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2023-04-03 | CVD Graphene Contacts for Lateral Heterostructure MoS${_2}$ Field Effect Transistors | Intensive research is carried out on two-dimensional materials, in particular
molybdenum disulfide, towards high-performance transistors for integrated
circuits. Fabricating transistors with ohmic contacts is challenging due to the
high Schottky barrier that severely limits the transistors' performance.
Graphene-based heterostructures can be used in addition or as a substitute for
unsuitable metals. We present lateral heterostructure transistors made of
scalable chemical vapor-deposited molybdenum disulfide and chemical
vapor-deposited graphene with low contact resistances of about 9
k${\Omega}$${\mu}$m and high on/off current ratios of 10${^8}$. We also present
a theoretical model calibrated on our experiments showing further potential for
scaling transistors and contact areas into the few nanometers range and the
possibility of a strong performance enhancement by means of layer optimizations
that would make transistors promising for use in future logic circuits. | 2304.01177v2 |
2023-09-21 | Superconductivity in Compositionally-Complex Cuprates with the YBa$_2$Cu$_3$O$_{7-x}$ Structure | High-temperature superconductivity is reported in a series of
compositionally-complex cuprates with varying degrees of size and spin
disorder. Three compositions of Y-site alloyed YBa$_2$Cu$_3$O$_{7-x}$, i.e.,
(5Y)BCO, were prepared using solid-state methods with different sets of rare
earth ions on the Y-site. Synchrotron X-ray diffraction and energy-dispersive
X-ray spectroscopy confirm these samples have high phase-purity and homogeneous
mixing of the Y-site elements. The superconducting phase transition was probed
using electrical resistivity and AC magnetometry measurements, which reveal the
transition temperature, T$_C$, is greater than 91 K for all series when near
optimal oxygen doping. Importantly, these T$_C$ values are only $\approx$1$\%$
suppressed relative to pure YBCO (T$_C$ = 93 K). This result highlights the
robustness of pairing in the YBCO structure to specific types of disorder. In
addition, the chemical flexibility of compositionally-complex cuprates allows
spin and lattice disorder to be decoupled to a degree not previously possible
in high-temperature superconductors. This feature makes compositionally-complex
cuprates a uniquely well-suited materials platform for studying proposed
pairing interactions in cuprates. | 2309.12535v1 |
2023-12-11 | Local laser-induced solid-phase recrystallization of phosphorus-implanted Si/SiGe heterostructures for contacts below 4.2 K | Si/SiGe heterostructures are of high interest for high mobility transistor
and qubit applications, specifically for operations below 4.2 K. In order to
optimize parameters such as charge mobility, built-in strain, electrostatic
disorder, charge noise and valley splitting, these heterostructures require Ge
concentration profiles close to mono-layer precision. Ohmic contacts to undoped
heterostructures are usually facilitated by a global annealing step activating
implanted dopants, but compromising the carefully engineered layer stack due to
atom diffusion and strain relaxation in the active device region. We
demonstrate a local laser-based annealing process for recrystallization of
ion-implanted contacts in SiGe, greatly reducing the thermal load on the active
device area. To quickly adapt this process to the constantly evolving
heterostructures, we deploy a calibration procedure based exclusively on
optical inspection at room-temperature. We measure the electron mobility and
contact resistance of laser annealed Hall bars at temperatures below 4.2 K and
obtain values similar or superior than that of a globally annealed reference
samples. This highlights the usefulness of laser-based annealing to take full
advantage of high-performance Si/SiGe heterostructures. | 2312.06267v1 |
2023-03-13 | Unusual magnetotransport and anomalous Hall effect in quasi-two-dimensional van der Waals ferromagnet Fe$_4$GeTe$_2$ | Fe$_4$GeTe$_2$, an itinerant vdW ferromagnet (FM) having Curie temperature
(T$_C$) close to room temperature ($\sim 270$ K), exhibits another transition
(T$_{SR}$ $\sim$ 120 K) where the easy axis of magnetization changes from
in-plane to the out-of-plane direction in addition to T$_C$. Here, we have
studied the magnetotransport in a multilayer Hall bar device fabricated on 300
nm Si/SiO$_2$ substrate. Interestingly, the zero field resistivity shows a
negligible change in resistivity near T$_C$ unlike the typical metallic FM,
whereas, it exhibits a dramatic fall below T$_{SR}$. Also, the resistivity
shows a weak anomaly at T $ \sim $ 38 K (T$_Q$), below which the resistivity
shows a quadratic temperature dependence according to the Fermi liquid
behavior. Temperature-dependent Hall data exhibits important consequences. The
ordinary Hall coefficient changes sign near T$_{SR}$ indicating the change in
majority carriers. In a similar manner, the magnetoresistance (MR) data shows
significantly large negative MR near T$_{SR}$ and becomes positive below T$_Q$.
The observations of anomaly in the resistivity, sign-change of the ordinary
Hall coefficient and maximum negative MR near T$_{SR}$, together suggest a
possible Fermi surface reconstruction associated with the spin reorientation
transition. Furthermore, analysis of the Hall data reveals a significant
anomalous Hall conductivity (AHC) from $\sim 123 \Omega^{-1}$ cm$^{-1}$ (at T
$\approx$ 5 K) to the maximum value of $\sim 366 \Omega^{-1}$ cm$^{-1}$ near
T$_{SR}$. While the low-temperature part may originate due to the intrinsic KL
mechanism, our analysis indicates that the temperature-dependent AHC is
primarily appearing due to the side-jump mechanism as a result of the spin-flip
electron-magnon scattering. Our study demonstrates an interplay between
magnetism and band topology and its consequence on electron transport in
Fe$_4$GeTe$_2$. | 2303.07440v1 |
2020-03-04 | Adaptive phase field modelling of crack propagation in orthotropic functionally graded materials | In this work, we extend the recently proposed adaptive phase field method to
model fracture in orthotropic functionally graded materials (FGMs). A recovery
type error indicator combined with quadtree decomposition is employed for
adaptive mesh refinement. The proposed approach is capable of capturing the
fracture process with a localized mesh refinement that provides notable gains
in computational efficiency. The implementation is validated against
experimental data and other numerical experiments on orthotropic materials with
different material orientations. The results reveal an increase in the
stiffness and the maximum force with increasing material orientation angle. The
study is then extended to the analysis of orthotropic FGMs. It is observed
that, if the gradation in fracture properties is neglected, the material
gradient plays a secondary role, with the fracture behaviour being dominated by
the orthotropy of the material. However, when the toughness increases along the
crack propagation path, a substantial gain in fracture resistance is observed. | 2003.04689v1 |
2020-09-04 | Impact of interfaces on the radiation response and underlying defect recovery mechanisms in nanostructured Cu-Fe-Ag | Newest developments in nuclear fission and fusion technology as well as
planned long-distance space missions demand novel materials to withstand harsh,
irradiative environments. Radiation-induced hardening and embrittlement are a
concern that can lead to failure of materials deployed in these applications.
Here the underlying mechanisms are accommodation and clustering of lattice
defects created by the incident radiation particles. Interfaces, such as free
surfaces, phase and grain boundaries, are known for trapping and annihilating
defects and therefore preventing these radiation-induced defects from forming
clusters. In this work, differently structured nanocomposite materials based on
Cu-Fe-Ag were fabricated using a novel solid-state route, combining severe
plastic deformation with thermal and electrochemical treatments. The influence
of different interface types and spacings on radiation effects in these
materials was investigated using nanoindentation. Interface-rich bulk
nanocomposites showed a slight decrease in hardness after irradiation, whereas
the properties of a nanoporous material remain mostly unchanged. An explanation
for this different material behavior and its link to recovery mechanisms at
interfaces is attempted in this work, paving a concept towards radiation
resistant materials. | 2009.02039v1 |
2021-03-05 | Effect of spoke design and material nonlinearity on non-pneumatic tire stiffness and durability performance | Non-pneumatic tire has been widely used due to their advantages of no
run-flat, no need of air maintenance, low rolling resistance, and improvement
of passengers comfort due to its better shock absorption. It has variety of
application in the military vehicle, earthmovers, lunar rover, stair climbing
vehicles etc. Recently UPTIS (Unique Puncture-Proof Tire System) non pneumatic
tire has been introduced for passenger vehicles. In this study three different
design configuration Tweel, Honeycomb and newly developed UPTIS have been
compared. Effect of Polyurethane (PU) material nonlinearity have also been
introduced by applying 5 different nonlinear PU material property in the
spokes. The combined analysis of the PU material nonlinearity and spoke design
configuration on the overall tire stiffness and spoke damage prediction is
analysed using 3-Dimensional FEM simulations performed in ANSYS 16.0. It has
been observed that Mooney Rivlin 5-parameter model is best to capture all 5
studied PU materials the nonlinearity. Effect of material nonlinearity on
various spoke designs have been studied. The best combination of spoke design
and the use of nonlinear material have been suggested in terms of riding
comfort, tire stiffness and durability performance. | 2103.03637v1 |
2023-01-24 | Seebeck-driven transverse thermoelectric generation in magnetic hybrid bulk materials | The Seebeck-driven transverse thermoelectric generation in
magnetic/thermoelectric hybrid materials (STTG) has been investigated in
all-bulk hybrid materials. The transverse thermopower in a ferromagnetic
Co$_2$MnGa/thermoelectric $n$-type Si hybrid bulk material with the adjusted
dimensions reaches 16.0 $\mu$V/K at room temperature with the aid of the STTG
contribution, which is much larger than the anomalous Nernst coefficient of the
Co$_2$MnGa slab (6.8 $\mu$V/K). Although this transverse thermopower is smaller
than the value for previously reported thin-film-based hybrid materials, the
hybrid bulk materials exhibit much larger electrical power owing to their small
internal resistance. This demonstration confirms the validity of STTG in bulk
materials and clarifies its potential as a thermal energy harvester. | 2301.09903v1 |
2023-01-24 | Accelerate & Actualize: Can 2D Materials Bridge the Gap Between Neuromorphic Hardware and the Human Brain? | Two-dimensional (2D) materials present an exciting opportunity for devices
and systems beyond the von Neumann computing architecture paradigm due to their
diversity of electronic structure, physical properties, and atomically-thin,
van der Waals structures that enable ease of integration with conventional
electronic materials and silicon-based hardware. All major classes of
non-volatile memory (NVM) devices have been demonstrated using 2D materials,
including their operation as synaptic devices for applications in neuromorphic
computing hardware. Their atomically-thin structure, superior physical
properties, i.e., mechanical strength, electrical and thermal conductivity, as
well as gate-tunable electronic properties provide performance advantages and
novel functionality in NVM devices and systems. However, device performance and
variability as compared to incumbent materials and technology remain major
concerns for real applications. Ultimately, the progress of 2D materials as a
novel class of electronic materials and specifically their application in the
area of neuromorphic electronics will depend on their scalable synthesis in
thin-film form with desired crystal quality, defect density, and phase purity. | 2301.10277v1 |
2023-07-19 | Emergence of high-temperature superconducting phase in the pressurized La3Ni2O7 crystals | The recent report of pressure-induced structure transition and signature of
superconductivity with Tc = 80 K above 14 GPa in the La3Ni2O7 crystals has
garnered considerable attention. To further elaborate this discovery, we
carried out comprehensive resistance measurements on the La3Ni2O7 crystals
grown with the optical-image floating zone furnace under oxygen pressure (15
bar) by using the diamond anvil cell (DAC) and cubic anvil cell (CAC), which
employs the solid and liquid pressure transmitting medium, respectively. For
the sample #1 measured in DAC, it exhibits a semiconducting-like behavior with
large resistance at low pressures and becomes metallic gradually upon
compression. At the pressures P >= 13.7 GPa, we observed the appearance of
resistance drop as large as ~50% around 70 K, which evolves into a kink-like
anomaly at pressures above 40 GPa and shifts to lower temperatures gradually
with increasing magnetic field. These observations are consistent with the
recent report mentioned above. On the other hand, the sample #2 measured in CAC
retains the metallic behavior in the investigated pressure range up to 15 GPa.
The hump-like anomaly in resistance around ~130 K at ambient pressure
disappears at P >= 2 GPa. In the pressure range from 11 to 15 GPa, we observed
the gradual development of a shoulder-like anomaly in resistance at low
temperatures, which evolves into a pronounced drop of resistance by 98% below
62 K at 15 GPa, reaching a temperature-independent resistance of 20 uOhm below
20 K. Similarly, this resistance anomaly can be shifted to lower temperatures
progressively by applying external magnetic fields, resembling a typical
superconducting transition. | 2307.09865v1 |
2023-07-27 | High-temperature superconductivity with zero-resistance and strange metal behavior in La$_{3}$Ni$_{2}$O$_{7-δ}$ | Recently signatures of superconductivity were observed close to 80 K in \LN\
under pressure. This discovery positions \LN\ as the first bulk nickelate with
high-temperature superconductivity, but the lack of zero resistance presents a
significant drawback for validating the findings. Here we report pressure
measurements up to over 30 GPa using a liquid pressure medium and show that
single crystals of \LNO\ do exhibit zero resistance. We find that \LNO\ remains
metallic under applied pressures, suggesting the absence of a metal-insulator
transition proximate to the superconductivity. Analysis of the normal state
$T$-linear resistance suggests an intricate link between this strange metal
behaviour and superconductivity, whereby at high pressures both the linear
resistance coefficient and superconducting transition are slowly suppressed by
pressure, while at intermediate pressures both the superconductivity and
strange metal behaviour appear disrupted, possibly due to a nearby structural
instability. The association between strange metal behaviour and
high-temperature superconductivity is very much in line with diverse classes of
unconventional superconductors, including the cuprates and Fe-based
superconductors. Understanding the superconductivity of \LNO\ evidently
requires further revealing the interplay of strange metal behaviour,
superconductivity, as well as possible competing electronic or structural
phases. | 2307.14819v2 |
2023-08-04 | Stopping microfluidic flow | We present a cross-comparison of three stop-flow configurations--such as
low-pressure (LSF), high-pressure open-circuit (OC-HSF), and high-pressure
short-circuit (SC-HSF) stop-flow--to rapidly bring a high flow velocity within
a microchannel to a standstill. The average velocities inside the microchannels
were reduced from > 1 m/s to < 10 um/s within 2s of initiating the stop-flow.
