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2022-04-26 | Nonreciprocal electrical transport in multiferroic semiconductor (Ge,Mn)Te | We have investigated the nonreciprocal electrical transport, that is a
nonlinear resistance effect depending on the current direction, in multiferroic
Rashba semiconductor (Ge,Mn)Te. Due to coexistence of ferromagnetic and
ferroelectric orders, (Ge,Mn)Te provides a unique platform for exploring the
nonreciprocal electrical transport in a bulk form. (Ge,Mn)Te thin films shows a
large nonreciprocal resistance compared to GeTe, the nonmagnetic counterpart
with the same crystal structure. The magnetic-field-angle dependence of the
nonreciprocal resistance is maximized when magnetic field is orthogonal to both
current and electric polarization, in accord with the symmetry argument. From
the analysis of temperature and magnetic field dependence, we deduce that
inelastic scatterings of electrons mediated by magnons dominantly contribute to
the observed nonreciprocal response. Furthermore, the nonreciprocal resistance
is significantly enhanced by lowering hole density. The Fermi level dependence
is attributed to the deformation of the Rashba band in which the spin-momentum
locked single Fermi surface appears by exchange field from the in-plane
magnetization. The present study provides a key insight to the mechanisms of
novel transport phenomena caused by the interplay of ferroelectric and
ferromagnetic orders in a semiconductor. | 2204.12427v1 |
2022-08-17 | Transport properties of dipole skyrmions in amorphous FeGd multilayers | The transport response of dipole skyrmions in amorphous centrosymmetric Fe/Gd
multilayer is investigated by temperature and field-dependent resistivity
measurements collected in three current and magnetic field configurations. It
is shown that a dipole skyrmion lattice phase may form at certain temperatures
leading to a unique signature of the polar longitudinal resistivity. This
signature differs from the conventional field-varying parabolic response
associated with stripe phases, which transition to a disordered skyrmion phase
under applied fields. Transport measurements under different field history
protocols reveal that the anomaly in the polar longitudinal resistivity appears
under specific field history reversal processes. Our experimental results are
reproduced using micromagnetic simulations that show the anomaly in the polar
longitudinal resistivity is related to a domain wall reconfiguration that
occurs as the domain morphology transitions from disordered stripes to a
skyrmion lattice under applied perpendicular fields. | 2208.08487v1 |
2023-01-18 | Synthesizing $2h/e^2$ resistance plateau at the first Landau level confined in a quantum point contact | A comprehensive understanding of quantum Hall edge transmission, especially
the hole-conjugate of a Laughlin state such as a $2/3$ state, is critical for
advancing fundamental quantum Hall physics and enhancing the design of quantum
Hall edge interferometry. In this study, we report a robust intermediate
$2h/e^2$ resistance quantization in a quantum point contact (QPC) when the bulk
is set at the fractional filling $2/3$ quantum Hall state. Our results suggest
the occurrence of two equilibration processes. First, the co-propagating $1/3$
edges moving along a soft QPC arm confining potential fully equilibrate and act
as a single $2/3$ edge mode. Second, the $2/3$ edge mode is further
equilibrated with an integer $1$ edge mode formed in the QPC. The complete
mixing between them results in a diagonal resistance value quantized at
$2h/e^2$. Similar processes occur for a bulk filling $5/3$, leading to an
intermediate $(2/3)h/e^2$ resistance quantization. | 2301.07488v2 |
2024-02-28 | Impact of etches on thin-film single-crystal niobium resonators | A single crystal niobium thin film was grown using molecular beam epitaxy on
a c-plane sapphire wafer. Several samples were fabricated into dc resistivity
test devices and coplanar waveguide resonator chips using the same
microfabrication procedures and solvent cleans. The samples were then subject
to different acid cleaning treatments using different combinations of piranha,
hydrofluoric acid, and buffered oxide etch solutions. The different samples
expressed changes in dc resistivity in the normal and superconducting states
such that the low temperature resistivities changed by more than 100\%, and the
residual resistivity ratio dropped by a factor of 2. The internal quality
factor of coplanar waveguide resonators measured near 5~GHz also showed
significant variation at single photon powers ranging from 1.4$\times 10^6$ to
less than 60$\times 10^3$. These changes correlate with the formation of
surface crystallites that appear to be hydrocarbons. All observations are
consistent with hydrogen diffusing into the niobium film at levels below the
saturation threshold that is needed to observe niobium hydrides. | 2402.18051v1 |
2024-05-02 | Optimization of reactively sputtered Mn3GaN films based on resistivity measurements | Mn-based nitrides with antiperovskite structures have several properties that
can be utilised for antiferromagnetic spintronics. Their magnetic properties
depend on the structural quality, composition and doping of the cubic
antiperovskite structure. Such nitride thin films are usually produced by
reactive physical vapour deposition, where the deposition rate of N can only be
controlled by the N2 gas flow. We show that the tuning of the N content can be
optimised using low temperature resistivity measurements, which serve as an
indicator of the degree of structural disorder. Several Mn3GaNx films were
prepared by reactive magnetron sputtering under different N2 gas flows. Under
optimised conditions, we obtain films that exhibit a metal-like temperature
dependence, a vanishing logarithmic increase in resistivity towards zero, the
highest resistivity ratio and a lattice contraction of 0.4 % along the growth
direction when heated above that of the N\'eel temperature in agreement with
the bulk samples. | 2405.01203v1 |
2004-02-04 | A simple estimate of the electron hopping energy in the Bechgaard salts | Starting from a previously derived theoretical expression for the electrical
conductivity of the Bechgaard salts,and experimental data on the resistivity of
these materials,it is shown how a valueof the electron hopping energy can be
determined.Values obtained in this way are in agreement with estimates made by
more complicated methods. | 0402127v1 |
2019-09-12 | Gallium Nitride FET Model | We have presented an analytical physics-based compact model of GaN power FET,
which can accurately describe the I-V characteristics in all operation modes.
The model considers the source-drain resistance, different interface trap
densities and self-heating effects. | 1909.05702v1 |
2019-09-18 | A current-voltage characteristic of photoresistance. A plane case | The formation of photo-electron current in the volume of semi-conductor
material is investigated in this article. A plane case when a material is
uniformly illuminated by light is considered. The current-voltage
characteristic of a photo resistance is obtained in analytical form. | 1909.09135v1 |
2019-10-04 | The Test of Topological Property of YbB6 | Topological insulator is a recently discovered class of material with
topologically protected surface state. YbB6 is predicted to be moderately
correlated Z2 topological insulator similar to SmB6. Here, I experimentally
test the resistance property of bulk YbB6 to verify its topological property.
By changing the thickness of YbB6, I found out that although the data curves
did not completely conform to the theory of topology, the experimental
observation to the overall trend showed a similar topological phenomenon. | 1910.02056v1 |
2020-01-25 | Exfoliation of Two-Dimensional Nanosheets of Metal Diborides | The metal diborides are a class of ceramic materials with crystal structures
consisting of hexagonal sheets of boron atoms alternating with planes of metal
atoms held together with mixed character ionic/covalent bonds. Many of the
metal diborides are ultrahigh temperature ceramics like HfB$_2$, TaB$_2$, and
ZrB$_2$, which have melting points above 3000$^\circ$C, high mechanical
hardness and strength at high temperatures, and high chemical resistance, while
MgB$_2$ is a superconductor with a transition temperature of 39 K. Here we
demonstrate that this diverse family of non-van der Waals materials can be
processed into stable dispersions of two-dimensional (2D) nanosheets using
ultrasonication-assisted exfoliation. We generate 2D nanosheets of the metal
diborides AlB$_2$, CrB$_2$, HfB$_2$, MgB$_2$, NbB$_2$, TaB$_2$, TiB$_2$, and
ZrB$_2$, and use electron and scanning probe microscopies to characterize their
structures, morphologies, and compositions. The exfoliated layers span up to
micrometers in lateral dimension and reach thicknesses down to 2-3 nm, while
retaining their hexagonal atomic structure and chemical composition. We exploit
the convenient solution-phase dispersions of exfoliated CrB$_2$ nanosheets to
incorporate them directly into polymer composites. In contrast to the hard and
brittle bulk CrB$_2$, we find that CrB$_2$ nanocomposites remain very flexible
and simultaneously provide increases in the elastic modulus and the ultimate
tensile strength of the polymer. The successful liquid-phase production of 2D
metal diborides enables their processing using scalable low-temperature
solution-phase methods, extending their use to previously unexplored
applications, and reveals a new family of non-van der Waals materials that can
be efficiently exfoliated into 2D forms. | 2001.09237v1 |
2005-10-07 | Collective Spin and Charge Excitations in (Sr,La)_{14-x}Ca_xCu_{24}O_{41} Quantum Spin Ladders | We study magnetic and electronic properties of two-leg ladder materials. We
observed a two-magnon (2M) resonance which we analyze in terms of symmetry,
relaxation and resonance properties. Our findings were contrasted to 2M Raman
measurements in other magnetic crystals. This comparison made us suggest that
the spin-spin correlations in a self-doped two leg ladder may have a modulated
component besides the exponential decay characteristic of a spin liquid ground
state. We found that the 2M intensity resonates at the Mott gap energy.
Interplane Sr substitution for Ca introduces strong disorder leading to
inhomogeneous broadening of the 2M resonance. The doped holes in the spin
liquid ground state further dilute the magnetic correlations, suppressing the
spectral weight of this excitation. At high Ca concentrations are
superconducting under pressure and hole pairing was proposed to be a robust
feature of doped ladders. The measured dielectric response in the microwave
region, the low energy Raman data, the non-linear transport properties along
with soft x-ray scattering allowed us to conclude that the ground state for a
wide range of Ca concentrations (x < 12) is characterized by charge density
wave correlations. This state seems to be driven not by phonons but by Coulomb
forces and many-body effects. We highlighted the similarity in the finite
frequency Raman response as opposed to the very different behavior of the DC
resistivity between undoped and doped ladders. We found that at high Ca
concentrations the carrier relaxation is characterized by the same large
activation energy (~2000 K) as in the self-doped compound. This observation
prompted us to suggest an unconventional metallic transport driven by
collective electronic response. | 0510193v1 |
2009-02-13 | Nanoscale Electronic Inhomogeneity in In2Se3 Nanoribbons Revealed by Microwave Impedance Microscopy | Driven by interactions due to the charge, spin, orbital, and lattice degrees
of freedom, nanoscale inhomogeneity has emerged as a new theme for materials
with novel properties near multiphase boundaries. As vividly demonstrated in
complex metal oxides and chalcogenides, these microscopic phases are of great
scientific and technological importance for research in high-temperature
superconductors, colossal magnetoresistance effect, phase-change memories, and
domain switching operations. Direct imaging on dielectric properties of these
local phases, however, presents a big challenge for existing scanning probe
techniques. Here, we report the observation of electronic inhomogeneity in
indium selenide (In2Se3) nanoribbons by near-field scanning microwave impedance
microscopy. Multiple phases with local resistivity spanning six orders of
magnitude are identified as the coexistence of superlattice, simple hexagonal
lattice and amorphous structures with 100nm inhomogeneous length scale,
consistent with high-resolution transmission electron microscope studies. The
atomic-force-microscope-compatible microwave probe is able to perform
quantitative sub-surface electronic study in a noninvasive manner. Finally, the
phase change memory function in In2Se3 nanoribbon devices can be locally
recorded with big signal of opposite signs. | 0902.2255v1 |
2009-10-01 | Template engineering of Co-doped BaFe2As2 single-crystal thin films | Understanding new superconductors requires high-quality epitaxial thin films
to explore intrinsic electromagnetic properties, control grain boundaries and
strain effects, and evaluate device applications. So far superconducting
properties of ferropnictide thin films appear compromised by imperfect
epitaxial growth and poor connectivity of the superconducting phase. Here we
report novel template engineering using single-crystal intermediate layers of
(001) SrTiO3 and BaTiO3 grown on various perovskite substrates that enables
genuine epitaxial films of Co-doped BaFe2As2 with high transition temperature
(zero resistivity Tc of 21.5K), small transition widths (delta Tc = 1.3K),
superior Jc of 4.5 MA/cm2 (4.2K, self field) and strong c-axis flux pinning.
Implementing SrTiO3 or BaTiO3 templates to match the alkaline earth layer in
the Ba-122 with the alkaline earth-oxygen layer in the templates opens new
avenues for epitaxial growth of ferropnictides on multi-functional single
crystal substrates. Beyond superconductors, it provides a framework for growing
heteroepitaxial intermetallic compounds on various substrates by matching
interfacial layers between templates and thin film overlayers. | 0910.0268v1 |
2009-11-19 | Large transport critical currents of powder-in-tube Sr0.6K0.4Fe2As2/Ag superconducting wires and tapes | We report significant transport critical currents firstly achieved in
Sr0.6K0.4Fe2As2 wires and tapes with a Tc = 34 K, which were fabricated through
an in-situ powder-in-tube process. Silver was used as a chemical addition as
well as a sheath material. Transport measurements were performed by a standard
four-probe resistive method. All the wire and tape samples have shown transport
properties. Critical current density Jc was enhanced upon silver addition, and
at 4.2 K, a best Jc of ~1200 A/cm^2 (Ic = 9 A) was achieved for 20 % silver
added tapes, which is the highest in iron-based wires and tapes so far. The Jc
is almost field independent between 1 T and 10 T, exhibiting a strong vortex
pinning. Such a high transport critical current density is attributed to the
absence of reaction layer between the silver sheath and superconducting core,
as well as an improved connectivity between grains. We also identify a
weak-link behavior from the creep drop of Jc at low fields and a hysteretic
phenomenon. Finally, we found that compared to Fe, Ta and Nb tubes, Ag was the
best sheath material for the fabrication of high-performance 122 type pnictide
wires and tapes. | 0911.3701v1 |
2010-10-05 | Hierarchical formation of bulgeless galaxies: Why outflows have low angular momentum | Using high resolution, fully cosmological smoothed particle hydro-dynamical
simulations of dwarf galaxies in a Lambda cold dark matter Universe, we show
how baryons attain a final angular momentum distribution which allows pure disc
galaxies to form. Blowing out substantial amounts of gas through supernovae and
stellar winds, which is well supported observationally, is a key ingredient in
forming bulgeless discs. We outline why galactic outflows preferentially remove
low angular momentum material, and show that this is a natural result when
structure forms in a cold dark matter cosmology. The driving factors are a) the
mean angular momentum of accreted material increases with time, b) lower
potentials at early times, c) the existence of an extended reservoir of high
angular momentum gas which is not within star forming regions, meaning that
only gas from the inner region (low angular momentum gas) is expelled and d)
the tendency for outflows to follow the path of least resistance which is
perpendicular to the disc. We also show that outflows are enhanced during
mergers, thus expelling much of the gas which has lost its angular momentum
during these events, and preventing the formation of "classical", merger driven
bulges in low mass systems. Stars formed prior to such mergers form a diffuse,
extended stellar halo component. | 1010.1004v3 |
2014-02-13 | On the spheroidized carbide dissolution and elemental partitioning in a high carbon bearing steel 100Cr6 | We report on the characterization of high carbon bearing steel 100Cr6 using
electron microscopy and atom probe tomography in combination with
multi-component diffusion simulations (DICTRA). Scanning electron micrographs
show that around 14 vol.% spheroidized carbides are formed during soft
annealing and only 3 vol.% remain after dissolution into the austenitic matrix
by austenitization at 1123 K (850 {\deg}C) for 300 s. The spheroidized
particles are identified as (Fe, Cr)3C by transmission electron microscopy.
