publicationDate stringlengths 10 10 | title stringlengths 17 233 | abstract stringlengths 20 3.22k | id stringlengths 9 12 |
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2023-12-07 | Energy dissipation in astrophysical simulations: results of the Orszag-Tang test problem | The magnetic field through the magnetic reconnection process affects the
dynamics and structure of astrophysical systems. Numerical simulations are the
tools to study the evolution of these systems. However, the resolution,
dimensions, resistivity, and turbulence of the system are some important
parameters to take into account in the simulations. In this paper, we
investigate the evolution of magnetic energy in astrophysical simulations by
performing a standard test problem for MHD codes, Orszag-Tang. We estimate the
numerical dissipation in the simulations using state-of-the-art numerical
simulation code in astrophysics, PLUTO. The estimated numerical resistivity in
2D simulations corresponds to the Lundquist number $\approx 10^{4}$ in the
resolution of $512\times512$ grid cells. It is also shown that the plasmoid
unstable reconnection layer can be resolved with sufficient resolutions. Our
analysis demonstrates that in non-relativistic magnetohydrodynamics
simulations, magnetic and kinetic energies undergo conversion into internal
energy, resulting in plasma heating. | 2312.06675v1 |
2024-01-10 | Some aspects of resistive-to-normal state transition by direct and microwave currents in superconducting thin films with phase slip lines | Based on analysis of current-voltage characteristics and imaging of the
resistive state of thin-film tin strips using the low-temperature laser
scanning microscopy (LTLSM), the process of destruction of superconductivity by
current and microwave irradiation with the formation and spatial rearrangement
of the order parameter phase slip lines, and their transformation into discrete
localized normal domains is shown. The prospects of LTLSM are considered form
the point of view of study of the high-frequency properties of superconducting
structures and spatial characteristics in the pre-critical state for
instrumental applications. | 2401.05333v1 |
2024-02-24 | A simple model of globally magnetized accretion discs | We present an analytic, quasi-local dynamo model for accretion discs threaded
by net, vertical magnetic flux. In a simple slab geometry and ignoring
stochastic mean-field dynamo effects, we calculate the large-scale field
resulting from the balance between kinematic field amplification and turbulent
resistive diffusion. The ability of the disc to accumulate magnetic flux is
sensitive to a single parameter dependent on the ratio of the vertical
resistive diffusion time to the Alfv\'en crossing time, and we show how the
saturation levels of magnetorotational and other instabilities can govern disc
structure and evolution. Under wide-ranging conditions, inflow is governed by
large-scale magnetic stresses rather than internal viscous stress. We present
models of such "magnetically boosted" discs and show that they lack a radiation
pressure-dominated zone. Our model can account for "magnetically elevated"
discs as well as instances of midplane outflow and field reversals with height
that have been seen in some global simulations. Using the time-dependent
features of our model, we find that the incorporation of dynamo effects into
disc structure can lead to steady or episodic "magnetically arrested discs"
(MADs) that maximize the concentration of magnetic flux in their central
regions. | 2402.15657v1 |
2024-02-28 | Stability studies of sealed Resistive Plate Chambers | The phase-out of hydro-fluorocarbons, owing to their high Global Warming
Power, affecting the main gas used in Resistive Plate Chambers (RPCs),
tetrafluoroethane C$_2$H$_2$F$_4$, has increased operational difficulties on
existing systems and imposes strong restrictions on its use in new systems.
This has motivated a new line of R\&D on sealed RPCs: RPCs that do not
require a continuous gas flow for their operation and dispense the use of very
complex and expensive re-circulation and/or recycling gas systems. At the
moment it is not clear whether this solution can cover all fields of
application normally allocated to RPCs, but it seems that it could be
considered as a valid option for low particle flux triggering/tracking of
particles, e.g. in cosmic ray or rare event experiments.
In this work, we demonstrate the feasibility of a small telescope for
atmospheric muon tracking consisting of four $300$~x~$300$~mm$^2$ sealed RPCs
with gas gap widths of $1$~mm, $1.5$~mm and $2$~mm. The results suggest that it
is possible to operate this type of detectors for extended periods of time
(more than five months) with its main characteristics, efficiency, average
charge and streamer probability, without apparent degradation and similar to a
RPC operated in continuous gas flow. | 2402.18663v1 |
2024-03-06 | Foot Shape-Dependent Resistive Force Model for Bipedal Walkers on Granular Terrains | Legged robots have demonstrated high efficiency and effectiveness in
unstructured and dynamic environments. However, it is still challenging for
legged robots to achieve rapid and efficient locomotion on deformable, yielding
substrates, such as granular terrains. We present an enhanced resistive force
model for bipedal walkers on soft granular terrains by introducing effective
intrusion depth correction. The enhanced force model captures fundamental
kinetic results considering the robot foot shape, walking gait speed variation,
and energy expense. The model is validated by extensive foot intrusion
experiments with a bipedal robot. The results confirm the model accuracy on the
given type of granular terrains. The model can be further integrated with the
motion control of bipedal robotic walkers. | 2403.03460v1 |
2024-03-13 | Lithographically Defined Zerogap Strain Sensors | Metal thin films on soft polymers provide a unique opportunity for
resistance-based strain sensors. A mechanical mismatch between the conductive
film and the flexible substrate causes cracks to open and close, changing the
electrical resistance as a function of strain. However, the very randomness of
the formation, shape, length, orientation, and distance between adjacent cracks
limits the sensing range as well as repeatability. Herein, we present a
breakthrough: the Zerogap Strain Sensor, whereby lithography eliminates
randomness and violent tearing process inherent in conventional crack sensors
and allows for short periodicity between gaps with gentle sidewall contacts,
critical in high strain sensing enabling operation over an unprecedently wide
range. Our sensor achieves a gauge factor of over 15,000 at {\epsilon}ext=18%,
the highest known value. With the uniform gaps of three-to-ten thousand
nanometer widths characterized by periodicity and strain, this approach has far
reaching implications for future strain sensors whose range is limited only by
that of the flexible substrate, with non-violent operations that always remain
below the tensile limit of the metal. | 2403.08288v1 |
2024-03-19 | Revisiting shear stress tensor evolution: Non-resistive magnetohydrodynamics with momentum-dependent relaxation time | This study aims to develop second-order relativistic viscous
magnetohydrodynamics (MHD) derived from kinetic theory within an extended
relaxation time approximation (momentum/energy dependent) for the collision
kernel. The investigation involves a detailed examination of shear stress
tensor evolution equations and associated transport coefficients. The Boltzmann
equation is solved using a Chapman-Enskog-like gradient expansion for a
charge-conserved conformal system, incorporating a momentum-dependent
relaxation time. The derived relativistic non-resistive, viscous second-order
MHD equations for the shear stress tensor reveal significant modifications in
the coupling with dissipative charge current and magnetic field due to the
momentum dependence of the relaxation time. By utilizing a power law
parametrization to quantify the momentum dependence of the relaxation time, the
anisotropic magnetic field-dependent shear coefficients in the Navier-Stokes
limit have been investigated. The resulting viscous coefficients are seen to be
sensitive to the momentum dependence of the relaxation time. | 2403.13160v2 |
2024-03-20 | Experience gained about Resistive Plate Chambers ageing from the ALICE Muon TRigger/IDentifier detector | The ALICE Muon IDentifier is composed of 72 single-gap bakelite Resistive
Plate Chambers, which have been operational since 2009 in maxi-avalanche mode
(discrimination threshold:7 mV without amplification) with a
Tetrafluoroethane/Isobutane/Sulfur Hexafluoride gas mixture, undergoing
counting rates of the order of tens of Hz/cm^2. In this talk, the long-term
performance and stability of the RPC system will be discussed, in terms of
efficiency, dark current and dark rate. An assessment of potential signs of
ageing observed on the detectors will be presented, together with a summary of
the most common hardware problems experienced. | 2403.13618v1 |
2024-03-25 | Improving Diffusion Models's Data-Corruption Resistance using Scheduled Pseudo-Huber Loss | Diffusion models are known to be vulnerable to outliers in training data. In
this paper we study an alternative diffusion loss function, which can preserve
the high quality of generated data like the original squared $L_{2}$ loss while
at the same time being robust to outliers. We propose to use pseudo-Huber loss
function with a time-dependent parameter to allow for the trade-off between
robustness on the most vulnerable early reverse-diffusion steps and fine
details restoration on the final steps. We show that pseudo-Huber loss with the
time-dependent parameter exhibits better performance on corrupted datasets in
both image and audio domains. In addition, the loss function we propose can
potentially help diffusion models to resist dataset corruption while not
requiring data filtering or purification compared to conventional training
algorithms. | 2403.16728v1 |
2024-04-01 | Giant and negative magnetoresistances in conical magnets | We study magnetotransport in conical helimagnet crystals. Spin dependent
magnetoresistance exhibits dramatic properties for high and low electron
concentrations at different temperatures. For spin up electrons we find
negative magnetoresistance despite only considering a single carrier type. For
spin down electrons we observe giant magnetoresistance due to depletion of spin
down electrons with an applied magnetic field. For spin up carriers, the
magnetoresistance is negative, due to the increase in charge carriers with a
magnetic field. In addition, we investigate spin dependent Hall effect. If a
magnetic field reaches some critical value for spin down electrons, giant Hall
resistance occurs, i.e., Hall current vanishes. This effect is explained by the
absence of spin down carriers. For spin up carriers, the Hall constant
dramatically decreases with field, due to the increase in spin up electron
density. Because of the giant spin dependent magnetoresistance and Hall
resistivity, conical helimagnets could be useful in spin switching devices. | 2404.01401v1 |
2024-04-02 | Integer and fractional quantum anomalous Hall effects in pentalayer graphene | We critically analyze the recently reported observation of integer (IQAHE)
and fractional (FQAHE) quantum anomalous Hall effects at zero applied magnetic
field in pentalayer graphene. Our quantitative activation and variable range
hopping transport analysis of the experimental data reveals that the observed
IQAHE and FQAHE at different fillings all have similar excitation gaps of the
order of $5-10$ K. In addition, we also find that the observed FQAHE manifests
a large hidden background contact series resistance >10 k$\Omega$ of unknown
origin whereas this contact resistance is much smaller ~500 $\Omega$ in the
observed IQAHE. Both of these findings are surprising as well as inconsistent
with the well-established phenomenology of the corresponding high-field integer
and fractional quantum Hall effects in 2D semiconductor systems. | 2404.02192v1 |
2024-05-06 | Polynomial lower bound on the effective resistance for the one-dimensional critical long-range percolation | In this work, we study the critical long-range percolation on $\mathbb{Z}$,
where an edge connects $i$ and $j$ independently with probability
$1-\exp\{-\beta |i-j|^{-2}\}$ for some fixed $\beta>0$. Viewing this as a
random electric network where each edge has a unit conductance, we show that
with high probability the effective resistances from the origin 0 to $[-N,
N]^c$ and from the interval $[-N,N]$ to $[-2N,2N]^c$ (conditioned on no edge
joining $[-N,N]$ and $[-2N,2N]^c$) both have a polynomial lower bound in $N$.
Our bound holds for all $\beta>0$ and thus rules out a potential phase
transition (around $\beta = 1$) which seemed to be a reasonable possibility. | 2405.03460v1 |
2018-09-07 | Mapping the depleted area of silicon diodes using a micro-focused X-ray beam | For the Phase-II Upgrade of the ATLAS detector at CERN, the current ATLAS
Inner Detector will be replaced with the ATLAS Inner Tracker. The ATLAS Inner
Tracker will be an all-silicon detector, consisting of a pixel tracker and a
strip tracker. Sensors for the ITk strip tracker are required to have a low
leakage current up to bias voltages of -700 V to maintain a low noise and power
dissipation. In order to minimise sensor leakage currents, particularly in the
high-radiation environment inside the ATLAS detector, sensors are foreseen to
be operated at low temperatures and to be manufactured from wafers with a high
bulk resistivity of several k{\Omega} cm. Simulations showed the electric field
inside sensors with high bulk resistivity to extend towards the sensor edge,
which could lead to increased surface currents for narrow dicing edges. In
order to map the electric field inside biased silicon sensors with high bulk
resistivity, three diodes from ATLAS silicon strip sensor prototype wafers were
studied with a monochromatic, micro-focused X-ray beam at the Diamond Light
Source. For all devices under investigation, the electric field inside the
diode was mapped and its dependence on the applied bias voltage was studied.
The findings showed that the electric field in each diode under investigation
extended beyond its bias ring and reached the dicing edge. | 1809.02667v2 |
2020-02-28 | Accelerating and Stopping Resistance Drift in Phase Change Memory Cells via High Electric Field Stress | We observed resistance drift in 125 K - 300 K temperature range in melt
quenched amorphous Ge2Sb2Te5 line-cells with length x width x thickness = ~500
nm x ~100 nm x ~ 50 nm. Drift coefficients measured using small voltage sweeps
appear to decrease from 0.12 +/- 0.029 at 300 K to 0.075 +/- 0.006 at 125 K.
The current-voltage characteristics of the amorphized cells measured in the 85
K - 300 K using high-voltage sweeps (0 to ~25 V) show a combination of a
linear, low-field exponential and high-field exponential conduction mechanisms,
all of which are strong functions of temperature. The very first high-voltage
sweep after amorphization (with electric fields up to ~70% of the breakdown
field) shows clear hysteresis in the current-voltage characteristics due to
accelerated drift, while the consecutive sweeps show stable characteristics.
Stabilization was achieved with 50 nA compliance current (current densities
~104 A/cm^2), preventing appreciable self-heating in the cells. The observed
acceleration and stoppage of the resistance drift with the application of high
electric fields is attributed to changes in the electrostatic potential profile
within amorphous Ge2Sb2Te5 due to trapped charges, reducing tunneling current.
Stable current-voltage characteristics are used to extract carrier activation
energies for the conduction mechanisms in 85 K - 300 K temperature range. The
carrier activation energy associated with linear current-voltage response is
extracted to be 331 +/- 5 meV in 200 - 300 K range, while carrier activation
energies of 233 +/- 2 meV and 109 +/- 5 meV are extracted in 85 K to 300 K
range for the mechanisms that give exponential current-voltage responses. | 2002.12487v1 |
2021-10-18 | Negative differential resistance state in the free-flux-flow regime of driven vortices in a single crystal of 2H-NbS$_2$ | Time series measurements in 2H-NbS$_2$ crystal had unravelled a drive induced
transition wherein the critical current (Ic) changes from a low to a high Ic
jammed vortex state, via a negative differential resistance (NDR) transition.