The performance of the three stop-flow configurations was assessed by measuring
the residual flow velocities within microchannels having three
orders-of-magnitude different flow resistances. The LSF configuration
outperformed the OC-HSF and SC-HSF configurations within the high flow
resistance microchannel, and resulted in a residual velocity of < 10 um/s. The
OC-HSF configuration resulted in a residual velocity of < 150 um/s within a low
flow resistance microchannel. The SC-HSF configuration resulted in a residual
velocity of < 200 um/s across the three orders-of-magnitude different flow
resistance microchannels, and < 100 um/s for the low flow resistance channel.
We hypothesized that the residual velocity resulted from the compliance in the
fluidic circuit, which was further investigated by varying the elasticity of
the microchannel walls and the connecting tubing. A numerical model was
developed to estimate the expanded volumes of the compliant microchannel and
connecting tubings under a pressure gradient and to calculate the distance
traveled by the sample fluid. A comparison of the numerically and
experimentally obtained traveling distances confirmed our hypothesis that the
residual velocities were an outcome of the compliance in the fluidic circuit.
Therefore, a configuration with minimal fluidic circuit compliance resulted in
the least residual velocity. | 2308.02386v2 |
2009-06-02 | The Evolution of Gas Clouds Falling in the Magnetized Galactic Halo: High Velocity Clouds (HVCs) Originated in the Galactic Fountain | In the Galactic fountain scenario, supernovae and/or stellar winds propel
material into the Galactic halo. As the material cools, it condenses into
clouds. By using FLASH three-dimensional magnetohydrodynamic simulations, we
model and study the dynamical evolution of these gas clouds after they form and
begin to fall toward the Galactic plane. In our simulations, we assume that the
gas clouds form at a height of z=5 kpc above the Galactic midplane, then begin
to fall from rest. We investigate how the cloud's evolution, dynamics, and
interaction with the interstellar medium (ISM) are affected by the initial mass
of the cloud. We find that clouds with sufficiently large initial densities (>
0.1 hydrogen atoms per cc) accelerate sufficiently and maintain sufficiently
large column densities as to be observed and identified as high-velocity clouds
(HVCs) even if the ISM is weakly magnetized (1.3 micro Gauss). We also
investigate the effects of various possible magnetic field configurations. As
expected, the ISM's resistance is greatest when the magnetic field is strong
and perpendicular to the motion of the cloud. The trajectory of the cloud is
guided by the magnetic field lines in cases where the magnetic field is
oriented diagonal to the Galactic plane. The model cloud simulations show that
the interactions between the cloud and the ISM can be understood via analogy to
the shock tube problem which involves shock and rarefaction waves. We also
discuss accelerated ambient gas, streamers of material ablated from the clouds,
and the cloud's evolution from a sphere-shaped to a disk- or cigar-shaped
object. | 0906.0613v1 |
2018-10-30 | THz-TDS time-trace analysis for the extraction of material and metamaterial parameters | We report on a method and an associated open source software, Fit@TDS,
working on an average personal computer. The method is based on the fitting of
a time-trace data of a terahertz time-domain-spectroscopy system enabling the
retrieval of the refractive index of a dielectric sample and the resonance
parameters of a metasurface (quality factor, absorption losses, etc.). The
software includes commonly used methods where the refractive index is extracted
from frequency domain data. However, these methods are limited, for instance in
case of a high noise level or when an absorption peak saturates the absorption
spectrum bringing the signal to the noise level. Our software allows to use a
new method where the refractive indices are directly fitted from the
time-trace. The idea is to model a material or a metamaterial through
parametric physical models (Drude-Lorentz model and time-domain coupled mode
theory) and to implement the subsequent refractive index in the propagation
model to simulate the time-trace. Then, an optimization algorithm is used to
retrieve the parameters of the model corresponding to the studied
material/metamaterial. In this paper, we explain the method and test it on
fictitious samples to probe the feasibility and reliability of the proposed
model. Finally, we used Fit@TDS on real samples of high resistivity silicon,
lactose and gold metasurface on quartz to show the capacity of our method | 1810.12567v4 |
2020-05-28 | Graphene-TiS$_3$ heterojunction for selective polar vapor sensing at room temperature | In this work, the room temperature polar vapor sensing behavior of two
dimentional (2D) heterojunction Graphene-TiS3 materials and TiS3 nanoribbons is
investigated. TiS3 nanoribbons were synthesized via chemical vapor transport
(CVT) and their structure was investigated by scanning electron microscopy
(SEM), high resolution transmission electron microscopy (HRTEM), energy
dispersive X- ray spectroscopy (EDS), X-ray diffraction (XRD), Raman
spectroscopy and Fourier transform infrared spectroscopy (FT-IR) analysis. The
gas sensing performance of the TiS3 nanoribbons was assessed through the
observed changes in their electronic behavior. Sensing devices were fabricated
with gold contacts and with lithographically patterned graphene (Gr) electrodes
in a 2D heterojunction Gr-TiS3-Gr architecture.. It is observed that the gold
contacted TiS3 device has a rather linear I-V behavior while the Gr-TiS3-Gr
heterojunction forms a contact with a higher Schottky barrier (250 meV). I-V
responses of the sensors were recorded at room temperature with a relative
humidity of 55% and for different ethanol vapor concentrations (varying from 2
to 20 ppm). I-V plots indicated an increase in the resistance of Gr-TiS3-Gr by
the adsorption of water and ethanol molecules with relatively high sensing
response (~3353% at 2 ppm). Our results reveal that selective and stable
responses to a low concentration of ethanol vapor (2 ppm) can be achieved at
room temperature with transient response and recovery times of around 6 s and
40 s, respectively. Our proposed design demonstrate a new approach for
selective molecular recognition using polar interactions between analyte vapors
and heterojunctions of 2D-materials. | 2005.14245v1 |
2022-01-06 | A quantitative assessment of Imaging High-Z and Medium-Z materials using Muon Scattering Tomography | Muon Scattering Tomography (MST) has been shown to be a powerful technique
for the non-invasive imaging of high-shielded objects. We present here the
application of the MST technique to investigate two types of nuclear waste
packages, a small-steel drum and a large nuclear waste cask, namely, a CASTOR
V/52. We have developed a quantitative method using the contrast-to-noise ratio
(CNR) to evaluate the performance of an MST detector system in differentiating
between high-, medium-, and low-Z materials inside nuclear waste packages with
different shielding types. This study reveals that our MST detector system is
able to differentiate between a (10 $\times$ 10 $\times$ 10 cm$^3$) uranium
cube, embedded within a concrete matrix inside the small-steel drum, and
regions of background signal in six hours of muon exposure time with a CNR
value of 3.1$\pm$0.2. During our investigation of the highly-shielded cask, the
reconstructed images of the cask contents indicated the ability of our system
to detect irregular baskets, such as empty baskets, with a CNR value of
5.0$\pm$0.3 after 30 days of muon exposure. These studies were done using a
Monte Carlo simulation tuned to the performance of resistive plate chambers
(RPCs) based muon tomography system built by the University of Bristol, which
had a reported position resolution of 350 micron.
Here we also report the dependence of the performance on the position
resolution. We argue that using a combination of RPC and drift chambers (DC)
detectors with 700 micron and 4 mm position resolutions respectively is able to
generate tomographic images of well-shielded materials in a few hours of muon
exposure time. With these position resolutions, our system needs six hours of
muon exposure time to produce a good quality image of a cube of uranium with
side-length of 10 cm shielded by a concrete matrix with CNR value of
2.4$\pm$0.25. | 2201.01877v3 |
2023-03-07 | Stacking disorder and thermal transport properties of $α$-RuCl$_3$ | $\alpha$-RuCl$_3$, a well-known candidate material for Kitaev quantum spin
liquid, is prone to stacking disorder due to the weak van der Waals bonding
between the honeycomb layers. After a decade of intensive experimental and
theoretical studies, the detailed correlation between stacking degree of
freedom, structure transition, magnetic and thermal transport properties
remains unresolved. In this work, we reveal the effects of a small amount of
stacking disorder inherent even in high quality $\alpha$-RuCl$_3$ crystals.
This small amount of stacking disorder results in the variation of the magnetic
ordering temperature, suppresses the structure transition and thermal
conductivity. Crystals with minimal amount of stacking disorder have a
T$_N>$7.4\,K and exhibit a well-defined structure transition around 140\,K upon
cooling. For those with more stacking faults and a T$_N$ below 7\,K, the
structure transition occurs well below 140\,K upon cooling and is incomplete,
manifested by the diffuse streaks and the coexistence of both high temperature
and low temperature phases down to the lowest measurement temperature. Both
types of crystals exhibit oscillatory field dependent thermal conductivity and
a plateau-like feature in thermal Hall resistivity in the field-induced quantum
spin liquid state. However, $\alpha$-RuCl$_3$ crystals with minimal amount of
stacking disorder have a higher thermal conductivity that pushes the thermal
Hall conductivity to be closer to the half-integer quantized value. These
findings demonstrate a strong correlation between layer stacking, structure
transition, magnetic and thermal transport properties, underscoring the
importance of interlayer coupling in $\alpha$-RuCl$_3$ despite the weak van der
Waals bonding. | 2303.03682v2 |
2023-12-27 | Combined effect of SiC and carbon on sintering kinetics, microstructure and mechanical properties of fine-grained binderless tungsten carbide | The study investigates the density, phase composition, microstructure and
mechanical properties (microhardness, fracture toughness) of binderless WC +
SiC and WC + SiC + C ceramics obtained by Spark Plasma Sintering (SPS).
Nanopowders of a-WC produced by DC arc plasma chemical synthesis were used as
raw materials. Powder compositions for sintering contained graphite (0.3, 0.5%
wt.) or b-SiC (1, 3, 5% wt.) with 0.3% wt. graphite. It was shown that WC + 1%
wt. SiC + 0.3% wt.C ceramics have a homogeneous fine-grained microstructure,
high relative density, increased microhardness and Palmquist fracture toughness
(Indentation Fracture Resistance). The kinetics of the initial sintering stage
of WC + C and WC + C + SiC powder compositions was also analyzed using
high-temperature dilatometry at the conventional pressureless sintering (CPS)
conditions. The CPS and SPS activation energies of WC + SiC powder at the
intensive shrinkage stage were determined using the Young-Cutler model. The CPS
activation energies of WC, WC + C and WC + C + SiC powder compositions are
close to the activation energy of diffusion of the carbon C along the a-WC
grain boundaries. The SPS activation energies of WC + C and WC+ C + SiC powder
compositions turn out to be lower than the activation energy of the C of a-WC
grain boundary. | 2312.16579v1 |
2022-07-26 | Hysteresis-Free High Mobility Graphene Encapsulated in Tungsten Disulfide | High mobility is a crucial requirement for a large variety of electronic
device applications. The state-of-the-art for high quality graphene devices is
based on heterostructures made with graphene encapsulated in $>80\,$nm-thick
flakes of hexagonal boron nitride (hBN). Unfortunately, scaling up multilayer
hBN while precisely controlling the number of layers remains an elusive
challenge, resulting in a rough material unable to enhance the mobility of
graphene. This leads to the pursuit of alternative, scalable materials, which
can be simultaneously used as substrate and encapsulant for graphene. Tungsten
disulfide (WS$_2$) is a transition metal dichalcogenide, which was successfully
grown in large ($\sim$mm-size) multi-layers by chemical vapour deposition.