Atom probe analyses reveal the redistribution and partitioning behaviors of
elements, i.e. C, Si, Mn, Cr, Fe in both, the spheroidized carbides and the
bainitic matrix in the sample isothermally heat-treated at 773 K (500 {\deg}C)
after austenitization. A homogeneous distribution of C and gradual gradient of
Cr was detected within the spheroidized carbides. Due to its limited
diffusivity in (Fe, Cr)3C, Cr exhibits a maximum concentration at the surface
of spheroidized carbides (16 at.%) and decreases gradually from surface towards
the core down to a level of about 2 at.%. The atom probe results also indicate
that the partially dissolved spheroidized carbides during austenitization may
serve as nucleation sites for intermediate temperature cementite within
bainite, which results in a relatively softer surface and harder core in
spheroidized particles. This microstructure may contribute to the good wear
resistance and fatigue properties | 1402.3315v1 |
2014-05-23 | Proof-of-principle of a new geometry for sampling calorimetry using inorganic scintillator plates | A novel geometry for a sampling calorimeter employing inorganic scintillators
as an active medium is presented. To overcome the mechanical challenges of
construction, an innovative light collection geometry has been pioneered, that
minimises the complexity of construction. First test results are presented,
demonstrating a successful signal extraction. The geometry consists of a
sampling calorimeter with passive absorber layers interleaved with layers of an
active medium made of inorganic scintillating crystals. Wavelength-shifting
(WLS) fibres run along the four long, chamfered edges of the stack,
transporting the light to photodetectors at the rear. To maximise the amount of
scintillation light reaching the WLS fibres, the scintillator chamfers are
depolished. It is shown herein that this concept is working for cerium fluoride
(CeF$_3$) as a scintillator. Coupled to it, several different types of
materials have been tested as WLS medium. In particular, materials that might
be sufficiently resistant to the High-Luminosity Large Hadron Collider
radiation environment, such as cerium-doped Lutetium-Yttrium Orthosilicate
(LYSO) and cerium-doped quartz, are compared to conventional plastic WLS
fibres. Finally, an outlook is presented on the possible optimisation of the
different components, and the construction and commissioning of a full
calorimeter cell prototype is presented. | 1405.6202v1 |
2015-02-10 | Emergence of the minority hole with high mobility on the electrical transport in the Fe-pnictides Ba(Fe$_{1-x}$Mn$_x$As)$_2$ | In Fe pnictide (Pn) superconducting materials, neither Mn- nor Cr- doping to
the Fe site induces superconductivity, even though hole carriers are generated.
This is in strong contrast with the superconductivity appearing when holes are
introduced by alkali metal substitution on the insulating blocking layers. We
investigate in detail the effects of Mn doping on magneto-transport properties
in Ba(Fe$_{1-x}$Mn$_x$As)$_2$ for elucidating the intrinsic reason. The
negative Hall coefficient for $x$ = 0 estimated in the low magnetic field ($B$)
regime gradually increases as $x$ increases, and its sign changes to a positive
one at $x$ = 0.020. Hall resistivities as well as simultaneous interpretation
using the magnetoconductivity tensor including both longitudinal and transverse
transport components clarify that minority holes with high mobility are
generated by the Mn doping via spin density wave (SDW) transition at low
temperatures, while original majority electrons and holes residing in the
parabolic-like Fermi surfaces (FSs) of the semimetallic Ba(FeAs)$_2$ are
negligibly affected. Present results indicate that the mechanism of hole doping
in Ba(Fe$_{1-x}$Mn$_x$As)$_2$ is greatly different from that of the other
superconducting FePns family. | 1502.02845v3 |
2015-08-16 | Surface States Engineering of Metal/MoS2 Contacts Using Sulfur Treatment for Reduced Contact Resistance and Variability | Variability and lack of control in the nature of contacts between metal/MoS2
interface is a major bottleneck in the realisation of high-performance devices
based on layered materials for several applications. In this letter, we report
on the reduction in Schottky barrier height at metal/MoS2 interface by
engineering the surface states through sulphur treatment. Electrical
characteristics for back-gated MoS2 field effect transistor structures were
investigated for two high work-function metal contacts Ni and Pd. Contacts on
MoS2 treated with sulphur exhibited significant improvements in Ohmic nature
with concomitant reduction in variability compared to those on untreated MoS2
films leading to a 2x increase in extracted mobility. X-ray Photoelectron
Spectroscopy (XPS) measurements, Raman Spectroscopy and comparison of threshold
voltages indicated absence of additional doping or structural changes due to
sulphur treatment. The Schottky barrier heights were extracted from
temperature-dependent transfer characteristics based on the thermionic current
model. A reduction in barrier height of 80 and 135 meV extracted for Ni/MoS2
and Pd/MoS2 contacts respectively is hence attributed to the increase in
surface states (or stronger Fermi level pinning) due to sulphur treatment. The
corresponding charge neutrality levels at metal/MoS2 interface, were extracted
to be 0.16 eV (0.17 eV) below the conduction band before (after) Sulphur
treatment. This first report of surface states engineering in MoS2 leading to
superior contacts is expected to significantly benefit the entire class of
devices based on layered 2D materials. | 1508.03795v2 |
2016-06-28 | Method for Transferring High-Mobility CVD-Grown Graphene with Perfluoropolymers | The transfer of graphene grown by chemical vapor deposition (CVD) using
amorphous polymers represents a widely implemented method for graphene-based
electronic device fabrication. However, the most commonly used polymer,
poly(methyl methacrylate) (PMMA), leaves a residue on the graphene that limits
the mobility. Here we report a method for graphene transfer and patterning that
employs a perfluoropolymer---Hyflon---as a transfer handle and to protect
graphene against contamination from photoresists or other polymers. CVD-grown
graphene transferred this way onto LaAlO$_3$/SrTiO$_3$ heterostructures is
atomically clean, with high mobility (~30,000 cm$^2$V$^{-1}$s$^{-1}$) near the
Dirac point at 2 K and clear, quantized Hall and magneto-resistance. Local
control of the LaAlO$_3$/SrTiO$_3$ interfacial metal-insulator
transition---through the graphene---is preserved with this transfer method. The
use of perfluoropolymers such as Hyflon with CVD-grown graphene and other 2D
materials can readily be implemented with other polymers or photoresists. | 1606.08802v1 |
2016-10-31 | High Electric Field Carrier Transport and Power Dissipation in Multilayer Black Phosphorus Field Effect Transistor with Dielectric Engineering | This study addresses high electric field transport in multilayer black
phosphorus (BP) field effect transistors (FETs) with self-heating and thermal
spreading by dielectric engineering. Interestingly, we found that multilayer BP
device on a SiO2 substrate exhibited a maximum current density of 3.3 x 10E10
A/m2 at an electric field of 5.58 MV/m, several times higher than multilayer
MoS2. Our breakdown thermometry analysis revealed that self-heating was impeded
along BP-dielectric interface, resulting in a thermal plateau inside the
channel and eventual Joule breakdown. Using a size-dependent electro-thermal
transport model, we extracted an interfacial thermal conductance of 1-10 MW/m2
K for the BP-dielectric interfaces. By using hBN as a dielectric material for
BP instead of thermally resistive SiO2 (about 1.4 W/m K), we observed a 3 fold
increase in breakdown power density and a relatively higher electric field
endurance together with efficient and homogenous thermal spreading because hBN
had superior structural and thermal compatibility with BP. We further confirmed
our results based on micro-Raman spectroscopy and atomic force microscopy, and
observed that BP devices on hBN exhibited centrally localized hotspots with a
breakdown temperature of 600K, while the BP device on SiO2 exhibited a hotspot
in the vicinity of the electrode at 520K. | 1610.09951v1 |
2017-02-02 | High-Pressure Synthesis and Characterization of $β$-GeSe - A Semiconductor with Six-Rings in an Uncommon Boat Conformation | Two-dimensional materials have significant potential for the development of
new devices. Here we report the electronic and structural properties of
$\beta$-GeSe, a previously unreported polymorph of GeSe, with a unique crystal
structure that displays strong two-dimensional structural features.
$\beta$-GeSe is made at high pressure and temperature and is stable under
ambient conditions. We compare it to its structural and electronic relatives
$\alpha$-GeSe and black phosphorus. The $\beta$ form of GeSe displays a boat
conformation for its Ge-Se six-ring, while the previously known $\alpha$ form,
and black phosphorus, display the more common chair conformation for their
six-rings. Electronic structure calculations indicate that $\beta$-GeSe is a
semiconductor, with an approximate bulk band gap of $\Delta~\approx$ 0.5 eV,
and, in its monolayer form, $\Delta~\approx$ 0.9 eV. These values fall between
those of $\alpha$-GeSe and black phosphorus, making $\beta$-GeSe a promising
candidate for future applications. The resistivity of our $\beta$-GeSe crystals
measured in-plane is on the order of $\rho \approx$ 1 $\Omega$cm, while being
essentially temperature independent. | 1702.00715v1 |
2017-09-21 | Effect of nanostructure on thermoelectric properties of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ in 300-600 K range | In oxide materials, nanostructuring effect has been found very promising
approach for the enhancement of \textit{figure-of-merit}, \textit{ZT}. In the
present work, we have synthesized La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) compound
using sol-gel method and samples of crystallite size of 34, 41, and 49 nm were
obtained by giving different heat treatment. Seebeck coefficient ($\alpha$),
electrical resistivity ($\rho$), and thermal conductivity ($\kappa$)
measurements were carried out in 300-600 K temperature range. The systematic
change in the values of $\alpha$ from $\sim$ -19 $\mu$V/K to $\sim$ -24
$\mu$V/K and drastic reduction in the values of $\kappa$ from $\sim$0.88 W/mK
to $\sim$0.23 W/mK are observed as crystallite size is reduced from 49 nm to 34
nm at $\sim$600 K. Also, fall in the values of $\rho$ in the paramagnetic (PM)
insulator phase (400-600 K) are effectively responsible for the increasing
trend in the values of \textit{ZT} at high temperature. For the crystallite
size of 41 nm, value of \textit{ZT} at 600 K was found to be $\sim$0.017, which
can be further increased up to $\sim$0.045 around 650 K temperature. The
predicted value of \textit{ZT} suggests that LSMO can be suitable oxide
material for thermoelectric applications at high temperature. | 1709.07205v2 |
2017-12-17 | Short range order in the quantum XXZ honeycomb lattice material BaCo$_2$(PO$_4$)$_2$ | We present observations of highly frustrated quasi two-dimensional (2D)
magnetic correlations in the honeycomb lattice layers of the S$_{eff}$ = 1/2
compound $\gamma$-BaCo$_2$(PO$_4$)$_2$ ($\gamma$-BCPO). Specific heat shows a
broad peak comprised of two weak kink features at $T_{N1} \sim$ 6 K and $T_{N2}
\sim$ 3.5 K, the relative weights of which can be modified by sample annealing.
Neutron powder diffraction measurements reveal short range quasi-2D order that
is established below $T_{N1}$ and $T_{N2}$, at which two separate,
incompatible, short range magnetic orders onset: commensurate antiferromagnetic
correlations with correlation length $\xi_c = 60\pm2$ \AA \ ($T_{N1}$) and in
quasi-2D helical domains with $\xi_h = 350 \pm 11$ \AA \ ($T_{N2}$). The ac
magnetic susceptibility response lacks frequency dependence, ruling out spin
freezing. Inelastic neutron scattering data on $\gamma$-BCPO is compared with
linear spin wave theory, and two separate parameter regions of the XXZ
$J_1$-$J_2$-$J_3$ model with ferromagnetic nearest-neighbor exchange $J_1$ are
favored, both near regions of high classical degeneracy. High energy coherent
excitations ($\sim 10$ meV) persist up to at least 40 K, suggesting strong
in-plane correlations persist above $T_N$. These data show that $\gamma$-BCPO
is a rare highly frustrated, quasi-2D S$_{eff}$ = 1/2 honeycomb lattice
material which resists long range magnetic order and spin freezing. | 1712.06208v2 |
2018-02-22 | Touch Sensors with Overlapping Signals: Concept Investigation on Planar Sensors with Resistive or Optical Transduction | Traditional methods for achieving high localization accuracy on tactile
sensors usually involve a matrix of miniaturized individual sensors distributed
on the area of interest. This approach usually comes at a price of increased
complexity in fabrication and circuitry, and can be hard to adapt to non-planar
geometries. We propose a method where sensing terminals are embedded in a
volume of soft material. Mechanical strain in this material results in a
measurable signal between any two given terminals. By having multiple terminals
and pairing them against each other in all possible combinations, we obtain a
rich signal set using few wires. We mine this data to learn the mapping between
the signals we extract and the contact parameters of interest. Our approach is
general enough that it can be applied with different transduction methods, and
achieves high accuracy in identifying indentation location and depth. Moreover,
this method lends itself to simple fabrication techniques and makes no
assumption about the underlying geometry, potentially simplifying future
integration in robot hands. | 1802.08209v2 |
2018-11-27 | Unifying Description of Competing Orders in Two Dimensional Quantum Magnets | Quantum magnets provide the simplest example of strongly interacting quantum
matter, yet they continue to resist a comprehensive understanding above one
spatial dimension (1D). In 1D, a key ingredient to progress is Luttinger liquid
theory which provides a unified description. Here we explore a promising
analogous framework in two dimensions, the Dirac spin liquid (DSL), which can
be constructed on several different lattices. The DSL is a version of Quantum
Electrodynamics ( QED$_3$) with four flavors of Dirac fermions coupled to
photons. Importantly, its excitations also include magnetic monopoles that
drive confinement. By calculating the complete action of symmetries on
monopoles on the square, honeycomb, triangular and kagom\`e lattices, we answer
previously open key questions. We find that the stability of the DSL is
enhanced on the triangular and kagom\`e lattices as compared to the bipartite
(square and honeycomb) lattices. We obtain the universal signatures of the DSL
on the triangular and kagom\`e lattices, including those that result from
monopole excitations, which serve as a guide to numerics and to experiments on
existing materials. Interestingly, the familiar 120 degree magnetic orders on
these lattices can be obtained from monopole proliferation. Even when unstable,
the Dirac spin liquid unifies multiple ordered states which could help organize
the plethora of phases observed in strongly correlated two-dimensional
materials. | 1811.11186v2 |
2017-03-18 | The Effect of Temperature on Cu-K-In-Se Thin Films | Films of Cu-K-In-Se were co-evaporated at varied K/(K+Cu) compositions and
substrate temperatures (with constant (K+Cu)/In ~ 0.85). Increased Na
composition on the substrate's surface and decreased growth temperature were
both found to favor Cu1-xKxInSe2 (CKIS) alloy formation, relative to
mixed-phase CuInSe2 + KInSe2 formation. Structures from X-ray diffraction
(XRD), band gaps, resistivities, minority carrier lifetimes and carrier
concentrations from time-resolved photoluminescence were in agreement with
previous reports, where low K/(K+Cu) composition films exhibited properties
promising for photovoltaic (PV) absorbers. Films grown at 400-500 C were then
annealed to 600 C under Se, which caused K loss by evaporation in proportion to
initial K/(K+Cu) composition. Similar to growth temperature, annealing drove
CKIS alloy consumption and CuInSe2 + KInSe2 production, as evidenced by high
temperature XRD. Annealing also decomposed KInSe2 and formed K2In12Se19. At
high temperature the KInSe2 crystal lattice gradually contracted as temperature
and time increased, as well as just time. Evaporative loss of K during
annealing could accompany the generation of vacancies on K lattice sites, and
may explain the KInSe2 lattice contraction. This knowledge of Cu-K-In-Se
material chemistry may be used to predict and control minor phase impurities in
Cu(In,Ga)(Se,S)2 PV absorbers-where impurities below typical detection limits
may have played a role in recent world record PV efficiencies that utilized KF
post-deposition treatments. | 1703.06366v1 |
2019-02-26 | Bonding and oxidation protection of Ti${_2}$AlC and Cr${_2}$AlC for a Ni-based Superalloy | Alumina forming, oxidation and thermal shock resistant MAX phases are of a
high interest for high temperature applications. Herein we report, on bonding
and resulting interactions between a Ni-based superalloy, NSA, and two alumina
forming MAX phases. The diffusion couples Cr${_2}$AlC/Inconel-718/Ti${_2}$AlC
were assembled and heated to 1000 or 1100{\deg}C in a vacuum hot press under
loads corresponding to stresses of either 2 MPa or 20 MPa. The resulting
interfaces were examined using X-ray diffraction, scanning electron microscopy
and energy-dispersive X-ray spectroscopy. Good bonding between Cr${_2}$AlC and
NSA was achieved after hot pressing at 1000{\deg}C and a contact pressure of
only 2 MPa; in the case of Ti${_2}$AlC a higher temperature (1100{\deg}C) and
pressure (20 MPa) were needed. In both cases, a diffusion bond was realized
with no evidence of interfacial damage or cracking after cooling to room
temperature. Twenty thermal cycles from room temperature to 1000{\deg}C showed
that Ti${_2}$AlC is a poor oxidation barrier for Inconel-718. However, in the
case of Cr${_2}$AlC no cracks, delamination nor surface degradation were
observed, suggesting that this material could be used to protect Inconel-718
from oxidation. | 1902.10001v2 |
2020-04-22 | Deformation of micrometer and mm-sized Fe2.4wt.%Si single- and bi-crystals with a high angle grain boundary at room temperature | Plasticity in body-centred cubic (BCC) metals, including dislocation
interactions at grain boundaries, is much less understood than in face-centred
cubic (FCC) metals. At low temperatures additional resistance to dislocation
motion due to the Peierls barrier becomes important, which increases the
complexity of plasticity. Iron-silicon steel is an interesting, model BCC
material since the evolution of the dislocation structure in
specifically-oriented grains and at particular grain boundaries have
far-reaching effects not only on the deformation behaviour but also on the
magnetic properties, which are important in its final application as electrical
steel. In this study, two different orientations of micropillars (1, 2, 4
microns in diameter) and macropillars (2500 microns) and their corresponding bi
crystals are analysed after compression experiments with respect to the effect
of size on strength and dislocation structures. Using different experimental
methods, such as slip trace analysis, plane tilt analysis and cross-sectional
EBSD, we show that direct slip transmission occurs, and different slip systems
are active in the bi-crystals compared to their single-crystal counterparts.
