to study the effect of resistivity on accreting black holes,
accounting for dissipative effects as reconnection. | 1907.07197v2 |
2020-06-23 | Thermal interfacial resistance and nanolayer effect on the thermal conductivity of Al2O3-CO2 nanofluid: A Molecular Dynamics approach | Nanofluids are known to have significantly different thermal properties
relative to the corresponding conventional fluids. Heat transfer at the
solid-fluid interface affects the thermal properties of nanofluids. The current
work helps in understanding the role of two nanoscale phenomena, namely
ordering of fluid layer around the nanoparticle (nanolayer) and thermal
resistance at the interface of solid-fluid in the enhancement of thermal
conductivity of Al2O3 - CO2 nanofluid. In this study, molecular dynamics (MD)
simulations have been used to study the thermal interfacial resistance by
transient non-equilibrium heat technique and nanolayer formed between Al2O3
nanoparticle (np) and surrounded CO2 molecules in the gaseous and supercritical
phase. The nanoparticle diameter (dNP) is varied between 2 and 5 nm to
investigate the size effect on thermal interfacial resistance (TIR) and thermal
conductivity of nanofluid and the results indicate that the TIR for larger
diameters is relatively high in both the phases. The study of the effect of
surface wettability and temperature on TIR reveals that the resistance
decreases with increase in interaction strength and temperature, but is
entirely independent at higher temperatures, in both gaseous and supercritical
nanofluid. A density distribution study of the nanolayer and the monolayer
around the nanoparticle revealed that the latter is more ordered in smaller
diameter with less thermal resistance. However, nanolayer study reveals that
the nanoparticle with bigger diameters are more suitable for the
cooling/heating purpose, as the system with larger diameters has higher thermal
conductivity. Results show that the nanolayer plays a significant role in
determining the effective thermal conductivity of the nanofluid, while the
influence of TIR appears negligible compared to the nanolayer. | 2006.12805v2 |
2022-03-12 | Parameterization of hydrodynamic friction in a model for sheared suspensions of rough particles | We propose a method to parameterize a coarse grained model for the
hydrodynamic friction between nearly touching rough spheres in suspension
flows. The frictional resistance due to surface roughness primarily alters the
sliding and rolling modes of motion of rough particles. Stokesian dynamics
simulations incorporating a near-field pairwise resistance model accounting for
these enhanced frictional modes were employed to compute particle trajectories
in shear flow. In this model, the resistance to sliding and rolling modes of
motion are augmented from a weakly diverging log$(1/h)$ form for smooth spheres
to a strongly diverging $1/h$ form for rough spheres to account for the
additional resistance due to squeezing flows between surface asperities, where
$h$ is the mean surface separation between particles. We determine new bounds
on the relative magnitude of the augmentations to the resistance to different
modes of motion using inequality constraints reflecting the positive
definiteness of the Stokes resistance tensor for a pair of rough particles.
Using the simulations of a particle pair in a shear flow, a simple model for
angular rotation rate of the pair centerline is computed as a function of its
orientation in the shear flow and the free parameters of the hydrodynamic
resistance model: the friction coupling strength, $\alpha$, and friction
coupling range, $h_0$. Values of $\alpha$ and $h_0$ for real-world rough
particles can then be inferred by matching the pair rotation rate in the model
to experimental observations when a dilute rough particle suspension is
subjected to a linear shear flow. The same model is used to calculate the
hydrodynamic contribution to the high frequency viscosity of rough particle
suspensions. For different $\alpha$ and $h_0$, we observe that the viscosity
diverges differently depending on $h_0$. | 2203.06300v1 |
2003-11-17 | Collapse of Magnetized Singular Isothermal Toroids: I. Non-Rotating Case | We study numerically the collapse of non-rotating, self-gravitating,
magnetized, singular isothermal toroids characterized by sound speed, $a$, and
level of magnetic to thermal support, $H_0$. In qualitative agreement with
previous treatments by Galli & Shu and other workers, we find that the
infalling material is deflected by the field lines towards the equatorial
plane, creating a high-density, flattened structure -- a pseudodisk. The
pseudodisk contracts dynamically in the radial direction, dragging the field
lines threading it into a highly pinched configuration that resembles a split
monopole. The oppositely directed field lines across the midplane and the large
implied stresses may play a role in how magnetic flux is lost in the actual
situation in the presence of finite resistivity or ambipolar diffusion. The
infall rate into the central regions is given to 5% uncertainty by the formula,
$\dot M = (1+H_0)a^3/G$, where $G$ is the universal gravitational constant,
anticipated by semi-analytical studies of the self-similar gravitational
collapses of the singular isothermal sphere and isopedically magnetized disks.