However, the resistance \textit{vs} gate voltage characteristics when gating
graphene through WS$_2$ exhibit largely hysteretic shifts of the charge
neutrality point (CNP) in the order of $\Delta n\sim$2.6$\cdot$10$^{11}$
cm$^{-2}$, hindering the use of WS$_2$ as a reliable encapsulant. The
hysteresis originates due to the charge traps from sulfur vacancies present in
WS$_2$. In this work, we report for the first time the use of WS$_2$ as a
substrate and the overcoming of hysteresis issues by chemically treating WS$_2$
with a super-acid, which passivates these vacancies and strips the surface from
contaminants. The hysteresis is significantly reduced below the noise level by
at least a factor five (to $\Delta n<$5$\cdot$10$^{10}$ cm$^{-2}$) and,
simultaneously, the room-temperature mobility of WS$_2$-encapsulated graphene
is as high as $\sim$6.2$\cdot$10$^{4}$ cm$^{-2}$V$^{-1}$s$^{-1}$ at a carrier
density $n$ $\sim$1$\cdot$ 10$^{12}$ cm$^{-2}$. Our results promote WS$_2$ to a
valid alternative to hBN as encapsulant for high-performance graphene devices. | 2207.12836v1 |
2000-06-23 | Temperature-Dependence of the Resistivity of a Dilute 2D Electron System in High Parallel Magnetic Field | We report measurements of the resistance of silicon MOSFETs as a function of
temperature in high parallel magnetic fields where the 2D system of electrons
has been shown to be fully spin-polarized. A magnetic field suppresses the
metallic behavior observed in the absence of a magnetic field. In a field of
10.8 T, insulating behavior is found for densities up to n_s approximately 1.35
x 10^{11} cm^{-2} or 1.5 n_c; above this density the resistance is a very weak
function of temperature, varying less than 10% between 0.25 K and 1.90 K. At
low densities the resistance goes to infinity more rapidly as the temperature
is reduced than in zero field and the magnetoresistance diverges as T goes to
0. | 0006379v4 |
2002-10-08 | Resistivity of dilute 2D electrons in an undoped GaAs heterostructure | We report resistivity measurements from 0.03 K to 10 K in a dilute high
mobility 2D electron system. Using an undoped GaAs/AlGaAs heterojunction in a
gated field-effect transistor geometry, a wide range of densities, $0.16 \times
10^{10} {cm}^{-2}$ to $7.5 \times 10^{10} {cm}^{-2}$, are explored. For high
densities, the results are quantitatively shown to be due to scattering by
acoustic phonons and impurities. In an intermediate range of densities, a peak
in the resistivity is observed for temperatures below 1 K. This non-monotonic
resistivity can be understood by considering the known scattering mechanisms of
phonons, bulk and interface ionized impurities. Still lower densities appear
insulating to the lowest temperature measured. | 0210155v1 |
2003-09-11 | Superconductivity on the localization threshold and magnetic-field-tuned superconductor-insulator transition in TiN films | Temperature- and magnetic-field dependent measurements of the resistance of
ultrathin superconducting TiN films are presented. The analysis of the
temperature dependence of the zero field resistance indicates an underlying
insulating behavior, when the contribution of Aslamasov-Larkin fluctuations is
taken into account. This demonstrates the possibility of coexistence of the
superconducting and insulating phases and of a direct transition from the one
to the other. The scaling behavior of magnetic field data is in accordance with
a superconductor-insulator transition (SIT) driven by quantum phase
fluctuations in two-dimensional superconductor. The temperature dependence of
the isomagnetic resistance data on the high-field side of the SIT has been
analyzed and the presence of an insulating phase was confirmed. A transition
from the insulating to a metallic phase is found at high magnetic fields, where
the zero-temperature asymptotic value of the resistance being equal to h/e^2. | 0309281v2 |
2003-10-02 | Electrical resistivity of the Ti4O7 Magneli phase under high pressure | We have measured resistivity as a function of temperature and pressure of
Ti4O7 twinned crystals using different contact configurations. Pressures over
4kbar depress the localization of bipolarons and allow the study of the
electrical conduction of the bipolaronic phase down to low temperatures. For
pressures P > 40 kbar the bipolaron formation transition is suppressed and a
nearly pressure independent behavior is obtained for the resistivity. We
observed an anisotropic conduction. When current is injected parallel to the
principal axis, a metallic conduction with interacting carrier effects is
predominant. A superconducting state was not obtained down to 1.2 K, although
evidences of the proximity of a quantum critical point were noticed. While when
current is injected non-parallel to the crystal's principal axis, we obtained a
logarithmic divergence of the resistivity at low temperatures. For this case,
our results for the high pressure regime can be interpreted in the framework of
interacting carriers (polarons or bipolarons) scattered by Two Level Systems. | 0310048v1 |
2004-03-17 | Interaction Correction to the Longitudinal Conductivity and Hall Resistivity in High Quality Two-Dimensional GaAs Electron and Hole Systems | We present a systematic study of the corrections to both the longitudinal
conductivity and Hall resistivity due to electron-electron interactions in high
quality GaAs systems using the recent theory of Zala et al. [Phys. Rev. B 64,
214204 (2001)]. We demonstrate that the interaction corrections to the
longitudinal conductivity and Hall resistivity predicted by the theory are
consistent with each other. This suggests that the anomalous metallic drop in
resistivity at B=0 is due to interaction effects and supports the theory of
Zala et al. | 0403411v1 |
2005-05-19 | Universal scaling behavior of the c-axis resistivity of high-temperature superconductors | We propose and show that the c-axis transport in high-temperature
superconductors is controlled by the pseudogap energy and the c-axis
resistivity satisfies a universal scaling law in the pseudogap phase. We
derived approximately a scaling function for the c-axis resistivity and found
that it fits well with the experimental data of
Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$,
and YBa$_2$Cu$_3$O$_{7-\delta}$. Our works reveals the physical origin of the
semiconductor-like behavior of the c-axis resistivity and suggests that the
c-axis hopping is predominantly coherent. | 0505480v2 |
2006-07-27 | Effect of DC electric field on longitudinal resistance of two dimensional electrons in a magnetic field | The effect of a DC electric field on the longitudinal resistance of highly
mobile two dimensional electrons in heavily doped GaAs quantum wells is studied
at different magnetic fields and temperatures. Strong suppression of the
resistance by the electric field is observed in magnetic fields at which the
Landau quantization of electron motion occurs. The phenomenon survives at high
temperature where Shubnikov de Haas oscillations are absent. The scale of the
electric fields essential for the effect is found to be proportional to
temperature in the low temperature limit. We suggest that the strong reduction
of the longitudinal resistance is the result of a nontrivial change in the
distribution function of 2D electrons induced by the DC electric field.
Comparison of the data with recent theory yields the inelastic electron-electon
scattering time $\tau_{in}$ and the quantum scattering time $\tau_q$ of 2D
electrons at high temperatures, a regime where previous methods were not
successful. | 0607741v1 |
2009-05-07 | Correlation between linear resistivity and Tc in organic and pnictide superconductors | A linear temperature dependence of the electrical resistivity as T -> 0 is
the hallmark of quantum criticality in heavy-fermion metals and the archetypal
normal-state property of high-Tc superconductors, yet in both cases it remains
unexplained. We report a linear resistivity on the border of spin-density-wave
order in the organic superconductor (TMTSF)2X (X = PF6, ClO4), whose strength
scales with the superconducting temperature Tc. This scaling, also present in
the pnictide superconductors, reveals an intimate connection between linear-T
scattering and pairing, shown by renormalization group theory to arise from
antiferromagnetic fluctuations, enhanced by the interference of superconducting
correlations. Our results suggest that linear resistivity in general may be a
consequence of such interference and pairing in overdoped high-Tc cuprates is
driven by antiferromagnetic fluctuations, as in organic and pnictide
superconductors. | 0905.0964v1 |
2010-02-25 | Signatures of pressure induced superconductivity in insulating Bi2212 | We have performed several high pressure electrical resistance experiments on
Bi1.98Sr2.06Y0.68Cu2O8, an insulating parent compound of the high-Tc Bi2212
family of copper oxide superconductors. We find a resistive anomaly, a downturn
at low temperature, that onsets with applied pressure in the 20-40 kbar range.
Through both resistance and magnetoresistance measurements, we identify this
anomaly as a signature of induced superconductivity. Resistance to higher
pressures decreases Tc, giving a maximum of 10 K. The higher pressure
measurements exhibit a strong sensitivity to the hydrostaticity of the pressure
environment. We make comparisons to the pressure induced superconductivity now
ubiquitous in the iron arsenides. | 1002.4672v1 |
2010-05-10 | Analysis and Design of Ultra Thin Electromagnetic Absorbers Comprising Resistively Loaded High Impedance Surfaces | High-Impedance Surfaces (HIS) comprising lossy Frequency Selective Surfaces
(FSS) are employed to design thin electromagnetic absorbers. The structure,
despite its typical resonant behavior, is able to perform a very wideband
absorption in a reduced thickness. Losses in the frequency selective surface
are introduced by printing the periodic pattern through resistive inks and
hence avoiding the typical soldering of a large number of lumped resistors. The
effect of the surface resistance of the FSS and dielectric substrate
characteristics on the input impedance of the absorber is discussed by means of
a circuital model. It is shown that the optimum value of surface resistance is
affected both by substrate parameters (thickness and permittivity) and by FSS
element shape. The equivalent circuit model is then used to introduce the
working principles of the narrowband and the wideband absorbing structure and
to derive the best-suited element for wideband absorption. Finally, the
experimental validation of the presented structures is presented. | 1005.1553v1 |
2011-07-05 | Resistive Solutions for Pulsar Magnetospheres | The current state of the art in the modeling of pulsar magnetospheres invokes
either the vacuum or force-free limits for the magnetospheric plasma. Neither
of these limits can simultaneously account for both the plasma currents and the
accelerating electric fields that are needed to explain the morphology and
spectra of high-energy emission from pulsars. To better understand the
structure of such magnetospheres, we combine accelerating fields and force-free
solutions by considering models of magnetospheres filled with resistive plasma.
We formulate Ohm's Law in the minimal velocity fluid frame and construct a
family of resistive solutions that smoothly bridges the gap between the vacuum
and the force-free magnetosphere solutions. The spin-down luminosity, open
field line potential drop, and the fraction of open field lines all transition
between the vacuum and force-free values as the plasma conductivity varies from
zero to infinity. For fixed inclination angle, we find that the spin-down
luminosity depends linearly on the open field line potential drop. We consider
the implications of our resistive solutions for the spin down of intermittent
pulsars and sub-pulse drift phenomena in radio pulsars. | 1107.0979v2 |
2011-11-02 | Development and Performance of spark-resistant Micromegas Detectors | The Muon ATLAS MicroMegas Activity (MAMMA) focuses on the development and
testing of large-area muon detectors based on the bulk-Micromegas technology.
These detectors are candidates for the upgrade of the ATLAS Muon System in view
of the luminosity upgrade of Large Hadron Collider at CERN (sLHC). They will
combine trigger and precision measurement capability in a single device. A
novel protection scheme using resistive strips above the readout electrode has
been developed. The response and sparking properties of resistive Micromegas
detectors were successfully tested in a mixed (neutron and gamma) high
radiation field, in a X-ray test facility, in hadron beams, and in the ATLAS
cavern. Finally, we introduced a 2-dimensional readout structure in the
resistive Micromegas and studied the detector response with X-rays. | 1111.0426v1 |
2011-11-09 | Nonequilibrium phenomena in high Landau levels | Developments in the physics of 2D electron systems during the last decade
have revealed a new class of nonequilibrium phenomena in the presence of a
moderately strong magnetic field. The hallmark of these phenomena is
magnetoresistance oscillations generated by the external forces that drive the
electron system out of equilibrium. The rich set of dramatic phenomena of this
kind, discovered in high mobility semiconductor nanostructures, includes, in
particular, microwave radiation-induced resistance oscillations and
zero-resistance states, as well as Hall field-induced resistance oscillations
and associated zero-differential resistance states. We review the experimental
manifestations of these phenomena and the unified theoretical framework for
describing them in terms of a quantum kinetic equation. The survey contains
also a thorough discussion of the magnetotransport properties of 2D electrons
in the linear response regime, as well as an outlook on future directions,
including related nonequilibrium phenomena in other 2D electron systems. | 1111.2176v3 |
2011-12-12 | Resistivity Calculations for Cuprate Superconductor Systems using an Electronic Phase Separation | The resistivity as function of temperature of high temperature
superconductors is very unusual and despite its importance lacks an unified
theoretical explanation. It is linear with the temperature for overdoped
compounds but it falls more quickly as the doping level decreases, and for
weakly doped samples it has a minimum, increases like an insulator before it
drops to zero at low temperatures. We show that this overall behavior can be
explained by calculations using an electronic phase segregation into two main
component phases with low and high densities. The total resistivity is
calculated by the various contributions through several random picking
processes of the local resistivities and using the Random Resistor Network
approach. | 1112.2631v1 |
2017-11-19 | Global current circuit structure in a resistive pulsar magnetosphere model | Pulsar magnetospheres have strong magnetic fields and large amounts of
plasma. The structures of these magnetospheres are studied using force-free
electrodynamics. To understand pulsar magnetospheres, discussions must include
their outer region. However, force-free electrodynamics is limited in it does
not handle dissipation. Therefore, a resistive pulsar magnetic field model is
needed. To break the ideal magnetohydrodynamic (MHD) condition $E \cdot B = 0$,
Ohm's law is used. In this work, I introduce resistivity depending upon the
distance from the star and obtain a self-consistent steady state by time
integration. Poloidal current circuits form in the magnetosphere while the
toroidal magnetic-field region expands beyond the light cylinder and the
Poynting flux radiation appears. High electric resistivity causes a large space
scale poloidal current circuit and the magnetosphere radiates a larger Poynting
flux than the linear increase outside of the light cylinder radius. The formed
poloidal current circuit has width, which grows with the electric conductivity.
This result contributes to a more concrete dissipative pulsar magnetosphere
model. | 1711.07471v1 |
2018-10-10 | Test particles in relativistic resistive magnetohydrodynamics | The Black Hole Accretion Code (BHAC) has recently been extended with the
ability to evolve charged test particles according to the Lorentz force within
resistive relativistic magnetohydrodynamics simulations. We apply this method
to evolve particles in a reconnecting current sheet that forms due to the
coalescence of two magnetic flux tubes in 2D Minkowski spacetime. This is the
first analysis of charged test particle evolution in resistive relativistic
magnetohydrodynamics simulations. The energy distributions of an ensemble of
100.000 electrons are analyzed, as well as the acceleration of particles in the
plasmoids that form in the reconnection layer. The effect of the Lundquist
number, magnetization, and plasma-$\beta$ on the particle energy distribution
is explored for a range of astrophysically relevant parameters. We find that
electrons accelerate to non-thermal energies in the thin current sheets in all
cases. We find two separate acceleration regimes: An exponential increase of
the Lorentz factor during the island coalescence where the acceleration depends
linearly on the resistivity and a nonlinear phase with high variability. These
results are relevant for determining energy distributions and acceleration
sites obtaining radiation maps in large-scale magnetohydrodynamics simulations
of black hole accretion disks and jets. | 1810.04323v2 |
2019-07-07 | First demonstration of 200, 100, and 50 um pitch Resistive AC-Coupled Silicon Detectors (RSD) with 100% fill-factor for 4D particle tracking | We designed, produced, and tested RSD (Resistive AC-Coupled Silicon
Detectors) devices, an evolution of the standard LGAD (Low-Gain Avalanche
Diode) technology where a resistive n-type implant and a coupling dielectric
layer have been implemented. The first feature works as a resistive sheet,
freezing the multiplied charges, while the second one acts as a capacitive
coupling for readout pads. We succeeded in the challenging goal of obtaining
very fine pitch (50, 100, and 200 um) while maintaining the signal waveforms
suitable for high timing and 4D-tracking performances, as in the standard
LGAD-based devices. | 1907.03314v3 |
2020-09-01 | On resistive spiking of fungi | We study long-term electrical resistance dynamics in mycelium and fruit
bodies of oyster fungi P. ostreatus. A nearly homogeneous sheet of mycelium on
the surface of a growth substrate exhibits trains of resistance spikes. The
average width of spikes is c.~23~min and the average amplitude is c.~1~kOhm.
The distance between neighbouring spikes in a train of spikes is c.~30~min.