However, in spite of direct transmission and a very high transmission factor,
dislocation pile-up at the grain boundary is also observed at early stages of
deformation. Moreover, an effect of size scaling with the pillar size in single
crystals and the grain size in bi-crystals is found, which is consistent with
investigations elsewhere in FCC metals. | 2004.10598v2 |
2022-02-01 | Strong Edge Stress in Molecularly Thin Organic$-$Inorganic Hybrid Ruddlesden$-$Popper Perovskites and Modulations of Their Edge Electronic Properties | Organic$-$inorganic hybrid Ruddlesden$-$Popper perovskites (HRPPs) have
gained much attention for optoelectronic applications due to their high
moisture resistance, good processibility under ambient conditions, and long
functional lifetimes. Recent success in isolating molecularly thin hybrid
perovskite nanosheets and their intriguing edge phenomena have raised the need
for understanding the role of edges and the properties that dictate their
fundamental behaviours. In this work, we perform a prototypical study on the
edge effects in ultrathin hybrid perovskites by considering monolayer
(BA)$_2$PbI$_4$ as a representative system. Based on first-principles
simulations of nanoribbon models, we show that in addition to significant
distortions of the octahedra network at the edges, strong edge stresses are
also present in the material. Structural instabilities that arise from the edge
stress could drive the relaxation process and dominate the morphological
response of edges in practice. A clear downward shift of the bands at the
narrower ribbons, as indicative of the edge effect, facilitates the separation
of photo-excited carriers (electrons move towards the edge and holes move
towards the interior part of the nanosheet). Moreover, the desorption energy of
the organic molecule can also be much lower at the free edges, making it easier
for functionalization and/or substitution events to take place. The findings
reported in this work elucidate the underlying mechanisms responsible for edge
states in HRPPs and will be important in guiding the rational design and
development of high-performance layer$-$edge devices. | 2202.00296v1 |
2016-03-11 | Quantum Critical Behavior in a Concentrated Ternary Solid Solution | Quantum critical behavior has been associated with some of the most exotic
emergent states of matter including high-temperature superconductivity. Much of
the research into quantum critical point (QCP) physics has been hampered by the
lack of model systems simple enough to be analyzed by theory. Here, we show
that the concentrated solid solution fcc alloys, including the so-called
high-entropy alloys, are ideal model systems to study the effects of chemical
disorder on emergent properties near a quantum critical region. The face
centered cubic (fcc) alloy NiCoCrx with x near 1 is found to be close to the Cr
concentration where the ferromagnetic transition temperature, Tc, goes to 0.
Near this composition these alloys exhibit a resistivity linear in temperature
to 2 K, a linear magnetoresistance, an excess -TlnT contribution to the low
temperature heat capacity and excess low temperature entropy. All of the low
temperature electrical, magnetic and thermodynamic properties of the alloys
with compositions near x near 1 are not typical of a Fermi liquid and suggest
strong magnetic fluctuations associated with a quantum critical region. The
limit of extreme chemical disorder in these simple fcc materials thus provides
a novel and unique platform to study quantum critical behavior in a highly
tunable system. | 1603.03781v1 |
2019-03-09 | Dry transfer method for suspended graphene on lift-off-resist: simple ballistic devices with Fabry-Pérot interference | We demonstrate a fabrication scheme for clean suspended structures using
chemical-vapor-deposition-grown graphene and a dry transfer method on
lift-off-resist-coated substrates to facilitate suspended graphene
nanoelectronic devices for technology applications. It encompasses the demands
for scalable fabrication as well as for ultra-fast response due to weak
coupling to environment. The fabricated devices exhibited initially a weak
field-effect response with substantial positive ($p$) doping which transformed
into weak negative ($n$) doping upon current annealing at the temperature of 4
Kelvin. With increased annealing current, $n$-doping gradually decreased while
the Dirac peak position approached zero in gate voltage. An ultra-low residual
charge density of $9\times10^8 \mathrm{ \ cm^{-2}}$ and a mobility of $1.9
\times 10^5 \mathrm{\ cm^2/Vs}$ were observed. Our samples display clear
Fabry-P\'{e}rot (FP) conductance oscillation which indicates ballistic electron
transport. The spacing of the FP oscillations are found to depend on the charge
density in a manner that agrees with theoretical modeling based on Klein
tunneling of Dirac particles. The ultra-low residual charge, the FP
oscillations with density dependent period, and the high mobility prove
excellent quality of our suspended graphene devices. Owing to its simplicity,
scalability and robustness, this fabrication scheme enhances possibilities for
production of suspended, high-quality, two-dimensional-material structures for
novel electronic applications. | 1903.03780v1 |
2019-12-11 | Quantum oscillations and electronic structures in large Chern number semimetal RhSn | We report the magnetoresistance, Hall effect, de Haas-van Alphen (dHvA)
oscillations and the electronic structures of single crystal RhSn, which is a
typical material of CoSi family holding a large Chern number. The large
unsaturated magnetoresistance is observed with B//[001]. The Hall resistivity
curve indicates that RhSn is a multi-band system with high mobility. Evident
quantum oscillations have been observed, from which the light effective masses
are extracted. Ten fundamental frequencies are extracted after the fast Fourier
transform analysis of the dHvA oscillations with B//[001] configuration. The
two low frequencies F$_1$ and F$_2$ do not change obviously and the two high
frequencies F$_9$ and F$_{10}$ evolve into four when B rotates from B//[001] to
B//[110], which is consistent with the band structure in the first-principles
calculations with spin-orbit coupling (SOC). The extracted Berry phases of the
relative pockets show a good agreement with the Chern number $\pm4$ (with SOC)
in the first-principles calculations. Above all, our studies indicate that RhSn
is an ideal platform to study the unconventional chiral fermions and the
surface states. | 1912.05148v1 |
2019-12-11 | Bulk and Two-dimensional Silver and Copper Monohalides: A Unique Class of Materials with Modest Ionicity/Covalency and Ferroelasticity/Multiferroicity | Silver and copper monohalides can be viewed as a class of compounds in the
neutral zone between predominantly covalent and ionic compounds, thereby
exhibiting neither strong ionicity nor strong covalency. We show ab initio
calculation evidence that silver and copper monohalides entail relatively low
transition barriers between the non-polar rock-salt phase and the polar
zinc-blende phase, due largely to their unique chemical nature of modest
iconicity or covalency. Notably, the low transition barriers endow both
monohalides with novel mechanical and electronic properties, i.e., coupled
ferroelasticity and ferroelectricity with large polarizations and relatively
low switching barriers at ambient conditions. Several halides even possess very
similar lattice constants and structures as the prevailing semiconductors such
as silicon, thereby enabling epitaxial growth on silicon. Moreover, based on
extensive structural search, we find that the most stable two-dimensional (2D)
polymorphs of the monolayer halides are close or even greater in energy than
their bulk counterparts, a feature not usually seen in the family of rock-salt
or zinc-blende semiconductors. The low transition barrier between zinc-blende
phase and 2D phase is predicted. Moreover, several 2D monolayer halides also
exhibit multiferroicity with coupled ferroelasticity or ferroelectricity,
thereby rendering their potential applications as high-density integrated
memories for efficient data reading and writing. Their surfaces, covered by
halides, also provide oxidation resistance and give low cleave energy from
layered structure, suggesting high likelihood of experimental synthesis of
these 2D polymorphs. | 1912.05172v1 |
2015-04-14 | Carrier Transport at the Metal-MoS2 Interface | This study illustrates the nature of electronic transport and its transition
from one mechanism to another between a metal electrode and MoS2 channel
interface in a field effect transistor (FET) device. Interestingly,
measurements of the contact resistance (Rc) as a function of temperature
indicate a transition in the carrier transport across the energy barrier from a
thermionic emission at a high temperature to tunneling at a low temperature.
Furthermore, at a low temperature, the nature of the tunneling behavior is
ascertained by the current-voltage dependency that helps us feature direct
tunneling at a low bias and Fowler-Nordheim tunneling at a high bias for a
Pd-MoS2 contact due to the effective barrier shape modulation by biasing. In
contrast, only direct tunneling is observed for a Cr-MoS2 contact over the
entire applied bias range. In addition, simple analytical calculations were
carried out to extract Rc at the gating range, and the results are consistent
with the experimental data. Our results describe the transition in carrier
transport mechanisms across a metal-MoS2 interface, and this information
provides guidance for the design of future flexible, transparent electronic
devices based on 2-dimensional materials. | 1504.03466v1 |
2018-12-28 | Nanoscale Self-Healing Mechanisms in Shape Memory Ceramics | Shape memory (SM) ceramics, such as yttria-stabilized tetragonal zirconia
(YSTZ), are a unique family of SM materials that offer unique properties
including ultra-high operating temperature, and high resistance to chemical
corrosion and oxidation. However, formation of defects is usually observed in
SM ceramics during manufacturing and/or by mechanical deformation. To fully
take advantage of the SM properties of these ceramics, it is necessary to fully
understand the nano-structural evolution of defects under external stimuli. In
this study, defect closure behaviors in YSTZ nanopillars are investigated by
atomistic simulations. Two characteristic orientations of [011-] and [001] are
selected to represent the dominant deformation mechanisms of phase
transformation and dislocation migration, respectively. With the presence of
crack and void, the strength and yield strain of nanopillars are noted to
decrease significantly, especially for [011-]-oriented YSTZ nanopillars. Volume
expansion associated with the tetragonal to monoclinic phase transformation is
observed to promote healing of crack and void. Atom stress analyses reveal
stress concentrations along the newly formed monoclinic phase bands. A critical
crack width is identified, less than which the crack can be fully closed in
compression. Size effect study reveals that an increase in nanopillar size has
a positive effect on crack self-healing behavior. For [001]-oriented YSTZ
nanopillars, dislocation migration leads to formations of an amorphous phase,
which also assist the crack and void closure process. The revealed crack/void
healing mechanisms may provide a path for mitigating internal defects that
influences the mechanical properties and deformation mechanisms of SM ceramics. | 1812.11136v1 |
2019-06-24 | Melting of vortex lattice in magnetic superconductor $\mathrm{Rb}\mathrm{Eu}\mathrm{Fe}_{4}\mathrm{As}_{4}$ | The iron-based superconductors are characterized by strong fluctuations due
to high transition temperatures and small coherence lengths. We investigate
fluctuation behavior in the magnetic iron-pnictide superconductor
$\mathrm{Rb}\mathrm{Eu}\mathrm{Fe}_{4}\mathrm{As}_{4}$ by calorimetry and
transport. We find that the broadening of the specific-heat transition in
magnetic fields is very well described by the lowest-Landau-level scaling. We
report calorimetric and transport observations for vortex-lattice melting,
which is seen as a sharp drop of the resistivity and a step of the specific
heat at the magnetic-field-dependent temperature. The melting line in the
temperature/magnetic-field plane lies noticeably below the upper-critical-field
line and its location is in quantitative agreement with theoretical predictions
without fitting parameters. Finally, we compare the melting behavior of
$\mathrm{Rb}\mathrm{Eu}\mathrm{Fe}_{4}\mathrm{As}_{4}$ with other
superconducting materials showing that thermal fluctuations of vortices are not
as prevalent as in the high-temperature superconducting cuprates, yet they
still noticeably influence the properties of the vortex matter. | 1906.10236v2 |
2020-01-23 | Magnetic-field-induced robust zero Hall plateau state in MnBi$_2$Te$_4$ Chern insulator | The intrinsic antiferromagnetic topological insulator MnBi2Te4 provides an
ideal platform for exploring exotic topological quantum phenomena. Recently,
the Chern insulator and axion insulator phases have been realized in few-layer
MnBi2Te4 devices at low magnetic field regime. However, the fate of MnBi2Te4 in
high magnetic field has never been explored in experiment. In this work, we
report transport studies of exfoliated MnBi2Te4 flakes in pulsed magnetic
fields up to 61.5 T. In the high-field limit, the Chern insulator phase with
Chern number C = -1 evolves into a robust zero Hall resistance plateau state.