The introduction of finite initial rotation results in a complex interplay
between pseudodisk and true (Keplerian) disk formation that is examined in a
companion paper. | 0311376v1 |
2006-02-01 | From Protoplanets to Protolife: The Emergence and Maintenance of Life | Despite great advances in our understanding of the formation of the Solar
System, the evolution of the Earth, and the chemical basis for life, we are not
much closer than the ancient Greeks to an answer of whether life has arisen and
persisted on any other planet. The origin of life as a planetary phenomenon
will probably resist successful explanation as long as we lack an early record
of its evolution and additional examples. It is widely thought that the
geologic record shows that life emerged quickly after the end of prolonged
bombardment of the Earth. New data and simulations contradict that view and
suggest that more than half a billion years of unrecorded Earth history may
have elapsed between the origin of life and LUCA. The impact-driven exchange of
material between the inner planets may have allowed earliest life to be more
cosmopolitan. Indeed, terrestrial life may not have originated on the Earth, or
even on any planet. Smaller bodies, e.g. the parent bodies of primitive
meteorites, offer alternative environments for the origin of life in our Solar
System. The search for past or present life on Mars is an obvious path to
greater enlightenment. The subsurface oceans of some icy satellites of the
outer planets represent the best locales to search for an independent origin of
life in the Solar System because of the high dynamical barriers for transfer,
intense radiation at their surfaces, and thick ice crusts. The ``ultimate''
answer to the abundance of life in the Cosmos will remain the domain of
speculation until we develop observatories capable of detecting habitable
planets - and signs of life - around the nearest million or so stars. | 0602008v1 |
2000-05-10 | Magnetic, thermal, and transport properties on single crystals of antiferromagnetic Kondo-lattice Ce2PdSi3 | Magnetization, heat capacity, electrical resistivity, thermoelectric power,
and Hall effect have been investigated on single-crystalline Ce_2PdSi_3. This
compound is shown to order antiferromagnetically below N'eel temperature (T_N)
\~3 K. The Sommerfeld coefficient far below T_N is found to be about 110 mJ/K^2
mol Ce, which indicates the heavy-fermion character of this compound. The
transport and magnetic properties exhibit large anisotropy with an interplay
between crystalline-electric-field (CEF) and Kondo effects. The sign of
thermoelectric power is opposite for different directions at high temperatures
and the ordinary Hall coefficient is anisotropic with opposite sign for
different geometries, indicating the anisotropic Fermi surface. The CEF
analysis from the temperature dependence of magnetic susceptibility suggests
that the ground state is |+/-1/2>. The first and the second excited CEF doublet
levels are found to be located at about 30 and 130 K, respectively. The Kondo
temperature is estimated to be the same order as T_N, indicating the presence
of a delicate competition between the Kondo effect and magnetic order. | 0005164v1 |
2001-06-03 | Transport processes in metal-insulator granular layers | The non-equilibrium tunnel transport processes are considered in a square
lattice of metallic nanogranules embedded into insulating host. Based on a
simple model with three possible charging states (+,-, or 0) of a granule and
three kinetic processes (creation or recombination of a +- pair, and charge
transfer) between neighbor granules, the mean-field kinetic theory is
developed, which takes into account the interplay between the charging energy
and temperature and between the applied electric field and Coulomb fields by
non-compensated charge density. The resulting charge and current distributions
are found to depend essentially on the particular conditions in a granular
layer, namely, in a free area (FA) or in contact areas (CA) under macroscopic
metallic contacts. Thus, a steady state dc transport is only compatible with
zero charge density and ohmic resistivity within FA, but charge accumulation
and non-ohmic behavior are necessary for conduction over CA. The approximate
analytic solutions are obtained for characteristic regimes (low or high charge
density) of such conduction. Also non- stationary processes are considered,
displaying a peculiar combination of two strongly different relaxation times.
The comparison is done with the available transport data on similar
experimental systems. | 0106034v2 |
2002-03-06 | Concentration Dependence of Superconductivity and Order-Disorder Transition in the Hexagonal Rubidium Tungsten Bronze RbxWO3. Interfacial and bulk properties | We revisited the problem of the stability of the superconducting state in
RbxWO3 and identified the main causes of the contradictory data previously
published. We have shown that the ordering of the Rb vacancies in the
nonstoichiometric compounds have a major detrimental effect on the
superconducting temperature Tc.The order-disorder transition is first order
only near x = 0.25, where it cannot be quenched effectively and Tc is reduced
below 1K. We found that the high Tc's which were sometimes deduced from
resistivity measurements, and attributed to compounds with .25 < x < .30, are
to be ascribed to interfacial superconductivity which generates spectacular
non-linear effects. We also clarified the effect of acid etching and set more
precisely the low-rubidium-content boundary of the hexagonal phase.This work
makes clear that Tc would increase continuously (from 2 K to 5.5 K) as we
approach this boundary (x = 0.20), if no ordering would take place - as its is
approximately the case in CsxWO3. This behaviour is reminiscent of the
tetragonal tungsten bronze NaxWO3 and asks the same question : what mechanism
is responsible for this large increase of Tc despite the considerable
associated reduction of the electron density of state ? By reviewing the other
available data on these bronzes we conclude that the theoretical models which
are able to answer this question are probably those where the instability of
the lattice plays a major role and, particularly, the model which call upon
local structural excitations (LSE), associated with the missing alkali atoms. | 0203120v1 |
2003-04-19 | Transport properties of granular metals at low temperatures | We investigate transport in a granular metallic system at large tunneling
conductance between the grains, $g_T\gg 1$. 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. | 0304448v1 |
2004-03-31 | Correlation of microwave surface impedance of MgB_2 thin film with material parameters and a temperature niche for microwave applications | Two issues related to the microwave surface impedance Z_s of MgB_2 thin film
are discussed in this Letter, both being significant for potential microwave
applications. At first, a correlation between Zs and Alpha = Xi/l was found,
where Xi is the coherence length, and l is the mean free path. The surface
resistance Rs decreases with Alpha at moderate and large values of Alpha and
saturates when Alpha approaches one. The values of the penetration depth at
zero temperature Lamda(0) for different films could be well fitted by Lamda_L
(1+Alpha)^(1/2), yielding a London penetration depth Lamda_L of 33.6 nm. The
second issue is to find a temperature niche for possible microwave
applications. Between 10K and 15K, our best MgB_2 films possess the lowest Rs
values compared with other superconductors such as NbN, Nb3Sn and the
high-temperature superconductor YBCO. | 0403755v1 |
2004-10-21 | Effect of nanometer-sized B powder on phase formation of polycrystalline MgB2 | The size effect of the raw B powder on the MgB2 phase formation has been
studied by the technique of in-situ high temperature resistivity (HT-rT)
measurement. The onset temperature, Tonset, and the completion temperature,
TPF, of the phase formation are determined directly during the ongoing thermal
process. These two temperatures, Tonset and TPF of the sample synthesized using
nanometer B and Mg powders (NanoB-MgB2) are 440 C and 490 C, respectively, the
same as those of the sample using micrometer B and nanometer Mg powders
(MicroB-MgB2). This indicates that the phase formation temperature of MgB2 do
not depend on the B powder size. On the other hand, the upper limit of the
sintering temperature, TN, above which the sample loses superconductivity, is
below 750 C for NanoB-MgB2, much lower than 980 C for the MgB2 prepared using
micron-sized B powder and millimeter sized Mg powder (DM-MgB2). In comparison