Typically there are 4-6 spikes in a train of spikes. Two types of resistance
spikes trains are found in fruit bodies: low frequency and high amplitude
(28~min spike width, 1.6~kOhm amplitude, 57~min distance between spikes) and
high frequency and low amplitude (10~min width, 0.6~kOhm amplitude, 44~min
distance between spikes). The findings could be applied in monitoring of
physiological states of fungi and future development of living electronic
devices and sensors. | 2009.00292v1 |
2020-09-21 | Contact resistance assessment and high-frequency performance projection of black phosphorus field-effect transistor technologies | In this work, an evaluation of the contact quality of black phosphorus (BP)
field-effect transistors (FETs) from different technologies previously reported
is performed by means of an efficient and reliable contact resistance
extraction methodology based on individual device practical characteristics. A
good agreement is achieved between the extracted values with the Y-function
method used here and reference values obtained with other methods considering
internal values as well as with more expensive methods involving fabricated
test structures. The method enables a direct evaluation of different steps in
the same technology and it embraces the temperature dependence of the contact
characteristics. Channel phenomena have no impact on the extracted contact
resistance values. High-frequency performance projections are obtained for
fabricated devices based on the extracted contact resistance. | 2009.09661v3 |
2012-06-25 | Performances of silicone coated high resistive bakelite RPC | Performances of several single gap (gas gap 2 mm) prototype Resistive Plate
Chambers (RPC) made of high resistive ({\rho} \sim 1010 - 1012 {\Omega} cm)
bakelite, commercially available in India have been studied in recent times. To
make the inner electrode surfaces smooth, a thin coating of silicone has been
applied. An efficiency > 90% and time resolution \sim 2 ns (FWHM) have been
obtained for both the streamer and the avalanche mode. The induced charge
distributions of those silicone coated RPC are studied and the results are
presented. A numerical study on the effect of surface roughness of the
resistive electrodes on the electric field of the device has been carried out
using Garfield-neBEM code. A few results for a simplified model representing
surface roughness, measured using a surface profilometer for the bakelite
surfaces, have also been presented. | 1206.5627v1 |
2022-08-18 | Observation of Fermi liquid phase with broken symmetry in a single crystalline nanorod of Pr$_2$Ir$_2$O$_7$ | We report experimental evidence of emergent broken symmetry Fermi liquid
state in an isolated single crystalline nanorod of $\rm Pr_2 Ir_2 O_7$. We find
clear signature of the onset of the Fermi liquid behavior at low temperature
marked by the sign inversion of magnetoresistance from negative at high
temperature, characteristic of incoherent Kondo scattering, to positive as well
as a $\rm T^2$ dependence of resistivity at low temperature. A resistive
anomaly is observed, which is accompanied by thermal hysteresis in the presence
of magnetic field, suggesting itinerant metamagnetism. The observed high field
negative magnetoresistance with quadratic field dependence at low temperature,
which is most likely due to suppression of itinerant spin fluctuation, and the
irreversibility of the magneto-resistive properties in the Fermi liquid regime
suggest existence of an unusual state with broken spin rotation and time
reversal symmetry, hallmark of `hastatic' order. The major features of such
temperature dependence of resistivity and magnetoresistance can be explained in
a phenomenological model incorporating two distinct hybridization channels,
which is physically consistent with the possibility of the formation of the
`hastatic' Fermi liquid phase. | 2208.08811v1 |
2022-09-16 | Prediction of Cross-Fitness for Adaptive Evolution to Different Environmental Conditions: Consequence of Phenotypic Dimensional Reduction | How adaptive evolution to one environmental stress improves or suppresses
adaptation to another is an important problem in evolutionary biology. For
instance, in microbiology, the evolution of bacteria to be resistant to
different antibiotics is a critical issue that has been investigated as
cross-resistance. In fact, recent experiments on bacteria have suggested that
the cross-resistance of their evolution to various stressful environments can
be predicted by the changes to their transcriptome upon application of stress.
However, there are no studies so far that explain a possible theoretical
relationship between cross-resistance and changes in the transcriptome, which
causes high-dimensional changes to cell phenotype. Here, we show that a
correlation exists between fitness change in stress tolerance evolution and
response to the environment, using a cellular model with a high-dimensional
phenotype and establishing the relationship theoretically. The present results
allow for the prediction of evolution from transcriptome information in
response to different stresses before evolution. The relevance of this to
microbiological evolution experiments is discussed. | 2209.07756v2 |
2023-09-29 | Pressure-induced superconductivity in polycrystalline La3Ni2O7 | We synthesized polycrystalline La3Ni2O7 samples by using the sol-gel method
without post-annealing under high oxygen pressure, and then measured
temperature-dependent resistivity under various hydrostatic pressures up to
14.5 GPa in a cubic anvil cell apparatus. We find that the density-wave-like
anomaly in resistivity is progressively suppressed with increasing pressure and
the resistivity drop corresponding to the onset of superconductivity emerges at
pressure as low as 7 GPa. Zero resistivity is achieved at 9 GPa below 6.6 K,
which increases quickly with pressure to 35.6 K at 14.5 GPa. The observation of
zero-resistance state in the polycrystalline La3Ni2O7 samples under high
pressures not only corroborates the recent report of superconductivity in the
pressurized La3Ni2O7 crystals but also facilitates further studies on this
emerging family of nickelate high-Tc superconductors. | 2309.17378v2 |
2005-10-12 | Magnetodielectric effect without multiferroic coupling | The existence of a magnetodielectric (magnetocapacitance) effect is often
used as a test for multiferroic behavior in new material systems. However,
strong magnetodielectric effects can also be achieved through a combination of
magnetoresistance and the Maxwell-Wagner effect, unrelated to multiferroic
coupling. The fact that this resistive magnetocapacitance does not require
multiferroic materials may be advantageous for some practical applications.
Conversely, it also implies that magnetocapacitance per se is not sufficient to
establish multiferroic coupling. | 0510313v1 |
2007-09-24 | External Control of a Metal-Insulator Transition in GaMnAs Wires | Quantum transport in disordered ferromagnetic (III,Mn)V semiconductors is
studied theoretically. Mesoscopic wires exhibit an Anderson disorder-induced
metal-insulator transition that can be controlled by a weak external magnetic
field. This metal-insulator transition should also occur in other materials
with large anisotropic magneto resistance effects. The transition can be useful
for studies of zero-temperature quantum critical phase transitions and
fundamental material properties. | 0709.3847v2 |
2008-01-28 | Evidence for hyperconductivity and thermal superconductivity | Physical explanation of hyperconductivity and thermal superconductivity
existence is done in given article on the basis of inherent atomic nuclei
oscillations in atoms of materials which are connected with electrons and
phonons and in accordance with the well known Bardeen-Cooper-Schrieffer
superconductivity theory.
It is shown that hyperconductivity is the self-supporting, independent
physical phenomenon which is caused by oscillations of atomic nuclei in atoms
of materials and the minimal temperature of its existence does not reach
absolute zero temperature. Hyperconductivity represents the typical dynamic
condition of a material with zero electrical and zero thermal resistances. | 0801.4212v1 |
2009-11-24 | The possible superconductivity at 109 K in YBaCuO materials | The new YBaCuO superconductors are synthesized by using the standard solid
state reaction method as Y5-6-11, Y7-9-16, Y5-8-13, Y7-11-18, Y156, Y3-8-11,
and Y13-20-23. We find that all material obtained are shown the Meissner effect
at 77 K. The resistivity measurements are used by the four-probe method .The
Y 7-11-18 has the highest onset as 109 K . The XRD spectra are shown that
they have the same crystal structure as Y123 with some impurities peaks . | 0911.4524v1 |
2012-12-20 | Thermoelectricity in ternary rare-earth systems | Crystallographic data, Seebeck coefficient, electrical resistance and thermal
conductivity are reported for a large number of rare-earth compounds,
manifestations of the Kondo effect being discussed. In more detail,
thermoelectric properties of Yb3Co4Ge13 and Yb3Co4Sn13 compounds and
Yb2CeCo4Ge13 and Yb2.3La0.7Co4Ge13 solid solutions are presented. | 1212.4995v1 |
2015-05-07 | Computer modelling of hafnium doping in lithium niobate | Lithium niobate, LiNbO3, is an important technological material with good
electro-optic, acousto-optic, elasto-optic, piezoelectric and nonlinear
properties. Doping LiNbO3 with hafnium, Hf has been shown to improve the
resistance of the material to optical damage. Computer modelling provides a
useful means of determining the properties of doped and undoped LiNbO3,
including its defect chemistry, and the effect of doping on the structure. In
this paper, Hf doped LiNbO3 has been modelled, and the final defect
configurations are found to be consistent with experimental results. | 1505.01661v2 |
2022-11-24 | Anisotropic magnetoresistance: materials, models and applications | Resistance of certain (conductive and otherwise isotropic) ferromagnets turns
out to exhibit anisotropy with respect to the direction of magnetisation:
R$_\parallel$ different from R$_\perp$ with reference to the electric current
direction. This century-old phenomenon is reviewed both from the perspective of
materials and physical mechanisms involved. More recently, this effect has also
been extended to antiferromagnets. This opens the possibility for industrial
applications reaching far beyond the current ones, e.g. hard drive read heads
or position sensors. | 2212.03700v2 |
2024-05-15 | The Detection of Unconventional Quantum Oscillations in Insulating 2D Materials | In strongly correlated quantum materials, electrons behave in ways that often
extend beyond the confines of conventional Fermi-liquid theory. Interesting
results include the observation of low-temperature metallic behavior in systems
that are highly resistive. Here we provide an overview of experiments in which
insulators exhibit characteristics of a metal such as the Shubnikov de Haas
like quantum oscillations, focusing on recent findings in the correlated
insulating states of two-dimensional WTe2. We discuss the status of current
research, clarify the debates and challenges in interpreting the experiments,
rule out extrinsic explanations and discuss promising future directions. | 2405.09666v2 |
1998-06-30 | Transport, optical and electronic properties of the half metal CrO2 | The electronic structure of CrO_2 is critically discussed in terms of the
relation of existing experimental data and well converged LSDA and GGA
calculations of the electronic structure and transport properties of this half
metal magnet, with a particular emphasis on optical properties. We find only
moderate manifestations of many body effects. Renormalization of the density of
states is not large and is in the typical for transition metals range. We find
substantial deviations from Drude behavior in the far-infrared optical
conductivity. These appear because of the unusually low energy of interband
optical transitions. The calculated mass renormalization is found to be rather
sensitive to the exchange-correlation functional used and varies from 10%
(LSDA) to 90% (GGA), using the latest specific heat data. We also find that
dressing of the electrons by spin fluctuations, because of their high energy,
renormalizes the interband optical transition at as high as 4 eV by about 20%.
Although we find no clear indications of strong correlations of the Hubbard
type, strong electron-magnon scattering related to the half metallic band
structure is present and this leads to a nontrivial temperature dependence of
the resistivity and some renormalization of the electron spectra. | 9806378v1 |
2002-08-14 | Nonlinear microwave response of epitaxial YBaCuO films of varying oxygen content on MgO substrates | We have investigated the nonlinear microwave properties of electron-beam
coevaporated YBaCuO films on MgO, using stripline resonators at 2.3 GHz and
temperatures 1.7-80 K. The oxygen content of the films ranged from strongly
underdoped to overdoped. Above 20 K, the nonlinear response of the resonators
was dominated by the superconductor. We could establish clear correlations
between the nonlinear surface resistance, two-tone intermodulation (IMD), and
oxygen content of the films, which indicate that the superconducting order
parameter is important for the nonlinearities. An exponential rather than a
power-law representation of the nonlinear current-voltage relation would be
required to explain our results phenomenologically. Below 20 K, the dielectric
loss tangent of MgO dominated the nonlinear response of the resonators. With
increasing power, the dissipation losses decreased markedly, accompanied by
enhanced IMD. The surface reactance passed through a shallow minimum at about 5
K, independent of power. We attribute these effects to resonant absorption by
impurity states in MgO. | 0208285v1 |
2004-04-13 | Quantum Transparency of Anderson Insulator Junctions: Statistics of Transmission Eigenvalues, Shot Noise, and Proximity Conductance | We investigate quantum transport through strongly disordered barriers, made
of a material with exceptionally high resistivity that behaves as an Anderson
insulator or a ``bad metal'' in the bulk, by analyzing the distribution of
Landauer transmission eigenvalues for a junction where such barrier is attached
to two clean metallic leads. We find that scaling of the transmission
eigenvalue distribution with the junction thickness (starting from the single
interface limit) always predicts a non-zero probability to find high
transmission channels even in relatively thick barriers. Using this
distribution, we compute the zero frequency shot noise power (as well as its
sample-to-sample fluctuations) and demonstrate how it provides a single number
characterization of non-trivial transmission properties of different types of
disordered barriers. The appearance of open conducting channels, whose
transmission eigenvalue is close to one, and corresponding violent mesoscopic
fluctuations of transport quantities explain at least some of the peculiar
zero-bias anomalies in the Anderson-insulator/superconductor junctions observed
in recent experiments [Phys. Rev. B {\bf 61}, 13037 (2000)]. Our findings are
also relevant for the understanding of the role of defects that can undermine
quality of thin tunnel barriers made of conventional band-insulators. | 0404271v2 |
2004-06-17 | Phase formation of polycrystalline MgB2 at low temperature using nanometer Mg powder | The MgB2 superconductor synthesized in a flowing argon atmosphere using
nanometer magnesium powder as the raw materials, denoted as Nano-MgB2, has been
studied by the technique of in-situ high temperature resistance measurement
(HT-RT measurement). The MgB2 phase is identified to form within the
temperature range of 430 to 490 C, which is much lower than that with the MgB2
sample fabricated in the same gas environment using the micron-sized magnesium
powder, denoted as Micro-MgB2, reported previously. The sample density of the
Nano-MgB2 reaches 1.7 g/cm3 with a crystal porosity structure less than a
micrometer, as determined by the scanning electron microscope (SEM) images,
while the Micro-MgB2 has a much more porous structure with corresponding
density of 1.0 g/cm3. This indicates that the Mg raw particle size, besides the
sintering temperature, is a crucial factor for the formation of high density
MgB2 sample, even at the temperature much lower than that of the Mg melting,
650 C. The X-ray diffraction (XRD) pattern shows a good MgB2 phase with small
amount of MgO and Mg and the transition temperature, TC, of the Nano-MgB2 was
determined as 39 K by the temperature dependent magnetization measurement
(M-T), indicating the existence of a good superconducting property. | 0406398v1 |
2004-08-27 | Spin reorientation and in-plane magnetoresistance of lightly doped La_{2-x}Sr_{x}CuO_{4} in magnetic fields up to 55 T | The magnetoresistance (MR) in the in-plane resistivity is measured in
magnetic fields up to 55 T in lightly doped La_{2-x}Sr_{x}CuO_{4} in the N\'eel
state (x = 0.01) and in the spin-glass state (x = 0.03) using high-quality
untwinned single crystals. In both cases, a large negative MR is observed to
appear when the magnetic order is established. For x = 0.01, it is found that
the MR is indicative of a one-step transition into a high-field
weak-ferromagnetic state at around 20 T when the magnetic field is applied from
the spin easy axis (b axis), which means that there is no spin-flop transition
in the N\'eel state of this material; this is contrary to a previous report,
but is natural in light of the peculiar in-plane magnetic susceptibility
anisotropy recently found in this system. In the spin-glass state, we observe
that the large (up to \sim20%) negative MR saturates at around 40 T, and this
MR is found to be essentially isotropic when the magnetic field is rotated
within the ab plane. Our data show that the large negative MR is inherent to
LSCO in a magnetically ordered state, in which the weak-ferromagnetic (WF)
moment becomes well-defined; we discuss that the observed MR is essentially due
to the reorientation of the WF moments towards the magnetic field direction
both in the N\'eel state and in the spin-glass state. | 0408604v1 |
2005-06-27 | Strong-coupling theory of high-temperature superconductivity and colossal magnetoresistance | We argue that the extension of the BCS theory to the strong-coupling regime
describes the high-temperature superconductivity of cuprates and the colossal
magnetoresistance (CMR) of ferromagnetic oxides if the phonon dressing of
carriers and strong attractive correlations are taken into account. The
long-range Froehlich electron-phonon interaction has been identified as the
most essential in cuprates providing "superlight" lattice polarons and
bipolarons. Here some kinetic, magnetic, and more recent thermomagnetic normal
state measurements are interpreted in the framework of the strong-coupling
theory, including the Nernst effect and normal state diamagnetism. Remarkably,
a similar strong-coupling approach offers a simple explanation of CMR in
ferromagnetic oxides. The pairing of oxygen holes into heavy bipolarons in the
paramagnetic phase and their magnetic pair-breaking in the ferromagnetic phase
account for the first-order ferromagnetic phase transition, CMR, isotope
effects, and pseudogaps in doped manganites. Here we propose an explanation of
the phase coexistence and describe the shape of resistivity of manganites near
the transition in the framework of the strong-coupling approach. | 0506706v3 |
2006-06-05 | Phenomenological theory of current driven exchange switching in ferromagnetic nanojunctions | Phenomenological approach is developed in the theory of spin-valve type
ferromagnetic junctions to describe exchange switching by current flowing
perpendicular to interfaces. Forward and backward current switching effects are
described and they may be principally different in nature. Mobile electron
spins are considered as being free in all the contacting ferromagnetic layers.