Nonlocal transport measurements and theoretical calculations demonstrate that
the charge transport in the zero Hall plateau state is conducted by two
counter-propagating edge states that arise from the combined effects of Landau
levels and large Zeeman effect in strong magnetic fields. Our result
demonstrates the intricate interplay among intrinsic magnetic order, external
magnetic field, and nontrivial band topology in MnBi2Te4. | 2001.08401v2 |
2020-04-14 | Highly Conducting Spaced TiO$_2$ Nanotubes Enable Defined Conformal Coating with Nanocrystalline Nb$_2$O$_5$ and High Performance Supercapacitor Applications | In this work, we report on the electrochemical behavior of nitrided spaced
TiO$_2$ nanotubes conformally coated with a nanocrystalline Nb$_2$O$_5$ layer
and find for these hierarchical structures an excellent supercapacitor
performance. Highly aligned conductive 1D electrodes were obtained by a three
step process: i) growth of self-organized nanotubes with defined and adjustable
intertube spacing, ii) conformal Nb$_2$O$_5$ decoration in the tube interspace
(while providing full electrolyte access to the entire active area), and iii)
high temperature nitridation. Key is the growth of a nanotube array with
regular tube-to-tube interspacing that enables an optimized decoration with
secondary materials such as Nb$_2$O$_5$. We observe an increase in electrode
capacitance from 158 $\mu$F cm-2 for bare TiO$_2$ NTs, to 1536 $\mu$F cm-2 for
TiO$_2$/Nb$_2$O$_5$ NTs, to finally 37 mF cm-2 for Nb$_2$O$_5$ decorated and
then nitrided nanotubes. This drastic increase can be ascribed firstly to the
defined spacing established between the tube arrays that then allows for a
conformal coating with a secondary active coating. Secondly, nitridation causes
a drastic increase of the electron conductivity of the entire scaffold and thus
reduces resistive losses. | 2005.01603v1 |
2020-10-13 | High-pressure synthesis of Ba$_2$RhO$_4$, a rhodate analogue of the layered perovskite Sr-ruthenate | A new layered perovskite-type oxide Ba$_2$RhO$_4$ was synthesized by a
high-pressure technique with the support of convex-hull calculations. The
crystal and electronic structure were studied by both experimental and
computational tools. Structural refinements for powder x-ray diffraction data
showed that Ba$_2$RhO$_4$ crystallizes in a K$_2$NiF$_4$-type structure,
isostructural to Sr$_2$RuO$_4$ and Ba$_2$IrO$_4$. Magnetic, resistivity, and
specific heat measurements for polycrystalline samples of Ba$_2$RhO$_4$
indicate that the system can be characterized as a correlated metal. Despite
the close similarity to its Sr$_2$RuO$_4$ counterpart in the electronic
specific heat coefficient and the Wilson ratio, Ba$_2$RhO$_4$ shows no
signature of superconductivity down to 0.16 K. Whereas the Fermi surface
topology has reminiscent pieces of Sr$_2$RuO$_4$, an electron-like
e$_g$-($d_{x^2-y^2}$) band descends below the Fermi level, making of this
compound unique also as a metallic counterpart of the spin-orbit-coupled Mott
insulator Ba$_2$IrO$_4$. | 2010.06556v1 |
2021-02-15 | Magic thickness of 25 Å makes periodic metal-insulator transitions | Novel quantum phenomena, including high-temperature superconductivity,
topological properties, and charge/spin density waves, appear in
low-dimensional conductive materials. It is possible to artificially create
low-dimensional systems by fabricating ultrathin films, quantum wires, or
quantum dots with flat interfaces. Some experiments have been performed on
ultrathin compounds of strongly correlated electron systems. However, since it
is technically difficult to control multiple elements precisely, most of the
properties of artificially fabricated low-dimensional compounds fall into
uncharted territory. Here we show that extraordinary metal-insulator
transitions that oscillate depending on the scale occur in CaRuO_3 films with a
thickness of around several unit cells. We grow high-crystalline CaRuO_3
ultrathin films, whose surface roughness is controlled at 199 pm, by molecular
beam epitaxy. We observe that resistivity oscillates with a magic thickness of
25 {\AA}, which changes by 3 and 9 orders of magnitude at room temperature and
at low temperature, respectively. These changes are much larger than quantum
size effects. We also confirm the same periodicity with photoelectron
spectroscopy by etching the ultrathin film. Considering the large energy,
periodicity and anisotropy, we conclude that the oscillating transitions
originate from the commensurability of Mott insulation triggered by Peierls
instability arising from a dual restriction on the dimensions in wavenumber
space and real space. We have shown the possibility of producing new functional
materials by controlling film thickness on electron correlated compounds at the
picometer level. | 2102.07323v1 |
2021-06-17 | Synthesis of Murunskite Single Crystals: A Bridge Between Cuprates and Pnictides | Numerous contemporary investigations in condensed matter physics are devoted
to high temperature (high-$T_c$ ) cuprate superconductors. Despite its unique
effulgence among research subjects, the enigma of the high-$T_c$ mechanism
still persists. One way to advance its understanding is to discover and study
new analogous systems. Here we begin a novel exploration of the natural mineral
murunskite, K$_2$FeCu$_3$S$_4$, as an interpolation compound between cuprates
and ferropnictides, the only known high-$T_c$ superconductors at ambient
pressure. Because in-depth studies can be carried out only on single crystals,
we have mastered the synthesis and growth of high quality specimens. Similar to
the cuprate parent compounds, these show semiconducting behavior in resistivity
and optical transmittance, and an antiferromagnetic ordering at 100 K.
Spectroscopy (XPS) and calculations (DFT) concur that the sulfur 3$p$ orbitals
are partially open, making them accessible for charge manipulation, which is a
prerequisite for superconductivity in analogous layered structures. DFT
indicates that the valence band is more cuprate-like, while the conduction band
is more pnictide-like. With appropriate doping strategies, this parent compound
promises exciting future developments. | 2106.09555v1 |
2021-09-14 | Creep properties and deformation mechanisms of single-crystalline $γ^\prime$-strengthened superalloys in dependence of the Co/Ni ratio | Co-base superalloys are considered as promising high temperature materials
besides the well-established Ni-base superalloys. However, Ni appears to be an
indispensable alloying element also in Co-base superalloys. To address the
influence of the base elements on the deformation behavior, high-temperature
compressive creep experiments were performed on a single crystal alloy series
that was designed to exhibit a varying Co/Ni ratio and a constant Al, W and Cr
content. Creep tests were performed at 900 {\deg}C and 250 MPa and the
resulting microstructures and defect configurations were characterized via
electron microscopy. The minimum creep rates differ by more than one order of
magnitude with changing Co/Ni ratio. An intermediate CoNi-base alloy exhibits
the overall highest creep strength. Several strengthening contributions like
solid solution strengthening of the $\gamma$ phase, effective diffusion
coefficients or stacking fault energies were quantified. Precipitate shearing
mechanisms differ significantly when the base element content is varied. While
the Ni-rich superalloys exhibit SISF and SESF shearing, the Co-rich alloys
develop extended APBs when the $\gamma^\prime$ phase is cut. This is mainly
attributed to a difference in planar fault energies, caused by a changing
segregation behavior. As result, it is assumed that the shearing resistivity
and the occurring deformation mechanisms in the $\gamma^\prime$ phase are
crucial for the creep properties of the investigated alloy series. | 2109.06767v1 |
2022-06-29 | Tailoring of rhenium oxidation state in ReOx thin films during reactive HiPIMS deposition process and following annealing | Bulk rhenium trioxide (ReO3) has an unusually high electrical conductivity
and, being nanosized, has promising catalytic properties. However, the
production of pure ReO3 thin films is challenging due to the difficulty to
stabilize rhenium in a 6+ oxidation state. Here we present a novel approach for
the deposition of ReOx (x = 1.6-2.9) thin films using reactive high power
impulse magnetron sputtering (r-HiPIMS) from a metallic rhenium target in a
mixed Ar/O2 atmosphere. The thin films were deposited in the gas-sustained
self-sputtering regime, observed during r-HiPIMS process according to current
waveforms. The influence of the substrate temperature, the oxygen-to-argon flow
ratio and post-annealing at 250 {\deg}C in the air for 3 h on the properties of
the films were studied. The as-deposited films have an X-ray amorphous
structure (a-ReOx) when deposited at room temperature while a nano-crystalline
\b{eta}-ReO2 phase when deposited at elevated temperatures (150 or 250
{\deg}C). The amorphous a-ReOx can be converted into the crystalline ReO3 with
a lattice parameter of 3.75 {\AA} upon annealing in the air. The surface
morphology of the films is dense without detectable voids when elevated
substrate temperatures are used. Various Re oxidation states are observed on
the surface of the films in different ratios depending on the deposition
parameters. All samples exhibit electrical resistivity on the order of 10-3
Ohmxcm and optical properties typical for thin metallic films. | 2206.14665v1 |
2022-06-02 | A mechanically strong and ductile soft magnet with extremely low coercivity | Soft magnetic materials (SMMs) serve in electrical applications and
sustainable energy supply, allowing magnetic flux variation in response to
changes in applied magnetic field, at low energy loss1. The electrification of
transport, households and manufacturing leads to an increase in energy
consumption due to hysteresis losses2. Therefore, minimizing coercivity, which
scales these losses, is crucial3. Yet, meeting this target alone is not enough:
SMMs in electrical engines must withstand severe mechanical loads, i.e., the
alloys need high strength and ductility4. This is a fundamental design
challenge, as most methods that enhance strength introduce stress fields that
can pin magnetic domains, thus increasing coercivity and hysteretic losses5.
Here, we introduce an approach to overcome this dilemma. We have designed a
Fe-Co-Ni-Ta-Al multicomponent alloy with ferromagnetic matrix and paramagnetic
coherent nanoparticles (~91 nm size, ~55% volume fraction). They impede
dislocation motion, enhancing strength and ductility. Their small size, low
coherency and small magnetostatic energy create an interaction volume below the
magnetic domain wall width, leading to minimal domain wall pinning, thus
maintaining the soft magnetic properties. The alloy has a tensile strength of
1336 MPa at 54% tensile elongation, extremely low coercivity of 78 A/m (<1 Oe),
moderate saturation magnetization of 100 Am2/kg, and high electrical
resistivity of 103 {\mu}{\Omega} u Ohm cm. | 2207.05686v1 |
2022-07-26 | Cr$_3$X$_4$ (X=Se, Te) monolayers as new platform to realize robust spin filter, spin diode and spin valve | Two-dimensional ferromagnetic (FM) half-metals are promising candidates for
advanced spintronic devices with small-size and high-capacity. Motivated by
recent report on controlling synthesis of FM Cr$_3$Te$_4$ nanosheet, herein, to
explore the potential application in spintronics, we designed spintronic
devices based on Cr$_3$X$_4$ (X=Se, Te) monolayers and investigated their spin
transport properties. We found that Cr$_3$Te$_4$ monolayer based device shows
spin filtering and dual spin diode effect when applying bias voltage, while
Cr$_3$S$_4$ monolayer is an excellent platform to realize a spin valve. The
different transport properties are primarily ascribed to the semiconducting
spin channel, which is close to and away from the Fermi level in Cr$_3$Te$_4$
and Cr$_3$Se$_4$ monolayers, respectively. Interestingly, the current in
monolayer Cr$_3$Se$_4$ based device also displays a negative differential
resistance effect (NDRE) and a high magnetoresistance ratio (up to 2*10$^3$).
Moreover, we found thermally induced spin filtering effect and NDRE in
Cr$_3$Se$_4$ junction when applying temperature gradient instead of bias
voltage. These theoretical findings highlight the potential of Cr$_3$X$_4$
(X=Se, Te) monolayers in spintronic applications and put forward realistic
materials to realize nanosale spintronic device. | 2207.12679v1 |
2022-10-26 | Ion-beam Assisted Sputtering of Titanium Nitride Thin Films | Titanium nitride is a material of interest for many superconducting devices
such as nanowire microwave resonators and photon detectors. Thus, controlling
the growth of TiN thin films with desirable properties is of high importance.
In previous work on niobium nitride, ion beam-assisted sputtering (IBAS)
reduced nitrogen sensitivity during deposition in tandem with an increase in
nominal critical temperature. We have deposited thin films of titanium nitride
by both, the conventional method of DC reactive magnetron sputtering and the
IBAS method and compare their superconducting critical temperatures Tc as
functions of thickness, sheet resistance, and nitrogen flow rate. We perform
electrical and structural characterizations by electric transport and X-ray
diffraction measurements. Compared to the conventional method of reactive
sputtering, the IBAS technique has demonstrated a 10% increase in nominal
critical temperature and 33% reduced sensitivity to nitrogen flow, without
noticeable variation in the lattice structure. Additionally, we explore the
behavior of superconducting Tc in ultra-thin films. Trends in films grown at
high nitrogen concentrations follow predictions of mean-field theory in
disordered films and show suppression of superconducting Tc due to geometric
effects, while nitride films grown at low nitrogen concentrations strongly
deviate from the theoretical models. | 2210.15065v3 |
2023-02-14 | MgF$_2$ as an effective additive for improving ionic conductivity of ceramic solid electrolytes | As typical solid-state electrolytes (SSEs),
{Na}$_{1+x}${Zr}$_2${Si}$_{x}${P}$_{3-x}${O}$_{12}$ NASICONs provide an ideal
platform for solid-state batteries (SSBs) that display higher safety and
accommodate higher energy densities. The critical points for achieving SSBs
with higher efficiencies are to improve essentially the ionic conductivity and
to reduce largely the interfacial resistance between SSEs and cathode
materials, which would necessitate extremely high level of craftsmanship and
high-pressure equipment. An alternative to higher-performance and lower-cost
SSBs is additive manufacturing. Here, we report on an effective additive,
MgF$_2$, which was used in synthesizing NASICONs, resulting in SSEs with fewer
defects and higher performance. With an addition of mere 1 wt$\%$ MgF$_2$
additive, the total room-temperature ionic conductivity of the NASICON
electrolyte reaches up to 2.03 mS cm$^{-1}$, improved up to $\sim$ 181.3$\%$,
with an activation energy of 0.277 eV. Meanwhile, the stability of the Na
plating/stripping behavior in symmetric cells increases from 236 to 654 h. We
tried to reveal the microscopic origins of the higher ionic conductivity of
MgF$_2$-doped NASICONs by comprehensive in-house characterizations. Our study
discovers a novel MgF$_2$ additive and provides an efficient way to prepare
higher-performance SSEs, making it possible to fabricate lower-cost SSBs in
industries. | 2302.07264v1 |
2023-07-03 | Low temperature dynamic polaron liquid in a manganite exhibiting colossal magnetoresistance | Polarons - fermionic charge carriers bearing a strong companion lattice
deformation - exhibit a natural tendency for self-localization due to the
recursive interaction between electrons and the lattice. While polarons are
ubiquitous in insulators, how they evolve in transitions to metallic and
superconducting states in quantum materials remains an open question. Here, we
use resonant inelastic x-ray scattering (RIXS) to track the electron-lattice
coupling in the colossal magneto-resistive bi-layer manganite
La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ across its metal-to-insulator transition. The
response in the insulating high-temperature state features harmonic emissions
of a dispersionless oxygen phonon at small energy transfer. Upon cooling into
the metallic state, we observe a drastic redistribution of spectral weight from
the region of these harmonic emissions to a broad high energy continuum. In
concert with theoretical calculations, we show that this evolution implies a
shift in electron-lattice coupling from static to dynamic lattice distortions
that leads to a distinct polaronic ground state in the low temperature metallic
phase - a dynamic polaron liquid. | 2307.00718v3 |
2023-09-24 | FeCo Nanowire-Strontium Ferrite Powder Composites for Permanent Magnets with High-Energy Products | Due to the issues associated with rare-earth elements, there arises a strong
need for magnets with properties between those of ferrites and rare-earth
magnets that could substitute the latter in selected applications. Here, we
produce a high remanent magnetization composite bonded magnet by mixing FeCo
nanowire powders with hexaferrite particles. In the first step, metallic
nanowires with diameters between 30 and 100 nm and length of at least 2 {\mu}m
are fabricated by electrodeposition. The oriented as-synthesized nanowires show
remanence ratios above 0.76 and coercivities above 199 kA/m and resist core
oxidation up to 300 {\deg}C due to the existence of a > 8 nm thin oxide
passivating shell. In the second step, a composite powder is fabricated by
mixing the nanowires with hexaferrite particles. After the optimal nanowire
diameter and composite composition are selected, a bonded magnet is produced.