with the sample directly sintered at 650 C < TN, an interesting, irreversible
transformation in the crystal structure of the MgB2 phase was observed with the
sample going through the stages of initial sintering at 750 C, then
re-sintering at 650 C in an Mg-rich environment after the processes of
regrinding and pressing. Possible explanation of the observed properties is
discussed. | 0410533v1 |
2005-03-18 | Aharonov-Bohm electron interferometer in the integer quantum Hall regime | We report experiments on a quantum electron interferometer fabricated from
high mobility, low density GaAs/AlGaAs heterostructure material. In this
device, a nearly circular electron island is defined by four front gates
deposited in etched trenches. The island is separated from the 2D electron bulk
by two nearly open constrictions. In the quantum Hall regime, two
counterpropagating edge channels are coupled by tunneling in the constrictions,
thus forming a closed electron interference path.For several fixed front gate
voltages, we observe periodic Aharonov-Bohm interference oscillations in
four-terminal resistance as a function of the enclosed flux. The oscillation
period DeltaB gives the area of the interference path S via quantization
condition S=h/eDeltaB. We experimentally determine the dependence of S on the
front gate voltage, and find that the Aharonov-Bohm quantization condition does
not require significant corrections due to the confining potential. These
results can be interpreted as a constant integrated compressibility of the
island with respect to the front gates. We also analyze experimental results
using two classical electrostatics models: one modeling the 2D electron density
due to depletion from an etch trench, and another modeling the gate voltage
dependence of the electron density profile in the island. | 0503456v2 |
2006-09-05 | Electrical and Thermal Transport in Metallic Single-Wall Carbon Nanotubes on Insulating Substrates | We analyze transport in metallic single-wall carbon nanotubes (SWNTs) on
insulating substrates over the bias range up to electrical breakdown in air. To
account for Joule self-heating, a temperature-dependent Landauer model for
electrical transport is coupled with the heat conduction equation along the
nanotube. The electrical breakdown voltage of SWNTs in air is found to scale
linearly with their length, approximately as 5 V/um; we use this to deduce a
thermal conductance between SWNT and substrate g ~ 0.17 +/- 0.03 W/K/m per tube
length, which appears limited by the SWNT-substrate interface rather than the
thermal properties of the substrate itself. We examine the phonon scattering
mechanisms limiting electron transport, and find the strong temperature
dependence of the optical phonon absorption rate to have a remarkable influence
on the electrical resistance of micron-length nanotubes. Further analysis
reveals that unlike in typical metals, electrons are responsible for less than
15% of the total thermal conductivity of metallic nanotubes around room
temperature, and this contribution decreases at high bias or higher
temperatures. For interconnect applications of metallic SWNTs, significant
self-heating may be avoided if power densities are limited below 5 uW/um, or if
the SWNT-surrounding thermal interface is optimized. | 0609075v2 |
2009-09-14 | Progress in the development of a S RETGEM-based detector for an early forest fire warning system | In this paper we present a prototype of a Strip Resistive Thick GEM
photosensitive gaseous detector filled with Ne and ethylferrocene vapours at a
total pressure of 1 atm for an early forest fire detection system. Tests show
that it is one hundred times more sensitive than the best commercial
ultraviolet flame detectors and therefore, it is able to reliably detect a
flame of 1.5x1.5x1.5 m3 at a distance of about 1km. An additional and unique
feature of this detector is its imaging capability, which in combination with
other techniques, may significantly reduce false fire alarms when operating in
an automatic mode.
Preliminary results conducted with air filled photosensitive gaseous
detectors are also presented. The approach main advantages include both the
simplicity of manufacturing and affordability of construction materials such as
plastics and glues specifically reducing detector production cost. The
sensitivity of these air filled detectors at certain conditions may be as high
as those filled with Ne and EF. Long term test results of such sealed detectors
indicate a significant progress in this direction.
We believe that our detectors utilized in addition to other flame and smoke
sensors will exceptionally increase the sensitivity of forest fire detection
systems. Our future efforts will be focused on attempts to commercialize such
detectors utilizing our aforementioned findings. | 0909.2480v1 |
2009-09-28 | Hall effect in superconducting Fe(Se0.5Te0.5) thin films | The Hall effect is investigated for eight superconducting Fe(Se_0.5_Te_0.5_)
thin films grown on MgO and LaSrAlO_4_ substrates with different transition
temperatures (T_c_). The normal Hall coefficients (R_H_) have positive values
with magnitude of 1 - 1.5 x 10^-3^ cm^3^/C at room temperature for the all
samples. With decreasing temperature, we find two characteristic types of
behavior in R_H_(T) depending on T_c_. For thin films with lower T_c_
(typically T_c_ < 5 K), R_H_ start decreasing approximately below T = 250 K
toward a negative side, some of which shows sign reversal at T = 50 - 60 K, but
turns positive toward T = 0 K. On the other hand for the films with higher T_c_
(typically T_c_ > 9 K), R_ H_ leaves almost unchanged down to T = 100 K, and
then starts decreasing toward a negative side. Around the temperatures when
R_H_ changes its sign from positive to negative, obvious nonlinearity is
observed in the field-dependence of Hall resistance as to keep the low-field
R_H_ positive while the high-field R_H_ negative. Thus the electronic state
just above T_c_ is characterized by n_e_ (electron density) > n_h_ (hole
density) with keeping \mu_e_ < \mu_h_. These results suggest the dominance of
electron density to the hole density is an essential factor for the occurence
of superconductivity in Fe-chalcogenide superconductors. | 0909.4985v3 |
2010-11-14 | Tunneling anisotropic magnetoresistance of NiFe/IrMn/MgO/Pt stack: An antiferromagnet based spin-valve | Spin-valve is a microelectronic device in which high and low resistance
states are realized by utilizing both charge and spin of carriers. Spin-valve
structures used in modern hard drive read-heads and magnetic random access
memories comprise two ferromagnetic (FM) electrodes whose relative
magnetization orientations can be switched between parallel and antiparallel
configurations, yielding the desired giant or tunneling magnetoresistance
effect. In this paper we demonstrate >100$% spin-valve-like signal in a
NiFe/IrMn/MgO/Pt stack with an antiferromagnet (AFM) on one side and a
non-magnetic metal on the other side of the tunnel barrier. FM moments in NiFe
are reversed by external fields <50mT and the exchange-spring effect of NiFe on
IrMn induces rotation of AFM moments in IrMn which is detected by the measured
tunneling anisotropic magnetoresistance (TAMR). Our work demonstrates a
spintronic element whose transport characteristics are governed by an AFM. It
demonstrates that sensitivity to low magnetic fields can be combined with
large, spin-orbit coupling induced magneto-transport anisotropy using a single
magnetic electrode. The AFM-TAMR provides means to study magnetic
characteristics of AFM films by an electronic transport measurement. | 1011.3188v1 |
2011-09-09 | Study of ageing in Al-Mg-Si alloys by positron annihilation spectroscopy | In many common Al-Mg-Si alloys (6000 series) intermediate storage at or near
'room temperature' after solutionising leads to pronounced changes of the
precipitation kinetics during the ensuing artificial ageing step at \approx
180{\deg}C. This is not only an annoyance in production, but also a challenge
for researchers. We studied the kinetics of natural 'room temperature' ageing
(NA) in Al-Mg-Si alloys by means of various different techniques, namely
electrical resistivity and hardness measurement, thermoanalysis and positron
lifetime and Doppler broadening (DB) spectroscopy to identify the stages in
which the negative effect of NA on artificial ageing might appear. Positron
lifetime measurements were carried out in a fast mode, allowing us to measure
average lifetimes in below 1 minute. DB measurements were carried out with a
single detector and a 68Ge positron source by employing high momentum analysis.