Here, using multiple current (I) - voltage (V) measurement cycles, we explore
the statistical nature of observing the NDR transition in the free-flux-flow
(FF) regime in a single crystal of 2H-NbS$_2$. The probability of observing the
NDR transition always remains finite for a vortex state created with either
fast or slow rate of magnetic field. The probability of observing the NDR
transition in the FF regime is found to systematically increase with magnetic
field (B) in weak collective pinning regime. In the strong pinning regime, the
said probability becomes B-independent. We show that the higher Ic state is
unique and cannot be accessed via any conventional route. While the I-V curves
do not distinguish between zero field cooled (ZFC) and field cooled (FC) modes
of preparing the vortex state, the probability for observing an NDR transition
has different B-dependences for the vortex matter prepared in the ZFC and FC
modes. We find that the NDR occurs in a high dissipation regime, where the flow
resistivity is well above the theoretical value expected in the FF regime. We
understand our results on the basis of a rapid drop in vortex viscosity at high
drives in 2H-NbS$_2$, which triggers a rapid increase in the vortex velocity
and reorganization in the moving vortex matter leading to a dynamical unstable
vortex flow. This dynamical instability leads to the NDR transition into a high
entropy vortex state with high Ic. | 2110.09379v2 |
2020-11-28 | Development of polar nematic fluids with giant-\k{appa} dielectric properties | Super-high-\k{appa} materials that exhibit exceptionally high dielectric
permittivity are recognized as potential candidates for a wide range of
next-generation photonic and electronic devices. Generally, the high
dielectricity for achieving a high-\k{appa} state requires a low symmetry of
materials so that most of the discovered high-\k{appa} materials are
symmetry-broken crystals. There are scarce reports on fluidic high-\k{appa}
dielectrics. Here we demonstrate a rational molecular design, supported by
machine-learning analyses, that introduces high polarity to asymmetric
molecules, successfully realizing super-high-\k{appa} fluid materials
(dielectric permittivity, {\epsilon} > 104) and strong second harmonic
generation with macroscopic spontaneous polar ordering. The polar structures
are confirmed to be identical for all the synthesized materials. Our
experiments and computational calculation reveal the unique orientational
structures coupled with the emerging polarity. Furthermore, adopting this
strategy to high-molecular-weight systems additionally extends the novel
material category from monomer to polar polymer materials, creating polar soft
matters with spontaneous symmetry breaking. | 2011.14099v1 |
2019-02-26 | The Evidence of Cathodic Micro-discharges during Plasma Electrolytic Oxidation Process | Plasma electrolytic oxidation (PEO) processing of EV 31 magnesium alloy has
been carried out in fluoride containing electrolyte under bipolar pulse current
regime. Unusual PEO cathodic micro-discharges have been observed and
investigated. It is shown that the cathodic micro-discharges exhibit a
collective intermittent behavior which is discussed in terms of charge
accumulations at the layer/electrolyte and layer/metal interfaces. Optical
emission spectroscopy is used to determine the electron density (typ. 10 15
cm-3) and the electron temperature (typ. 7500 K) while the role of F-anions on
the appearance of cathodic micro-discharges is pointed out. Plasma Electrolytic
Oxidation (PEO) is a promising plasma-assisted surface treatment of light
metallic alloys (e.g. Al, Mg, Ti). Although the PEO process makes it possible
to grow oxide coatings with interesting corrosion and wear resistant
properties, the physical mechanisms of coating growth are not yet completely
understood. Typically, the process consists in applying a high voltage
difference between a metallic piece and a counter-electrode which are both
immersed in an electrolyte bath. Compare to anodizing, the main differences
concern the electrolyte composition and the current and voltage ranges which
are at least one order of magnitude higher in PEO 1. These significant
differences in current and voltage imply the dielectric breakdown and
consequently the appearance of micro-discharges on the surface of the sample
under processing. Those micro-discharges are recognized as being the main
contributors to the formation of a dielectric porous crystalline oxide coating.
2 Nevertheless, the breakdown mechanism that governs the appearance of those
micro-discharges is still under investigation. Hussein et al. 3 proposed a
mechanism with three different plasma formation processes based on differences
in plasma chemical composition. The results of Jovovi{\'c} et al. 4,5
concerning physical properties of the plasma seem to corroborate this
mechanism, and also point out the importance of the substrate material in the
plasma composition. 6 Compared with DC conducted PEO process, using a bipolar
pulsed DC or AC current supply gives supplementary control latitude through the
current waveform parameters. The effect of these parameter on the
micro-discharges behavior has been investigated in several previous works.
2,3,7,8 One of the main results of these studies is the absence of
micro-discharge during the cathodic current half-period. 9-11 Even if the
cathodic half-period has an obvious effect on the efficiency of PEO as well as
on the coating growth and composition, the micro-plasmas appear only in anodic
half-period. Sah et al. 8 have observed the cathodic breakdown of an oxide
layer but at very high current density (10 kA.dm-${}^2$), and after several
steps of sample preparation. Several models of micro-discharges appearance in
AC current have already been proposed. 1,2,8,12,13 Though cathodic
micro-discharges have never been observed within usual process conditions, the
present study aims at defining suitable conditions to promote cathodic
micro-discharges and at studying the main characteristics of these
micro-plasmas. | 1902.09828v1 |
2019-07-05 | Materials databases: the need for open, interoperable databases with standardized data and rich metadata | Driven by the recent rapid increase in the number of materials databases
published (open and commercial), I discuss here some perspectives on the
growing need for standardized, interoperable, open databases. The field of
computational materials discovery is quickly expanding, and recent advances in
data mining, high throughput screening, and machine learning highlight the
potential of open databases. | 1907.02791v1 |
2016-01-07 | First principles search for $n$-type oxide, nitride, and sulfide thermoelectrics | Oxides have many potentially desirable characteristics for thermoelectric
applications, including low cost and stability at high temperatures, but thus
far there are few known high $zT$ $n$-type oxide thermoelectrics. In this work,
we use high-throughput first principles calculations to screen transition metal
oxides, nitrides, and sulfides for candidate materials with high power factors
and low thermal conductivity. We find a variety of promising materials, and we
investigate these materials in detail in order to understand the mechanisms
that cause them to have high power factors. These materials all combine a high
density of states near the Fermi level with dispersive bands, reducing the
trade-off between the Seebeck coefficient and the electrical conductivity, but
they do so for several different reasons. In addition, our calculations
indicate that many of our candidate materials have low thermal conductivity. | 1601.01622v2 |
2014-09-03 | Mechanics of freely-suspended ultrathin layered materials | The study of atomically thin two-dimensional materials is a young and rapidly
growing field. In the past years, a great advance in the study of the
remarkable electrical and optical properties of 2D materials fabricated by
exfoliation of bulk layered materials has been achieved. Due to the
extraordinary mechanical properties of these atomically thin materials, they
also hold a great promise for future applications such as flexible electronics.
For example, this family of materials can sustain very large deformations
without breaking. Due to the combination of small dimensions, high Young's
modulus and high crystallinity of 2D materials, they have attracted the
attention of the field of nanomechanical systems as high frequency and high
quality factor resonators. In this article, we review experiments on static and
dynamic response of 2D materials. We provide an overview and comparison of the
mechanics of different materials, and highlight the unique properties of these
thin crystalline layers. We conclude with an outlook of the mechanics of 2D
materials and future research directions such as the coupling of the mechanical
deformation to their electronic structure. | 1409.1173v2 |
2015-01-18 | Ultra-high mechanical stretchability and controllable topological phase transitions in two-dimensional arsenic | The mechanical stretchability is the magnitude of strain which a material can
suffer before it breaks. Materials with high mechanical stretchability, which
can reversibly withstand extreme mechanical deformation and cover arbitrary
surfaces and movable parts, are used for stretchable display devices, broadband
photonic tuning and aberration-free optical imaging. Strain can be utilised to
control the band structures of materials and can even be utilised to induce a
topological phase transition, driving the normal insulators to topological
non-trivial materials with non-zero Chern number or Z2 number. Here, we propose
a new two-dimensional topological material with ultra-high mechanical
stretchability - the ditch-like 2D arsenic. This new anisotropic material
possesses a large Poisson's ratio 1.049, which is larger than any other
reported inorganic materials and has a ultra-high stretchability 44% along the
armchair direction, which is unprecedent in inorganic materials as far as we
know. Its minimum bend radius of this material can be as low as 0.66 nm, which
is comparable to the radius of carbon-nanotube. Such mechanical properties make
this new material be a stretchable semiconductor which could be used to
construct flexible display devices and stretchable sensors. Axial strain will
make a conspicuous affect on the band structure of the system, and a proper
strain along the zigzag direction will drive the 2D arsenic into the
topological insulator in which the topological edge state can host
dissipation-less spin current and spin transfer toque, which are useful in
spintronics devices such as dissipation transistor, interconnect channels and
spin valve devices. | 1501.04350v1 |
2022-04-02 | Inventory of high-quality flat-band van der Waals materials | More is left to do in the field of flat bands besides proposing theoretical
models. One unexplored area is the flat bands featured in the van der Waals
(vdW) materials. Exploring more flat-band material candidates and moving the
promising materials toward applications have been well recognized as the
cornerstones for the next-generation high-efficiency devices. Here, we utilize
a powerful high-throughput tool to screen desired vdW materials based on the
Inorganic Crystal Structure Database. Through layers of filtration, we obtained
861 potential monolayers from 4997 vdW materials. Significantly, it is the
first example to introduce flat-band electronic properties in the vdW materials
and propose three families of representative flat-band materials by mapping
two-dimensional (2D) flat-band lattice models. Unlike existing screening
schemes, a simple, universal rule, i.e., 2D flat-band score criterion, is first
proposed to efficiently identify 229 high-quality flat-band candidates, and
guidance is provided to diagnose the quality of 2D flat bands. All these
efforts to screen experimental available flat-band candidates will certainly
motivate continuing exploration towards the realization of this class of
special materials and their applications in material science. | 2204.00810v1 |
2017-02-25 | Electronic conduction properties of indium tin oxide: single-particle and many-body transport | Indium tin oxide (Sn-doped In$_2$O$_{3-\delta}$ or ITO) is an interesting and
technologically important transparent conducting oxide. This class of material
has been extensively studied for decades, with research efforts focusing on the
application aspects. The fundamental issues of the electronic conduction
properties of ITO from 300 K down to low temperatures have rarely been
addressed. Studies of the electrical-transport properties over a wide range of
temperature are essential to unraveling the underlying electronic dynamics and
microscopic electronic parameters. We show that one can learn rich physics in
ITO material, including the semi-classical Boltzmann transport, the
quantum-interference electron transport, and the electron-electron interaction
effects in the presence of disorder and granularity. To reveal the
opportunities that the ITO material provides for fundamental research, we
demonstrate a variety of charge transport properties in different forms of ITO
structures, including homogeneous polycrystalline films, homogeneous
single-crystalline nanowires, and inhomogeneous ultrathin films. We not only
address new physics phenomena that arise in ITO but also illustrate the
versatility of the stable ITO material forms for potential applications. We
emphasize that, microscopically, the rich electronic conduction properties of
ITO originate from the inherited free-electron-like energy bandstructure and
low-carrier concentration (as compared with that in typical metals)
characteristics of this class of material. Furthermore, a low carrier
concentration leads to slow electron-phonon relaxation, which causes ($i$) a
small residual resistance ratio, ($ii$) a linear electron diffusion
thermoelectric power in a wide temperature range 1-300 K, and ($iii$) a weak
electron dephasing rate. We focus our discussion on the metallic-like ITO
material. | 1702.07845v1 |
2018-06-11 | Coincident Molecular Auxeticity and Negative Order Parameter in a Liquid Crystal Elastomer | "Auxetic" materials have the counter-intuitive property of expanding rather
than contracting perpendicular to an applied stretch, formally they have
negative Poisson's Ratios (PRs).[1,2] This results in properties such as
enhanced energy absorption and indentation resistance, which means that
auxetics have potential for applications in areas from aerospace to biomedical
industries.[3,4] Existing synthetic auxetics are all created by carefully
structuring porous geometries from positive PR materials. Crucially, their
geometry causes the auxeticity.[3,4] The necessary porosity weakens the
material compared to the bulk and the structure must be engineered, for
example, by using resource-intensive additive manufacturing processes.[1,5] A
longstanding goal for researchers has been the development of a synthetic
material that has intrinsic auxetic behaviour. Such "molecular auxetics" would
avoid porosity-weakening and their very existence implies chemical
tuneability.[1,4-9] However molecular auxeticity has never previously been
proven for a synthetic material.[6,7] Here we present a synthetic molecular
auxetic based on a monodomain liquid crystal elastomer (LCE). When stressed
perpendicular to the alignment direction, the LCE becomes auxetic at strains
greater than approximately 0.8 with a minimum PR of -0.8. The critical strain
for auxeticity coincides with the occurrence of a negative liquid crystal order
parameter (LCOP). We show the auxeticity agrees with theoretical predictions
derived from the Warner and Terentjev theory of LCEs.[10] This demonstration of
a synthetic molecular auxetic represents the origin of a new approach to
producing molecular auxetics with a range of physical properties and functional
behaviours. Further, it demonstrates a novel feature of LCEs and a route for
realisation of the molecular auxetic technologies that have been proposed over
the years. | 1807.03608v1 |
2022-05-11 | Effect of magnetic phase coexistence on spin-phonon coupling and magnetoelectric effect in polycrystalline Sm0.5Y0.5Fe0.58Mn0.42O3 | The polycrystalline co-doped samples of Sm0.5Y0.5Fe0.58Mn0.42O3 were prepared
by solid-state reaction route and its various physical properties with their
correlations have been investigated. The dc magnetization measurements on the
sample revealed a weak ferromagnetic (WFM) transition at TN=361 K that is
followed by an incomplete spin reorientation (SR) transition at TSR1= 348 K. A
first order magnetic transition (FOMT) around 292 K completes the spin
reorientation transition and the material enters into a nearly collinear
antiferromagnetic (AFM) state for T < 260 K. The compound exhibited
magnetization reversal below the compensation temperature (Tcomp) = 92 K at low
measured field of 100 Oe. At further low temperature below 71 K, the compound
also exhibited Zero-field cooled memory effects confirming a reentrant
spinglass state formation. Robust magnetodielectric (MD) magnetoelectric
coupling has been established in the present material through field dependent
dielectric and resistivity measurements. True ferroelectric transition with a
considerable value of saturation polarization (= 0.06 micro C/cm2 at 15 K) have
been found in the specimen below TFE= 108 K. We observed an intense spin-phonon
coupling (SPC) across TSR and TN from the temperature dependent Raman
spectroscopy and is responsible for the intrinsic magnetoelectric effect. This
SPC also stabilizes the ferroelectric state below TFE in the material. The
delicate interplay of the lattice (Phonons), charge and spins governs the
observed features in the investigated physical properties of the material that
makes the specimen a promising multifunctional material. | 2205.05291v2 |
2023-08-18 | Magnon Diffusion Length and Longitudinal Spin Seebeck Effect in Vanadium Tetracyanoethylene (V[TCNE]$_x$, $x \sim 2$) | Spintronic, spin caloritronic, and magnonic phenomena arise from complex
interactions between charge, spin, and structural degrees of freedom that are
challenging to model and even more difficult to predict. This situation is
compounded by the relative scarcity of magnetically-ordered materials with
relevant functionality, leaving the field strongly constrained to work with a
handful of well-studied systems that do not encompass the full phase space of
phenomenology predicted by fundamental theory. Here we present an important
advance in this coupled theory-experiment challenge, wherein we extend existing
theories of the spin Seebeck effect (SSE) to explicitly include the
temperature-dependence of magnon non-conserving processes. This expanded theory
quantitatively describes the low-temperature behavior of SSE signals previously
measured in the mainstay material yttrium iron garnet (YIG) and predicts a new
regime for magnonic and spintronic materials that have low saturation
magnetization, $M_S$, and ultra-low damping. Finally, we validate this
prediction by directly observing the spin Seebeck resistance (SSR) in the
molecule-based ferrimagnetic semiconductor vanadium tetracyanoethylene
(V[TCNE]$_x$, $x \sim 2$). These results validate the expanded theory, yielding
SSR signals comparable in magnitude to YIG and extracted magnon diffusion
length ($\lambda_m>1$ $\mu$ m) and magnon lifetime for V[TCNE]$_x$
($\tau_{th}\approx 1-10$ $\mu$ s) exceeding YIG ($\tau_{th}\sim 10$ ns).