The resulting magnet presents a 20% increase in remanence and an enhancement of
the energy product of 48% with respect to a pure hexaferrite (strontium
ferrite) magnet. These results put nanowire-ferrite composites at the forefront
as candidate materials for alternative magnets for substitution of rare earths
in applications that operate with moderate magnet performance. | 2309.13724v1 |
2023-11-01 | Transport and electrical properties of cryogenic thermoelectric FeSb2: the effect of isoelectronic and hole doping | Thermoelectric materials operating at cryogenic temperatures are in high
demand for efficient cooling and power generation in applications ranging from
superconductors to quantum computing. The narrow band-gap semiconductor FeSb2,
known for its colossal Seebeck coefficient, holds promise for such
applications, provided its thermal conductivity value can be reduced. This
study investigates the impact of isoelectronic substitution (Bi) and hole
doping (Pb) at the Sb site on the transport properties of FeSb2, with a
particular focus on thermal conductivity (\k{appa}). Polycrystalline FeSb2
powder, along with Bi- and Pb-doped samples, were synthesized using a simple
co-precipitation approach, followed by thermal treatment in an H2 atmosphere.
XRD and SEM analysis confirms the formation of the desired phase pre- and
post-consolidation using spark plasma sintering (SPS). The consolidation
process resulted in a high compaction density and the formation of
submicrometer-sized grains, as substantiated by electron backscattered
diffraction (EBSD) analysis. Substituting 1% of Bi and Pb at the Sb site
successfully suppressed the thermal conductivity (\k{appa}) from ~15 W/m-K in
pure FeSb2 to ~10 and ~8.7 W/m-K, respectively. Importantly, resistivity
measurements revealed a metal-to-insulator transition at around 6.5 K in
undoped FeSb2 and isoelectronically Bi-substituted FeSb2, suggesting the
existence of metallic surface states and provides valuable evidence for the
perplexing topological behavior exhibited by FeSb2. | 2311.00326v1 |
2023-12-13 | Computational design of NDR tunnel diodes with high peak-to-valley current ratio based on two-dimensional cold metals: The case of NbSi$_2$N$_4$/HfSi$_2$N$_4$/NbSi$_2$N$_4$ lateral heterojunction diode | Cold metals have recently gained attention as a promising platform for
innovative devices, such as tunnel diodes with negative differential resistance
(NDR) and field-effect transistors with subthreshold swings below the
thermionic limit. Recently discovered two-dimensional (2D) MA$_2$Z$_4$ (M = Ti,
Zr, Hf, Nb, Ta; A = Si, Ge; Z = N, P) compounds exhibit both cold metallic and
semiconducting behavior. In this work, we present a computational study of
lateral heterojunction tunnel diodes based on 2D NbSi$_2$N$_4$ and
HfSi$_2$N$_4$ compounds. Employing density functional theory combined with a
nonequilibrium Green function method, we investigate the current-voltage
($I$-$V$) characteristics of lateral tunnel diodes with varying barrier
thicknesses in both zigzag and armchair orientations. We find that tunnel
diodes in the zigzag orientation exhibit significantly higher peak current
densities, while those in the armchair orientation display larger
peak-to-valley current ratios (PVCRs) compared to the zigzag orientation. Our
findings suggest that MA$_2$Z$_4$ materials are promising candidates for
realizing NDR tunnel diodes with high PVCR values, which could have potential
applications in memory, logic circuits, and other electronic devices. | 2312.08473v2 |
2024-01-29 | Increasing the Collection Efficiency in Selenium Thin-Film Solar Cells Using a Closed-Space Annealing Strategy | Elemental selenium is a promising wide-bandgap ($E_\mathrm{G}\approx$ 1.95
eV) photovoltaic material for the next generation of thin-film solar cells. To
realize high-efficiency selenium solar cells, it is crucial to optimize the
crystallization process of the selenium thin-film photoabsorber. However, the
high vapor pressure of selenium restricts the processing conditions to a
compromise between the growth of large crystal grains and the formation of
pinholes. In this study, we introduce a closed-space annealing (CSA) strategy
designed to suppress the sublimation of selenium, enabling thermal annealing
processes at higher temperatures and for longer periods of time. As a result,
we consistently improve carrier collection and the overall photovoltaic device
performance in our selenium solar cells. By characterizing the carrier dynamics
in our devices, we conclude that the observed improvements result from a
reduction in charge transfer resistance rather than an increase in carrier
diffusion length. The CSA strategy is a promising method for controlling
surface morphology and roughness without reducing crystal grain sizes, which
paves the way for further advancements in the efficiency and reproducibility of
selenium thin-film solar cells. | 2401.15936v1 |
2024-02-10 | Cylindrical compression of thin wires by irradiation with a Joule-class short pulse laser | Equation of state measurements at Jovian or stellar conditions are currently
conducted by dynamic shock compression driven by multi-kilojoule multi-beam
nanosecond-duration lasers. These experiments require precise design of the
target and specific tailoring of the spatial and temporal laser profiles to
reach the highest pressures. At the same time, the studies are limited by the
low repetition rate of the lasers. Here, we show that by the irradiation of a
thin wire with single beam Joule-class short-pulse laser, a converging
cylindrical shock is generated compressing the wire material to conditions
relevant for the above applications. The shockwave was observed using Phase
Contrast Imaging employing a hard X-ray Free Electron Laser with unprecedented
temporal and spatial sensitivity. The data collected for Cu wires is in
agreement with hydrodynamic simulations of an ablative shock launched by a
highly-impulsive and transient resistive heating of the wire surface. The
subsequent cylindrical shockwave travels towards the wire axis and is predicted
to reach a compression factor of 9 and pressures above 800 Mbar. Simulations
for astrophysical relevant materials underline the potential of this
compression technique as a new tool for high energy density studies at high
repetition rates. | 2402.06983v1 |
2024-03-04 | Quantum Hall Transport Measurements of Lateral p-n Junctions Formed via Precise Spatial Photodoping of Graphene/hBN Heterostructures | Heterostructures composed of 2-dimensional (2D) materials are spatially dope
in-operando to modify devices for custom functionalities, such as lateral p-n-p
junctions. After optically photodoping an hBN/Graphene/hBN heterostructure,
detailed magnetotransport measurements including quantum Hall transport show
several clear electronic regimes. In the p+-p-p+ and n-n+-n configurations, we
see clear quantization of the longitudinal resistance. Using the
Landauer-Buttiker model we elucidate the nature of the electrostatic profile at
the interface between the doped regions. In the p-n-p configuration, due to the
heavily graded junction profile that completely separates the p- and n-Landau
level edge states from interacting, an "insulating" state is observed that is
not common and has not been measured in previous quantum Hall transport
measurements of graphene pnJ devices in high magnetic fields. This insulating
state is promising as the basis for a high-performance graphene switching
device with a good ON/OFF ratio. In principle, these doping and measurement
techniques can be applied to any other 2D heterostructure encapsulated within
an hBN sandwich to understand the quality of the electrostatic interface
between doped regions. | 2403.01998v1 |
2020-12-05 | A15 Nb$_3$Si -- A "high" Tc superconductor synthesized at a pressure of one megabar and metastable at ambient conditions | A15 Nb$_3$Si is, until now, the only high temperature superconductor produced
at high pressure (~110 GPa) that has been successfully brought back to room
pressure conditions in a metastable condition. Based on the current great
interest in trying to create metastable-at-room-pressure high temperature
superconductors produced at high pressure, we have restudied explosively
compressed A15 Nb$_3$Si and its production from tetragonal Nb$_3$Si. First,
diamond anvil cell pressure measurements up to 88 GPa were performed on
explosively compressed A15 Nb$_3$Si material to trace Tc as a function of
pressure. Tc is suppressed to ~ 5.2 K at 88 GPa. Then, using these Tc (P) data
for A15 Nb$_3$Si, pressures up to 92 GPa were applied at room temperature
(which increased to 120 GPa at 5 K) on tetragonal Nb$_3$Si. Measurements of the
resistivity gave no indication of any A15 structure production, i.e., no
indications of the superconductivity characteristic of A15 Nb$_3$Si. This is in
contrast to the explosive compression (up to P~110 GPa) of tetragonal Nb$_3$Si,
which produced 50-70% A15 material, Tc = 18 K at ambient pressure, in a 1981
Los Alamos National Laboratory experiment. Our theoretical calculations show
that A15 Nb$_3$Si has an enthalpy vs the tetragonal structure that is 0.07
eV/atom smaller at 100 GPa, implying that the accompanying high temperature
(1000 deg C) caused by explosive compression is necessary to successfully drive
the reaction kinetics of the tetragonal -> A15 Nb$_3$Si structural
transformation. Annealing experiments on the A15 explosively compressed
material reaching time scales of 39 years are consistent with this viewpoint. | 2012.02905v2 |
2006-03-17 | Second Core Formation and High Speed Jets: Resistive MHD Nested Grid Simulations | The stellar core formation and high speed jets driven by the formed core are
studied by using three-dimensional resistive MHD nested grid simulations.
Starting with a Bonnor-Ebert isothermal cloud rotating in a uniform magnetic
field, we calculate the cloud evolution from the molecular cloud core (n = 10^6
cm^-3, r_c = 4.6 times 10^4 AU) to the stellar core (n \simeq 10^23 cm^-3, r_c
\simeq 1 solar radius). We resolve cloud structure over 7 orders of magnitude
in spatial extent and over 17 orders of magnitude in density contrast. For
comparison, we calculate two models: resistive and ideal MHD models. Both
models have the same initial condition, but the former includes dissipation
process of magnetic field while the latter does not. The magnetic fluxes in
resistive MHD model are extracted from the first core during 10^12 cm^-3 < n <
10^16 cm^-3 by Ohmic dissipation. Magnetic flux density of the formed stellar
core (n \simeq 10^20 cm^-3) in resistive MHD model is two orders of magnitude
smaller than that in ideal MHD model. Since magnetic braking is less effective
in resistive MHD model, rapidly rotating stellar core (the second core) is
formed. After stellar core formation, the magnetic field of the core is largely
amplified both by magneto-rotational instability and the shearing motion
between the stellar core and ambient medium. As a consequence, high speed
(simeq 45 km,s^-1) jets are driven by the second core, which results in strong
mass ejection. A cocoon-like structure around the second core also forms with
clear bow shocks. | 0603456v1 |
2018-05-07 | Appearance of ferromagnetism property for Si nano-polycrystalline body and vanishing of electrical resistances at local high frequencies | Reduction in the skin effect for the sintered Si nanopolycrystalline body as
an electricity conductor at a high frequency due to its nano-structure was
studied. Singular vanishing of electrical resistances near a local high
magnetic harmonic frequency of a few MHz was observed. This phenomenon has not
been observed for conventional ferromagnetic metals. The measured electrical
resistances changed to almost 0 m{\Omega} at room temperature. At the same
time, negative resistance of the sintered Si nano-polycrystalline body was
observed. It will be applicable to electronic transmittance lines or
semiconductors. Numerical calculation was also performed on the electrical
resistance with frequency dependency while considering the electric field and
magnetic field in the sintered Si nanopolycrystalline body. The calculation
could explain the variation of the relative permittivity of the Si
nanopolycrystalline and the phenomenon for vanishing the resistivity at
frequency of MHz theoretically. Reduced Si nanoparticles from SiO2 powder were
synthesized by laser ablation in liquid. A Si nano-polycrystalline body made of
the reduced Si nanoparticles was fabricated. It was found by measuring the
magnetization property of the body that the sintered Si nano-polycrystalline
body has ferromagnetism. High-density dangling bonds cause the sintered Si
nanopolycrystalline to have ferromagnetism. In this study, the density of the
unpaired electrons in the sintered Si nanopolycrystalline was observed using
ESR. It has been clarified that the Si nanopowder and the sintered Si
nanopolycrystalline have numerous dangling bonds. Both densities of the
dangling bonds were evaluated. | 1805.02312v1 |
2019-03-26 | The micro-RWELL layouts for high particle rate | The $\mu$-RWELL is a single-amplification stage resistive Micro-Pattern
Gaseous Detector (MPGD). The detector amplification element is realized with a
single copper-clad polyimide foil micro-patterned with a blind hole (well)
matrix and embedded in the readout PCB through a thin Diamond-Like-Carbon (DLC)
sputtered resistive film. The introduction of the resistive layer, suppressing
the transition from streamer to spark, allows to achieve large gains
($\geq$10$^4$) with a single amplification stage, while partially reducing the
capability to stand high particle fluxes. The simplest resistive layout,
designed for low-rate applications, is based on a single-resistive layer with
edge grounding. At high particle fluxes this layout suffers of a non-uniform
response. In order to get rid of such a limitation different current evacuation
geometries have been designed. In this work we report the study of the
performance of several high rate resistive layouts tested at the CERN H8-SpS
and PSI $\pi$M1 beam test facilities. These layouts fulfill the requirements
for the detectors at the HL-LHC and for the experiments at the next generation
colliders FCC-ee/hh and CepC. | 1903.11017v2 |
2018-04-15 | $\mathrm{Co_2Fe_{1-x}Cr_xSi}$ Heusler Alloys : A promising material for spintronics application | In this article, we investigated the effect of Cr substitution in place of Fe
on the structural, magnetic and transport properties of $\mathrm{Co_2FeSi}$
alloy. A comprehensive structural analysis is done using X-ray diffraction
(XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy.