The various measurements show that NA is much more complex than anticipated and
at least four different stages can be distinguished. The nature of these stages
cannot be given with certainty, but a possible sequence includes vacancy
diffusion to individual solute atoms, nucleation of solute clusters, Mg
agglomeration to clusters and coarsening or ordering of such clusters. Positron
lifetime measurements after more complex ageing treatments involving storage at
0{\deg}C, 20{\deg}C and 180{\deg}C have also been carried out and help to
understand the mechanisms involved. | 1109.2019v2 |
2012-11-20 | Enhanced superconducting performance of melt quenched Bi2Sr2CaCu2O8 (Bi-2212) superconductor | We scrutinize the enhanced superconducting performance of melt quench Bismuth
based Bi2Sr2CaCu2O8 (Bi-2212) superconductor. The superconducting properties of
melt quenched Bi-2212 (Bi2212-MQ) sample are compared with non-melted Bi2212-NM
and Bi1.4Pb0.6Sr2Ca2Cu3O10 (Bi-2223). Crystal structure and morphology of the
samples are studied using X-ray diffraction and Scanning Electron Microscopy
(SEM) techniques. The high field (14T) magneto-transport and DC/AC magnetic
susceptibility techniques are extensively used to study the superconducting
properties of the investigated samples. The superconducting critical
temperature (Tc) and upper critical field (Hc2) as well as thermally activated
flux flow (TAFF) activation energy are estimated from the magneto-resistive
[R(T)H] measurements. Both DC magnetization and amplitude dependent AC
susceptibility measurements are used to determine the field and temperature
dependence of critical current density (Jc) for studied samples. On the other
hand, the frequency dependent AC susceptibility is used for estimating flux
creep activation energy. It is found that melt quenching significantly enhances
the superconducting properties of granular Bi-2212 superconductor. The results
are interpreted in terms of better alignment and inter-connectivity of the
grains along with reduction of grain boundaries for Bi2212-MQ sample. | 1211.4681v2 |
2013-07-27 | Strain-induced effects on the magnetic and electronic properties of epitaxial Fe$_{1-x}$Co$_{x}$Si thin films | We have investigated the Co-doping dependence of the structural, transport,
and magnetic properties of \epsilon-FeCoSi epilayers grown by molecular beam
epitaxy on silicon (111) substrates. Low energy electron diffraction, atomic
force microscopy, X-ray diffraction, and high resolution transmission electron
microscopy studies have confirmed the growth of phase-pure, defect-free
\epsilon-FeCoSi epitaxial films with a surface roughness of ~1 nm. These
epilayers are strained due to lattice mismatch with the substrate, deforming
the cubic B20 lattice so that it becomes rhombohedral. The temperature
dependence of the resistivity changes as the Co concentration is increased,
being semiconducting-like for low $x$ and metallic-like for x \gtrsim 0.3. The
films exhibit the positive linear magnetoresistance that is characteristic of
\epsilon-FeCoSi below their magnetic ordering temperatures $T_\mathrm{ord}$, as
well as the huge anomalous Hall effect of order several \mu\Omega cm. The
ordering temperatures are higher than those observed in bulk, up to 77 K for x
= 0.4. The saturation magnetic moment of the films varies as a function of Co
doping, with a contribution of ~1 \mu_{B}/ Co atom for x \lesssim 0.25. When
taken in combination with the carrier density derived from the ordinary Hall
effect, this signifies a highly spin-polarised electron gas in the low x,
semiconducting regime. | 1307.7301v1 |
2013-08-05 | Enhancement of superconductivity near the pressure-induced semiconductor-metal transition in BiS2-based compounds LnO(0.5)F(0.5)BiS2 (Ln = La, Ce, Pr, Nd) | Measurements of electrical resistivity were performed between 3 and 300 K at
various pressures up to 2.8 GPa on the BiS2-based superconductors
LnO0.5F0.5BiS2 (Ln = Pr, Nd). At lower pressures, PrO0.5F0.5BiS2 and
NdO0.5F0.5BiS2 exhibit superconductivity with critical temperatures Tc of 3.5
and3.9 K, respectively. As pressure is increased, both compounds undergo a
transition at a pressure Pt from a low Tc superconducting phase to a high Tc
superconducting phase in which Tc reaches maximum values of 7.6 and 6.4 K for
PrO0.5F0.5BiS2 and NdO0.5F0.5BiS2, respectively. The pressure-induced
transition is characterized by a rapid increase in Tc within a small range in
pressure of ~0.3 GPa for both compounds. In the normal state of PrO0.5F0.5BiS2,
the transition pressure Pt correlates with the pressure where the suppression
of semiconducting behaviour saturates. In the normal state of NdO0.5F0.5BiS2,
Pt is coincident with a semiconductor-metal transition. This behaviour is
similar to the results recently reported for the LnO0.5F0.5BiS2 (Ln = La, Ce)
compounds. We observe that Pt and the size of the jump in Tc between the two
superconducting phases both scale with the lanthanide element in LnO0.5F0.5BiS2
(Ln = La, Ce, Pr, Nd). | 1308.1072v3 |
2014-02-13 | Colloidal pattern replication through contact photolithography operated in a "Talbot-Fabry-Perot" regime | We detail on a continuous colloidal pattern replication by using contact
photolithography. Chrome on quartz masks are fabricated using colloidal
nanosphere lithography and subsequently used as photolithography stamps.