Joint action of the following two current effects is investigated: the
nonequilibrium longitudinal spin-injection effective field and the transverse
spin-transfer surface torque. Dispersion relation for fluctuations is derived
and solved for a junction model having spatially localized spin transfer
torque: depth of the torque penetration into the free layer is assumed much
smaller than the total free layer thickness. Some critical value of the well
known Gilbert damping constant is established for the first time. Spin transfer
torque dominates in the instability threshold determination for small enough
damping constants, while the spin-injection effective field dominates for high
damping. Fine interplay between spin transfer torque and spin injection is
necessary to provide a hysteretic behavior of the resistance versus current
dependence. The state diagram building up shows the possibility of
non-stationary (time dependent) nonlinear states arising due to instability
development. Calculations lead to the instability rise time values of the order
of 0.1 ns. Spin wave resonance frequency spectrum softening occurs under the
current growing to the instability threshold. Magnetization fluctuations above
the threshold rise oscillating with time for low damping, but rise
aperiodically and much more rapid for high damping. | 0606102v2 |
2007-02-28 | A local field emission study of partially aligned carbon-nanotubes by AFM probe | We report on the application of Atomic Force Microscopy (AFM) for studying
the Field Emission (FE) properties of a dense array of long and vertically
quasi-aligned multi-walled carbon nanotubes grown by catalytic Chemical Vapor
Deposition on a silicon substrate. The use of nanometric probes enables local
field emission measurements allowing investigation of effects non detectable
with a conventional parallel plate setup, where the emission current is
averaged on a large sample area. The micrometric inter-electrode distance let
achieve high electric fields with a modest voltage source. Those features
allowed us to characterize field emission for macroscopic electric fields up to
250 V/$\mu$m and attain current densities larger than 10$^5$ A/cm$^2$. FE
behaviour is analyzed in the framework of the Fowler-Nordheim theory. A field
enhancement factor $\gamma \approx$ 40-50 and a turn-on field $E_{turn-on}
\sim$15 V/$\mu$m at an inter-electrode distance of 1 $\mu$m are estimated.
Current saturation observed at high voltages in the I-V characteristics is
explained in terms of a series resistance of the order of M$\Omega$. Additional
effects as electrical conditioning, CNT degradation, response to laser
irradiation and time stability are investigated and discussed. | 0702682v1 |
2007-11-09 | Defect healing at room temperature in pentacene thin films and improved transistor performance | We report on a healing of defects at room temperature in the organic
semiconductor pentacene. This peculiar effect is a direct consequence of the
weak intermolecular interaction which is characteristic of organic
semiconductors. Pentacene thin-film transistors were fabricated and
characterized by in situ gated four-terminal measurements. Under high vacuum
conditions (base pressure of order 10E-8 mbar), the device performance is found
to improve with time. The effective field-effect mobility increases by as much
as a factor of two and mobilities up to 0.45 cm2/Vs were achieved. In addition,
the contact resistance decreases by more than an order of magnitude and there
is a significant reduction in current hysteresis. Oxygen/nitrogen exposure and
annealing experiments show the improvement of the electronic parameters to be
driven by a thermally promoted process and not by chemical doping. In order to
extract the spectral density of trap states from the transistor
characteristics, we have implemented a powerful scheme which allows for a
calculation of the trap densities with high accuracy in a straightforward
fashion. We show the performance improvement to be due to a reduction in the
density of shallow traps <0.15 eV from the valence band edge, while the
energetically deeper traps are essentially unaffected. This work contributes to
an understanding of the shallow traps in organic semiconductors and identifies
structural point defects within the grains of the polycrystalline thin films as
a major cause. | 0711.1457v1 |
2008-04-05 | Strain rate effects in the mechanical response of polymer anchored carbon nanotube foams | Super-compressible foam-like carbon nanotube films have been reported to
exhibit highly nonlinear viscoelastic behaviour in compression similar to soft
tissue. Their unique combination of light weight and exceptional electrical,
thermal and mechanical properties have helped identify them as viable building
blocks for more complex nanosystems and as stand-alone structures for a variety
of different applications. In the as-grown state, their mechanical performance
is limited by the weak adhesion between the tubes, controlled by the van der
Waals forces, and the substrate allowing the forests to split easily and to
have low resistance in shear. Under axial compression loading carbon nanotubes
have demonstrated bending, buckling8 and fracture9 (or a combination of the
above) depending on the loading conditions and on the number of loading cycles.
In this work, we partially anchor dense vertically aligned foam-like forests of
carbon nanotubes on a thin, flexible polymer layer to provide structural
stability, and report the mechanical response of such systems as a function of
the strain rate. We test the sample under quasi-static indentation loading and
under impact loading and report a variable nonlinear response and different
elastic recovery with varying strain rates. A Bauschinger-like effect is
observed at very low strain rates while buckling and the formation of permanent
defects in the tube structure is reported at very high strain rates. Using
high-resolution transmission microscopy | 0804.0868v1 |
2008-04-10 | Electrical transport and ferromagnetism in Ga1-xMnxAs synthesized by ion implantation and pulsed-laser melting | We present a detailed investigation of the magnetic and magnetotransport
properties of thin films of ferromagnetic Ga1-xMnxAs synthesized using ion
implantation and pulsed-laser melting (II-PLM). The field and
temperature-dependent magnetization, magnetic anisotropy, temperature-dependent
resistivity, magnetoresistance, and Hall effect of II-PLM Ga1-xMnxAs films have
all of the characteristic signatures of the strong p-d interaction of holes and
Mn ions observed in the dilute hole-mediated ferromagnetic phase. The
ferromagnetic and electrical transport properties of II-PLM films correspond to
the peak substitutional Mn concentration meaning that the non-uniform Mn depth
distribution is unimportant in determining the film properties. Good
quantitative agreement is found with films grown by low temperature molecular
beam epitaxy (LT-MBE) and having the similar substitutional Mn_Ga composition.
Additionally, we demonstrate that II-PLM Ga1-xMnxAs films are free from
interstitial Mn_I because of the high temperature processing. At high Mn
implantation doses the kinetics of solute redistribution during solidification
alone determine the maximum resulting Mn_Ga concentration. Uniaxial anisotropy
between in-plane [-110]and [110] directions is present in II-PLM Ga1-xMnxAs
giving evidence for this being an intrinsic property of the carrier-mediated
ferromagnetic phase. | 0804.1612v1 |
2010-01-26 | Signature of checkerboard fluctuations in the phonon spectra of a possible polaronic metal La1.2Sr1.8Mn2O7 | Charge carriers in low-doped semiconductors may distort the atomic lattice
and trap themselves forming so-called small polarons. High carrier
concentrations can lead to short range ordered polarons (large polarons) and
even to long range charge and orbital order. Both systems should be insulating
with a large electrical resistivity, which decreases with increasing
temperature. However, photoemission measurements recently found a polaronic
pseudogap, in a metallic phase of La2-2xSr1+2xMn2O7. This layered manganite is
famous for colossal magnetoresistance (CMR) associated with a phase transition
from this low-temperature metallic phase to a high temperature insulating
phase. Broad charge order peaks due to large polarons observed by neutron and
x-ray scattering in the insulating phase disappear when La2-2xSr1+2xMn2O7
becomes metallic. We report results of inelastic neutron scattering
measurements showing that polarons remain inside the metallic phase as
fluctuations that strongly broaden and soften certain phonons near the wave
vectors where the charge order peaks appeared in the insulating phase. Our
findings imply that polaronic signatures in metals may generally come from a
competing insulating charge-ordered phase. It is highly relevant to cuprate
superconductors with both a pseudogap, and a similar phonon effect associated
with a competing stripe order. | 1001.4636v1 |
2010-04-28 | Synthesis, Structure and Properties of Tetragonal Sr2M3As2O2 (M3 = Mn3, Mn2Cu and MnZn2) Compounds Containing Alternating CuO2-Type and FeAs-Type Layers | Polycrystalline samples of Sr2Mn2CuAs2O2, Sr2Mn3As2O2, and Sr2Zn2MnAs2O2 were
synthesized. Their temperature- and applied magnetic field-dependent
structural, transport, thermal, and magnetic properties were characterized by
means of x-ray and neutron diffraction, electrical resistivity rho, heat
capacity, magnetization and magnetic susceptibility measurements. These
compounds have a body-centered-tetragonal crystal structure (space group
I4/mmm) that consists of MO2 (M = Zn and/or Mn) oxide layers similar to the
CuO2 layers in high superconducting transition temperature Tc cuprate
superconductors, and intermetallic MAs (M = Cu and/or Mn) layers similar to the
FeAs layers in high-Tc pnictides. These two types of layers alternate along the
crystallographic c-axis and are separated by Sr atoms. The site occupancies of
Mn, Cu and Zn were studied using Rietveld refinements of x-ray and neutron
powder diffraction data. The temperature dependences of rho suggest metallic
character for Sr2Mn2CuAs2O2 and semiconducting character for Sr2Mn3As2O2 and
Sr2Zn2MnAs2O2. Sr2Mn2CuAs2O2 is inferred to be a ferrimagnet with a Curie
temperature TC = 95(1) K. Remarkably, we find that the magnetic ground state
structure changes from a G-type antiferromagnetic structure in Sr2Mn3As2O2 to
an A-type ferrimagnetic structure in Sr2Mn2CuAs2O2 in which the Mn ions in each
layer are ferromagnetically aligned, but are antiferromagnetically aligned
between layers. | 1004.5038v1 |
2011-06-21 | Transport properties and anisotropy in rare earth doped CaFe2As2 single crystals with Tc above 40 K | In this paper we report the superconductivity above 40 K in the electron
doping single crystal Ca1-xRexFe2As2 (Re = La, Ce, Pr). The x-ray diffraction
patterns indicate high crystalline quality and c-axis orientation. the
resistivity anomaly in the parent compound CaFe2As2 is completely suppressed by
partial replacement of Ca by rare earth and a superconducting transition
reaches as high as 43 K, which is higher than the value in electron doping
FeAs-122 compounds by substituting Fe ions with transition metal, even
surpasses the highest values observed in hole doping systems with a transition
temperature up to 38 K. The upper critical field has been determined with the
magnetic field along ab-plane and c-axis, yielding the anisotropy of 2~3.