Surprisingly, these properties persist to room temperature despite relatively
low spin wave stiffness (exchange). This identification of a new regime for
highly efficient SSE-active materials opens the door to a new class of magnetic
materials for spintronic and magnonic applications. | 2308.09752v1 |
1998-08-25 | Unconventional Transition from Metallic to Insulating Resistivity in the Spin-ladder Compound (Sr,Ca)$_{14}$Cu$_{24}$O$_{41}$ | Spin-ladder compounds make interesting analogs of the high-temperature
superconductors, because they contain layers of nearly one-dimensional
"ladders" consisting of a square array of copper and oxygen atoms. Increasing
the number of legs in the ladders provides a step-wise approach toward the
two-dimensional copper-oxygen plane, that structure believed to be a key to
high temperature superconductivity. Short-range spin correlations in ladders
have been predicted to lead to formation of hole pairs favorable for
superconductivity, once enough holes are introduced onto the ladders by doping.
Indeed, superconductivity has been discovered in the two-leg ladder compound
(Sr,Ca)$_{14}$Cu$_{24}$O$_{41}$ under high pressure. Here we show that charge
transport in the non-superconducting state of (Sr,Ca)$_{14}$Cu$_{24}$O$_{41}$
shares three distinct regimes in common with high-temperature superconductors,
including an unexplained insulating behavior at low temperatures in which the
resistivity increases as the logarithm of the temperature. These observations
suggest that the logarithmic divergence arises from a new localization
mechanism common to the ladder compounds and the high-temperature
superconductors, which may arise from nearly one-dimensional charge transport
in the presence of a spin gap. | 9808284v2 |
2000-02-17 | Effect of Magnetic field on the Pseudogap Phenomena in High-Tc Cuprates | We theoretically investigate the effect of magnetic field on the pseudogap
phenomena in High-Tc cuprates.
The obtained results well explain the experimental results including their
doping dependences.
In our previous paper (J. Phys. Soc. Jpn. 68 (1999) 2999.), we have shown
that the pseudogap phenomena observed in High-Tc cuprates are naturally
understood as a precursor of the strong coupling superconductivity. On the
other hand, there is an interpretation for the recent high field NMR
measurements to be an evidence denying the pairing scenarios for the pseudogap.
In this paper, we investigate the magnetic field dependence of NMR $1/T_{1}T$
on the basis of our formalism and show the interpretation to be inappropriate.
The results indicate that the value of the characteristic magnetic field
$B_{{\rm ch}}$ is remarkably large in case of the strong coupling
superconductivity, especially near the pseudogap onset temperature $T^{*}$.
Therefore, the magnetic field dependences can not be observed and $T^{*}$ does
not vary when the strong pseudogap anomaly is observed. On the other hand,
$B_{{\rm ch}}$ is small in the comparatively weak coupling case and $T^{*}$
varies when the weak pseudogap phenomena are observed.
These results properly explain the high magnetic field NMR experiments
continuously from under-doped to over-doped cuprates.
Moreover, we discuss the transport phenomena in the pseudogap phase. The
behaviors of the in-plane resistivity, the Hall coefficient and the c-axis
resistivity in the pseudogap phase are naturally understood by considering the
d-wave pseudogap. | 0002274v1 |
2018-11-15 | High Kinetic Inductance Microwave Resonators Made by He-Beam Assisted Deposition of Tungsten Nanowires | We evaluate the performance of hybrid microwave resonators made by combining
sputtered Nb thin films with Tungsten nanowires grown with a He-beam induced
deposition technique. Depending on growth conditions the nanowires have a
typical width $w\in[35-75]$~nm and thickness $t\in[5-40]$~nm. We observe a high
normal state resistance $R_{sq}\in [65-150]$ $\Omega/sq$ which together with a
critical temperature $T_c\in[4-6]~K$ ensure a high kinetic inductance making
the resonator strongly nonlinear. Both lumped and coplanar waveguide resonators
were fabricated and measured at low temperature exhibiting internal quality
factors up to $3990$ at $4.5$~GHz in the few photon regime. Analyzing the wire
length, temperature and microwave power dependence we extracted a kinetic
inductance for the W nanowire of $L_K\approx15$ pH/sq, which is 250 times
higher than the geometrical inductance, and a Kerr non-linearity as high as
$K_{W,He}/2\pi=200 \pm 120$~Hz/photon at $4.5$~GHz. The nanowires made with the
helium focused ion beam are thus versatile objects to engineer compact, high
impedance, superconducting environments with a mask and resist free direct
write process. | 1811.06496v2 |
2017-05-16 | Scale-invariant magnetoresistance in a cuprate superconductor | The anomalous metallic state in high-temperature superconducting cuprates is
masked by the onset of superconductivity near a quantum critical point. Use of
high magnetic fields to suppress superconductivity has enabled a detailed study
of the ground state in these systems. Yet, the direct effect of strong magnetic
fields on the metallic behavior at low temperatures is poorly understood,
especially near critical doping, $x=0.19$. Here we report a high-field
magnetoresistance study of thin films of \LSCO cuprates in close vicinity to
critical doping, $0.161\leq x\leq0.190$. We find that the metallic state
exposed by suppressing superconductivity is characterized by a
magnetoresistance that is linear in magnetic field up to the highest measured
fields of $80$T. The slope of the linear-in-field resistivity is
temperature-independent at very high fields. It mirrors the magnitude and
doping evolution of the linear-in-temperature resistivity that has been
ascribed to Planckian dissipation near a quantum critical point. This
establishes true scale-invariant conductivity as the signature of the strange
metal state in the high-temperature superconducting cuprates. | 1705.05806v2 |
2022-11-22 | Development of ultra-low mass and high-rate capable RPC based on Diamond-Like Carbon electrodes for MEG II experiment | A new type of resistive plate chamber with thin-film electrodes based on
diamond-like carbon is under development for background identification in the
MEG II experiment. Installed in a low-momentum and high-intensity muon beam,
the detector is required to have extremely low mass and a high rate capability.
A single-layer prototype detector with 2 cm $\times$ 2 cm size was constructed
and evaluated to have a high rate capability of 1 MHz/cm$^2$ low-momentum
muons. For a higher rate capability and scalability of the detector size, the
electrodes to supply high voltage were segmented at a 1 cm pitch by
implementing a conductive pattern on diamond-like carbon. Using the new
electrodes, a four-layer prototype detector was constructed and evaluated to
have a 46% detection efficiency with only a single layer active at a rate of
$\cal O$(10 kHz). The result with the new electrodes is promising to achieve
the required detection efficiency of 90% at a rate of 4 MHz/cm$^2$ with all the
layers active. | 2211.12172v2 |
2010-05-05 | Review of Best Practice Methods for Determining an Electrode Material's Performance for Ultracapacitors | Ultracapacitors are rapidly being adopted for use for a wide range of
electrical energy storage applications. While ultracapacitors are able to
deliver high rates of charge and discharge, they are limited in the amount of
energy stored. The capacity of ultracapacitors is largely determined by the
electrode material and as a result, research to improve the performance of
electrode materials has dramatically increased. While test methods for packaged
ultracapacitors are well developed, it is often not feasible for the materials
scientist to assemble full sized, packaged cells to test electrode materials.
Methodology to reliably measure a material's performance for ultracapacitor
electrode use is not well standardized with the different techniques currently
being used yielding widely varying results. In this manuscript, we review the
best practice test methods that accurately predict a materials performance, yet
are flexible and quick enough to accommodate a wide range of material sample
types and amounts. | 1005.0805v2 |
2016-07-22 | How to Characterize Thermal Transport Capability of 2D Materials Fairly? - Sheet Thermal Conductance and the Choice of Thickness | Ever since the discovery of the record-high thermal conductivity of single
layer graphene, thermal transport capability of monolayer 2D materials has been
under constant spotlight. Since thermal conductivity is an intensive property
for 3D materials and the thickness of 2D materials is not well defined,
different definitions of thickness in literature have led to ambiguity towards
predicting thermal conductivity values and thus in understanding the heat
transfer capability of different monolayer 2D materials. We argue that if
conventional definition of thermal conductivity should be used as the quantity
to compare the heat transfer capability of various monolayer 2D materials, then
the same thickness should be used. Alternatively, to circumvent the problem of
ambiguous thickness completely, we also suggest that a "sheet thermal
conductance" to be defined as an intensive 2D material property when
characterizing the heat transfer capability of 2D materials. When converting
literature thermal conductivity values of monolayer materials to this new
property, some new features that were not displayed when using different
thicknesses show up. | 1607.06542v2 |
2018-02-05 | Plasmonic physics of 2D crystalline materials | Collective modes of doped two-dimensional crystalline materials, namely
graphene, MoS$_2$ and phosphorene, both monolayer and bilayer structures, are
explored using the density functional theory simulations together with the
random phase approximation. The many-body dielectric functions of the materials
are calculated using an {\it ab initio} based model involving
material-realistic physical properties. Having calculated the electron
energy-loss, we calculate the collective modes of each material considering the
in-phase and out-of-phase modes for bilayer structures. Furthermore, owing to
many band structures and intreband transitions, we also find high-energy
excitations in the systems. We explain that the material-specific dielectric
function considering the polarizability of the crystalline material such as
MoS$_2$ are needed to obtain realistic plasmon dispersions. For each material
studied here, we find different collective modes and describe their physical
origins. | 1802.01291v1 |
2018-07-01 | Correlated materials design: prospects and challenges | The design of correlated materials challenges researchers to combine the
maturing, high throughput framework of DFT-based materials design with the
rapidly-developing first-principles theory for correlated electron systems. We
review the field of correlated materials, distinguishing two broad classes of
correlation effects, static and dynamics, and describe methodologies to take
them into account. We introduce a material design workflow, and illustrate it
via examples in several materials classes, including superconductors, charge
ordering materials and systems near an electronically driven metal to insulator
transition, highlighting the interplay between theory and experiment with a
view towards finding new materials. We review the statistical formulation of
the errors of currently available methods to estimate formation energies.
Correlation effects have to be considered in all the material design steps.
These include bridging between structure and property, obtaining the correct
structure and predicting material stability. We introduce a post-processing
strategy to take them into account. | 1807.00398v2 |
2020-05-09 | Learning Properties of Ordered and Disordered Materials from Multi-fidelity Data | Predicting the properties of a material from the arrangement of its atoms is
a fundamental goal in materials science. While machine learning has emerged in
recent years as a new paradigm to provide rapid predictions of materials
properties, their practical utility is limited by the scarcity of high-fidelity
data. Here, we develop multi-fidelity graph networks as a universal approach to
achieve accurate predictions of materials properties with small data sizes. As
a proof of concept, we show that the inclusion of low-fidelity
Perdew-Burke-Ernzerhof band gaps greatly enhances the resolution of latent
structural features in materials graphs, leading to a 22-45\% decrease in the
mean absolute errors of experimental band gap predictions. We further
demonstrate that learned elemental embeddings in materials graph networks
provide a natural approach to model disorder in materials, addressing a
fundamental gap in the computational prediction of materials properties. | 2005.04338v3 |
2020-08-24 | Inverse Design of Composite Metal Oxide Optical Materials based on Deep Transfer Learning | Optical materials with special optical properties are widely used in a broad
span of technologies, from computer displays to solar energy utilization
leading to large dataset accumulated from years of extensive materials
synthesis and optical characterization. Previously, machine learning models
have been developed to predict the optical absorption spectrum from a materials
characterization image or vice versa. Herein we propose TLOpt, a transfer
learning based inverse optical materials design algorithm for suggesting
material compositions with a desired target light absorption spectrum. Our
approach is based on the combination of a deep neural network model and global
optimization algorithms including a genetic algorithm and Bayesian
optimization. A transfer learning strategy is employed to solve the small
dataset issue in training the neural network predictor of optical absorption
spectrum using the Magpie materials composition descriptor. Our extensive
experiments show that our algorithm can inverse design the materials
composition with stoichiometry with high accuracy. | 2008.10618v1 |
2022-04-04 | Hole-doping induced ferromagnetism in 2D materials | Two-dimensional (2D) ferromagnetic materials are considered as promising
candidates for the future generations of spintronic devices. Yet, 2D materials
with intrinsic ferromagnetism are scarce. High-throughput first-principles
simulations are performed in order to screen 2D materials that present a
non-magnetic to a ferromagnetic transition upon hole doping. A global
evolutionary search is subsequently performed, in order to identify alternative
possible atomic structures of the eligible candidates, and 122 materials
exhibiting a hole-doping induced ferromagnetism are identified. Their energetic
and dynamic stability, as well as their magnetic properties under hole doping
are investigated systematically. Half of these 2D materials are metal halides,
followed by chalcogenides, oxides and nitrides, some of them having predicted
Curie temperatures above 300 K. The exchange interactions responsible for the
ferromagnetic order in these 2D materials are also discussed. This work not
only provides theoretical insights into hole-doped 2D ferromagnetic materials,
but also enriches the family of 2D magnetic materials for possible spintronic
applications. | 2204.01551v2 |
2023-03-29 | A Comprehensive and Versatile Multimodal Deep Learning Approach for Predicting Diverse Properties of Advanced Materials | We present a multimodal deep learning (MDL) framework for predicting physical
properties of a 10-dimensional acrylic polymer composite material by merging
physical attributes and chemical data. Our MDL model comprises four modules,
including three generative deep learning models for material structure
characterization and a fourth model for property prediction. Our approach
handles an 18-dimensional complexity, with 10 compositional inputs and 8
property outputs, successfully predicting 913,680 property data points across
114,210 composition conditions. This level of complexity is unprecedented in
computational materials science, particularly for materials with undefined
structures. We propose a framework to analyze the high-dimensional information
space for inverse material design, demonstrating flexibility and adaptability
to various materials and scales, provided sufficient data is available. This
study advances future research on different materials and the development of
more sophisticated models, drawing us closer to the ultimate goal of predicting
all properties of all materials. | 2303.16412v1 |
2024-04-03 | Construction of Functional Materials Knowledge Graph in Multidisciplinary Materials Science via Large Language Model | The convergence of materials science and artificial intelligence has unlocked
new opportunities for gathering, analyzing, and generating novel materials
sourced from extensive scientific literature. Despite the potential benefits,
persistent challenges such as manual annotation, precise extraction, and
traceability issues remain. Large language models have emerged as promising
solutions to address these obstacles. This paper introduces Functional
Materials Knowledge Graph (FMKG), a multidisciplinary materials science
knowledge graph. Through the utilization of advanced natural language
processing techniques, extracting millions of entities to form triples from a
corpus comprising all high-quality research papers published in the last
decade. It organizes unstructured information into nine distinct labels,
covering Name, Formula, Acronym, Structure/Phase, Properties, Descriptor,
Synthesis, Characterization Method, Application, and Domain, seamlessly
integrating papers' Digital Object Identifiers. As the latest structured
database for functional materials, FMKG acts as a powerful catalyst for
expediting the development of functional materials and a fundation for building
a more comprehensive material knowledge graph using full paper text.