Hexagonal pattern arrangements with different dimensions (980, 620 and 480 nm,
using colloidal particles with respective diameters) have been studied. When
the mask and the imaged resist layer were in intimate contact, a high fidelity
pattern replica was obtained after photolithography exposure and processing. In
turn, the presence of an air-gap in between has been found to affect the
projected image onto the photoresist layer, strongly dependent on the mask
feature size and air-gap height. Pattern replication, inversion and
hybridization was achieved for 980 nm-period mask; no hybridization for the 620
nm; and only pattern replication for the 480 nm. These results are interpreted
in the framework of a "Talbot-Fabry-Perot" effect. Numerical simulations
corroborate with the experimental findings providing insight into the involved
processes highlighting the important parameters affecting the exposure pattern.
The approach allows complex subwavelength patterning and is relevant for a 3D
layer-by-layer printing. | 1402.3117v1 |
2014-02-28 | Exploring of point-contact spectra of Ba1-xNaxFe2As2 in the normal and superconducting state | We present study of derivatives of current-voltage I(V) characteristics of
point-contacts (PCs) based on Ba{1-x}Na{x}Fe2As2 (x=0.25) in the normal and
superconducting state. The detailed analysis of dV/dI(V) data (also given in
Appendix A) shows that the thermal regime, when temperature increases with a
voltage at a rate of about 1.8 K/mV, is realized in the investigated PCs at
least at high biases V above the superconducting (SC) gap \Delta. In this case,
specific resistivity \rho (T) in PC core is responsible for a peculiar dV/dI(V)
behavior, while a pronounced asymmetry of dV/dI(V) is caused by large value of
thermopower in this material. A reproducible zero-bias minima detected on
dV/dI(V) at low biases in the range \pm(6--9)mV well below the SC critical
temperature T_c could be connected with the manifestation of the SC gap \Delta.
Evaluation of these Andreev-reflection-like structures on dV/dI(V) points out
to the preferred value of 2\Delta/kT_c \approx 6. The expected second gap
features on dV/dI(V) are hard to resolve unambiguously, likely due to impurity
scattering, spatial inhomogeneity and transition to the mentioned thermal
regime as the bias further increases. Suggestions are made how to separate
spectroscopic features in dV/dI(V) from those caused by the thermal regime. | 1402.7233v1 |
2014-03-16 | Electrical Contacts to Three-Dimensional Arrays of Carbon Nanotubes | We use numerical simulations to investigate the properties of metal contacts
to three-dimensional arrays of carbon nanotubes (CNTs). For undoped arrays
top-contacted with high or low work function metals, electrostatic screening is
very strong, resulting in a small Schottky barrier for current injection in the
top layer and large Schottky barriers for current injection in the deeper
layers. As a consequence, the majority of the current flows through the top
layer of the array. Our simulations show that doping of the CNT array can
alleviate this problem, even without direct contact to each tube in the array;
however, we find that the charge transfer length is unusually long in arrays
and increases with the number of CNT layers under the contact. We also show
that a bottom gate can modulate the contact resistance, but only very weakly.
These results are important for the design of electronic and optoelectronic
devices based on CNT arrays, because they suggest that increasing the thickness
of the array does little to improve the device performance unless the film is
strongly doped at the contacts and the contact is long, or unless each tube in
the array is directly contacted by the metal. | 1403.3942v1 |
2014-06-05 | Giant overlap between the magnetic and superconducting phases of CeAu2Si2 under pressure | High pressure provides a powerful means for exploring unconventional
superconductivity which appears mostly on the border of magnetism. Here we
report the discovery of pressure-induced heavy fermion superconductivity up to
2.5 K in the antiferromanget CeAu2Si2 (TN ~ 10 K). Remarkably, the magnetic and
superconducting phases are found to overlap across an unprecedentedly wide
pressure interval from 11.8 to 22.3 GPa. Moreover, both the bulk Tc and TM are
strongly enhanced when increasing the pressure from 16.7 to 20.2 GPa. Tc
reaches a maximum at a pressure slightly below pc ~ 22.5 GPa, at which magnetic
order disappears. Furthermore, the scaling behavior of the resistivity provides
evidence for a continuous delocalization of the Ce 4f-electrons associated with
a critical endpoint lying just above pc. We show that the maximum Tc of
CeAu2Si2 actually occurs at almost the same unit-cell volume as that of
CeCu2Si2 and CeCu2Ge2, and when the Kondo and crystal field splitting energies
becomes comparable. Dynamical mean-filed theory calculations suggest that the
peculiar behavior in pressurized CeAu2Si2 might be related to its Ce 4f-orbital
occupancy. Our results not only provide a unique example of the interplay
between superconductivity and magnetism, but also underline the role of orbital
physics in understanding Ce-based heavy fermion systems. | 1406.1438v2 |
2014-09-10 | Proximity effects at the interface of a superconductor and a topological insulator in NbN - Bi_2Se_3 thin film bilayers | In a search for a simple proximity system of a topological insulator and a
superconductor for studying the role of surface versus bulk effects by gating,
we report here on a first step toward this goal, namely the choice of such a
system and its characterization. We chose to work with thin film bilayers of
grainy 5 nm thick NbN films as the superconductor, overlayed with 20 nm thick