Hall-effect measurements indicate that the conduction in this material is
dominated by electron like charge carriers. Our results explicitly demonstrate
the feasibility of inducing superconductivity in Ca122 compounds via electron
doping using aliovalent rare earth substitution into the alkaline earth site,
which should add more ingredients to the underlying physics of the iron-based
superconductors. | 1106.4208v1 |
2012-02-13 | Graphene-on-Diamond Devices with Enhanced Current-Carrying Capacity: Carbon sp2-on-sp3 Technology | Graphene demonstrated potential for practical applications owing to its
excellent electronic and thermal properties. Typical graphene field-effect
transistors and interconnects built on conventional SiO2/Si substrates reveal
the breakdown current density on the order of 1 uA/nm2 (i.e. 10^8 A/cm2) which
is ~100\times larger than the fundamental limit for the metals but still
smaller than the maximum achieved in carbon nanotubes. We show that by
replacing SiO2 with synthetic diamond one can substantially increase the
current-carrying capacity of graphene to as high as ~18 uA/nm2 even at ambient
conditions. Our results indicate that graphene's current-induced breakdown is
thermally activated. We also found that the current carrying capacity of
graphene can be improved not only on the single-crystal diamond substrates but
also on an inexpensive ultrananocrystalline diamond, which can be produced in a
process compatible with a conventional Si technology. The latter was attributed
to the decreased thermal resistance of the ultrananocrystalline diamond layer
at elevated temperatures. The obtained results are important for graphene's
applications in high-frequency transistors, interconnects, transparent
electrodes and can lead to the new planar sp2-on-sp3 carbon-on-carbon
technology. | 1202.2886v1 |
2012-07-12 | Structure and electronic transport in graphene wrinkles | Wrinkling is a ubiquitous phenomenon in two-dimensional membranes. In
particular, in the large-scale growth of graphene on metallic substrates, high
densities of wrinkles are commonly observed. Despite their prevalence and
potential impact on large-scale graphene electronics, relatively little is
known about their structural morphology and electronic properties. Surveying
the graphene landscape using atomic force microscopy, we found that wrinkles
reach a certain maximum height before folding over. Calculations of the
energetics explain the morphological transition, and indicate that the tall
ripples are collapsed into narrow standing wrinkles by van der Waals forces,
analogous to large-diameter nanotubes. Quantum transport calculations show that
conductance through these collapsed wrinkle structures is limited mainly by a
density-of-states bottleneck and by interlayer tunneling across the collapsed
bilayer region. Also through systematic measurements across large numbers of
devices with wide folded wrinkles, we find a distinct anisotropy in their
electrical resistivity, consistent with our transport simulations. These
results highlight the coupling between morphology and electronic properties,
which has important practical implications for large-scale high-speed graphene
electronics. | 1207.2994v1 |
2012-09-06 | Terahertz time-domain spectroscopic ellipsometry: Instrumentation and calibration | We present a new instrumentation and calibration procedure for terahertz
time-domain spectroscopic ellipsometry (THz-TDSE) that is a newly established
characterization technique. The experimental setup is capable of providing
arbitrary angle of incidence in the range of $15^\circ$--$85^\circ$ in the
reflection geometry, and with no need for realignment. The setup is also
configurable easily into transmission geometry. For this setup, we successfully
used hollow core photonic band gap fiber with no pre-chirping in order to
deliver a femtosecond laser into a THz photoconductive antenna detector, which
is the first demonstration of this kind. The proposed calibration scheme can
compensate for the non-ideality of the polarization response of the THz
photoconductive antenna detector as well as that of wire grid polarizers used
in the setup. In the calibration scheme, the ellipsometric parameters are
obtained through a regression algorithm which we have adapted from the
conventional regression calibration method developed for rotating element
optical ellipsometers, and used here for the first time for THz-TDSE. As a
proof-of-principle demonstration, results are presented for a high resistivity
silicon substrate as well as an opaque Si substrate with a high phosphorus
concentration. We also demonstrate the capacity to measure a few micron thick
grown thermal oxide on top of Si. Each sample was characterized by THz-TDSE in
reflection geometry with different angle of incidence. | 1209.1294v2 |
2013-05-09 | Fermilab experience of post-annealing losses in SRF niobium cavities due to furnace contamination and the ways to its mitigation: a pathway to processing simplification and quality factor improvement | We investigate the effect of high temperature treatments followed by only
high-pressure water rinse (HPR) of superconducting radio frequency (SRF)
niobium cavities. The objective is to provide a cost effective alternative to
the typical cavity processing sequence, by eliminating the material removal
step post furnace treatment while preserving or improving the RF performance.
The studies have been conducted in the temperature range 800-1000C for
different conditions of the starting substrate: large grain and fine grain,
electro-polished (EP) and centrifugal barrel polished (CBP) to mirror finish.
An interesting effect of the grain size on the performances is found. Cavity
results and samples characterization show that furnace contaminants cause poor
cavity performance, and a practical solution is found to prevent surface
contamination. Extraordinary values of residual resistances ~ 1 nOhm and below
are then consistently achieved for the contamination-free cavities. These
results lead to a more cost-effective processing and improved RF performance,
and, in conjunction with CBP, open a potential pathway to acid-free processing. | 1305.2182v2 |
2013-06-03 | Electrical switching dynamics and broadband microwave characteristics of VO2 RF devices | Vanadium dioxide is a correlated electron system that features a
metal-insulator phase transition (MIT) above room temperature and is of
interest in high speed switching devices. Here, we integrate VO2 into
two-terminal coplanar waveguides and demonstrate a large resistance modulation
of the same magnitude (>10^3) in both electrically (i.e. by bias voltage,
referred to as E-MIT) and thermally (T-MIT) driven transitions. We examine
transient switching characteristics of the E-MIT and observe two
distinguishable time scales for switching. We find an abrupt jump in
conductivity with a rise time of the order of 10 ns followed by an oscillatory
damping to steady state on the order of several {\mu}s. We characterize the RF
power response in the On state and find that high RF input power drives VO2
further into the metallic phase, indicating that electromagnetic
radiation-switching of the phase transition may be possible. We measure
S-parameter RF properties up to 13.5 GHz. Insertion loss is markedly flat at
2.95 dB across the frequency range in the On state and sufficient isolation of
over 25 dB is observed in the Off state. We are able to simulate the RF
response accurately using both lumped element and 3D electromagnetic models.
Extrapolation of our results suggests that optimizing device geometry can
reduce insertion loss further and maintain broadband flatness up to 40 GHz. | 1306.0292v1 |
2013-07-16 | ZnS/Diamond Composite Coatings for Infrared Transmission Applications Formed by the Aerosol Deposition Method | The deposition of nano-crystalline ZnS/diamond composite protective coatings
on silicon, sapphire, and ZnS substrates, as a preliminary step to coating
infrared transparent ZnS substrates from powder mixtures by the aerosol
deposition method is presented. Advantages of the aerosol deposition method
include the ability to form dense, nanocrystalline films up to hundreds of
microns thick at room temperature and at a high deposition rate on a variety of
substrates. Deposition is achieved by creating a pressure gradient that
accelerates micrometer-scale particles in an aerosol to high velocity.
Upon impact with the target substrate the particles fracture and embed.
Continued deposition forms the thick compacted film. Deposition from an
aerosolized mixture of ZnS and diamond powders onto all targets results in
linear trend from apparent sputter erosion of the substrate at 100% diamond to
formation of a film with increasing fractions of ZnS. The crossover from
abrasion to film formation on sapphire occurs above about 50% ZnS and a mixture
of 90% ZnS and 10% diamond forms a well-adhered film of about 0.7 \mu m
thickness at a rate of 0.14 \mu m/min. Resulting films are characterized by
scanning electron microscopy, profilometry, infrared transmission spectroscopy,
and x-ray photoemission spectroscopy. These initial films mark progress toward
the future goal of coating ZnS substrates for abrasion resistance. | 1307.4319v1 |
2013-10-01 | Unexpected Giant Superconducting Fluctuation and Anomalous Semiconducting Normal State in NdO1-xFxBi1-yS2 Single Crystals | The BiS2-based superconductors were discovered recently. The
superconductivity has been proved by many other groups. Since the previous
experiments were all done on polycrystalline samples, therefore there remains a
concern whether the superconductivity is really derived from the materials
intrinsically or from some secondary phases. Experiments on single crystals are
highly desired. In this paper, we report the successful growth of the
NdO1-xFxBi1-yS2 single crystals. Resistive and magnetic measurements reveal
that the bulk superconducting transition occurs at about 5 K, while an
unexpected giant superconducting fluctuation appears at temperatures as high as
2-4 kBTC. Analysis based on the anisotropic Ginzbaug-Landau theory gives an
anisotropy of 30-45. Two gap features with magnitudes of about 3.5+-0.3 meV and
7.5+-1 meV were observed by scanning tunneling spectroscopy. The smaller gap is
associated with the bulk superconducting transition at about 5 K yielding a
huge ratio 2Delta_s/kBTc =16.8, the larger gap remains up to about 26 K. The
normal state recovered by applying a high magnetic field shows an anomalous
semiconducting behavior. All these suggest that the superconductivity in this
newly discovered superconductor cannot be formatted into the BCS theory. | 1310.0377v1 |
2014-06-26 | Electron localization and possible phase separation in the absence of a charge density wave in single-phase 1T-VS$_2$ | We report on a systematic study of the structural, magnetic and transport
properties of high-purity 1T-VS$_2$ powder samples prepared under high
pressure. The results differ notably from those previously obtained by
de-intercalating Li from LiVS$_2$. First, no Charge Density Wave (CDW) is found
by transmission electron microscopy down to 94 K. Though, \textit{ab initio}
phonon calculations unveil a latent CDW instability driven by an acoustic
phonon softening at the wave vector ${\bf q}_{CDW} \approx$ (0.21,0.21,0)
previously reported in de-intercalated samples. A further indication of latent
lattice instability is given by an anomalous expansion of the V-S bond distance
at low temperature. Second, infrared optical absorption and electrical
resistivity measurements give evidence of non metallic properties, consistent
with the observation of no CDW phase. On the other hand, magnetic
susceptibility and NMR data suggest the coexistence of localized moments with
metallic carriers, in agreement with \textit{ab initio} band structure
calculations. This discrepancy is reconciled by a picture of electron
localization induced by disorder or electronic correlations leading to a phase
separation of metallic and non-metallic domains in the nm scale. We conclude
that 1T-VS$_2$ is at the verge of a CDW transition and suggest that residual
electronic doping in Li de-intercalated samples stabilizes a uniform CDW phase
with metallic properties. | 1406.6945v1 |
2014-11-07 | Upgrade of the ALICE Inner Tracking System | During the Long Shutdown 2 of the LHC in 2018/2019, the ALICE experiment
plans the installation of a novel Inner Tracking System. It will replace the
current six layer detector system with a seven layer detector using Monolithic
Active Pixel Sensors. The upgraded Inner Tracking System will have
significantly improved tracking and vertexing capabilities, as well as readout
rate to cope with the expected increased Pb-Pb luminosity of the LHC. The
choice of Monolithic Active Pixel Sensors has been driven by the specific
requirements of ALICE as a heavy ion experiment dealing with rare processes at
low transverse momenta. This leads to stringent requirements on the material
budget of 0.3$% X/X_{0}$ per layer for the three innermost layers. Furthermore,
the detector will see large hit densities of $\sim 19
\mathrm{cm}^{-2}/\mathrm{event}$ on average for minimum-bias events in the
inner most layer and has to stand moderate radiation loads of 700 kRad TID and
$1\times 10^{13}$ 1 MeV n$_\mathrm{eq}/\mathrm{cm}^{2}$ NIEL at maximum. The
Monolithic Active Pixel Sensor detectors are manufactured using the TowerJazz
0.18 $\mu$m CMOS Imaging Sensor process on wafers with a high-resistivity
epitaxial layer. This contribution summarises the recent R&D activities and
focuses on results on the large-scale pixel sensor prototypes. | 1411.1802v2 |
2014-12-26 | High Pressure Induced Binding Between Linear Carbon Chains and Nanotubes | Recent studies of single-walled carbon nanotubes (CNTs) in aqueous media have
showed that water can significantly affect the tube mechanical properties. CNTs
under hydrostatic compression can preserve their elastic properties up to large
pressure values, while exhibiting exceptional resistance to mechanical
loadings. It was experimentally observed that CNTs with encapsulated linear
carbon chains (LCCs), when subjected to high hydrostatic pressure values,
present irreversible red shifts in some of their vibrational frequencies. In
order to address the cause of this phenomenon, we have carried out fully
atomistic reactive (ReaxFF) molecular dynamics (MD) simulations for model
structures mimicking the experimental conditions. We have considered the cases
of finite and infinite (cyclic boundary conditions) CNTs filled with LCCs (LCC
inside CNTs) of different lengths (from 9 up to 40 atoms). Our results show
that increasing the hydrostatic pressure causes the CNT to be deformed in an
inhomogeneous way due to the LCC presence. The LCC-CNT interface regions
exhibit convex curvatures, which results in more reactive sites, thus favoring
the formation of covalent chemical bonds between the chain and the nanotube.
This process is irreversible with the newly formed bonds continuing to exist
even after releasing the external pressure and causing an irreversibly red
shift in the chain vibrational modes from 1850 to 1500 cm$^{-1}$. | 1412.7966v1 |
2015-01-29 | Superconducting properties of sulfur-doped iron selenide | The recent discovery of high-temperature superconductivity in single-layer
iron selenide has generated significant experimental interest for optimizing
the superconducting properties of iron-based superconductors through the
lattice modification. For simulating the similar effect by changing the
chemical composition due to S doping, we investigate the superconducting
properties of high-quality single crystals of FeSe$_{1-x}$S$_{x}$ ($x$=0, 0.04,
0.09, and 0.11) using magnetization, resistivity, the London penetration depth,
and low temperature specific heat measurements. We show that the introduction
of S to FeSe enhances the superconducting transition temperature $T_{c}$,
anisotropy, upper critical field $H_{c2}$, and critical current density
$J_{c}$. The upper critical field $H_{c2}(T)$ and its anisotropy are strongly
temperature dependent, indicating a multiband superconductivity in this system.
Through the measurements and analysis of the London penetration depth $\lambda
_{ab}(T)$ and specific heat, we show clear evidence for strong coupling two-gap
$s$-wave superconductivity. The temperature-dependence of $\lambda _{ab}(T)$
calculated from the lower critical field and electronic specific heat can be
well described by using a two-band model with $s$-wave-like gaps. We find that
a $d$-wave and single-gap BCS theory under the weak-coupling approach can not
describe our experiments. The change of specific heat induced by the magnetic
field can be understood only in terms of multiband superconductivity. | 1501.07346v1 |
2015-09-28 | Thermoelectric properties of rare earth filled type-I like Clathrate, Dy8Al16Si30 | Type-I clathrates with a cage structure are known to be of importance for
thermoelectric applications as the cage can be filled with a guest atom which
leads to reduced thermal conductivity. Among the type-I clathrates, Si-based
alloys are of relevance for high temperature application and most importantly
because they are made of earth abundant elements. Dysprosium, Dy has been
chosen as the guest atom because of its large mass and small size compared to
divalent alkali metal ion. The Dy8Al16Si30, DAS, alloy has been synthesized by
arc melting of pure elements followed by annealing at 780 K for 7 days.