Furthermore, our research lays the groundwork for practical text-mining-based
knowledge management systems, not only in intricate materials systems but also
applicable to other specialized domains. | 2404.03080v1 |
2024-04-09 | Deep-Learning Database of Density Functional Theory Hamiltonians for Twisted Materials | Moir\'e-twisted materials have garnered significant research interest due to
their distinctive properties and intriguing physics. However, conducting
first-principles studies on such materials faces challenges, notably the
formidable computational cost associated with simulating ultra-large twisted
structures. This obstacle impedes the construction of a twisted materials
database crucial for datadriven materials discovery. Here, by using
high-throughput calculations and state-of-the-art neural network methods, we
construct a Deep-learning Database of density functional theory (DFT)
Hamiltonians for Twisted materials named DDHT. The DDHT database comprises
trained neural-network models of over a hundred homo-bilayer and hetero-bilayer
moir\'e-twisted materials. These models enable accurate prediction of the DFT
Hamiltonian for these materials across arbitrary twist angles, with an averaged
mean absolute error of approximately 1.0 meV or lower. The database facilitates
the exploration of flat bands and correlated materials platforms within
ultra-large twisted structures. | 2404.06449v1 |
2020-03-15 | New quantum phases of matter: Topological Materials | In this article, we provide an overview of the basic concepts of novel
topological materials. This new class of materials developed by combining the
Weyl/Dirac fermionic electron states and magnetism, provide a materials-science
platform to test predictions of the laws of topological physics. Owing to their
dissipationless transport, these materials hold high promises for technological
applications in quantum computing and spintronics devices. | 2003.06835v1 |
2019-06-11 | Towards Photoferroic Materials by Design: Recent Progresses and Perspective | The use of photoferroic materials that combine ferroelectric and light
harvesting properties in a photovoltaic device is a promising route to
significantly improve the efficiency of solar cells. These materials do not
require the formation of a p-n junction and can produce photovoltages well
above the value of the band gap, because of the spontaneous intrinsic
polarization and the formation of domain walls. In this perspective, we discuss
the recent experimental progresses and challenges for the synthesis of these
materials and the theoretical discovery of novel photoferroic materials using a
high-throughput approach. | 1906.04490v1 |
2023-11-06 | Cross-Plane Thermal Transport in Layered Materials | The cross-plane (across-layers) phonon thermal transport of five diverse,
layered semiconductors is investigated by accounting for higher-order
four-phonon scattering, phonon renormalization, and multi-channel thermal
transport. For materials having relatively large cross-plane thermal
conductivity (AlB6, MoS2, and MoSi2N4), phonons contributing to cross-plane
conductivity have an order of magnitude larger mean free path than that for the
basal-plane thermal transport, whereas the opposite effect is observed for
materials with low thermal conductivity (MoO3 and KCuSe). The contribution from
the wave-like coherent transport channel is less than 5% in all considered
materials. Our work unravels the contrasting role of nano-structuring on the
basal- vs. cross-plane thermal conductivity of low and high thermal
conductivity layered materials. | 2311.03144v1 |
2024-03-15 | MADAS -- A Python framework for assessing similarity in materials-science data | Computational materials science produces large quantities of data, both in
terms of high-throughput calculations and individual studies. Extracting
knowledge from this large and heterogeneous pool of data is challenging due to
the wide variety of computational methods and approximations, resulting in
significant veracity in the sheer amount of available data. Here, we present
MADAS, a Python framework for computing similarity relations between material
properties. It can be used to automate the download of data from various
sources, compute descriptors and similarities between materials, analyze the
relationship between materials through their properties, and can incorporate a
variety of existing machine learning methods. We explain the design of the
package and demonstrate its power with representative examples. | 2403.10470v1 |
2017-02-18 | Tunable Hyperbolic Dispersion and Negative Refraction in Natural Electride Materials | Hyperbolic (or indefinite) materials have attracted significant attention due
to their unique capabilities for engineering electromagnetic space and
controlling light propagation. A current challenge is to find a hyperbolic
material with wide working frequency window, low energy loss, and easy
controllability. Here, we propose that naturally existing electride materials
could serve as high-performance hyperbolic medium. Taking the electride Ca$_2$N
as a concrete example and using first-principles calculations, we show that the
material is hyperbolic over a wide frequency window from short-wavelength to
near infrared. More importantly, it is almost lossless in the window. We
clarify the physical origin of these remarkable properties, and show its
all-angle negative refraction effect. Moreover, we find that the optical
properties can be effectively tuned by strain. With moderate strain, the
material can even be switched between elliptic and hyperbolic for a particular
frequency. Our result points out a new route toward high-performance natural
hyperbolic materials, and it offers realistic materials and novel methods to
achieve controllable hyperbolic dispersion with great potential for
applications. | 1702.05602v1 |
2018-04-23 | Gaussian Material Synthesis | We present a learning-based system for rapid mass-scale material synthesis
that is useful for novice and expert users alike. The user preferences are
learned via Gaussian Process Regression and can be easily sampled for new
recommendations. Typically, each recommendation takes 40-60 seconds to render
with global illumination, which makes this process impracticable for real-world
workflows. Our neural network eliminates this bottleneck by providing
high-quality image predictions in real time, after which it is possible to pick
the desired materials from a gallery and assign them to a scene in an intuitive
manner. Workflow timings against Disney's "principled" shader reveal that our
system scales well with the number of sought materials, thus empowering even
novice users to generate hundreds of high-quality material models without any
expertise in material modeling. Similarly, expert users experience a
significant decrease in the total modeling time when populating a scene with
materials. Furthermore, our proposed solution also offers controllable
recommendations and a novel latent space variant generation step to enable the
real-time fine-tuning of materials without requiring any domain expertise. | 1804.08369v1 |
2022-02-10 | Topogivity: A Machine-Learned Chemical Rule for Discovering Topological Materials | Topological materials present unconventional electronic properties that make
them attractive for both basic science and next-generation technological
applications. The majority of currently known topological materials have been
discovered using methods that involve symmetry-based analysis of the quantum
wavefunction. Here we use machine learning to develop a simple-to-use heuristic
chemical rule that diagnoses with a high accuracy whether a material is
topological using only its chemical formula. This heuristic rule is based on a
notion that we term topogivity, a machine-learned numerical value for each
element that loosely captures its tendency to form topological materials. We
next implement a high-throughput procedure for discovering topological
materials based on the heuristic topogivity-rule prediction followed by ab
initio validation. This way, we discover new topological materials that are not
diagnosable using symmetry indicators, including several that may be promising
for experimental observation. | 2202.05255v3 |
2021-09-11 | Highly Accurate, Reliable and Non-Contaminating Two-Dimensional Material Transfer System | The exotic properties of two-dimensional (2D) materials and 2D
heterostructures, built by forming heterogeneous multi-layered stacks, have
been widely explored across a number of subject matters following the goal to
invent, design, and improve applications enabled by 2D materials. To
successfully harvest these unique properties effectively and increase the yield
of manufacturing 2D material-based devices for achieving reliable and
repeatable results is the current challenge. The scientific community has
introduced various experimental transfer systems explained in detail for
exfoliated 2D materials, however, the field lacks statistical analysis and the
capability of producing a transfer technique enabling; i) high transfer
precision and yield, ii) cross-contamination free transfer, iii)
multi-substrate transfer, and iv) rapid prototyping without wet chemistry. Here
we introduce a novel 2D material deterministic transfer system and
experimentally show its high accuracy, reliability, repeatability, and
non-contaminating transfer features by demonstrating fabrication of 2D
material-based optoelectronic devices featuring novel device physics and unique
functionality. Such rapid and material-near prototyping capability can
accelerate not only layered material science in discovery but also engineering
innovations. | 2109.05158v2 |
2021-11-10 | Piezoelectric modulus prediction using machine learning and graph neural networks | Piezoelectric materials are widely used in all kinds of industries such as
electric cigarette lighters, diesel engines and x-ray shutters. However,
discovering high-performance and environmentally friendly (e.g. lead-free)
piezoelectric materials is a difficult problem due to the sophisticated
relationships from materials' composition/structures to the piezoelectric
effect. Compared to other material properties such as formation energy, band
gap, and bulk modulus, it is much more challenging to predict piezoelectric
coefficients. Here, we propose a comprehensive study on designing and
evaluating advanced machine learning models for predicting the piezoelectric
modulus from materials' composition and/or structures. We train the prediction
models based on extensive feature engineering combined with machine learning
models (Random Forest and Support Vector Machines) and automated feature
learning based on deep graph neural networks. Our SVM model with crystal
structure feature outperform other methods. We also use this model to predict
the piezoelectric coefficients for 12,680 materials from the Materials Project
database and report the top 20 potential high performance piezoelectric
materials. | 2111.05557v1 |
2023-06-25 | Discovering two-dimensional magnetic topological insulators by machine learning | Topological materials with unconventional electronic properties have been
investigated intensively for both fundamental and practical interests.
Thousands of topological materials have been identified by symmetry-based
analysis and ab initio calculations. However, the predicted magnetic
topological insulators with genuine full band gaps are rare. Here we employ
this database and supervisedly train neural networks to develop a heuristic
chemical rule for electronic topology diagnosis. The learned rule is
interpretable and diagnoses with a high accuracy whether a material is
topological using only its chemical formula and Hubbard $U$ parameter. We next
evaluate the model performance in several different regimes of materials.
Finally, we integrate machine-learned rule with ab initio calculations to
high-throughput screen for magnetic topological insulators in 2D material
database. We discover 6 new classes (15 materials) of Chern insulators, among
which 4 classes (7 materials) have full band gaps and may motivate for
experimental observation. We anticipate the machine-learned rule here can be
used as a guiding principle for inverse design and discovery of new topological
materials. | 2306.14155v2 |
2023-10-21 | Advances in Complex Oxide Quantum Materials Through New Approaches to Molecular Beam Epitaxy | Molecular beam epitaxy (MBE), a workhorse of the semiconductor industry, has
progressed rapidly in the last few decades in the development of novel
materials. Recent developments in condensed matter and materials physics have
seen the rise of many novel quantum materials that require ultra-clean and
high-quality samples for fundamental studies and applications. Novel
oxide-based quantum materials synthesized using MBE have advanced the
development of the field and materials. In this review, we discuss the recent
progress in new MBE techniques that have enabled synthesis of complex oxides
that exhibit "quantum" phenomena, including superconductivity and topological
electronic states. We show how these techniques have produced breakthroughs in
the synthesis of 4d and 5d oxide films and heterostructures that are of
particular interest as quantum materials. These new techniques in MBE offer a
bright future for the synthesis of ultra-high quality oxide quantum materials. | 2310.13902v1 |
2002-05-31 | Structure and Superconductivity in Zr-Stabilized, Nonstoichiometric Molybdenum Diboride | The structure and physical properties of the Zr-stabilized, nonstoichiometric
molybdenum diboride superconductor are reported. Good quality material of the
diboride structure type can only be obtained by partial substitution of Zr for
Mo, and the quenching of melts. The phase is best made with boron in excess of
the ideal 2:1 boron to metal ratio. Powder neutron diffraction measurements
show that the non-stoichiometry is accommodated by atom deficiency in the metal
layers. The diboride structure type exists for (Mo.96Zr.04)xB2 for x between
0.85 and 1.0. Electron diffraction shows that the stoichiometric material, x=1,
has a significant number of stacking faults. Tc increases from 5.9 to 8.2K with
the introduction of metal vacancies. Resistivity measurements indicate that
(Mo.96Zr.04).88B2 is a bad metal, and specific heat measurements show that
gamma= 4.4 mJ/mol K2, and that deltaC/gammaTc = 1.19. Preliminary boron isotope
effect measurements indicate an exponent 0.11(5). Analysis of the data in terms
of the electronic structure is reported, allowing an estimate of the
electron-phonon coupling constant, lamda = 0.1-0.3, making these weak-coupling
superconductors. Preliminary characterizations of the superconductivity in the
related phases NbxB2 and (Mo.96X.04).85B2 for X=Ti, and Hf are reported. | 0206006v1 |
2003-07-31 | The break up of heavy electrons at a quantum critical point | The point at absolute zero where matter becomes unstable to new forms of
order is called a quantum critical point (QCP). The quantum fluctuations
between order and disorder that develop at this point induce profound
transformations in the finite temperature electronic properties of the
material. Magnetic fields are ideal for tuning a material as close as possible
to a QCP, where the most intense effects of criticality can be studied. A
previous study on theheavy-electron material $YbRh_2Si_2$ found that near a
field-induced quantum critical point electrons move ever more slowly and
scatter off one-another with ever increasing probability, as indicated by a
divergence to infinity of the electron effective mass and cross-section. These
studies could not shed light on whether these properties were an artifact of
the applied field, or a more general feature of field-free QCPs. Here we report
that when Germanium-doped $YbRh_2Si_2$ is tuned away from a chemically induced
quantum critical point by magnetic fields there is a universal behavior in the
temperature dependence of the specific heat and resistivity: the characteristic
kinetic energy of electrons is directly proportional to the strength of the
applied field. We infer that all ballistic motion of electrons vanishes at a
QCP, forming a new class of conductor in which individual electrons decay into
collective current carrying motions of the electron fluid. | 0308001v1 |
2006-06-16 | Multiferroic tunnel junctions | Multiferroics are singular materials that can display simultaneously electric
and magnetic orders. Some of them can be ferroelectric and ferromagnetic and,
for example, provide the unique opportunity of encoding information
independently in electric polarization and magnetization to obtain four
different logic states. However, schemes allowing a simple electrical readout
of these different states have not been demonstrated so far. In this article,
we show that this can be achieved if a multiferroic material is used as the
tunnel barrier in a magnetic tunnel junction. We demonstrate that thin films of
ferromagnetic-ferroelectric La0.1Bi0.9MnO3 (LBMO) retain both ferroic
properties down to a thickness of only 2 nm. We have used such films as
spin-filtering tunnel barriers the magnetization and electric polarization of
which can be switched independently. In that case, the tunnel current across
the structure is controlled by both the magnetic and ferroelectric
configuration of the barrier, which gives rise to four distinct resistance
states. This can be explained by the combination of spin filtering by the
ferromagnetic LBMO barrier and the partial charge screening of electrical
charges at the barrier/electrode interfaces due to ferroelectricity. We
anticipate our results to be a starting point for more studies on the interplay
between ferroelectricity and spin-dependent tunneling, and for the use of
nanometric multiferroic elements in prototype devices. On a wider perspective,
they may open the way towards novel reconfigurable logic spintronics
architectures and to electrically controlled readout in quantum computing
schemes using the spin-filter effect. | 0606444v1 |
2006-10-29 | Detection of Individual Gas Molecules Absorbed on Graphene | The ultimate aspiration of any detection method is to achieve such a level of
sensitivity that individual quanta of a measured value can be resolved. In the
case of chemical sensors, the quantum is one atom or molecule. Such resolution
has so far been beyond the reach of any detection technique, including
solid-state gas sensors hailed for their exceptional sensitivity. The
fundamental reason limiting the resolution of such sensors is fluctuations due
to thermal motion of charges and defects which lead to intrinsic noise
exceeding the sought-after signal from individual molecules, usually by many
orders of magnitude. Here we show that micrometre-size sensors made from
graphene are capable of detecting individual events when a gas molecule
attaches to or detaches from graphenes surface. The adsorbed molecules change
the local carrier concentration in graphene one by one electron, which leads to
step-like changes in resistance. The achieved sensitivity is due to the fact
that graphene is an exceptionally low-noise material electronically, which
makes it a promising candidate not only for chemical detectors but also for
other applications where local probes sensitive to external charge, magnetic
field or mechanical strain are required. | 0610809v2 |
2007-11-26 | Experimental investigation on the microscopic structure of intrinsic paramagnetic point defects in amorphous silicon dioxide | In the present Ph.D. Thesis we report an experimental investigation on the
effects of gamma- and beta-ray irradiation and of subsequent thermal treatment
on many types of a-SiO2 materials, differing in the production methods, OH- and
Al-content, and oxygen deficiencies. Our main objective is to gain further
insight on the microscopic structures of the E'_gamma, E'_delta, E'_alpha and
triplet paramagnetic centers, which are among the most important and studied
class of radiation induced intrinsic point defects in a-SiO2. To pursue this
objective, we use prevalently the EPR spectroscopy. In particular, our work is
focused on the properties of the unpaired electrons wave functions involved in
the defects, and this aspect is mainly investigated through the study of the
EPR signals originating from the interaction of the unpaired electrons with
29Si magnetic nuclei (with nuclear spin I=1/2 and natural abundance 4.7 %). In
addition, in some cases of interest, OA measurements are also performed with
the aim to further characterize the electronic properties of the defects.