Quaternary Heusler compounds $\mathrm{Co_2Fe_{1-x}Cr_xSi}$ with Cr content (x =
0.1, 0.3, 0.5) were found to crystallize in cubic structure. The synchrotron
based EXAFS studies reveal that the anti-site disorder increases with the
increase in Cr concentration. The saturation magnetization values in all the
alloys are found to be less than those expected from the Slater-Pauling rule,
which may be due to the some inherent disorder. A detailed resistivity analysis
in the temperature range of 5-300 K is done, taking into account different
scattering mechanisms. The residual resistivity ratio is found to decrease with
increasing Cr concentration. A disorder induced resistivity minimum due to weak
localization effect is seen for x = 0.5. The resistivity measurements also
indicate that the half-metallic character survives upto 100 K for x = 0.1,
whereas the alloys with x= 0.3 and 0.5 show signature of half- metallic nature
even at higher temperatures. First principles calculation done with a more
robust exchange correlation functional (namely HSE-06) confirms the half
metallicity in the entire concentration range. Theoretically simulated band gap
and magnetic moments compliment the experimental findings and are compared
wherever possible. All these properties make $\mathrm{Co_2Fe_{1-x}Cr_xSi}$ a
promising material for spintronics application. | 1804.05321v1 |
2021-10-16 | Giant magnetoresistance and topological Hall effect in the EuGa4 antiferromagnet | We report on systematic temperature- and magnetic field-dependent studies of
the EuGa$_4$ binary compound, which crystallizes in a centrosymmetric
tetragonal BaAl$_4$-type structure with space group $I4/mmm$. The electronic
properties of EuGa$_4$ single crystals, with an antiferromagnetic (AFM)
transition at $T_\mathrm{N} \sim 16.4$ K, were characterized via electrical
resistivity and magnetization measurements. A giant nonsaturating
magnetoresistance was observed at low temperatures, reaching $\sim 7 \times
10^4$ % at 2 K in a magnetic field of 9 T. In the AFM state, EuGa$_4$ undergoes
a series of metamagnetic transitions in an applied magnetic field, clearly
manifested in its field-dependent electrical resistivity. Below $T_\mathrm{N}$,
in the $\sim$4-7 T field range, we observe also a clear hump-like anomaly in
the Hall resistivity which is part of the anomalous Hall resistivity. We
attribute such a hump-like feature to the topological Hall effect, usually
occurring in noncentrosymmetric materials known to host topological spin
textures (as e.g., magnetic skyrmions). Therefore, the family of materials with
a tetragonal BaAl$_4$-type structure, to which EuGa$_4$ and EuAl$_4$ belong,
seems to comprise suitable candidates on which one can study the interplay
among correlated-electron phenomena (such as charge-density wave or exotic
magnetism) with topological spin textures and topologically nontrivial bands. | 2110.08522v1 |
2016-01-09 | Field-induced resistivity plateau and unsaturated negative magnetoresistance in topological semimetal TaSb2 | Several prominent transport properties have been identified as key signatures
of topologicalmaterials. One is the resistivity plateau at low temperatures as
observed in several topological insulators (TIs), another is the negative
magnetoresistance (MR) when the applied magnetic field is parallel to the
current direction as observed in several topological semimetals (TSMs)
including Dirac semimetals (DSMs) and Weyl semimetals (WSMs). Usually, these
two exotic phenomena emerge in distinct materials with or without time reversal
symmetry (TRS), respectively. Here we report the discovery of a new member in
TSMs, TaSb2, which clearly exhibits both of these phenomena in a single
material. This compound crystallizes in a base-centered monoclinic,
centrosymmetric structure, and is metallic with a low carrier density in the
zero field. While applying magnetic field it exhibits insulating behavior
before appearance of a resistivity plateau below Tc =13 K. In the plateau
regime, the ultrahigh carrier mobility and extreme magnetoresistance (XMR) for
the field perpendicular to the current are observed as in DSMs and WSMs, in
addition to a quantum oscillation behavior with non-trivial Berry phases. In
contrast to the most known DSMs and WSMs, the negative MR in TaSb2 does not
saturate up to 9 T, which, together with the almost linear Hall resistivity,
manifests itself an electron-hole non-compensated TMS. These findings indicate
that the resistivity plateau could be a generic feature of topology-protected
metallic states even in the absence of TRS and compatible with the negative MR
depending on the field direction. Our experiment extends a materials basis
represented by TaSb2 as a new platform for future theoretical investigations
and device applications of topological materials. | 1601.02062v1 |
2011-01-31 | Encapsulation and Electronic Control of Epitaxial Graphene by Photosensitive Polymers and UV light | Electronic devices using epitaxial graphene on Silicon Carbide require
encapsulation to avoid uncontrolled doping by impurities deposited in ambient
conditions. Additionally, interaction of the graphene monolayer with the
substrate causes relatively high level of electron doping in this material,
which is rather difficult to change by electrostatic gating alone.
Here we describe one solution to these problems, allowing both encapsulation
and control of the carrier concentration in a wide range. We describe a novel
heterostructure based on epitaxial graphene grown on silicon carbide combined
with two polymers: a neutral spacer and a photoactive layer that provides
potent electron acceptors under UV light exposure. Unexposed, the same double
layer of polymers works well as capping material, improving the temporal
stability and uniformity of the doping level of the sample. By UV exposure of
this heterostructure we controlled electrical parameters of graphene in a
non-invasive, non-volatile, and reversible way, changing the carrier
concentration by a factor of 50. The electronic properties of the exposed SiC/
graphene/polymer heterostructures remained stable over many days at room
temperature, but heating the polymers above the glass transition reversed the
effect of light.
The newly developed photochemical gating has already helped us to improve the
robustness (large range of quantizing magnetic field, substantially higher
opera- tion temperature and significantly enhanced signal-to-noise ratio due to
significantly increased breakdown current) of a graphene resistance standard to
such a level that it starts to compete favorably with mature semiconductor
heterostructure standards. [2,3] | 1101.6014v1 |
2020-11-20 | Phase Transitions in Germanium Telluride Nanoparticle Phase-Change Materials Studied by Time-Resolved X-Ray Diffraction | Germanium telluride (GeTe), a phase-change material, is known to exhibit four
different structural phases: three at room temperature (one amorphous and two
crystalline, $\alpha$ and $\gamma$) and one at high temperature (crystalline
$\beta$). Because transitions between the amorphous and crystalline phases lead
to significant changes in material properties (e.g., refractive index and
resistivity), GeTe has been investigated as a phase-change material for
photonics, thermoelectrics, ferroelectrics, and spintronics. Consequently, the
temperature-dependent phase transitions in GeTe have been studied for bulk and
thin-film GeTe, both fabricated by sputtering. Colloidal synthesis of
nanoparticles offers a more flexible fabrication approach for amorphous and
crystalline GeTe. These nanoparticles are known to exhibit size-dependent
properties, such as an increased crystallization temperature for the
amorphous-to-$\alpha$ transition in sub-10\,nm GeTe particles. The
$\alpha$-to-$\beta$ phase transition is also expected to vary with size, but
this effect has not yet been investigated for GeTe. Here, we report
time-resolved X-ray diffraction of GeTe nanoparticles with different diameters
and from different synthetic protocols. We observe a non-volatile
amorphous-to-$\alpha$ transition between 210$^{\circ}$C and 240$^{\circ}$C and
a volatile $\alpha$-to-$\beta$ transition between 370$^{\circ}$C and
420$^{\circ}$C. The latter transition was reversible and repeatable. While the
transition temperatures are shifted relative to the values known for bulk GeTe,
the nanoparticle-based samples still exhibit the same structural phases
reported for sputtered GeTe. Thus, colloidal GeTe maintains the same general
phase behavior as bulk GeTe while allowing for more flexible and accessible
fabrication. Therefore, nanoparticle-based GeTe films show great potential for
applications, such as in active photonics. | 2011.10633v1 |
2022-08-23 | Electrochemical investigation of MoSeTe as an anode for sodium-ion batteries | Sodium ion batteries (SIBs) are considered as an efficient alternative for
lithium-ion batteries (LIBs) owing to the natural abundance and low cost of
sodium than lithium. In this context, the anode materials play a vital role in
rechargeable batteries to acquire high energy and power density. In order to
demonstrate transition metal dichalcogenide (TMD) as potential anode materials,
we have synthesized MoSeTe sample by conventional flux method, and the
structure and morphology are characterized using x-ray diffraction (XRD),
field-emission scanning electron microscopy (FESEM), transmission electron
microscopy (TEM), and Raman spectroscopy. These characterisations confirm the
hexagonal crystal symmetry with p63/mmc space group and layered morphology of
MoSeTe. We investigate the electrochemical performance of a MoSeTe as a
negative electrode (anode) for SIBs in the working potential range of 0.01 to
3.0~V. In a half-cell configuration, the MoSeTe as an anode and Na metal as
counter/reference electrode exhibits significant initial specific discharge
capacities of around 475 and 355 mAhg$^{-1}$ at current densities of 50 and 100
mAg$^{-1}$, respectively. However, the capacity degraded significantly like
$\approx$200~mAhg$^{-1}$ in 2nd cycle, but having $\approx$100\% Coulombic
efficiency, which suggest for further modification in this material to improve
its stability. The cyclic voltammetry (CV) study reveals the reversibility of
the material after 1st cycle, resulting no change in the initial peak
positions. The electrochemical impedance spectroscopy (EIS) measurements
affirms the smaller charge transfer resistance of fresh cells than the cells
after 10th cycle. Moreover, the extracted diffusion coefficient is found to be
of the order of 10$^{-14}$ cm$^2$s$^{-1}$. | 2208.10911v1 |
2024-05-13 | Reducing the oxygen contamination in conductive (Ti,Zr)N coatings via RF-bias assisted reactive sputtering | Ternary transition metal nitride coatings are promising for many applications
as they can offer improved hardness and oxidation resistance compared to binary
counterparts. A common challenge in the deposition of functional nitride thin
films is oxygen contamination. Even low amounts of oxygen contamination can
adversely affect the functional properties of the thin films. Here, we present
a practical approach for the growth of virtually oxygen-free (Ti, Zr)N thin
films. To cover the complete compositional range of (Ti,Zr)N coatings we employ
combinatorial reactive co-sputtering. The depositions are carried out with or
without applying a low-power radio-frequency (RF) bias voltage to the substrate
holder to study the possibility of decelerating energetic oxygen ions and
effectively reducing oxygen contamination in the growing film. High-throughput
structural analysis and functional property mapping are used to elucidate the
synthesis-property relationships. The structural analysis indicates solid
solution formation over the entire compositional range, as evidenced by
Vegardian lattice scaling, regardless of the applied RF substrate bias.
Irrespective of the composition of the films, the application of RF substrate
bias leads to a dramatic reduction of oxygen contamination, as demonstrated by
X-ray photoelectron spectroscopy (XPS) depth-profile mapping. This is reflected
in a significant improvement in the films' conductivity and hardness. We
demonstrate that the reduction in oxygen contamination is intrinsic to the
process and not due to changes in the microstructure. The approach presented
here is applicable to both conductive and insulating substrates and provides a
practical route to synthesize nitride thin films with improved purity that can
be applied in standard sputter chambers and on many different material systems. | 2405.07789v1 |
2018-11-03 | Pressure-induced Insulator to Metal Transition of Mixed Valence Compound Ce(O,F)SbS$_{2}$ | Transport properties of Ce$_{0.85}$F0.15SbS$_{2}$ and undoped CeOSbS$_{2}$
under high pressure were investigated experimentally and theoretically.
Electrical resistivity measurements of the Ce$_{0.85}$F0.15SbS$_{2}$ single
crystals were performed under various high pressures using a diamond anvil cell
with boron-doped diamond electrodes. The samples showed the insulator to metal
transition by applying high pressure up to 30-40 GPa. On the other hand, the
undoped CeOSbS$_{2}$ showed almost same transport property with the F-doped
sample under high pressure. The valence state analysis using X-ray
photoelectron spectroscopy revealed a simple valence state of Ce3+ in
Ce$_{0.85}$F0.15SbS$_{2}$ and mixed valence state between Ce3+ and Ce4+ in
undoped CeOSbS$_{2}$. The valence fluctuation in Ce carried out the comparable
transport nature in the both samples. A band calculation suggests that the
undoped CeOSbS$_{2}$ could be metallic under high pressure of 30 GPa in
accordance with the experimental results. A superior thermoelectric property of
power factor in CeOSbS$_{2}$ was estimated under high pressure around 20 GPa in
comparison with that of ambient pressure. | 1811.01151v1 |
2020-10-22 | A photogrammetric method for target monitoring inside the MEG II detector | An automatic target monitoring method based on photographs taken by a CMOS
photo-camera has been developed for the MEG II detector. The technique could be
adapted for other fixed-target experiments requiring good knowledge of their
target position to avoid biases and systematic errors in measuring the
trajectories of the outcoming particles. A CMOS-based, high resolution, high
radiation tolerant and high magnetic field resistant photo-camera was mounted
inside the MEG II detector at the Paul Scherrer Institute (Switzerland). MEG II
is used to search for lepton flavour violation in muon decays. The
photogrammetric method's challenges, affecting measurements of low momentum
particles' tracks, are high magnetic field of the spectrometer, high radiation
levels, tight space constraints, and the need to limit the material budget in
the tracking volume. The camera is focused on dot pattern drawn on the thin MEG
II target, about 1 m away from the detector endcaps where the photo-camera is
placed. Target movements and deformations are monitored by comparing images of
the dots taken at various times during the measurement. The images are acquired
with a Raspberry board and analyzed using a custom software. Global alignment
to the spectrometer is guaranteed by corner cubes placed on the target support.
As a result, the target monitoring fulfils the needs of the experiment. | 2010.11576v2 |
2021-02-04 | Disorder-robust high-field superconducting phase of FeSe single crystals | When exposed to high magnetic fields, certain materials manifest an exotic
superconducting (SC) phase that has attracted considerable attention. A
proposed explanation for the origin of the high-field SC phase is the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. This state is characterized by
inhomogeneous superconductivity, where the Cooper pairs have finite
center-of-mass momenta. Recently, the high-field SC phase was observed in FeSe,
and it was deemed to originate from the FFLO state. Here, we synthesize FeSe
single crystals with different levels of disorder. The level of disorder is
expressed by the ratio of the mean free path to the coherence length and ranges
between 35 and 1.2. The upper critical field \textit{B}$_{\rm{c}2}$ was
obtained by both resistivity and magnetic torque measurements over a wide range
of temperatures, which went as low as $\sim$0.5 K, and magnetic fields, which
went up to $\sim$38 T along the \textit{c} axis and in the \textit{ab} plane.