topological layer of $\rm Bi_2Se_3$ and compare the transport results to those
obtained on a 5 nm thick reference NbN film on the same wafer. Bilayers with
ex-situ and in-situ prepared $\rm NbN-Bi_2Se_3$ interfaces were studied and two
kinds of proximity effects were found. At high temperatures just below the
superconducting transition, all bilayers showed a conventional proximity effect
where the topological $\rm Bi_2Se_3$ suppresses the onset or mid-transition
$T_c$ of the superconducting NbN films by about 1 K. At low temperatures, a
cross-over of the resistance versus temperature curves of the bilayer and
reference NbN film occurs, where the bilayers show enhancement of $T_c(R=0)$,
$I_c$ (the supercurrent) and the Andreev conductance, as compared to the bare
NbN films. This indicates that superconductivity is induced in the $\rm
Bi_2Se_3$ layer at the interface region in between the NbN grains. Thus an
inverse proximity effect in the topological material is demonstrated. | 1409.2975v1 |
2015-02-15 | Contact doping, Klein tunneling, and asymmetry of shot noise in suspended graphene | The inherent asymmetry of the electric transport in graphene is attributed to
Klein tunneling across barriers defined by $\textit{pn}$-interfaces between
positively and negatively charged regions. By combining conductance and shot
noise experiments we determine the main characteristics of the tunneling
barrier (height and slope) in a high-quality suspended sample with Au/Cr/Au
contacts. We observe an asymmetric resistance $R_{\textrm{odd}}=100-70$
$\Omega$ across the Dirac point of the suspended graphene at carrier density
$|n_{\rm G}|=0.3-4 \cdot 10^{11}$ cm$^{-2}$, while the Fano factor displays a
non-monotonic asymmetry in the range $F_{\textrm{odd}} \sim 0.03 - 0.1$. Our
findings agree with analytical calculations based on the Dirac equation with a
trapezoidal barrier. Comparison between the model and the data yields the
barrier height for tunneling, an estimate of the thickness of the
$\textit{pn}$-interface $d < 20$ nm, and the contact region doping
corresponding to a Fermi level offset of $\sim - 18$ meV. The strength of
pinning of the Fermi level under the metallic contact is characterized in terms
of the contact capacitance $C_c=19 \times 10^{-6}$ F/cm$^2$. Additionally, we
show that the gate voltage corresponding to the Dirac point is given by the
work function difference between the backgate material and graphene. | 1502.04330v2 |
2015-10-06 | Spin Hall magnetoresistance as a probe for surface magnetization in Pt/CoFe$_2$O$_4$ bilayers | We study the spin Hall magnetoresistance (SMR) in Pt grown $\textit{in situ}$
on CoFe$_2$O$_4$ (CFO) ferrimagnetic insulating (FMI) films. A careful analysis
of the angle-dependent and field-dependent longitudinal magnetoresistance
indicates that the SMR contains a contribution that does not follow the bulk
magnetization of CFO but it is a fingerprint of the complex magnetism at the
surface of the CFO layer, thus signaling SMR as a tool for mapping surface
magnetization. A systematic study of the SMR for different temperatures and CFO
thicknesses gives us information impossible to obtain with any standard
magnetometry technique. On one hand, surface magnetization behaves
independently of the CFO thickness and does not saturate up to high fields,
evidencing that the surface has its own anisotropy. On the other hand,
characteristic zero-field magnetization steps are not present at the surface
while they are relevant in the bulk, strongly suggesting that antiphase
boundaries are the responsible of such intriguing features. In addition, a
contribution from ordinary magnetoresistance of Pt is identified, which is only
distinguishable due to the low resistivity of the $\textit{in-situ}$ grown Pt. | 1510.01449v2 |
2015-11-25 | Quantum oscillations of the topological surface states in low carrier concentration crystals of Bi$_{2-x}$Sb$_{x}$Te$_{3-y}$Se$_{y}$ | We report a high-field magnetotransport study on selected low-carrier
crystals of the topological insulator Bi$_{2-x}$Sb${_x}$Te$_{3-y}$Se$_{y}$.
Monochromatic Shubnikov - de Haas (SdH) oscillations are observed at 4.2~K and
their two-dimensional nature is confirmed by tilting the magnetic field with
respect to the sample surface. With help of Lifshitz-Kosevich theory, important
transport parameters of the surface states are obtained, including the carrier
density, cyclotron mass and mobility. For $(x,y)=(0.50,1.3)$ the Landau level
plot is analyzed in terms of a model based on a topological surface state in
the presence of a non-ideal linear dispersion relation and a Zeeman term with
$g_s = 70$ or $-54$. Input parameters were taken from the electronic dispersion
relation measured directly by angle resolved photoemission spectroscopy on
crystals from the same batch. The Hall resistivity of the same crystal
(thickness of 40~$\mu$m) is analyzed in a two-band model, from which we
conclude that the ratio of the surface conductance to the total conductance
amounts to 32~\%. | 1511.08186v1 |
2016-05-06 | Antiferromagnetism in trigonal SrMn2As2 and CaMn2As2 single crystals | Crystallographic, electronic transport, thermal and magnetic properties are
reported for SrMn2As2 and CaMn2As2 single crystals grown using Sn flux.
Rietveld refinements of powder x-ray diffraction data show that the two
compounds are isostructural and crystallize in the trigonal CaAl2Si2-type
structure (space group P-3m1), in agreement with the literature. Electrical
resistivity rho versus temperature T measurements demonstrate insulating ground
states for both compounds with activation energies of 85 meV for SrMn2As2 and
61 meV for CaMn2As2. In a local-moment picture, the Mn^{+2} 3d^5 ions are
expected to have high-spin S = 5/2 with spectroscopic splitting factor g = 2.