Furthermore, due to its relevance for electronics application, the charge state
of the defects is investigated by looking at the processes responsible for the
generation of the defects of interest. Once these information were gained, the
possible sites that can serve as precursors for defects formation are deduced,
with the definitive purpose to obtain in the future more radiation resistant
a-SiO2 materials in which the deleterious effects connected with the point
defects are significantly reduced. | 0711.4029v1 |
2011-06-16 | The Memory-Conservation Theory of Memristance | The memristor, the recently discovered fundamental circuit element, is of
great interest for neuromorphic computing, nonlinear electronics and computer
memory. It is usually modelled either using Chua's equations, which lack
material device properties, or using Strukov's phenomenological model (or
models derived from it), which deviates from Chua's definitions due to the lack
of a magnetic flux term. It is shown that by modelling the magnetostatics of
the memory-holding ionic current (oxygen vacancies in the Strukov memristor),
the memristor's magnetic flux can be identified as the flux arising from the
ions. This leads to a novel theory of memristance consisting of two components:
1. A memory function which describes how the memristance, as felt by the ions,
affects the conducting electrons located in the `on' part of the device; 2. A
conservation function which describes the time-varying resistance in the `off'
part of the device. This model allows for a straight-forward incorporation of
the ions within the electronic theory and relates Chua's constitutive
definition of a memristor with device material properties for the first time. | 1106.3170v3 |
2012-04-18 | Controlling bulk conductivity in topological insulators: Key role of anti-site defects | The binary Bi-chalchogenides, Bi2Ch3, are widely regarded as model examples
of a recently discovered new form of quantum matter, the three-dimensional
topological insulator (TI) [1-4]. These compounds host a single spin-helical
surface state which is guaranteed to be metallic due to time reversal symmetry,
and should be ideal materials with which to realize spintronic and quantum
computing applications of TIs [5]. However, the vast majority of such compounds
synthesized to date are not insulators at all, but rather have detrimental
metallic bulk conductivity [2, 3]. This is generally accepted to result from
unintentional doping by defects, although the nature of the defects responsible
across different compounds, as well as strategies to minimize their detrimental
role, are surprisingly poorly understood. Here, we present a comprehensive
survey of the defect landscape of Bi-chalchogenide TIs from first-principles
calculations. We find that fundamental differences in the energetics of native
defect formation in Te- and Se-containing TIs enables precise control of the
conductivity across the ternary Bi-Te-Se alloy system. From a systematic
angle-resolved photoemission (ARPES) investigation of such ternary alloys,
combined with bulk transport measurements, we demonstrate that this method can
be utilized to achieve true topological insulators, with only a single Dirac
cone surface state intersecting the chemical potential. Our microscopic
calculations reveal the key role of anti-site defects for achieving this, and
predict optimal growth conditions to realize maximally-resistive ternary TIs. | 1204.4063v1 |
2013-09-12 | Theoretical Field Limits for Multi-Layer Superconductors | The SIS structure---a thin superconducting film on a bulk superconductor
separated by a thin insulating film---was propsed as a method to protect
alternative SRF materials from flux penetration by enhancing the first critical
field $B_{c1}$. In this work, we show that in fact $B_{c1}$ = 0 for a SIS
structure. We calculate the superheating field $B_{sh}$, and we show that it
can be enhanced slightly using the SIS structure, but only for a small range of
film thicknesses and only if the film and the bulk are different materials. We
also show that using a multilayer instead of a single thick layer is
detrimental, as this decreases $B_{sh}$ of the film. We calculate the
dissipation due to vortex penetration above the $B_{sh}$ of the film, and find
that it is unmanageable for SRF applications. However, we find that if a
gradient in the phase of the order parameter is introduced, SIS structures may
be able to shield large DC and low frequency fields. We argue that the SIS
structure is not beneficial for SRF cavities, but due to recent experiments
showing low-surface-resistance performance above $B_{c1}$ in cavities made of
superconductors with small coherence lengths, we argue that enhancement of
$B_{c1}$ is not necessary, and that bulk films of alternative materials show
great promise. | 1309.3239v2 |
2014-07-08 | Competing magnetic phases and field-induced dynamics in DyRuAsO | Analysis of neutron diffraction, dc magnetization, ac magnetic
susceptibility, heat capacity, and electrical resistivity for DyRuAsO in an
applied magnetic field are presented at temperatures near and below those at
which the structural distortion (T_S = 25 K) and subsequent magnetic ordering
(T_N = 10.5 K) take place. Powder neutron diffraction is used to determine the
antiferromagnetic order of Dy moments of magnitude 7.6(1) mu_B in the absence
of a magnetic field, and demonstrate the reorientation of the moments into a
ferromagnetic configuration upon application of a magnetic field. Dy magnetism
is identified as the driving force for the structural distortion. The magnetic
structure of analogous TbRuAsO is also reported. Competition between the two
magnetically ordered states in DyRuAsO is found to produce unusual physical
properties in applied magnetic fields at low temperature. An additional phase
transition near T* = 3 K is observed in heat capacity and other properties in
fields greater than about 3 T. Magnetic fields of this magnitude also induce
spin-glass-like behavior including thermal and magnetic hysteresis, divergence
of zero-field-cooled and field-cooled magnetization, frequency dependent
anomalies in ac magnetic susceptibility, and slow relaxation of the
magnetization. This is remarkable since DyRuAsO is a stoichiometric material
with no disorder detected by neutron diffraction, and suggests analogies with
spin-ice compounds and related materials with strong geometric frustration. | 1407.2184v1 |
2014-07-13 | Gate-tunable Phase Transitions in 1T-TaS$_2$ | The ability to tune material properties using gate electric field is at the
heart of modern electronic technology. It is also a driving force behind recent
advances in two-dimensional systems, such as gate-electric-field induced
superconductivity and metal-insulator transition. Here we describe an ionic
field-effect transistor (termed "iFET"), which uses gate-controlled lithium ion
intercalation to modulate the material property of layered atomic crystal
1T-TaS$_2$. The extreme charge doping induced by the tunable ion intercalation
alters the energetics of various charge-ordered states in 1T-TaS$_2$, and
produces a series of phase transitions in thin-flake samples with reduced
dimensionality. We find that the charge-density-wave states in 1T-TaS$_2$ are
three-dimensional in nature, and completely collapse in the two-dimensional
limit defined by their critical thicknesses. Meanwhile the ionic gating induces
multiple phase transitions from Mott-insulator to metal in 1T-TaS$_2$ thin
flakes at low temperatures, with 5 orders of magnitude modulation in their
resistance. Superconductivity emerges in a textured charge-density-wave state
induced by ionic gating. Our method of gate-controlled intercalation of 2D
atomic crystals in the bulk limit opens up new possibilities in searching for
novel states of matter in the extreme charge-carrier-concentration limit. | 1407.3480v1 |
2014-12-16 | Characterization of 3 mm Glass Electrodes and Development of RPC Detectors for $INO-ICAL$ Experiment | India-based Neutrino Observatory (INO) is a multi-institutional facility,
planned to be built up in South India. The INO facility will host a 51 kton
magnetized Iron CALorimeter (ICAL) detector to study atmospheric muon
neutrinos. Iron plates have been chosen as the target material whereas
Resistive Plate Chambers (RPCs) have been chosen as the active detector element
for the ICAL experiment. Due to the large number of RPCs needed ($\sim$ 28,000
of $2~m \times 2~m$ in size) for ICAL experiment and for the long lifetime of
the experiment, it is necessary to perform a detailed $R\&D$ such that each and
every parameter of the detector performance can be optimized to improve the
physics output. In this paper, we report on the detailed material and
electrical properties studies for various types of glass electrodes available
locally. We also report on the performance studies carried out on the RPCs made
with these electrodes as well as the effect of gas composition and
environmental temperature on the detector performance. We also lay emphasis on
the usage of materials for RPC electrodes and the suitable enviormental
conditions applicable for operating the RPC detector for optimal physics output
at INO-ICAL experiment. | 1412.4998v1 |
2015-12-07 | Insulating phase in Sr$_2$IrO$_4$: An investigation using critical analysis and magnetocaloric effect | The nature of insulating phase in 5$d$ based Sr$_2$IrO$_4$ is quite debated
as the theoretical as well as experimental investigations have put forward
evidences in favor of both magnetically driven Slater-type and interaction
driven Mott-type insulator. To understand this insulating behavior, we have
investigated the nature of magnetic state in Sr$_2$IrO$_4$ through studying
critical exponents, low temperature thermal demagnetization and magnetocaloric
effect. The estimated critical exponents do not exactly match with any
universality class, however, the values obey the scaling behavior. The exponent
values suggest that spin interaction in present material is close to mean-field
model. The analysis of low temperature thermal demagnetization data, however,
shows dual presence of localized- and itinerant-type of magnetic interaction.
Moreover, field dependent change in magnetic entropy indicates magnetic
interaction is close to mean-field type. While this material shows an
insulating behavior across the magnetic transition, yet a distinct change in
slope in resistivity is observed around $T_c$. We infer that though the
insulating phase in Sr$_2$IrO$_4$ more close to be Slater-type but the
simultaneous presence of both Slater- and Mott-type is the likely scenario for
this material. | 1512.02041v1 |
2016-02-04 | Picosecond electric-field-induced threshold switching in phase-change materials | Many chalcogenide glasses undergo a breakdown in electronic resistance above
a critical field strength. Known as threshold switching, this mechanism enables
field-induced crystallization in emerging phase-change memory. Purely
electronic as well as crystal nucleation assisted models have been employed to
explain the electronic breakdown. Here, picosecond electric pulses are used to
excite amorphous Ag$_4$In$_3$Sb$_{67}$Te$_{26}$. Field-dependent reversible
changes in conductivity and pulse-driven crystallization are observed. The
present results show that threshold switching can take place within the
electric pulse on sub-picosecond time-scales - faster than crystals can
nucleate. This supports purely electronic models of threshold switching and
reveals potential applications as an ultrafast electronic switch. | 1602.01885v2 |
2016-10-17 | Structural, magnetic, electric, dielectric, and thermodynamic properties of multiferroic GeV4S8 | The lacunar spinel GeV4S8 undergoes orbital and ferroelectric ordering at the
Jahn-Teller transition around 30 K and exhibits antiferromagnetic order below
about 14 K. In addition to this orbitally driven ferroelectricity, lacunar
spinels are an interesting material class, as the vanadium ions form V4
clusters representing stable molecular entities with a common electron
distribution and a well-defined level scheme of molecular states resulting in a
unique spin state per V4 molecule. Here we report detailed x-ray, magnetic
susceptibility, electrical resistivity, heat capacity, thermal expansion, and
dielectric results to characterize the structural, electric, dielectric,
magnetic, and thermodynamic properties of this interesting material, which also
exhibits strong electronic correlations. From the magnetic susceptibility, we
determine a negative Curie-Weiss temperature, indicative for antiferromagnetic
exchange and a paramagnetic moment close to a spin S = 1 of the V4 molecular
clusters. The low-temperature heat capacity provides experimental evidence for
gapped magnon excitations. From the entropy release, we conclude about strong
correlations between magnetic order and lattice distortions. In addition, the
observed anomalies at the phase transitions also indicate strong coupling
between structural and electronic degrees of freedom. Utilizing dielectric
spectroscopy, we find the onset of significant dispersion effects at the polar
Jahn-Teller transition. The dispersion becomes fully suppressed again with the
onset of spin order. In addition, the temperature dependencies of dielectric
constant and specific heat possibly indicate a sequential appearance of orbital
and polar order. | 1610.05244v1 |
2017-02-15 | Negative Differential Resistance in Graphene Boron Nitride Heterostructure Controlled by Twist and Phonon-Scattering | Two-dimensional (2D) crystals, such as graphene, hexagonal boron nitride and
transitional metal dichalcogenides, have attracted tremendous amount of
attention over the past decade due to their extraordinary thermal, electrical
and optical properties, making them promising nano-materials for the
next-generation electronic systems. A large number of heterostructures have
been fabricated by stacking of various 2D materials to achieve different
functionalities. In this work, we simulate the electron transport properties of
a three-terminal multilayer heterostructure made from graphene nanoribbons
vertically sandwiching a boron nitride tunneling barrier. To investigate the
effects of the unavoidable misalignment in experiments, we introduce a tunable
angular misorientation between 2D layers to the modeled system. Current-Voltage
(I-V) characteristics of the device exhibit multiple NDR peaks originating from
distinct mechanisms. A unique NDR mechanism arising from the lattice mismatch
is captured and it depends on both the twisting angle and voltage bias.