In the high-field region parallel to the \textit{ab} plane, an unusual SC phase
was confirmed in all the crystals, and the phase was found to be robust against
disorder. This result suggests that the high-field SC phase in FeSe is not a
conventional FFLO state. | 2102.02353v2 |
2021-03-03 | On the early stages of precipitation during direct ageing of Alloy 718 | The Ni-based superalloy Alloy 718 is used in aircraft engines as
high-pressure turbine discs and must endure challenging demands on
high-temperature yield strength, creep-, and oxidation-resistance. Nanoscale
$\gamma^{\prime}$- and $\gamma^{\prime \prime}$-precipitates commonly found in
duplet and triplet co-precipitate morphologies provide high-temperature
strength under these harsh operating conditions. Direct ageing of Alloy 718 is
an attractive alternative manufacturing route known to increase the yield
strength at 650 $^{\deg}$C by at least +10 $\%$, by both retaining high
dislocation densities and changing the nanoscale co-precipitate morphology.
However, the detailed nucleation and growth mechanisms of the duplet and
triplet co-precipitate morphologies of $\gamma^{\prime}$ and $\gamma^{\prime
\prime}$ during the direct ageing process remain unknown. We provide a
correlative high-resolution microscopy approach using transmission electron
microscopy, high-angle annular dark-field imaging, and atom probe microscopy to
reveal the early stages of precipitation during direct ageing of Alloy 718.
Quantitative stereological analyses of the $\gamma^{\prime}$- and
$\gamma^{\prime \prime}$-precipitate dispersions as well as their chemical
compositions have allowed us to propose a qualitative model of the
microstructural evolution. It is shown that fine $\gamma^{\prime}$- and
$\gamma^{\prime \prime}$-precipitates nucleate homogeneously and grow
coherently. However, $\gamma^{\prime \prime}$-precipitates also nucleate
heterogeneously on dislocations and experience accelerated growth due to Nb
pipe diffusion. Moreover, the co-precipitation reactions are largely influenced
by solute availability and the potential for enrichment of Nb and rejection of
Al+Ti. | 2103.02763v1 |
2022-06-25 | High-Mobility Tri-Gate $β$-Ga$_2$O$_3$ MESFETs with a Power Figure of Merit over 0.9 GW/cm$^2$ | In this letter, fin-shape tri-gate $\beta$-Ga$_{2}$O$_{3}$ lateral MESFETs
are demonstrated with a high power figure of merit of 0.95 GW/cm$^{2}$ - a
record high for any $\beta$-Ga$_{2}$O$_{3}$ transistor to date. A
low-temperature undoped buffer-channel stack design is developed which
demonstrates record high Hall and drift electron mobilities in doped
$\beta$-Ga$_{2}$O$_{3}$ channels allowing for low ON resistances R$_{ON}$ in
$\beta$-Ga$_{2}$O$_{3}$ MESFETs. Fin-widths (W$_{fin}$) were 1.2-1.5 $\mu$m and
there were 25 fins (N$_{fin}$) per device with a trench depth of $\sim$1$\mu$m.
A $\beta$-Ga$_2$O$_3$ MESFET with a source-drain length of 6.4 $\mu$m exhibits
a high ON current (187 mA/mm), low R$_{ON}$ (20.5 $\Omega$.mm) and a high
average breakdown field (4.2 MV/cm). All devices show very low reverse leakage
until catastrophic breakdown for breakdown voltages scaled from 1.1kV to
$\sim$3kV. This work demonstrates the potential of channel engineering in
improving $\beta$-Ga$_{2}$O$_{3}$ device performance toward lower conduction
losses for low-to-medium voltage applications. | 2206.12539v2 |
2023-10-22 | Pressure-induced volumetric negative thermal expansion in CoZr2 superconductor | We investigate the thermal expansion and superconducting properties of a
CuAl2-type (tetragonal) superconductor CoZr2 under high pressures. We perform
high-pressure synchrotron X-ray diffraction in a pressure range of 2.9 GPa < P
< 10.4 GPa and discover that CoZr2 exhibits volumetric negative thermal
expansion under high pressures. Although the uniaxial positive thermal
expansion (PTE) along the a-axis is observed under ambient pressure, that is
suppressed by pressure, while the large uniaxial negative thermal expansion
(NTE) along the c-axis is maintained under the pressure regime. As a result of
a combination of the suppressed uniaxial PTE along the a-axis and uniaxial NTE
along the c-axis, volumetric negative thermal expansion is achieved under high
pressure in CoZr2. The mechanisms of volumetric NTE would be based on the
flexible crystal structure caused by the soft Co-Co bond as seen in the
iso-structural compound FeZr2, which exhibits uniaxial NTE along the c-axis. We
also perform high-pressure electrical resistance measurements of CoZr2 to
confirm the presence of superconductivity under the examined pressure regime in
the range of 0.03 GPa < P < 41.9 GPa. We confirm the presence of
superconductivity under all pressures and observe dome-like shape pressure
dependence of superconducting transition temperature. Because of the
coexistence of two phenomena, which are volumetric NTE and superconductivity,
in CoZr2 under high pressure, the coexistence would be achievable under ambient
pressure by tuning chemical compositions after our present observation. | 2310.14254v2 |
2023-10-22 | Superconductivity in the high-entropy ceramics Ti0.2Zr0.2Nb0.2Mo0.2Ta0.2Cx with possible nontrivial band topology | Topological superconductors have drawn significant interest from the
scientific community due to the accompanying Majorana fermions. Here, we report
the discovery of electronic structure and superconductivity in high-entropy
ceramics Ti0.2Zr0.2Nb0.2Mo0.2Ta0.2Cx (x = 1 and 0.8) combined with experiments
and first-principles calculations. The Ti0.2Zr0.2Nb0.2Mo0.2Ta0.2Cx high-entropy
ceramics show bulk type-II superconductivity with Tc about 4.00 K (x = 1) and
2.65 K (x = 0.8), respectively. The specific heat jump is equal to 1.45 (x = 1)
and 1.52 (x = 0.8), close to the expected value of 1.43 for the BCS
superconductor in the weak coupling limit. The high-pressure resistance
measurements show that a robust superconductivity against high physical
pressure in Ti0.2Zr0.2Nb0.2Mo0.2Ta0.2C, with a slight Tc variation of 0.3 K
within 82.5 GPa. Furthermore, the first-principles calculations indicate that
the Dirac-like point exists in the electronic band structures of
Ti0.2Zr0.2Nb0.2Mo0.2Ta0.2C, which is potentially a topological superconductor.
The Dirac-like point is mainly contributed by the d orbitals of transition
metals M and the p orbitals of C. The high-entropy ceramics provide an
excellent platform for the fabrication of novel quantum devices, and our study
may spark significant future physics investigations in this intriguing
material. | 2310.14271v1 |
2024-03-25 | In situ growth of hydrophilic nickel-cobalt layered double hydroxides nanosheets on biomass waste-derived porous carbon for high-performance hybrid supercapacitors | Rational design and cost-effective fabrication of layered double hydroxides
(LDHs) nanosheets with extraordinary electrochemical performance is a key
challenge for hybrid supercapacitors (HSCs). Herein, we report a facile in situ
growth methodology to eco-friendly synthesize hydrophilic NiCo-LDHs nanosheets
on biomass waste-derived porous carbon (BC) for robust high-performance HSC
cathode. The in situ growth process under ultrasonication realizes the rational
arrangement of NiCo-LDHs nanosheets on the surface of BC, which effectively
increases the specific surface area, promotes the electronic conductivity and
enhances the wettability of NiCo-LDHs nanosheets without affecting their
thickness values. With the beneficial effects of ultrathin thickness of LDHs
nanosheets (6.20 nm), large specific surface area (2324.1 m2 g-1), low charge
transfer resistance (1.65 ohm), and high wettability with electrolyte (34-35
degree), the obtained Ni2Co1-LDHs/BC50 electrode possesses an ultra-high
specific capacitance of 2390 F g-1 (956 C g-1) at 1 A g-1, which is superior to
most reported values. Furthermore, an assembled Ni2Co1-LDHs/BC50//YP-80F HSC
delivers a maximum specific energy of 52.47 Wh kg-1 at 375 W kg-1, and
maintains a high capacitance retention of 75.9% even after 4000 cycles. This
work provides a facile approach to fabricate LDHs nanosheets based cathode
materials for high-performance HSCs. | 2403.16506v1 |
1997-01-08 | Coupling between phonons and intrinsic Josephson oscillations in cuprate superconductors | The recently reported subgap structures observed in the current-voltage
characteristic of intrinsic Josephson junctions in the high-T_c superconductors
Tl_2Ba_2Ca_2Cu_3O_{10+\delta} and Bi_2Sr_2CaCu_2O_{8+\delta} are explained by
the coupling between c-axis phonons and Josephson oscillations. A model is
developed where c-axis lattice vibrations between adjacent superconducting
multilayers are excited by the Josephson oscillations in a resistive junction.
The voltages of the lowest structures correspond well to the frequencies of
longitudinal c-axis phonons with large oscillator strength in the two
materials, providing a new measurement technique for this quantity. | 9701049v2 |
1997-07-17 | Anomalous magnetotransport in wide quantum wells | We present magneto transport experiments of quasi 3D PbTe wide quantum wells.
A plateau-like structure in the Hall resistance is observed, which corresponds
to the Shubnikov de Haas oscillations in the same manner as known from the
quantum Hall effect. The onsets of plateaux in Rxy do not correspond to 2D
filling factors but coincide with the occupation of 3D (bulk-) Landau levels.
At the same time a non-local signal is observed which corresponds to the
structure in Rxx and Rxy and fulfils exactly the Onsager-Casimir relation
(Rij,kl(B) = Rkl,ij(-B)). We explain the behaviour in terms of edge channel
transport which is controlled by a permanent backscattering across a system of
"percolative EC - loops" in the bulk region. Long range potential fluctuations
with an amplitude of the order of the subband splitting are explained to play
an essential role in this electron system. | 9707173v1 |
1997-07-18 | Edge Channel Dominated Magnetotransport in PbTe Wide Parabolic Quantum Wells | In PbTe wide parabolic quantum wells (WPQW) a plateau-like structure is
observed in the Hall resistance, which corresponds to the Shubnikov-de Haas
oscillations in the same manner as known from the quantum Hall effect. At the
same time a non-local signal is observed which corresponds to the structure in
Rxx and Rxy. We find a striking correspondence between a standard quantum Hall
system and this quasi 3D WPQW system. | 9707188v1 |
1997-10-28 | Investigation of acceptor levels and hole scattering mechanisms in p-gallium selenide by means of transport measurements under pressure | The effect of pressure on acceptor levels and hole scattering mechanisms in
p-GaSe is investigated through Hall effect and resistivity measurements under
quasi-hydrostatic conditions up to 4 GPa. The pressure dependence of the hole
concentration is interpreted through a carrier statistics equation with a
single (nitrogen) or double (tin) acceptor whose ionization energies decrease
under pressure due to the dielectric constant increase. The pressure effect on
the hole mobility is also accounted for by considering the pressure
dependencies of both the phonon frequencies and the hole-phonon coupling
constants involved in the scattering rates. | 9710296v1 |
1998-12-24 | Electron Correlations in Molecular Systems | A short review of correlated electrons in molecular systems has been
performed. Main attention has been focussed on ET salts, which are the d=2
systems. They show the Mott transition in high temperatures and the transition
from the antiferromagnetic to the superconducting phase in low temperatures,
under a (chemical) pressure. Physical properties (the electrical resistivity,
the specific heat, the magnetic susceptibility, the photoemission spectra, the
optical conductivity) of ET salts have been compared with those ones in other
strongly correlated systems. The optical conductivity is described in the
framework of the Hubbard model, with a low frequency peak as an evidence for
the Abrikosov-Suhl resonance. | 9812392v1 |
1998-12-30 | Giant transverse magnetoresistance in an asymmetric system of three GaAs/AlGaAs quantum wells in a strong magnetic field at room temperature | The giant transverse magnetoresistance is observed in the case of
photoinduced nonequilibrium carriers in an asymmetric undoped system of three
GaAs/AlGaAs quantum wells at room temperature. In a magnetic field of 75 kOe,
the resistance of nanostructure being studied increases by a factor of 1.85.
The magnetoresistance depends quadratically on the magnetic field in low fields
and tends to saturation in high fields. This phenomenon is attributed to the
rearrangement of the electron wave function in magnetic field. Using the fact
that the incoherent part of the scattering probability for electron scattering
on impurities and bulk defects is proportional to the integral of the forth
power of the envelope wave function, the calculated field dependence of the
magnetoresistance is shown to be similar to that observed experimentally. | 9812426v1 |
1999-03-03 | Non linear flux flow in TiN superconducting thin film | We have studied the superconducting behavior of 100 nm Titanium Nitride (TiN)
thin film in a perpendicular magnetic field. We found a zero field transition
temperature of 4.6 K and a slope in the H-T plane of -0.745 T/K. At 4.2 K, we
have performed careful transport measurements by measuring both the
differential resistivity and voltage as a function of a DC current. Our results
are analyzed in the framework of linear and non linear flux flow behavior. In
particular, we have observed an electronic instability at high vortex
velocities and from its dependence with respect to the applied magnetic field,
we can exctract the inelastic scattering time and diffusion length of the
quasiparticles. | 9903060v2 |
1999-08-26 | Destruction of the Mott Insulating Ground State of Ca_2RuO_4 by a Structural Transition | We report a first-order phase transition at T_M=357 K in single crystal
Ca_2RuO_4, an isomorph to the superconductor Sr_2RuO_4. The discontinuous
decrease in electrical resistivity signals the near destruction of the Mott
insulating phase and is triggered by a structural transition from the low
temperature orthorhombic to a high temperature tetragonal phase. The magnetic
susceptibility, which is temperature dependent but not Curie-like decreases
abruptly at TM and becomes less temperature dependent. Unlike most insulator to
metal transitions, the system is not magnetically ordered in either phase,
though the Mott insulator phase is antiferromagnetic below T_N=110 K. | 9908390v1 |
1999-08-29 | Anomalous microwave response of high-temperature superconducting thin-film microstrip resonator in weak dc magnetic fields | We have studied an anomalous microwave (mw) response of superconducting
YBa_{2}Cu_{3}O_{7-delta} (YBCO) microstrip resonators in the presence of a weak
dc magnetic field, H_{dc}. The surface resistance (R_{s}) and reactance (X_{s})
show a correlated non-monotonic behaviour as a function of H_{dc}. R_{s} and
X_{s} were found to initially decrease with elevated H_{dc} and then increase
after H_{dc} reaches a crossover field, H_{c}, which is independent of the
amplitude and frequency of the input mw signal within the measurements. The
frequency dependence of R_{s} is almost linear at fixed H_{dc} with different
magnitudes (<H_{c}, =H_{c} and >H_{c}). The impedance plane analysis
demonstrates that r_{H}, which is defined as the ratio of the change in
R_{s}(H_{dc}) and that in X_{s}(H_{dc}), is about 0.6 at H_{dc}<H_{c} and 0.1
at H_{dc}>H_{c}. The H_{dc} dependence of the surface impedance is
qualitatively independent of the orientation of H_{dc}. | 9908429v1 |
2000-05-11 | Magnetic Field resulting from non-linear electrical transport in single crystals of charge-ordered Pr$_{0.63}$ Ca$_{0.37}$ MnO$_{3}$} | In this letter we report that the current induced destabilization of the
charge ordered (CO) state in a rare-earth manganite gives rise to regions with
ferromagnetic correlation. We did this experiment by measurement of the I-V
curves in single crystal of the CO system
Pr$_{0.63}$Ca$_{0.37}$MnO$_{3}$ and simultanously measuring the magnetization
of the current carrying conductor using a high T$_c$ SQUID working at T = 77K.