Structural characterization performed using XRD and SEM indicates presence of
both binary and ternary silicides, DySi2, DyAl2Si2 together with Al solid
solution and Si. The phase mixture remains unchanged even after annealing. The
microstructure has a typical dendritic structure with interdendritic phases,
signifying a slow, liquid transformation after arc melting. The Seebeck
coefficient is found to be positive, a p-type and increases with increasing
temperature both before and after annealing. The resistivity is found to be low
in the whole temperature range, 2 to 10 micro Ohm m and increases with
increasing temperature. The power factor in the as-prepared state is found to
be higher at all temperatures in the range 300 K to 700 K compared to annealed
state. The thermal conductivity however has been found to decrease on annealing
from an unusually high value of 100 W/m K to 50 W/m K. | 1509.08277v1 |
2016-01-06 | Observation of quantum Hall plateau-plateau transition and scaling behavior of the zeroth Landau level in graphene p-n-p junctions | We report distinctive magnetotransport properties of a graphene p-n-p
junction prepared by controlled diffusion of metallic contacts. In most cases,
materials deposited on a graphene surface introduce substantial carrier
scattering, which greatly reduces the high mobility of intrinsic graphene.
However, we show that an oxide layer only weakly perturbs the carrier
transport, which enables fabrication of a high-quality graphene p-n-p junction
through a one-step and resist-free method. The measured conductance-gate
voltage $(G-V_G)$ curves can be well described by a metal contact model, which
confirms the charge density depinning due to the oxide layer. The graphene
p-n-p junction samples exhibit pronounced quantum Hall effect, a well-defined
transition point of the zeroth Landau level (LL), and scaling behavior. The
scaling exponent obtained from the evolution of the zeroth LL width as a
function of temperature exhibits a relatively low value of
$\kappa=0.21\pm0.01$. Moreover, we calculate the energy level for the LLs based
on the distribution of plateau-plateau transition points, further validating
the assignment of the LL index of the QH plateau-plateau transition. | 1601.01155v1 |
2017-02-07 | Resonant spin transfer torque nano-oscillators | Spin transfer torque nano-oscillators are potential candidates for replacing
the traditional inductor based voltage controlled oscillators in modern
communication devices. Typical oscillator designs are based on trilayer
magnetic tunnel junctions which are disadvantaged by low power outputs and poor
conversion efficiencies. In this letter, we theoretically propose to use
resonant spin filtering in pentalayer magnetic tunnel junctions as a possible
route to alleviate these issues and present device designs geared toward a high
microwave output power and an efficient conversion of the d.c. input power. We
attribute these robust qualities to the resulting non-trivial spin current
profiles and the ultra high tunnel magnetoresistance, both arising from
resonant spin filtering. The device designs are based on the nonequilibrium
Green's function spin transport formalism self-consistently coupled with the
stochastic Landau-Lifshitz-Gilbert-Slonczewski's equation and the Poisson's
equation. We demonstrate that the proposed structures facilitate oscillator
designs featuring a large enhancement in microwave power of around $775\%$ and
an efficiency enhancement of over $1300\%$ in comparison with typical trilayer
designs. We also rationalize the optimum operating regions via an analysis of
the dynamic and static device resistances. This work sets stage for pentalyer
spin transfer torque nano-oscillator device designs that extenuate most of the
issues faced by the typical trilayer designs. | 1702.01869v1 |
2017-02-18 | Ultralow 1/f Noise in a Heterostructure of Superconducting Epitaxial Cobalt-Disilicide Thin Film on Silicon | High-precision resistance noise measurements indicate that the epitaxial
CoSi$_2$/Si hetero-structures at 150 K and 2 K (slightly above its
superconducting transition temperature $T_c$ of 1.54 K) exhibit an unusually
low 1/f noise level in the frequency range of 0.008-0.2 Hz. This corresponds to
an upper limit of Hooge constant $\gamma \leq 3 \times 10^{-6}$, about 100
times lower than that of single-crystalline aluminum films on SiO$_2$ capped Si
substrates. Supported by high-resolution cross-sectional transmission electron
microscopy studies, our analysis reveals that the 1/f noise is dominated by
excess interfacial Si atoms and their dimer reconstruction induced fluctuators.
Unbonded orbitals (i.e., dangling bonds) on excess Si atoms are intrinsically
rare at the epitaxial CoSi$_2$/Si(100) interface, giving limited
trapping-detrapping centers for localized charges. With its excellent
normal-state properties, CoSi$_2$ has been used in silicon-based integrated
circuits for decades. The intrinsically low noise properties discovered in this
work could be utilized for developing quiet qubits and scalable superconducting
circuits for future quantum computing. | 1702.05566v1 |
2017-06-03 | Electronic in-plane symmetry breaking at field-tuned quantum criticality in CeRhIn5 | Electronic nematics are exotic states of matter where electronic interactions
break a rotational symmetry of the underlying lattice, in analogy to the
directional alignment without translational order in nematic liquid crystals.
Intriguingly such phases appear in the copper- and iron-based superconductors,
and their role in establishing high-temperature superconductivity remains an
open question. Nematicity may take an active part, cooperating or competing
with superconductivity, or may appear accidentally in such systems. Here we
present experimental evidence for a phase of nematic character in the heavy
fermion superconductor CeRhIn5. We observe a field-induced breaking of the
electronic tetragonal symmetry of in the vicinity of an antiferromagnetic (AFM)
quantum phase transition at Hc~50T. This phase appears in out-of-plane fields
of H*~28T and is characterized by substantial in-plane resistivity anisotropy.
The anisotropy can be aligned by a small in-plane field component, with no
apparent connection to the underlying crystal structure. Furthermore no
anomalies are observed in the magnetic torque, suggesting the absence of
metamagnetic transitions in this field range. These observations are indicative
of an electronic nematic character of the high field state in CeRhIn5. The
appearance of nematic behavior in a phenotypical heavy fermion superconductor
highlights the interrelation of nematicity and unconventional
superconductivity, suggesting nematicity to be a commonality in such materials. | 1706.00963v1 |
2017-09-03 | Giant tunnel magnetoresistance with a single magnetic phase-transition electrode | Magnetic phase transition tunnel magnetoresistance (MPT-TMR) effect with a
single magnetic electrode has been investigated by first-principles
calculations. The calculations show that the MPT-TMR of FeRh/MgO/Cu tunnel
junction can be as high as hundreds of percent when the magnetic structure of
FeRh changes from G-type antiferromagnetic (GAFM) to ferromagnetic order. This
new type of MPT-TMR may be superior to the tunnel anisotropic magnetoresistance
because of its huge magneto-resistance effect and similar structural
simplicity. The main mechanism for the giant MPT-TMR can be attributed to the
formation of interface resonant states at GAFM-FeRh/MgO interface. A direct
FeRh/MgO interface is found to be necessary for achieving high MPT-TMR
experimentally. Moreover, we find the FeRh/MgO interface with FeRh in
ferromagnetic phase has nearly full spin-polarization due to the negligible
majority transmission and significantly different Fermi surface of two spin
channels. Thus, it may act as a highly efficient and tunable spin-injector. In
addition, electric field driven MPT of FeRh-based hetero-magnetic
nanostructures can be utilized to design various energy efficient tunnel
junction structures and the corresponding lower power consumption devices. Our
results will stimulate further experimental investigations of MPT-TMR and other
fascinating phenomenon of FeRh-based tunnel junctions that may be promising in
antiferromagnetic spintronics. | 1709.00687v2 |
2017-11-08 | X-ray photoelectron spectroscopy, Magnetotransport and Magnetization study of Nb2PdS5 superconductor | In the present report, we investigate various properties of the Nb2PdS5
superconductor. Scanning electron microscopy displayed slabs like laminar
growth of Nb2PdS5while X-ray photoelectron spectroscopy exhibited the
hybridization of Sulphur (2p) with both Palladium (3d)and Niobium (3d). High
field (140kOe) magneto-transport measurements revealed that superconductivity
(Tc onset =7K and Tc R = 0 = 6.2K) of the studied Nb2PdS5material is quite
robust against magnetic field with the upper critical field (Hc2) outside the
Pauli paramagnetic limit. Thermally activated flux flow (TAFF) of the compound
showed that resistivity curves follow Arrhenius behaviour. The activation
energy for Nb2PdS5 is found to decrease from 15.15meV at 10kOe to 2.35meV at
140kOe. Seemingly, the single vortex pinning is dominant in low field regions,
while collective pinning is dominant in high field region. The temperature
dependence of AC susceptibility confirmed the Tc at 6K, further varying
amplitude and frequency showed well coupled granular nature of
superconductivity. The lower critical field (Hc1) is extracted from DC
magnetisation measurements at various T below Tc. It is found that Hc1(T) of
Nb2PdS5 material seemingly follows the multiband nature of superconductivity. | 1711.02830v1 |
2017-11-10 | Thermodynamic Studies of \b{eta}-Ga2O3 Nanomembrane Field-Effect Transistors on a Sapphire Substrate | The self-heating effect is a severe issue for high-power semiconductor
devices, which degrades the electron mobility and saturation velocity, and also
affects the device reliability. On applying an ultrafast and high-resolution
thermoreflectance imaging technique, the direct self-heating effect and surface
temperature increase phenomenon are observed on novel top-gate \b{eta}-Ga2O3 on
insulator field-effect transistors. Here, we demonstrate that by utilizing a
higher thermal conductivity sapphire substrate rather than a SiO2/Si substrate,
the temperature rise above room temperature of \b{eta}-Ga2O3 on the insulator
field-effect transistor can be reduced by a factor of 3 and thereby the
self-heating effect is significantly reduced. Both thermoreflectance
characterization and simulation verify that the thermal resistance on the
sapphire substrate is less than 1/3 of that on the SiO2/Si substrate.
Therefore, maximum drain current density of 535 mA/mm is achieved on the
sapphire substrate, which is 70% higher than that on the SiO2/Si substrate due
to reduced self-heating. Integration of \b{eta}-Ga2O3 channel on a higher
thermal conductivity substrate opens a new route to address the low thermal
conductivity issue of \b{eta}-Ga2O3 for power electronics applications. | 1711.03672v1 |
2018-03-28 | Study of nitrogen ion doping of titanium dioxide films | This study reports on the properties of nitrogen doped titanium dioxide
$TiO_2$ thin films considering the application as transparent conducting oxide
(TCO). Sets of thin films were prepared by sputtering a titanium target under
oxygen atmosphere on a quartz substrate at 400 or 500{\deg}C. Films were then
doped at the same temperature by 150 eV nitrogen ions. The films were prepared
in Anatase phase which was maintained after doping. Up to 30at% nitrogen
concentration was obtained at the surface, as determined by in situ x-ray
photoelectron spectroscopy (XPS). Such high nitrogen concentration at the
surface lead to nitrogen diffusion into the bulk which reached about 25 nm.
Hall measurements indicate that average carrier density reached over $10^{19}
cm^{-3}$ with mobility in the range of $0.1$ to $1 cm^2V^{-1}s^{-1}$.
Resistivity about $3.10^{-1} \Omega cm$ could be obtained with 85% light
transmission at 550 nm. These results indicate that low energy implantation is
an effective technique for $TiO_2$ doping that allows an accurate control of
the doping process independently from the TiO2 preparation. Moreover, this
doping route seems promising to attain high doping levels without significantly
affecting the film structure. Such approach could be relevant for preparation
of $N:TiO_2$ transparent conduction electrodes (TCE). | 1803.10828v1 |
2018-06-15 | Quantum anomalous Hall multilayers grown by molecular beam epitaxy | Quantum anomalous Hall (QAH) effect is a quantum Hall effect that occurs
without the need of external magnetic field. A system composed of multiple
parallel QAH layers is an effective high Chern number QAH insulator and the key
to the applications of the dissipationless chiral edge channels in low energy
consumption electronics. Such a QAH multilayer can also be engineered into
other exotic topological phases such as a magnetic Weyl semimetal with only one
pair of Weyl points. This work reports the first experimental realization of
QAH multilayers in the superlattices composed of magnetically doped
(Bi,Sb)$_2$Te$_3$ topological insulator and CdSe normal insulator layers grown
by molecular beam epitaxy. The obtained multilayer samples show quantized Hall
resistance $h/Ne$$^2$, where $h$ is the Planck's constant, $e$ is the
elementary charge and $N$ is the number of the magnetic topological insulator
layers, resembling a high Chern number QAH insulator. | 1806.05923v1 |
2018-09-26 | Rare earth permanent magnets prepared by hot deformation process | Hot deformation process is one of the primary methods to produce anisotropic
rare earth permanent magnets. Firstly, rapidly quenched powder flakes with
nanocrystal structure are condensed into the full dense isotropic precursors by
hot pressing process. And then, the prepared isotropic precursors are hot
deformed to produce high-anisotropy uniaxial bulk rare earth permanent magnets,
in which the highly textured structure is obtained in the hot plastic
deformation process. The obtained hot-deformed magnets possess many advantages,
such as near net-shape, outstanding corrosion resistance and ultrafine-grain
structure. The noteworthy effects of preparation parameters employed in
hot-pressing and deformation processes on the magnetic properties and
microstructures characterizations are systemically summarized in this academic
monograph. As a near net-shape technique, hot deformation process has
noteworthy advantages in producing irregular shape magnets, especially for
radially oriented ring-shape magnets with high length-diameter ratio or thin
wall. The difficulties in producing crack-free, homogeneous and non-decentered
ring-shaped magnets are basically resolved through mold design, adjustment of
deformation parameters and application of theoretical simulation. Considering
the characteristics of hot-deformed magnets, such as the grain shapes and
sizes, anisotropic distribution of intergranular phases, etc., there is
practical significance to study and improve the mechanical, electric properties
and thermal stability to enlarge the applicable area of hot-deformed magnets or
ring-shaped magnets. | 1809.09838v1 |
2018-11-05 | SkyLogic - A proposal for a skyrmion logic device | This work proposes a novel logic device (SkyLogic) based on skyrmions, which
are magnetic vortex-like structures that have low depinning current density and
are robust to defects. A charge current sent through a polarizer ferromagnet
(P-FM) nucleates a skyrmion at the input end of an intra-gate FM interconnect
with perpendicular magnetic anisotropy (PMA-FM). The output end of the PMA--FM
forms the free layer of an MTJ stack. A spin Hall metal (SHM) is placed beneath
the PMA-FM. The skyrmion is propagated to the output end of the PMA-FM by
passing a charge current through the SHM. The resistance of the MTJ stack is
low (high) when a skyrmion is present (absent) in the free layer, thereby
realizing an inverter. A framework is developed to analyze the performance of
the SkyLogic device. A circuit-level technique is developed that counters the
transverse displacement of skyrmion in the PMA-FM and allows use of high
current densities for fast propagation. The design space exploration of the
PMA-FM material parameters is performed to obtain an optimal design point. At
the optimal point, we obtain an inverter delay of 434 ps with a switching
energy of 7.1 fJ. | 1811.02016v1 |
2019-09-06 | Electronic correlation determining correlated plasmons in Sb-doped Bi$_2$Se$_3$ | Electronic correlation is believed to play an important role in exotic
phenomena such as insulator-metal transition, colossal magneto resistance and
high temperature superconductivity in correlated electron systems. Recently, it
has been shown that electronic correlation may also be responsible for the
formation of unconventional plasmons. Herewith, using a combination of
angle-dependent spectroscopic ellipsometry, angle resolved photoemission
spectroscopy and Hall measurements all as a function of temperature supported
by first-principles calculations, the existence of low-loss high-energy
correlated plasmons accompanied by spectral weight transfer, a fingerprint of
electronic correlation, in topological insulator
(Bi$_{0.8}$Sb$_{0.2}$)$_2$Se$_3$ is revealed. Upon cooling, the density of free
charge carriers in the surface states decreases whereas those in the bulk
states increase, and that the newly-discovered correlated plasmons are key to
explaining this phenomenon. Our result shows the importance of electronic
correlation in determining new correlated plasmons and opens a new path in
engineering plasmonic-based topologically-insulating devices. | 1909.02703v1 |
2020-03-30 | Growth and transport properties of Mg3X2 (X = Sb, Bi) single crystals | The discovery of high thermoelectric performance in n-type polycrystalline
Mg3(Sb,Bi)2 based Zintl compounds has ignited intensive research interest.