Analytical expressions for the positions of the resonant peaks observed in I-V
characteristic are developed. To capture the slight degradation of PVR ratios
observed in experiments when temperature increases from 2K to 300K,
electron-photon scattering decoherence has been added to the simulation,
indicating a good agreement with experiment works as well as a robust
preservation of resonant tunneling feature. | 1702.04435v1 |
2017-09-12 | Electric field induced semiconductor-to-metal phase transition in vertical MoTe2 and Mo1-xWxTe2 devices | Over the past years, transition metal dichalcogenides (TMDs) have attracted
attention as potential building blocks for various electronic applications due
to their atomically thin nature. An exciting development is the recent success
in 'engineering' crystal phases of TMD compounds during the growth due to their
polymorphic character. Here, we report an electric field induced reversible
engineered phase transition in vertical 2H-MoTe2 devices, a crucial
experimental finding that enables electrical phase switching for these
ultra-thin layered materials. Scanning tunneling microscopy (STM) was utilized
to analyze the TMD crystalline structure after applying an electric field, and
scanning tunneling spectroscopy (STS) was employed to map a
semiconductor-to-metal phase transition on the nanoscale. In addition, direct
confirmation of a phase transition from 2H semiconductor to a distorted 2H'
metallic phase was obtained by scanning transmission electron microscopy
(STEM). MoTe2 and Mo1-xWxTe2 alloy based vertical resistive random access
memory (RRAM) cells were fabricated to demonstrate clear reproducible and
controlled switching with programming voltages that are tunable by the layer
thickness and that show a distinctly different trend for the binary compound if
compared to the ternary materials. | 1709.03835v1 |
2018-01-05 | Writing on Nanocrystals: Patterning Colloidal Inorganic Nanocrystal Films through Irradiation-Induced Chemical Transformations of Surface Ligands | In the past couple of decades, colloidal inorganic nanocrystals and, more
specifically, semiconductor quantum dots have emerged as crucial materials for
the development of nanoscience and nanotechnology, with applications in very
diverse areas such as optoelectronics and biotechnology. Films made of
inorganic NCs deposited on a substrate can be patterned by e-beam lithography,
altering the structure of their capping ligands and thus allowing exposed areas
to remain on the substrate while non-exposed areas are redispersed in a
solvent, as in a standard lift-off process. This methodology can be described
as a direct lithography process, since the exposure is performed directly on
the material of interest, in contrast with conventional lithography which uses
a polymeric resist as a mask for subsequent material deposition or etching. A
few reports from the late 1990 and early 2000 used such direct lithography to
fabricate electrical wires from metallic NCs. However, the poor conductivity
obtained through this process hindered the widespread use of the technique. In
the early 2010, the same method was used to define fluorescent patterns on QD
films, allowing for further applications in biosensing. For the past 2 ,3
years, direct lithography on NC films with e-beams and X rays has gone through
an important development as it has been demonstrated that it can tune further
transformations on the NCs, leading to more complex patternings and opening a
whole new set of possible applications. | 1801.01756v1 |
2018-04-09 | Pressure effects on the structural and superconducting transitions in La3Co4Sn13 | La3Co4Sn13 is a superconducting material with transition temperature at Tc =
2.70 K, which presents a superlattice structural transition at T* ~ 150 K, a
common feature for this class of compounds. However, for this material, it is
not clear that at T* the lattice distortions arise from a charge density wave
(CDW) or from a distinct microscopic origin. Interestingly, it has been
suggested in isostructural non-magnetic intermetallic compounds that T* can be
suppressed to zero temperature, by combining chemical and external pressure,
and a quantum critical point is argued to be observed near these critical
doping/pressure. Our study shows that application of pressure on
single-crystalline La3Co4Sn13 enhances Tc and decreases T*. We observe thermal
hysteresis loops for cooling/heating cycles around T* for P > 0.6 GPa, in
electrical resistivity measurements, which are not seen in x-ray diffraction
data. The hysteresis in electrical measurements may be due to the pinning of
the CDW phase to impurities/defects, while the superlattice structural
transition maintains its ambient pressure second-order transition nature under
pressure. From our experiments we estimate that T* vanishes at around 5.5 GPa,
though no quantum critical behavior is observed up to 2.53 GPa. | 1804.03215v2 |
2018-06-14 | Tunneling spin valves based on Fe$_3$GeTe$_2$/hBN/Fe$_3$GeTe$_2$ van der Waals heterostructures | Thin van der Waals (vdW) layered magnetic materials disclose the possibility
to realize vdW heterostructures with new functionalities. Here we report on the
realization and investigation of tunneling spin valves based on van der Waals
heterostructures consisting of an atomically thin hBN layer acting as tunnel
barrier and two exfoliated Fe3GeTe2 crystals acting as ferromagnetic
electrodes. Low-temperature anomalous Hall effect measurements show that thin
Fe3GeTe2 crystals are metallic ferromagnets with an easy axis perpendicular to
the layers, and a very sharp magnetization switching at magnetic field values
that depend slightly on their geometry. In Fe3GeTe2/hBN/Fe3GeTe2
heterostructures, we observe a textbook behavior of the tunneling resistance,
which is minimum (maximum) when the magnetization in the two electrodes is
parallel (antiparallel) to each other. The magnetoresistance is 160% at low
temperature, from which we determine the spin polarization of Fe3GeTe2 to be
0.66, corresponding to 83% and 17% of majority and minority carriers,
respectively. The measurements also show that, with increasing temperature, the
evolution of the spin polarization extracted from the tunneling
magnetoresistance is proportional to the temperature dependence of the
magnetization extracted from the analysis of the anomalous Hall conductivity.
This suggests that the magnetic properties of the surface are representative of
those of the bulk, as it may be expected for vdW materials. | 1806.05411v1 |
2018-06-18 | Mott transition and collective charge pinning in electron doped Sr2IrO4 | We studied the in-plane dynamic and static charge conductivity of electron
doped Sr2IrO4 using optical spectroscopy and DC transport measurements. The
optical conductivity indicates that the pristine material is an indirect
semiconductor with a direct Mott-gap of 0.55 eV. Upon substitution of 2% La per
formula unit the Mott-gap is suppressed except in a small fraction of the
material (15%) where the gap survives, and overall the material remains
insulating. Instead of a zero energy mode (or Drude peak) we observe a soft
collective mode (SCM) with a broad maximum at 40 meV. Doping to 10% increases
the strength of the SCM, and a zero-energy mode occurs together with metallic
DC conductivity. Further increase of the La substitution doesn't change the
spectral weight integral up to 3 eV. It does however result in a transfer of
the SCM spectral weight to the zero-energy mode, with a corresponding reduction
of the DC resistivity for all temperatures from 4 to 300 K. The presence of a
zero-energy mode signals that at least part of the Fermi surface remains
ungapped at low temperatures, whereas the SCM appears to be caused by pinning a
collective frozen state involving part of the doped electrons. | 1806.06937v1 |
2018-06-22 | Disorder control in crystalline GeSb2Te4 and its impact on characteristic length scales | Crystalline GeSb2Te4 (GST) is remarkable material, as it allows to
continuously tune the electrical resistance by orders of magnitude without
involving a phase transition or stoichiometric changes, just by altering the
short-range order. While well-ordered specimen are metallic, increasing amounts
of disorder can eventually lead to an insulating state with vanishing
conductivity in the 0K limit, but a similar number of charge carriers. These
observations make disordered GST one of the most promising candidates for the
realization of a true Anderson insulator. While so far the low-temperature
properties have mostly been studied in films of small grain size, here a
sputter-deposition process is employed that enables preparation of a large
variety of these GST states including metallic and truly insulating ones. By
growing films of GST on mica substrates, biaxially textured samples with huge
grain sizes are obtained. A series of these samples is employed for transport
measurements, as their electron mean free path can be altered by a factor of
20. Yet, the mean free path always remains more than an order of magnitude
smaller than the lateral grain size. This proves unequivocally that grain
boundaries play a negligible role for electron scattering, while intragrain
scattering, presumably by disordered vacancies, dominates. Most importantly,
these findings underline that the Anderson insulating state as well as the
system's evolution towards metallic conductivity are indeed intrinsic
properties of the material. | 1806.08636v1 |
2019-09-26 | Large-area implementation and critical evaluation of the material and fabrication aspects of a thin-film thermoelectric generator based on aluminum-doped zinc oxide | A large-area thermoelectric generator (TEG) utilizing a folded thin-film
concept is implemented and the performance evaluated for near room temperature
applications having modest temperature gradients (< 50 K). The TEGs with the
area of ~0.33 m^2 are shown capable of powering a wireless sensor node of
multiple sensors suitable e.g. for monitoring environmental variables in
buildings. The TEGs are based on a transparent, non-toxic and abundant
thermoelectric material, i.e. aluminium-doped zinc oxide (AZO), deposited on
flexible substrates. After folding, both the electrical current and heat flux
are in the plane of the thermoelectric thin-film. Heat leakage in the folded
TEG is shown to be minimal (close to that of air), enabling sufficient
temperature gradients without efficient heat sinks, contrary to the
conventional TEGs having the thermal flux and electrical current perpendicular
to the plane of the thermoelectric films. The long-term stability studies
reveal that there are no significant changes in the electrical or
thermoelectric properties of AZO over several months, while the contact
resistance between AZO and silver ink is an issue exhibiting a continuous
increase over time. The performance of the TEGs and technological implications
in relation to a state-of-the-art thermoelectric material are further assessed
via a computational study. | 1909.12042v2 |
2019-09-26 | Quantum effects in graphitic materials: Colossal magnetoresistance, Andreev reflections, Little-Parks effect, ferromagnetism, and granular superconductivity | Unlike the more common local conductance spectroscopy, nonlocal conductance
can differentiate between nontopological zero-energy modes localized around
inhomogeneities, and true Majorana edge modes in the topological phase. In
particular, negative nonlocal conductance is dominated by the crossed Andreev
reflection. In graphene, the Andreev reflection and the inter-band Klein
tunneling couple electron-like and hole-like states through the action of
either a superconducting (SC) pair potential or an electrostatic potential. We
are here probing quantum phenomena in modified graphitic samples. Four-point
contact transport measurements at cryogenic to room temperatures were conducted
using a Quantum Design Physical Property Measurement System. The observed
negative nonlocal differential conductance Gdiff probes the Andreev reflection
at the walls of the SC grains coupled by Josephson effect through the
semiconducting matrix. In addition, Gdiff shows the butterfly shape that is
characteristic to resistive random-access memory devices. In a magnetic field,
the Andreev reflection counters the effect of the otherwise lowered conduction.
At low temperatures, the magnetoresistance shows irreversible yet strong
colossal oscillations that are known to be quantum in nature. In addition, we
have found evidence for seemingly granular SC as well as ferromagnetism.
Moreover, the Little-Parks effect is revealed in both the classical
small-amplitude and the phase-slip driven large-amplitude oscillations in the
magnetoresistance. Thus, graphitic materials show potential for quantum
electronics applications, including rectification and topological states. | 1909.12145v1 |
2015-05-18 | Photoemission System with Polarized Hard X-rays for Probing Ground State Symmetry of Strongly Correlated Materials | We have developed a polarized hard X-ray photoemission (HAXPES) system to
study the ground-state symmetry of strongly correlated materials. The linear
polarization of the incoming X-ray beam is switched by the transmission-type
phase retarder composed of two diamond (100) crystals. The best degree of the
linear polarization $P_L$ is $-0.96$, containing the vertical polarization
component of 98%. A newly developed low temperature two-axis manipulator
enables easy polar and azimuthal rotations to select the detection direction of
photoelectrons. The lowest temperature achieved is 9 K, offering us a chance to
access the ground state even for the strongly correlated electron systems in
cubic symmetry. The co-axial sample monitoring system with the
long-working-distance microscope enables us to keep measuring the same region
on the sample surface before and after rotation procedures. Combining this
sample monitoring system with a micro-focused X-ray beam by means of an
ellipsoidal Kirkpatrick-Baez mirror (25 $\mu$m $\times$ 25 $\mu$m (FWHM)), we
have demonstrated the polarized valence-band HAXPES on NiO for voltage
application as resistive random access memories to reveal the origin of the
metallic spectral weight near the Fermi level. | 1505.04591v1 |
2017-01-02 | Thermoelectric power factor enhancement by spin-polarized currents - a nanowire case study | Thermoelectric (TE) measurements have been performed on the workhorses of
today's data storage devices, exhibiting either the giant or the anisotropic
magnetoresistance effect (GMR and AMR). The temperature-dependent (50-300 K)
and magnetic field-dependent (up to 1 T) TE power factor (PF) has been
determined for several Co-Ni alloy nanowires with varying Co:Ni ratios as well
as for Co-Ni/Cu multilayered nanowires with various Cu layer thicknesses, which
were all synthesized via a template-assisted electrodeposition process. A
systematic investigation of the resistivity, as well as the Seebeck
coefficient, is performed for Co-Ni alloy nanowires and Co-Ni/Cu multilayered
nanowires. At room temperature, measured values of TE PFs up to 3.6 mWK-2m-1
for AMR samples and 2.0 mWK-2m-1 for GMR nanowires are obtained. Furthermore,
the TE PF is found to increase by up to 13.1 % for AMR Co-Ni alloy nanowires
and by up to 52 % for GMR Co-Ni/Cu samples in an external applied magnetic
field. The magnetic nanowires exhibit TE PFs that are of the same order of
magnitude as TE PFs of Bi-Sb-Se-Te based thermoelectric materials and,
additionally, give the opportunity to adjust the TE power output to changing
loads and hotspots through external magnetic fields. | 1701.00404v1 |
2018-11-20 | Towards integrated metatronics: a holistic approach on precise optical and electrical properties of Indium Tin Oxide | The class of transparent conductive oxides includes the material indium tin
oxide (ITO) and has become a widely used material of modern every-day life such
as in touch screens of smart phones and watches, but also used as an optically
transparent low electrically-resistive contract in the photovoltaics industry.