Magnetic susceptibility chi and heat capacity measurements versus T reveal
antiferromagnetic (AFM) transitions at TN = 120(2) K and 62(3) K for SrMn2As2
and CaMn2As2, respectively. The anisotropic chi(T < TN) data indicate that the
hexagonal c axis is the hard axis and hence that the ordered Mn moments are
aligned in the ab plane. The chi(T) for both compounds and Cp(T) data for
SrMn2As2 show strong dynamic short-range AFM correlations from TN up to at
least 900 K, likely associated with quasi-two-dimensional connectivity of
strong AFM exchange interactions between the Mn spins within the corrugated
honeycomb Mn layers parallel to the ab plane. | 1605.02052v2 |
2016-06-13 | Observation of Spatial Charge and Spin Correlations in the 2D Fermi-Hubbard Model | Strong electron correlations lie at the origin of transformative phenomena
such as colossal magneto-resistance and high-temperature superconductivity.
Already near room temperature, doped copper oxide materials display remarkable
features such as a pseudo-gap and a "strange metal" phase with unusual
transport properties. The essence of this physics is believed to be captured by
the Fermi-Hubbard model of repulsively interacting, itinerant fermions on a
lattice. Here we report on the site-resolved observation of charge and spin
correlations in the two-dimensional (2D) Fermi-Hubbard model realized with
ultracold atoms. Antiferromagnetic spin correlations are maximal at
half-filling and weaken monotonically upon doping. Correlations between singly
charged sites are negative at large doping, revealing the Pauli and correlation
hole\textemdash a suppressed probability of finding two fermions near each
other. However, as the doping is reduced below a critical value, correlations
between such local magnetic moments become positive, signaling strong bunching
of doublons and holes. Excellent agreement with numerical linked-cluster
expansion (NLCE) and determinantal quantum Monte Carlo (DQMC) calculations is
found. Positive non-local moment correlations directly imply potential energy
fluctuations due to doublon-hole pairs, which should play an important role for
transport in the Fermi-Hubbard model. | 1606.04089v1 |
2016-06-24 | Epitaxial graphene homogeneity and quantum Hall effect in millimeter-scale devices | Quantized magnetotransport is observed in 5.6 x 5.6 mm^2 epitaxial graphene
devices, grown using highly constrained sublimation on the Si-face of SiC(0001)
at high temperature (1900 {\deg}C). The precise quantized Hall resistance of
Rxy = h/2e^2 is maintained up to record level of critical current Ixx = 0.72 mA
at T = 3.1 K and 9 T in a device where Raman microscopy reveals low and
homogeneous strain. Adsorption-induced molecular doping in a second device
reduced the carrier concentration close to the Dirac point(n ~ 1E10 (1/cm^2)),
where mobility of 43700 cm^2/Vs is measured over an area of 10 mm^2. Atomic
force, confocal optical, and Raman microscopies are used to characterize the
large-scale devices, and reveal improved SiC terrace topography and the
structure of the graphene layer. Our results show that the structural
uniformity of epitaxial graphene produced by face-to-graphite processing
contributes to millimeter-scale transport homogeneity, and will prove useful
for scientific and commercial applications. | 1606.07720v1 |
2016-07-11 | The electronic thermal conductivity of graphene | Graphene, as a semimetal with the largest known thermal conductivity, is an
ideal system to study the interplay between electronic and lattice
contributions to thermal transport. While the total electrical and thermal
conductivity have been extensively investigated, a detailed first-principles
study of its electronic thermal conductivity is still missing. Here, we first
characterize the electron-phonon intrinsic contribution to the electronic
thermal resistivity of graphene as a function of doping using electronic and
phonon dispersions and electron-phonon couplings calculated from first
principles at the level of density-functional theory and many-body perturbation
theory (GW). Then, we include extrinsic electron-impurity scattering using
low-temperature experimental estimates. Under these conditions, we find that
the in-plane electronic thermal conductivity of doped graphene is ~300 W/mK at
room temperature, independently of doping. This result is much larger than
expected, and comparable to the total thermal conductivity of typical metals,
contributing ~10 % to the total thermal conductivity of bulk graphene. Notably,
in samples whose physical or domain sizes are of the order of few micrometers
or smaller, the relative contribution coming from the electronic thermal
conductivity is more important than in the bulk limit, since lattice thermal
conductivity is much more sensitive to sample or grain size at these scales.
Last, when electron-impurity scattering effects are included, we find that the
electronic thermal conductivity is reduced by 30 to 70 %. We also find that the
Wiedemann-Franz law is broadly satisfied at low and high temperatures, but with
the largest deviations of 20-50 % around room temperature. | 1607.03037v1 |
2016-07-26 | Interface-driven topological Hall effect in SrRuO$_3$-SrIrO$_3$ bilayer | Electron transport coupled with magnetism has attracted attention over the
years as exemplified in anomalous Hall effect due to a Berry phase in momentum
space. Another type of unconventional Hall effect -- topological Hall effect,
originating from the real-space Berry phase, has recently become of great
importance in the context of magnetic skyrmions. We have observed topological
Hall effect in bilayers consisting of ferromagnetic SrRuO$_3$ and paramagnetic
SrIrO$_3$ over a wide region of both temperature and magnetic field. The
topological term rapidly decreases with the thickness of SrRuO$_3$, ending up
with the complete disappearance at 7 unit cells of SrRuO$_3$. Combined with
model calculation, we concluded that the topological Hall effect is driven by
interface Dzyaloshinskii-Moriya interaction, which is caused by both the broken
inversion symmetry and the strong spin-orbit coupling of SrIrO$_3$. Such
interaction is expected to realize the N\'{e}el-type magnetic skyrmion, of
which size is estimated to be $\sim$10 nm from the magnitude of topological
Hall resistivity. The results established that the high-quality oxide interface
enables us to tune the chirality of the system; this can be a step towards the
future topological electronics. | 1607.07536v1 |
2016-08-25 | Kelvin probe characterization of buried graphitic microchannels in single-crystal diamond | In this work, we present an investigation by Kelvin Probe Microscopy (KPM) of
buried graphitic microchannels fabricated in single-crystal diamond by direct
MeV ion microbeam writing. Metal deposition of variable-thickness masks was
adopted to implant channels with emerging endpoints and high temperature
annealing was performed in order to induce the graphitization of the
highly-damaged buried region. When an electrical current was flowing through
the biased buried channel, the structure was clearly evidenced by KPM maps of
the electrical potential of the surface region overlying the channel at
increasing distances from the grounded electrode. The KPM profiling shows
regions of opposite contrast located at different distances from the endpoints
of the channel. This effect is attributed to the different electrical
conduction properties of the surface and of the buried graphitic layer. The
model adopted to interpret these KPM maps and profiles proved to be suitable
for the electronic characterization of buried conductive channels, providing a
non-invasive method to measure the local resistivity with a micrometer
resolution. The results demonstrate the potential of the technique as a
powerful diagnostic tool to monitor the functionality of all-carbon
graphite/diamond devices to be fabricated by MeV ion beam lithography. | 1608.07126v1 |
2016-08-30 | Epitaxial thin films of pyrochlore iridate Bi_{2+x}Ir_{2-y}O_{7-delta}: structure, defects and transport properties | While pyrochlore iridate thin films are theoretically predicted to possess a
variety of emergent topological properties, experimental verification of these
predictions can be obstructed by the challenge in thin film growth. Here we
report on the pulsed laser deposition and characterization of thin films of a
representative pyrochlore compound Bi2Ir2O7. The films were epitaxially grown
on yttria-stabilized zirconia substrates and have lattice constants that are a
few percent larger than that of the bulk single crystals. The film composition
shows a strong dependence on the oxygen partial pressure.