We have found that the current induced destabilization of the CO state leads to
a regime of negative differential resistance which leads to a small enhancement
of the magnetization of the sample, indicating ferromagnetically aligned
moments. | 0005194v1 |
2000-06-22 | High field magnetotransport in composite conductors: the effective medium approximation revisited | The self consistent effective medium approximation (SEMA) is used to study
three-dimensional random conducting composites under the influence of a strong
magnetic field {\bf B}, in the case where all constituents exhibit isotropic
response. Asymptotic analysis is used to obtain almost closed form results for
the strong field magnetoresistance and Hall resistance in various types of two-
and three-constituent isotropic mixtures for the entire range of compositions.
Numerical solutions of the SEMA equations are also obtained, in some cases, and
compared with those results. In two-constituent
free-electron-metal/perfect-insulator mixtures, the magnetoresistance is
asymptotically proportional to $|{\bf B}|$ at {\em all concentrations above the
percolation threshold}. In three-constituent metal/insulator/superconductor
mixtures a line of critical points is found, where the strong field
magnetoresistance switches abruptly from saturating to non-saturating
dependence on $|{\bf B}|$, at a certain value of the
insulator-to-superconductor concentration ratio. This transition appears to be
related to the phenomenon of anisotropic percolation. | 0006351v1 |
2000-08-02 | The Zeno effect and an inter-layer pairing mechanism for high-temperature superconductivity in layered materials | Quantum Zeno Effect (QZE) is the suppression of the inter-subspace transition
by a relatively fast intra-subspace decoherence. Earlier, we had proposed a
QZE-based mechanism for the temperature-dependent normal-state c-axis
resistivity of the layered high-T$_c$ cuprate superconductors in which the
single-particle inter-layer tunneling is blocked by the strong intra-layer
decoherence (entanglement). We now argue that while the single-particle
inter-layer tunneling is thus blocked, the tunneling of the bosonic BCS-like
pairs must remain unblocked inasmuch as a BCS pairing condensate is an
eigenstate of the pair annihilation operator. This pair tunneling stabilizes | 0008023v1 |
2000-12-14 | Electronic Transmission Through Metallic Nanowires: Generalized Scattering Matrix Approach | An easy to implement and powerful method for the solution of 3D scattering
problems that can be well described by Helmholtz equation is presented. The
matrix algebra used provides excellent stability versus the number of junctions
as well as great computational speed. The matrix truncation method yields an
easy single-parameter convergence procedure. Subsequently, some aspects of the
electronic transport through metal nanowires are studied by the use of
Landauer's scattering approach to the conductance. We predict the existence of
current vortex-rings patterns due to sharp enough narrow-wide connections in
atomic size point contacts. Longitudinal resonances between scattering centers
provide a simple physical picture for the understanding of negative
differential resistance in ideal monoatomic contacts. Relatively long nanowires
with high geometrical perfection -like those recently observed by Transmission
Electron Microscopy- are modelled exhibiting resonant tunnelling and total
reflection at given incident energy intervals. | 0012250v1 |
2001-03-16 | Giant Magnetoresistance by Exchange Springs in DyFe$_2$/YFe$_2$ Superlattices | Magnetization and magnetoresistance measurements are reported for
antiferromagnetically coupled DyFe$_2$/YFe$_2$ multilayers in fields up to 23
T. We demonstrate that the formation of short exchange springs (~ 2 nm) in the
magnetically soft YFe$_2$ layers results in a giant magneto-resistance as high
as 32% in the spring region. It is shown that both the magnitude of the effect,
and its dependence on magnetic field, are in good agreement with the theory of
Levy and Zhang for giant magnetoresistance due to domain wall like structures. | 0103354v1 |
2001-04-05 | Fractal Nature and Scaling Exponent of Non-Drude Currents in Non-Fermi Liquids | In many oxides of the perovskite and pseudoperovskite families there are
phase transitions between insulating and normal metallic (Fermi liquid) phases
that are separated by an intermediate phase that is often called a non-Fermi
liquid (NFL). The dc resistivity of the intermediate or NFL phase often
exhibits a T temperature dependence, in contrast to the T2 dependence expected
from a bad normal metal. The same alloys exhibit a non-Drude (ND) w2alpha
frequency dependence, with alpha ~ 0.5, in contrast to the Drude dependence w2
characteristic of samples with the T2 behavior. Various attempts have been made
to modify the algebra of continuum Fermi liquid theory (FLT) to derive the ND
exponent alpha, but these have been based on artifices designed to explain only
this one parameter. The discrete filamentary model has been used to calculate
many properties of high temperature superconductors, and to explain the
asymmetric nature of the intermediate phase. Here it is used to derive a by the
same rules previously used for several other discrete relaxation calculations
that are in excellent agreement with other quite different experiments. The
results are: (cubic) perovskites, alpha = 0.45, and planar conductivity of
bilayered pseudoperovskites, alpha = 0.70. The corresponding experimental
values are (0.4, 0.5) and 0.7. | 0104095v1 |
2001-04-06 | Evidence for high inter-granular current flow in single-phase polycrystalline MgB2 superconductor | The distribution of magnetic field in single-phase polycrystalline bulk MgB2
has been measured using a Magneto-Optical (MO) technique for an external
magnetic field applied perpendicular to the sample surface. The MO studies
indicate that an inter-granular current network is readily established in this
material and the current is not limited by weak-linked grain boundaries. The
grain boundaries are observed to resist preferential magnetic field
penetration, with the inter-grain mechanism dominating the current flow in the
sample at temperatures up to 30K. The results provide clear evidence that the
intra-granular current flow is isotropic. A critical current density of ~10^4
Acm-2 was estimated at 30K in a field of 150mT from the MO measurements. These
results provide further evidence of the considerable potential for MgB2 for
engineering applications. | 0104114v1 |
2002-04-17 | Epitaxial Growth of La$_{1/3}$Sr$_{2/3}$FeO$_3$ thin films by laser ablation | We report on the synthesis of high quality La$_{1/3}$Sr$_{2/3}$FeO$_3$ (LSFO)
thin films using the pulsed laser deposition technique on both SrTiO$_3$ (STO)
and LaAlO$_3$ (LAO) substrates (100)-oriented. From X-Ray diffraction (XRD)
studies, we find that the films have an out-of-plane lattice parameter around
0.3865nm, almost independent of the substrate (i.e. the nature of the strains).
The transport properties reveal that, while LSFO films deposited on STO exhibit
an anomaly in the resistivity vs temperature at 180K (corresponding to the
charge-ordered transition and associated with a transition from a paramagnetic
to an antiferromagnetic state), the films grown on LAO display a very small
magnetoresistance behavior and present an hysteresis around 270K under the
application of a 4T magnetic field. The changes in transport properties between
both substrates are discussed and compared with the corresponding single
crystals. | 0204370v1 |
2002-05-24 | The Hall effect and hole densities in high Tc GaMnAs thin films | By studying the Hall effect in a series of low resistivity Ga1-xMnxAs
samples, accurate values for the hole density p, Mn concentration x, and Curie
temperature Tc are obtained over the range 0.015=<x=<0.08. The hole density
corresponds to 90% of the Mn concentration at low x, and has a maximum value of
1.0x10-27 m-3 when Tc=125K for x=0.06. This data allows the first meaningful
comparison of mean field predicted Curie temperatures with experiment over a
wide range of x. The theory is in qualitative agreement with experiment, but
overestimates Tc at large x and underestimates TC at low x. | 0205517v3 |
2002-06-19 | Transport and magnetic properties of LT annealed Ga1-xMnxAs | We present the results of low temperature (LT) annealing studies of
Ga1-xMnxAs epilayers grown by low temperature molecular beam epitaxy in a wide
range of Mn concentrations (0.01<x<0.084). Transport measurements in low and
high magnetic fields as well as SQUID measurements were performed on a wide
range of samples, serving to establish optimal conditions of annealing. Optimal
annealing procedure succeeded in the Curie temperatures higher than 110K. The
highest value of Curie temperature estimated from the maximum in the
temperature dependence of zero-field resistivity (Tr) was 127K. It is generally
observed that annealing leads to large changes in the magnetic and transport
properties of GaMnAs in the very narrow range of annealing temperature close to
the growth temperature. | 0206371v1 |
2002-07-08 | Magnetic behavior of single crystalline Ho$_2$PdSi$_3$ | The magnetic behavior of single-crystal Ho$_2$PdSi$_3$, crystallizing in an
AlB$_2$-derived hexagonal structure, is investigated by magnetic susceptibility
($\chi$) and electrical resistivity ($\rho$) measurements along two directions.
There is no dramatic anisotropy in the high temperature Curie-Weiss parameter
or in the $\rho$ and isothermal magnetization data, though there is a
noticeable anisotropy in the magnitude of $\rho$ between two perpendicular
orientations. The degree of anisotropy is overall less prominent than in the Gd
(which is an S-state ion!) and Tb analogues. A point of emphasis is that this
compound undergoes long range magnetic ordering below 8 K as in the case of
analogous Gd and Dy compounds. Considering this fact for these compounds with
well-localised f-orbital, the spin glass freezing noted for isomorphous U
compounds in the recent literature could be attributed to the role of the
f-ligand hybridization, rather than just Pd-Si disorder. | 0207199v1 |
2002-10-22 | Unipolar transport and shot noise in metal-semiconductor-metal structures | We carry out a self-consistent analytical theory of unipolar current and
noise properties of metal-semiconductor-metal structures made of highly
resistive semiconductors in the presence of an applied bias of arbitrary
strength. By including the effects of the diffusion current we succeed to study
the whole range of carrier injection conditions going from low level injection,
where the structure behaves as a linear resistor, to high level injection,
where the structure behaves as a space charge limited diode. We show that these
structures display shot noise at the highest voltages. Remarkably the crossover
from Nyquist noise to shot noise exhibits a complicate behavior with increasing
current where an initial square root dependence (double thermal noise) is
followed by a cubic power law. | 0210484v1 |
2002-11-14 | Synthesis and Properties of YbB2 | We report temperature and field dependent measurements of the magnetic
susceptibility, specific heat and resistivity of sintered YbB2 pellets,
prepared via two distinct reaction routes, utilizing different temperatures,
pressures and sintering times. Sample behavior is affected by the preparation
procedure, as a consequence of different secondary phases, most of which were
identified via x-ray diffraction. These experiments show that YbB2 is a metal
with the Yb atoms in or very close to their 3+ state. YbB2 appears to order
anti-ferromagnetically at TN ~ 5.6 K, which can be considered a relatively high
ordering temperature for an ytterbium-based intermetallic compound. | 0211288v1 |
2003-07-07 | Mn Interstitial Diffusion in (Ga,Mn)As | We present a combined theoretical and experimental study of the ferromagnetic
semiconductor (Ga,Mn)As which explains the remarkably large changes observed on
low temperature annealing. Careful control of the annealing conditions allows
us to obtain samples with ferromagnetic transition temperatures up to 159 K. Ab
initio calculations, and resistivity measurements during annealing, show that
the observed changes are due to out-diffusion of Mn interstitials towards the
surface, governed by an energy barrier of about 0.7-0.8 eV. The Mn interstitial
is a double donor resulting in compensation of charge carriers and suppression
of ferromagnetism. Electric fields induced by high concentrations of
substitutional Mn acceptors have a significant effect on the diffusion. | 0307140v1 |
2003-07-09 | Direct Observation of Long-Term Durability of Superconductivity in YBa$_2$Cu$_3$O$_7$-Ag$_2$O Composites | We report direct observation of long-term durability of superconductivity of
several YBa$_2$Cu$_3$O$_7$-Ag$_2$O composites that were first prepared and
studied almost 14 years ago [J. J. Lin {\it et al}., Jpn. J. Appl. Phys. {\bf
29}, 497 (1990)]. Remeasurements performed recently on both resistances and
magnetizations indicate a sharp critical transition temperature at 91 K. We
also find that such long-term environmental stability of high-temperature
superconductivity can only be achieved in YBa$_2$Cu$_3$O$_7$ with Ag$_2$O
addition, but not with pure Ag addition. | 0307187v1 |
2003-07-18 | Probing quasiparticle dynamics in Bi2Sr2CaCu2O(8+delta) with a driven Josephson vortex lattice | We show that the flux-flow transport of the Josephson vortex lattice (JVL) in
layered high-temperature superconductors provides a convenient probe for both
components of quasiparticle conductivity, $\sigma_{c}$ and $\sigma_{ab}$. We
found that the JVL flux-flow resistivity, $\rho_{ff}$, in a wide range of
magnetic fields is mainly determined by the in-plane dissipation. In the dense
lattice regime ($B>1$ T) $\rho_{ff}(B)$ dependence is well fitted by the
theoretical formula for that limit. That allows us to independently extract
from the experimental data the values of $\sigma_{c}$ and of the ratio $\sigma
_{ab}/(\sigma_{c}\gamma ^{4})$. The extracted temperature dependence $\sigma
_{ab}(T)$ is consistent with microwave data. The shape of the current-voltage
characteristics is also sensitive to the frequency dependence of $\sigma_{ab}$
and that allows us to estimate the quasiparticle relaxation time and relate it
to the impurity bandwidth using data obtained for the same crystal. | 0307450v1 |
2003-09-02 | The enhancement of phase separation aspect in electron doped manganite Ca0.8Sm0.16Nd0.04MnO3 | The complex lanthanide doping of electron manganites results in enhancement
of various phase separation effects in physical properties of these compounds.
Selecting Ca0.8Sm0.16Nd0.04MnO3 as a model case we show that the first order
structural phase transition from paramagnetic semi-metallic phase into
anti-ferromagnetic semi-metallic phase at TS ~ 158 +- 4 K is marked by an
abrupt decrease in magnetization, a step like anomaly DL/L = 10-4 in thermal
expansion and large latent heat DQ = 610 J/mol. In a certain temperature range
below TS, the high field magnetization exhibits hysteretic metamagnetic
behavior due to field-induced first order transformation. ac-susceptibility,
magnetization and resistivity data suggest rather a non-uniform state in
Ca0.8Sm0.16Nd0.04MnO3 at low temperatures. The metal - insulator transition
occurs at TMI ~112 +- 3 K, accompanied by a step-like increase in
magnetization. These features could be ascribed to "sponging" of electrons from
neighboring anti-ferromagnetic matrix by clusters undergoing the ferromagnetic
ordering. | 0309065v1 |
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