More recently ITO has shown epsilon-near-zero (ENZ) behavior in the
telecommunication frequency band enabling both strong index modulation and
other optically-exotic applications such as metatronics. However the ability to
precisely obtain targeted electrical and optical material properties in ITO is
still challenging due to complex intrinsic effects in ITO and as such no
integrated metatronic platform has been demonstrated to-date. Here we deliver
an extensive and accurate description process parameters of RF-sputtering,
showing a holistic control of the quality of ITO thin films in the visible and
particularly near-infrared spectral region. We further are able to
custom-engineer the ENZ point across the telecommunication band by explicitly
controlling the sputtering process conditions. Exploiting this control we
design a functional sub-wavelength-scale filter based on lumped
circuit-elements, towards the realization of integrated metatronic devices and
circuits. | 1811.08344v4 |
2018-11-30 | Thermal Transport Across Graphene Step Junctions | Step junctions are often present in layered materials, i.e. where
single-layer regions meet multi-layer regions, yet their effect on thermal
transport is not understood to date. Here, we measure heat flow across graphene
junctions (GJs) from monolayer to bilayer graphene, as well as bilayer to
four-layer graphene for the first time, in both heat flow directions. The
thermal conductance of the monolayer-bilayer GJ device ranges from ~0.5 to
9.1x10^8 Wm-2K-1 between 50 K to 300 K. Atomistic simulations of such GJ device
reveal that graphene layers are relatively decoupled, and the low thermal
conductance of the device is determined by the resistance between the two
dis-tinct graphene layers. In these conditions the junction plays a negligible
effect. To prove that the decoupling between layers controls thermal transport
in the junction, the heat flow in both directions was measured, showing no
evidence of thermal asymmetry or rectification (within experimental error
bars). For large-area graphene applications, this signifies that small bilayer
(or multilayer) islands have little or no contribution to overall thermal
transport. | 1811.12622v1 |
2019-05-29 | Physical mechanisms involved in the formation and operation of memory devices based on a monolayer of gold nanoparticles-polythiophene hybrid materials | Understanding the physical and chemical mechanisms occurring during the
forming process and operation of an organic resistive memory device is a major
issue for better performances. Various mechanisms were suggested in vertically
stacked memory structures, but the analysis remains indirect and needs
destructive characterization (e.g. cross-section to access the organic layers
sandwiched between electrodes). Here, we report a study on a planar, monolayer
thick, hybrid nanoparticle/molecule device (10 nm gold nanoparticles embedded
in an electro-generated poly(2-thienyl-3,4-(ethylenedioxy)thiophene) layer),
combining, in situ, on the same device, physical (scanning electron microscope,
physico-chemical (thermogravimetry and mass spectroscopy, Raman spectroscopy)
and electrical (temperature dependent current-voltage) characterizations. We
demonstrate that the forming process causes an increase in the gold particle
size, almost 4 times larger than the starting nanoparticles, and that the
organic layer undergoes a significant chemical rearrangement from a sp3 to sp2
amorphous carbon material. Temperature dependent electrical characterizations
of this nonvolatile memory confirm that the charge transport mechanism in the
device is consistent with a trap-filled space charge limited current in the off
state, the sp2 amorphous carbon material containing many electrically active
defects. | 1905.12719v1 |
2020-03-25 | Synergistically creating sulfur vacancies in semimetal-supported amorphous MoS2 for efficient hydrogen evolution | The presence of elemental vacancies in materials is inevitable according to
statistical thermodynamics, which will decide the chemical and physical
properties of the investigated system. However, the controlled manipulation of
vacancies for specific applications is a challenge. Here we report a facile
method for creating large concentrations of S vacancies in the inert basal
plane of MoS2 supported on semimetal CoMoP2. With a small applied potential, S
atoms can be removed in the form of H2S due to the optimized free energy of
formation. The existence of vacancies favors electron injection from the
electrode to the active site by decreasing the contact resistance. As a
consequence, the activity is increased by 221 % with the vacancy-rich MoS2 as
electrocatalyst for hydrogen evolution reaction (HER). A small overpotential of
75 mV is needed to deliver a current density of 10 mA cm-2, which is considered
among the best values achieved for MoS2. It is envisaged that this work may
provide a new strategy for utilizing the semimetal phase for structuring MoS2
into a multi-functional material. | 2003.11436v1 |
2020-04-01 | Physical properties and thermal stability of Fe5GeTe2 single crystals | The magnetic and transport properties of Fe-deficient Fe5GeTe2 single
crystals (Fe5-xGeTe2 with x~0.3) were studied and the impact of thermal
processing was explored. Quenching crystals from the growth temperature has
been previously shown to produce a metastable state that undergoes a strongly
hysteretic first-order transition upon cooling below ~100K. The first-order
transition impacts the magnetic properties, yielding an enhancement in the
Curie temperature T_C from 270 to 310K. In the present work, T_HT ~550K has
been identified as the temperature above which metastable crystals are obtained
via quenching. Diffraction experiments reveal a structural change at this
temperature, and significant stacking disorder occurs when samples are slowly
cooled through this temperature range. The transport properties are
demonstrated to be similar regardless of the crystal's thermal history. The
scattering of charge carriers appears to be dominated by moments fluctuating on
the Fe(1) sublattice, which remain dynamic down to 100-120K. Maxima in the
magnetoresistance and anomalous Hall resistance are observed near 120K. The
Hall and Seebeck coefficients are also impacted by magnetic ordering on the
Fe(1) sublattice. The data suggest that both electrons and holes contribute to
conduction above 120K, but that electrons dominate at lower temperature when
all of the Fe sublattices are magnetically ordered. This study demonstrates a
strong coupling of the magnetism and transport properties in Fe5-xGeTe2 and
complements the previous results that demonstrated strong magnetoelastic
coupling as the Fe(1) moments order. The published version of this manuscript
is DOI:10.1103/PhysRevMaterials.3.104401 (2019) | 2004.00494v1 |
2020-04-14 | Photoanodes Based on TiO$_2$ and $α$-Fe$_2$O$_3$ for Solar Water Splitting Superior Role of 1D Nanoarchitectures and of Combined Heterostructures | Solar driven photoelectrochemical water splitting (PEC-WS) using
semiconductor photoelectrodes represents a promising approach for a sustainable
and environmentally friendly production of renewable energy vectors and fuel
sources, such as dihydrogen (H2). In this context, titanium dioxide (TiO$_2$)
and iron oxide (hematite, $\alpha$-Fe$_2$O$_3$) are among the most investigated
candidates as photoanode materials, mainly owing to their resistance to
photocorrosion, non-toxicity, natural abundance, and low production cost. Major
drawbacks are, however, an inherently low electrical conductivity and a limited
hole diffusion length that significantly affect the performance of TiO$_2$ and
$\alpha$-Fe$_2$O$_3$ in PEC devices. To this regard, one-dimensional (1D)
nanostructuring is typically applied as it provides several superior features
such as a significant enlargement of the material surface area, extended
contact between the semiconductor and the electrolyte and, most remarkably,
preferential electrical transport that overall suppress charge carrier
recombination and improve TiO$_2$ and $\alpha$-Fe$_2$O$_3$
photo-electrocatalytic properties. The present review describes various
synthetic methods, properties and PEC applications of 1D-photoanodes
(nanotubes, nanorods, nanofibers, nanowires) based on titania, hematite, and on
$\alpha$-Fe$_2$O$_3$/TiO$_2$ heterostructures. Various routes towards
modification and enhancement of PEC activity of 1D photoanodes are also
discussed including doping, decoration with co-catalysts and heterojunction
engineering. Finally, the challenges related to the optimization of charge
transfer kinetics in both oxides are highlighted. | 2004.12844v1 |
2007-10-08 | Colossal dielectric constants in single-crystalline and ceramic CaCu3Ti4O12 investigated by broadband dielectric spectroscopy | In the present work the authors report results of broadband dielectric
spectroscopy on various samples of CaCu3Ti4O12, including so far only rarely
investigated single crystalline material. The measurements extend up to 1.3
GHz, covering more than nine frequency decades. We address the question of the
origin of the colossal dielectric constants and of the relaxational behavior in
this material, including the second relaxation reported in several recent
works. For this purpose, the dependence of the temperature- and
frequency-dependent dielectric properties on different tempering and surface
treatments of the samples and on ac-field amplitude are investigated. Broadband
spectra of a single crystal are analyzed by an equivalent circuit description,
assuming two highly resistive layers in series to the bulk. Good fits could be
achieved, including the second relaxation, which also shows up in single
crystals. The temperature- and frequency-dependent intrinsic conductivity of
CCTO is consistent with the Variable Range Hopping model. The second relaxation
is sensitive to surface treatment and, in contrast to the main relaxation, also
is strongly affected by the applied ac voltage. Concerning the origin of the
two insulating layers, we discuss a completely surface-related mechanism
assuming the formation of a metal-insulator diode and a combination of surface
and internal barriers. | 0710.1610v1 |
2017-04-13 | Extreme Magnetoresistance in Magnetic Rare Earth Monopnictides | The acute sensitivity of the electrical resistance of certain systems to
magnetic fields known as extreme magnetoresistance (XMR) has recently been
explored in a new materials context with topological semimetals. Exemplified by
WTe$_{2}$ and rare earth monopnictide La(Sb,Bi), these systems tend to be
non-magnetic, nearly compensated semimetals and represent a platform for large
magnetoresistance driven by intrinsic electronic structure. Here we explore
electronic transport in magnetic members of the latter family of semimetals and
find that XMR is strongly modulated by magnetic order. In particular, CeSb
exhibits XMR in excess of $1.6 \times 10^{6}$ % at fields of 9 T while the
magnetoresistance itself is non-monotonic across the various magnetic phases
and shows a transition from negative magnetoresistance to XMR with field above
magnetic ordering temperature $T_{N}$. The magnitude of the XMR is larger than
in other rare earth monopnictides including the non-magnetic members and
follows an non-saturating power law to fields above 30 T. We show that the
overall response can be understood as the modulation of conductivity by the Ce
orbital state and for intermediate temperatures can be characterized by an
effective medium model. Comparison to the orbitally quenched compound GdBi
supports the correlation of XMR with the onset of magnetic ordering and
compensation and highlights the unique combination of orbital inversion and
type-I magnetic ordering in CeSb in determining its large response. These
findings suggest a paradigm for magneto-orbital control of XMR and are relevant
to the understanding of rare earth-based correlated topological materials. | 1704.04226v1 |
2018-08-28 | Mott metal-insulator transitions in pressurized layered trichalcogenides | Transition metal phosphorous trichalcogenides, $M{\rm P}X_3$ ($M$ and $X$
being transition metal and chalcogen elements respectively), have been the
focus of substantial interest recently because of their possible magnetism in
the two-dimensional limit. Here we investigate material properties of the
compounds with $M$ = Mn and Ni employing $\textit{ab-initio}$ density
functional and dynamical mean-field calculations, especially their electronic
behavior under external pressure in the paramagnetic phase. Mott
metal-insulator transitions (MIT) are found to be a common feature for both
compounds, but their lattice structures show drastically different behaviors
depending on the relevant orbital degrees of freedom, i.e. $t_{\rm 2g}$ or
$e_{g}$. MnPS$_3$ undergoes an isosymmetric structural transition by forming
Mn-Mn dimers due to the strong direct overlap between the neighboring $t_{\rm
2g}$ orbitals, accompanied by a significant volume collapse and a spin-state
transition. In contrast, NiPS$_3$ and NiPSe$_3$, with their active $e_g$
orbital degrees of freedom, do not show a structural change at the MIT pressure
or deep in the metallic phase. Hence NiPS$_3$ and NiPSe$_3$ become rare
examples of materials hosting electronic bandwidth-controlled Mott MITs, thus
showing promise for ultrafast resistivity switching behavior. | 1808.09263v2 |
2019-03-01 | Unraveling Bulk and Grain Boundary Electrical Properties in La0.8Sr0.2Mn1-yO3 Thin Films | Grain boundaries in Sr-doped LaMnO3 thin films have been shown to strongly
influence the electronic and oxygen mass transport properties, being able to
profoundly modify the nature of the material. The unique behaviour of the grain
boundaries can be correlated with substantial modifications of the cation
concentration at the interfaces, which can be tuned by changing the overall
cationic ratio in the films. In this work, we study the electronic properties
of La0.8Sr0.2Mn1-yO3 thin films with variable Mn content. The influence of the
cationic composition on the grain boundary and grain bulk electronic properties
is elucidated by studying the manganese valence state evolution using
spectroscopy techniques and by confronting the electronic properties of
epitaxial and polycrystalline films. Substantial differences in the electronic
conduction mechanism are found in the presence of grain boundaries and
depending on the manganese content. Moreover, the unique defect chemistry of
the nanomaterial is elucidated by measuring the electrical resistance of the
thin films as a function of oxygen partial pressure, disclosing the importance
of the cationic local non-stoichiometry on the thin films behavior. | 1903.00235v1 |
2019-04-12 | Spin-dependent transport in van der Waals magnetic tunnel junctions with Fe3GeTe2 electrodes | Van der Waals (vdW) heterostructures, stacking different two-dimensional
materials, have opened up unprecedented opportunities to explore new physics
and device concepts. Especially interesting are recently discovered
two-dimensional magnetic vdW materials, providing new paradigms for spintronic
applications. Here, using density functional theory (DFT) calculations, we
investigate the spin-dependent electronic transport across vdW magnetic tunnel
junctions (MTJs) composed of Fe3GeTe2 ferromagnetic electrodes and a graphene
or hexagonal boron nitride (h-BN) spacer layer. For both types of junctions, we
find that the junction resistance changes by thousands of percent when the
magnetization of the electrodes is switched from parallel to antiparallel. Such
a giant tunneling magnetoresistance (TMR) effect is driven by dissimilar
electronic structure of the two spin-conducting channels in Fe3GeTe2, resulting
in a mismatch between the incoming and outgoing Bloch states in the electrodes
and thus suppressed transmission for an antiparallel-aligned MTJ. The vdW
bounding between electrodes and a spacer layer makes this result virtually
independent of the type of the spacer layer, making the predicted giant TMR
effect robust with respect to strain, lattice mismatch, interface distance and
other parameters which may vary in the experiment. We hope that our results
will further stimulate experimental studies of vdW MTJs and pave the way for
their applications in spintronics. | 1904.06098v1 |
2019-12-13 | Shallow impurity band in ZrNiSn | ZrNiSn and related half Heusler compounds are candidate materials for
efficient thermoelectric energy conversion with a reported thermoelectric
figure-of-merit of n-type ZrNiSn exceeding unity. Progress on p-type materials
has been more limited, which has been attributed to the presence of an impurity
band, possibly related to the presence of Ni interstitials in nominally vacant
4d position. The specific energetic position of this band, however, has not
been resolved. Here, we report results of a concerted theory-experiment
investigation for a nominally undoped ZrNiSn, based on measurements of
electrical resistivity, Hall coefficient, Seebeck coefficient and Nernst
coefficient, measured in a temperature range from 80 to 420 K. The results are
analyzed with a semi-analytical model combining a density functional theory
(DFT) description for ideal ZrNiSn, with a simple analytical correction for the
impurity band. The model provides a good quantitative agreement with
experiment, describing all salient features in the full temperature span for
the Hall, conductivity, and Seebeck measurements, while also reproducing key
trends in the Nernst results. This comparison pinpoints the impurity band edge
to 40 meV below the conduction band edge, which agrees well with a separate DFT
study of a supercell containing Ni interstitials. Moreover, we corroborate our
result with a separate study of ZrNiSn0.9Pb0.1 sample showing similar agreement
with an impurity band edge shifted to 32 meV below the conduction band. | 1912.06539v1 |
2020-12-23 | A general theory for anisotropic Kirchhoff-Love shells with in-plane bending of embedded fibers | This work presents a generalized Kirchhoff-Love shell theory that can
explicitly capture fiber-induced anisotropy not only in stretching and
out-of-plane bending, but also in in-plane bending. This setup is particularly
suitable for heterogeneous and fibrous materials such as textiles,
biomaterials, composites and pantographic structures. The presented theory is a
direct extension of classical Kirchhoff-Love shell theory to incorporate the
in-plane bending resistance of fibers. It also extends existing second-gradient
Kirchhoff-Love shell theory for initially straight fibers to initially curved
fibers. To describe the additional kinematics of multiple fiber families, a
so-called in-plane curvature tensor -- which is symmetric and of second order
-- is proposed. The effective stress tensor and the in-plane and out-of-plane
moment tensors are then identified from the mechanical power balance. These
tensors are all second order and symmetric in general. Constitutive equations
for hyperelastic materials are derived from different expressions of the
mechanical power balance. The weak form is also presented as it is required for
computational shell formulations based on rotation-free finite element
discretizations. | 2101.03122v3 |
2012-03-06 | Discrete modelling of capillary mechanisms in multi-phase granular media | A numerical study of multi-phase granular materials based upon
micro-mechanical modelling is proposed. Discrete element simulations are used
to investigate capillary induced effects on the friction properties of a
granular assembly in the pendular regime. Capillary forces are described at the
local scale through the Young-Laplace equation and are superimposed to the
standard dry particle interaction usually well simulated through an
elastic-plastic relationship. Both effects of the pressure difference between
liquid and gas phases and of the surface tension at the interface are
integrated into the interaction model. Hydraulic hysteresis is accounted for
based on the possible mechanism of formation and breakage of capillary menisci
at contacts. In order to upscale the interparticular model, triaxial loading
paths are simulated on a granular assembly and the results interpreted through
the Mohr-Coulomb criterion. The micro-mechanical approach is validated with a
capillary cohesion induced at the macroscopic scale. It is shown that
interparticular menisci contribute to the soil resistance by increasing normal
forces at contacts. In addition, more than the capillary pressure level or the
degree of saturation, our findings highlight the importance of the density
number of liquid bonds on the overall behaviour of the material. | 1203.1234v1 |
2015-06-12 | Current-limiting challenges for all-spin logic devices | All-spin logic device (ASLD) has attracted increasing interests as one of the
most promising post-CMOS device candidates, thanks to its low power,
non-volatility and logic-in-memory structure. Here we investigate the key
current-limiting factors and develop a physics-based model of ASLD through
nano-magnet switching, the spin transport properties and the breakdown
characteristic of channel. First, ASLD with perpendicular magnetic anisotropy
(PMA) nano-magnet is proposed to reduce the critical current (Ic0). Most
important, the spin transport efficiency can be enhanced by analyzing the
device structure, dimension, contact resistance as well as material parameters.