Density-functional-theory calculations indicate the existence of Bi_Ir antisite
defects, qualitatively consistent with the high Bi: Ir ratio found in the
films. Both Ir and Bi have oxidation states that are lower than their nominal
values, suggesting the existence of oxygen deficiency. The iridate thin films
show a variety of intriguing transport characteristics, including multiple
charge carriers, logarithmic dependence of resistance on temperature,
antilocalization corrections to conductance due to spin-orbit interactions, and
linear positive magnetoresistance. | 1608.08608v3 |
2017-10-04 | Formation, stratification, and mixing of the cores of Earth and Venus | Earth possesses a persistent, internally-generated magnetic field, whereas no
trace of a dynamo has been detected on Venus, at present or in the past,
although a high surface temperature and recent resurfacing events may have
removed paleomagnetic evidence. Whether or not a terrestrial body can sustain
an internally generated magnetic field by convection inside its metallic fluid
core is determined in part by its initial thermodynamic state and its
compositional structure, both of which are in turn set by the processes of
accretion and differentiation. Here we show that the cores of Earth- and
Venus-like planets should grow with stable compositional stratification unless
disturbed by late energetic impacts. They do so because higher abundances of
light elements are incorporated into the liquid metal that sinks to form the
core as the temperatures and pressures of metal-silicate equilibration increase
during accretion. We model this process and determine that this establishes a
stable stratification that resists convection and inhibits the onset of a
geodynamo. However, if a late energetic impact occurs, it could mechanically
stir the core creating a single homogenous region within which a long-lasting
geodynamo would operate. While Earth's accretion has been punctuated by a late
giant impact with likely enough energy to mix the core (e.g. the impact that
formed the Moon), we hypothesize that the accretion of Venus is characterized
by the absence of such energetic giant impacts and the preservation of its
primordial stratifications. | 1710.01770v1 |
2018-03-30 | Thermodynamics of Ion Separation by Electrosorption | We present a simple, top-down approach for the calculation of minimum energy
consumption of electrosorptive ion separation using variational form of the
(Gibbs) free energy. We focus and expand on the case of electrostatic
capacitive deionization (CDI), and the theoretical framework is independent of
details of the double-layer charge distribution and is applicable to any
thermodynamically consistent model, such as the Gouy-Chapman-Stern (GCS) and
modified Donnan (mD) models. We demonstrate that, under certain assumptions,
the minimum required electric work energy is indeed equivalent to the free
energy of separation. Using the theory, we define the thermodynamic efficiency
of CDI. We explore the thermodynamic efficiency of current experimental CDI
systems and show that these are currently very low, less than 1% for most
existing systems. We applied this knowledge and constructed and operated a CDI
cell to show that judicious selection of the materials, geometry, and process
parameters can be used to achieve a 9% thermodynamic efficiency (4.6 kT energy
per removed ion). This relatively high value is, to our knowledge, by far the
highest thermodynamic efficiency ever demonstrated for CDI. We hypothesize that
efficiency can be further improved by further reduction of CDI cell series
resistances and optimization of operational parameters. | 1803.11532v1 |
2018-05-22 | Non-saturating large magnetoresistance in semimetals | The rapidly expanding class of quantum materials known as {\emph{topological
semimetals}} (TSM) display unique transport properties, including a striking
dependence of resistivity on applied magnetic field, that are of great interest
for both scientific and technological reasons. However, experimental signatures
that can identify or discern the dominant mechanism and connect to available
theories are scarce. Here we present the magnetic susceptibility ($\chi$), the
tangent of the Hall angle ($\tan\theta_H$) along with magnetoresistance in four
different non-magnetic semimetals with high mobilities, NbP, TaP, NbSb$_2$ and
TaSb$_2$, all of which exhibit non-saturating large MR. We find that the
distinctly different temperature dependences, $\chi(T)$ and the values of
$\tan\theta_H$ in phosphides and antimonates serve as empirical criteria to
sort the MR from different origins: NbP and TaP being uncompensated semimetals
with linear dispersion, in which the non-saturating magnetoresistance arises
due to guiding center motion, while NbSb$_2$ and TaSb$_2$ being {\it
compensated} semimetals, with a magnetoresistance emerging from nearly perfect
charge compensation of two quadratic bands. Our results illustrate how a
combination of magnetotransport and susceptibility measurements may be used to
categorize the increasingly ubiquitous non-saturating large magnetoresistance
in TSMs. | 1805.08797v2 |
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