Furthermore, breakdown current density (JBR) of spin channel is studied for the
upper current limitation. As a result, we can deduce current-limiting
conditions and estimate energy dissipation. Based on the model, we demonstrate
ASLD with different structures and channel materials (graphene and copper).
Asymmetric structure is found to be the optimal option for current limitations.
Copper channel outperforms graphene in term of energy but seriously suffers
from breakdown current limit. By exploring the current limit and performance
tradeoffs, the optimization of ASLD is also discussed. This benchmarking model
of ASLD opens up new prospects for design and implementation of future
spintronics applications. | 1506.03946v2 |
2019-01-15 | Phenomenal magneto-elastoresistance of WTe$_{2}$: strain engineering of electronic and quantum transport properties | Elastoresistance describes the relative change of a material's resistance
when strained. It has two major contributions: strain induced geometric and
electronic changes. If the geometric factor dominates, like in ordinary metals
such as copper, the elastoresistance is positive and rather small, i.e.
typically of order 1. In a few materials, however, changes in electronic
structure dominate, which gives rise to larger and even negative values, such
as (-11) for Bi. Here, we report that the transition metal dichalcogenide
(TMDC) WTe$_{2}$ is a member of the second group, exhibiting a large and
non-monotonic elastoresistance that is about (-20) near 100 K and changes sign
at low temperatures. We discover that an applied magnetic field has a dramatic
effect on the elastoresistance in WTe$_{2}$: in the quantum regime at low
temperatures, it leads to quantum oscillations of the elastoresistance, that
ranges between (-80) to 120 within a field range of only half a Tesla. In the
semiclassical regime at intermediate temperatures, we find that the
elastoresistance rapidly increases and changes sign in a magnetic field. We
provide a semi-quantitative understanding of our experimental results using a
combination of first-principle and analytical low-energy model calculations.
Understanding bulk properties of TMDCs under uniaxial strain is an important
stepping stone toward strain engineering of 2D TMDCs. | 1901.05090v1 |
2019-10-26 | Magic continuum in twisted bilayer WSe2 | Emergent quantum phases driven by electronic interactions can manifest in
materials with narrowly dispersing, i.e. "flat", energy bands. Recently, flat
bands have been realized in a variety of graphene-based heterostructures using
the tuning parameters of twist angle, layer stacking and pressure, and
resulting in correlated insulator and superconducting states. Here we report
the experimental observation of similar correlated phenomena in twisted bilayer
tungsten diselenide (tWSe2), a semiconducting transition metal dichalcogenide
(TMD). Unlike twisted bilayer graphene where the flat band appears only within
a narrow range around a "magic angle", we observe correlated states over a
continuum of angles, spanning 4 degree to 5.1 degree. A Mott-like insulator
appears at half band filling that can be sensitively tuned with displacement
field. Hall measurements supported by ab initio calculations suggest that the
strength of the insulator is driven by the density of states at half filling,
consistent with a 2D Hubbard model in a regime of moderate interactions. At 5.1
degree twist, we observe evidence of superconductivity upon doping away from
half filling, reaching zero resistivity around 3 K. Our results establish
twisted bilayer TMDs as a model system to study interaction-driven phenomena in
flat bands with dynamically tunable interactions. | 1910.12147v1 |
2020-06-26 | YBCO-based non-volatile ReRAM tested in Low Earth Orbit | An YBCO-based test structure corresponding to the family of ReRAM devices
associated with the valence change mechanism is presented. We have
characterized its electrical response previous to its lift-off to a Low Earth
Orbit (LEO) using standard electronics and also with the dedicated LabOSat-01
controller. Similar results were obtained in both cases. After about 200 days
at LEO on board a small satellite, electrical tests started on the memory
device using the LabOSat-01 controller. We discuss the results of the first 150
tests, performed along a 433-day time interval in space. The memory device
remained operational despite the hostile conditions that involved launching,
lift-off vibrations, permanent thermal cycling and exposure to ionizing
radiation, with doses 3 orders of magnitude greater than the usual ones on
Earth. The device showed resistive switching and IV characteristics similar to
those measured on Earth, although with changes that follow a smooth drift in
time. A detailed study of the electrical transport mechanisms, based on
previous models that indicate the existence of various conducting mechanisms
through the metal-YBCO interface showed that the observed drift can be
associated with a local temperature drift at the LabOSat controller, with no
clear evidence that allows determining changes in the underlying microscopic
factors. These results show the reliability of complex-oxide non-volatile
ReRAM-based devices in order to operate under all the hostile conditions
encountered in space-borne applications. | 2006.15062v1 |
2020-07-22 | Machine Learning Potential for Hexagonal Boron Nitride Applied to Thermally and Mechanically Induced Rippling | We introduce an interatomic potential for hexagonal boron nitride (hBN) based
on the Gaussian approximation potential (GAP) machine learning methodology. The
potential is based on a training set of configurations collected from density
functional theory (DFT) simulations and is capable of treating bulk and
multilayer hBN as well as nanotubes of arbitrary chirality. The developed force
field faithfully reproduces the potential energy surface predicted by DFT while
improving the efficiency by several orders of magnitude. We test our potential
by comparing formation energies, geometrical properties, phonon dispersion
spectra and mechanical properties with respect to benchmark DFT calculations
and experiments. In addition, we use our model and a recently developed
graphene-GAP to analyse and compare thermally and mechanically induced rippling
in large scale two-dimensional (2D) hBN and graphene. Both materials show
almost identical scaling behaviour with an exponent of $\eta \approx 0.85$ for
the height fluctuations agreeing well with the theory of flexible membranes.
Based on its lower resistance to bending, however, hBN experiences slightly
larger out-of-plane deviations both at zero and finite applied external strain.
Upon compression a phase transition from incoherent ripple motion to
soliton-ripples is observed for both materials. Our potential is freely
available online at [http://www.libatoms.org]. | 2007.11448v3 |
2020-07-24 | Determination of the spin Hall angle, spin mixing conductance and spin diffusion length in Ir/CoFeB for spin-orbitronic devices | Iridium is a very promising material for spintronic applications due to its
interesting magnetic properties such as large RKKY exchange coupling as well as
its large spin-orbit coupling value. Ir is for instance used as a spacer layer
for perpendicular synthetic antiferromagnetic or ferrimagnet systems. However,
only a few studies of the spintronic parameters of this material have been
reported. In this paper, we present inverse spin Hall effect - spin pumping
ferromagnetic resonance measurements on CoFeB/Ir based bilayers to estimate the
values of the effective spin Hall angle, the spin diffusion length within
iridium, and the spin mixing conductance in the CoFeB/Ir bilayer. In order to
have reliable results, we performed the same experiments on CoFeB/Pt bilayers,
which behavior is well known due to numerous reported studies. Our experimental
results show that the spin diffusion length within iridium is 1.3 nm for
resistivity of 250 n$\Omega$.m, the spin mixing conductance $g_{eff}^{\uparrow
\downarrow}$ of the CoFeB/Ir interface is 30 nm$^{-2}$, and the spin Hall angle
of iridium has the same sign than the one of platinum and is evaluated at 26%
of the one of platinum. The value of the spin Hall angle found is 7.7% for Pt
and 2% for Ir. These relevant parameters shall be useful to consider Ir in new
concepts and devices combining spin-orbit torque and spin-transfer torque. | 2007.12413v1 |
2020-10-06 | Conductance Model for Single-Crystalline/Compact Metal Oxide Gas Sensing Layers in the Non-Degenerate Limit: Example of Epitaxial SnO$_2$(101) | Semiconducting metal oxide (SMOX)-based gas sensors are indispensable for
safety and health applications, e.g. explosive, toxic gas alarms, controls for
intake into car cabins and monitor for industrial processes. In the past, the
sensor community has been studying polycrystalline materials as sensors where
the porous and random microstructure of the SMOX does not allow a separation of
the phenomena involved in the sensing process. This lead to conduction models
that can model and predict the behavior of the overall response, but they were
not capable of giving fundamental information regarding the basic mechanisms
taking place. The study of epitaxial layers is the definite prove to clarify
the different aspects and contributions of the sensing mechanisms that are not
possible to do by studying a polycrystalline sample. A detailed analytical
model for n and p-type single-crystalline/compact metal oxide gas sensors was
developed that directly relates the conductance of the sample with changes in
the surface electrostatic potential. Combined DC resistance and work function
measurements were used in a compact SnO2 (101) layer in operando conditions
that allowed us to check the validity of our model in the region where
Boltzmann approximation holds to determine surface and bulk properties of the
material. | 2010.02962v1 |
2020-11-16 | Ferroelectric tunnel junctions | My research is dedicated to the electronic properties of functional oxides.
My activity specifically focuses on ferroelectric tunnel junctions in which an
ultrathin layer of ferroelectric material is intercalated between two metallic
electrodes. In these devices, polarization reversal induces large modifications
of the tunnel resistance, leading to a non-destructive readout of the
information. After a demonstration of the concept with scanning probe
microscopy techniques, I have been exploring the properties of ferroelectric
tunnel junctions with various ferroelectric materials (BaTiO3, BiFeO3, and
PVDF). I showed that these devices possess attractive properties for
applications as non-volatile binary memories. In addition, exploring the fact
that polarization usually reverses by the nucleation and propagation of
domains, I demonstrated a memristive behavior in the junctions associated to
non-uniform configurations of ferroelectric domains. Such ferroelectric
memristors can be used as artificial synapses in neuromorphic architectures.
Coupled to magnetic electrodes, the resulting multiferroic tunnel junctions
enable a non-volatile control of magnetism at the ferroelectric/electrode
interface and of the spin-polarized current associated. Besides this main
activity on tunnel devices, I explored the influence of ferroelectricity on
magnetic orders and on the properties of functional oxides. | 2011.07864v1 |
2021-02-09 | Quasi-quantized Hall response in bulk InAs | The quasi-quantized Hall effect (QQHE) is the three-dimensional (3D)
counterpart of the integer quantum Hall effect (QHE),exhibited only by
two-dimensional (2D) electron systems. It has recently been observed in layered
materials, consisting of stacks of weakly coupled 2D platelets that are yet
characterized by a 3D anisotropic Fermi surface. However, it is predicted that
the quasi-quantized 3D version of the 2D QHE should occur in a much broader
class of bulk materials, regardless of the underlying crystal structure. Here,
we compare the observation of quasi-quantized plateau-like features in the Hall
conductivity of then-type bulk semiconductor InAs with the predictions for the
3D QQHE in presence of parabolic electron bands. InAs takes form of a cubic
crystal without any low-dimensional substructure. The onset of the plateau-like
feature in the Hall conductivity scales with $\sqrt{2/3}k_{F}^{z}/\pi$ in units
of the conductance quantum and is accompanied by a Shubnikov-de Haas minimum in
the longitudinal resistivity, consistent wit the results of calculations. This
confirms the suggestion that the 3D QQHE may be a generic effect directly
observable in materials with small Fermi surfaces, placed in sufficiently
strong magnetic fields | 2102.04928v3 |
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