However, some fundamental questions concerning the anisotropic transport
properties and the origin of intrinsically low thermal conductivity are still
elusive, requiring the investigation of single crystals. In this work,
high-quality p-type Mg3Sb2 and Mg3Bi2 single crystals have been grown by using
a self-flux method. The electrical resistivity \r{ho} of Mg3Bi2 single crystal
displays an anisotropy with \r{ho} in-plane twice larger than out-of-plane. The
low-temperature heat capacity and lattice thermal conductivity of Mg3Sb2 and
Mg3Bi2 single crystals have been investigated by using the Debye-Callaway
model, from which the existence of low-lying vibration mode could be concluded.
Large Gr\"uneisen parameters and strong anharmonicity are found responsible for
the intrinsically low thermal conductivity. Moreover, grain boundary scattering
does not contribute significantly to suppress the lattice thermal conductivity
of polycrystalline Mg3Sb2. Our results provide insights into the intrinsic
transport properties of Mg3X2 and could pave a way to realize enhanced
thermoelectric performance in single-crystalline Mg3X2-based Zintl compounds. | 2003.13313v1 |
2017-03-13 | Extremely high magnetoresistance and conductivity in the type-II Weyl semimetals WP2 and MoP2 | The peculiar band structure of semimetals exhibiting Dirac and Weyl crossings
can lead to spectacular electronic properties such as large mobilities
accompanied by extremely high magnetoresistance. In particular, two closely
neighbouring Weyl points of the same chirality are protected from annihilation
by structural distortions or defects, thereby significantly reducing the
scattering probability between them. Here we present the electronic properties
of the transition metal diphosphides, WP2 and MoP2, that are type-II Weyl
semimetals with robust Weyl points. We present transport and angle resolved
photoemission spectroscopy measurements, and first principles calculations. Our
single crystals of WP2 display an extremely low residual low-temperature
resistivity of 3 nohm-cm accompanied by an enormous and highly anisotropic
magnetoresistance above 200 million % at 63 T and 2.5 K. These properties are
likely a consequence of the novel Weyl fermions expressed in this compound. We
observe a large suppression of charge carrier backscattering in WP2 from
transport measurements. | 1703.04527v3 |
2017-03-20 | Exceptional Anti-Icing Performance of Self-Impregnating Slippery Surfaces | A heat exchange interface at subzero temperature in a water vapor
environment, exhibits high probability of frost formation due to freezing
condensation, a factor that markedly decreases the heat transfer efficacy due
to the considerable thermal resistance of ice. Here we report a novel strategy
to delay ice nucleation on these types of solid-water vapor interfaces. With a
process-driven mechanism, a self-generated liquid intervening layer immiscible
to water, is deposited on a textured superhydrophobic surface and acts as a
barrier between the water vapor and the solid substrate. This liquid layer
imparts remarkable slippery conditions resulting in high mobility of condensing
water droplets. A large increase of the ensuing ice coverage time is shown
compared to the cases of standard smooth hydrophilic or textured
superhydrophobic surfaces. During deicing of these self-impregnating surfaces
we show an impressive tendency of ice fragments to skate expediting defrosting.
Robustness of such surfaces is also demonstrated by operating them under
subcooling for at least 490hr without a marked degradation. This is attributed
to the presence of the liquid intervening layer, which protects the substrate
from hydrolyzation enhancing longevity and sustaining heat transfer efficiency. | 1703.07349v1 |
2019-02-03 | Non-monotonic pressure dependence of high-field nematicity and magnetism in CeRhIn$_5$ | CeRhIn$_5$ provides a textbook example of quantum criticality in a heavy
fermion system: Pressure suppresses local-moment antiferromagnetic (AFM) order
and induces superconductivity in a dome around the associated quantum critical
point (QCP) near $p_{c} \approx 23\,$kbar. Strong magnetic fields also suppress
the AFM order at a field-induced QCP at $B_{\rm c}\approx 50\,$T. In its
vicinity, a nematic phase at $B^*\approx 28\,$T characterized by a large
in-plane resistivity anisotropy emerges. Here, we directly investigate the
interrelation between these phenomena via magnetoresistivity measurements under
high pressure. As pressure increases, the nematic transition shifts to higher
fields, until it vanishes just below $p_{\rm c}$. While pressure suppresses
magnetic order in zero field as $p_{\rm c}$ is approached, we find magnetism to
strengthen under strong magnetic fields due to suppression of the Kondo effect.
We reveal a strongly non-mean-field-like phase diagram, much richer than the
common local-moment description of CeRhIn$_5$ would suggest. | 1902.00970v3 |
2020-02-11 | Observation of an antiferromagnetic quantum critical point in high-purity LaNiO$_3$ | Amongst the rare-earth perovskite nickelates, LaNiO$_3$ (LNO) is an
exception. While the former have insulating and antiferromagnetic ground
states, LNO remains metallic and non-magnetic down to the lowest temperatures.
It is believed that LNO is a strange metal, on the verge of an
antiferromagnetic instability. Our work suggests that LNO is a quantum critical
metal, close to an antiferromagnetic quantum critical point (QCP). The QCP
behavior in LNO is manifested in epitaxial thin films with unprecedented high
purities. We find that the temperature and magnetic field dependences of the
resistivity of LNO at low temperatures are consistent with scatterings of
charge carriers from weak disorder and quantum fluctuations of an
antiferromagnetic nature. Furthermore, we find that the introduction of a small
concentration of magnetic impurities qualitatively changes the magnetotransport
properties of LNO, resembling that found in some heavy-fermion Kondo lattice
systems in the vicinity of an antiferromagnetic QCP. | 2002.04159v1 |
2020-09-18 | Nanoscale structural and electrical properties of graphene grown on AlGaN by catalyst-free chemical vapor deposition | The integration of graphene (Gr) with nitride semiconductors is highly
interesting for applications in high-power/high-frequency electronics and
optoelectronics. In this work, we demonstrated the direct growth of Gr on
Al0.5Ga0.5N/sapphire templates by propane (C3H8) chemical vapor deposition
(CVD) at temperature of 1350{\deg}C. After optimization of the C3H8 flow rate,
a uniform and conformal Gr coverage was achieved, which proved beneficial to
prevent degradation of AlGaN morphology. X-ray photoemission spectroscopy (XPS)
revealed Ga loss and partial oxidation of Al in the near-surface AlGaN region.
Such chemical modification of a 2 nm thick AlGaN surface region was confirmed
by cross-sectional scanning transmission electron microscopy (STEM) combined
with electron energy loss spectroscopy (EELS), which also showed the presence
of a bilayer of Gr with partial sp2/sp3 hybridization. Raman spectra indicated
that the deposited Gr is nanocrystalline (with domain size 7 nm) and
compressively strained. A Gr sheet resistance of 15.8 kOhm/sq was evaluated by
four-point-probe measurements, consistently with the nanocrystalline nature of
these films. Furthermore, nanoscale resolution current mapping by conductive
atomic force microscopy (C-AFM) indicated local variations of the Gr carrier
density at a mesoscopic scale, which can be ascribed to changes in the charge
transfer from the substrate due to local oxidation of AlGaN or to the presence
of Gr wrinkles. | 2009.08673v1 |
2017-07-28 | 3D laser printing by ultra-short laser pulses for micro-optical applications: towards telecom wavelengths | Three dimensional (3D) fast (< 0.5 hour) printing of micro-optical elements
down to sub-wavelength resolution over 100 micrometers footprint areas using
femtosecond (fs-)laser oscillator is presented. Using sub-1 nJ pulse energies,
optical vortex generators made of polymerised grating segments with an
azimuthally changing orientation have been fabricated in SZ2080 resist; width
of polymerised rods was ~150 nm and period 0.6-1 micrometers. Detailed phase
retardance analysis was carried out manually with Berek compensator (under a
white light illumination) and using an equivalent principle by an automated
Abrio implementation at 546 nm. Direct experimental measurements of retardance
was required since the period of the grating was comparable (or larger) than
the wavelength of visible light. By gold sputtering, transmission-type optical
vortex generators were turned into reflective ones with augmented retardance,
n.h defined by the form birefringence, n, and the height h = 2d where d is the
thickness of the polymerised structure. Retardance reached 315 nm as measured
with Berek compensator at visible wavelengths. Birefringent phase delays of 180
degrees (or half-wavelength) required for high purity vortex generators can be
made based on the proposed approach. Optical vortex generators for telecom
wavelengths with sub-wavelength patterns of azimuthally oriented gratings are
amenable by direct laser polymerisation. | 1707.09365v1 |
2017-08-14 | High-Kinetic Inductance Additive Manufactured Superconducting Microwave Cavity | Investigations into the microwave surface impedance of superconducting
resonators have led to the development of single photon counters that rely on
kinetic inductance for their operation. While concurrent progress in additive
manufacturing, `3D printing', opens up a previously inaccessible design space
for waveguide resonators. In this manuscript, we present results from the first
synthesis of these two technologies in a titanium, aluminum, vanadium
(Ti-6Al-4V) superconducting radio frequency resonator which exploits a design
unattainable through conventional fabrication means. We find that Ti-6Al-4V has
two distinct superconducting transition temperatures observable in heat
capacity measurements. The higher transition temperature is in agreement with
DC resistance measurements. While the lower transition temperature, not
previously known in literature, is consistent with the observed temperature
dependence of the superconducting microwave surface impedance. From the surface
reactance, we extract a London penetration depth of $8\pm3{\mu}$m - roughly an
order of magnitude larger than other titanium alloys and several orders of
magnitude larger than other conventional elemental superconductors. This large
London penetration depth suggests that Ti-6Al-4V may be a suitable material for
high kinetic inductance applications such as single photon counting or
parametric amplification used in quantum computing. | 1708.04273v1 |
2018-08-13 | Ultra-compact graphene plasmonic photodetector with the bandwidth over 110GHz | Graphene-based photodetectors, taking advantage of high carrier mobility and
broadband absorption in graphene, have recently experienced rapid development.
However, their performances with respect to the responsivity and bandwidth are
still limited by either weak light-graphene interaction or large
resistance-capacitance product. Here, we demonstrate a waveguide coupled
integrated graphene plasmonic photodetector on the silicon-on-insulator
platform. Benefiting from plasmonic enhanced graphene-light interactions and
subwavelength confinement of the optical energy, we present a small-footprint
graphene-plasmonic photodetector with bandwidth beyond 110GHz and intrinsic
responsivity of 360mA/W. Attributed to the unique electronic bandstructure of
graphene and its ultra-broadband absorption, the operational wavelength range
extending beyond mid-infrared, and possibly further, can be anticipated. Our
results show that the combination of graphene with plasmonic devices has great
potential to realize ultra-compact and high-speed optoelectronic devices for
graphene-based optical interconnects. | 1808.04815v3 |
2019-03-22 | Structure and mechanical behavior of ultrafine-grained aluminum-iron alloy stabilized by nanoscaled intermetallic particles | Ultrafine-grained aluminum alloys offer interesting multifunctional
properties with a combination of high strength, low electrical resistivity, and
low density. However, due to thermally induced grain coarsening, they typically
suffer from an intrinsic poor thermal stability. To overcome this drawback, an
Al-2%Fe alloy has been selected because of the low solubility of Fe in Al and
their highly positive enthalpy of mixing leading to the formation of stable
intermetallic particles. The two-phase alloy has been processed by severe
plastic deformation to achieve simultaneously submicrometer Al grains and a
uniform distribution of nanoscaled intermetallic particles. The influence of
the level of deformation on the microstructure has been investigated thanks to
transmission electron microscopy and atom probe tomography and it is shown that
for the highest strain a partial dissolution of the metastable Al6Fe particle
occurred leading to the formation of a Fe super saturated solid solution. The
thermal stability, and especially the precipitation of particles from the
ultrafine-grained solid solution and the way they pin grain boundaries has been
investigated both from static annealing and in-situ transmission electron
microscopy experiments. The correlation between microstructural features and
microhardness has been established to identify the various strengthening
contributions. Finally, it is 2 shown that ultrafine grained high purity Al
with less than 0.01 at. % Fe in solid solution could preserve a grain size only
300nm after 1h at 250$^\circ$C. | 1903.09391v1 |
2019-03-27 | Homogeneous hierarchical NiMoO4@NiMoO4 nanostructure as a high-performance anode material for electrochemical energy storage | Here we report the extraordinary electrochemical energy storage capability of
NiMoO4@NiMoO4 homogeneous hierarchical nanosheet-on-nanowire-arrays (SOWAs)
synthesized on nickel substrate by a two-stage hydrothermal process.
Comparatively speaking, the SOWAs electrode displays improved electrochemical
performances than the bare NiMoO4 nanowire arrays. Such improvements can be
ascribed to the characteristic homogeneous hierarchical structure which not
only effectively increases the active surface areas for fast charge transfer,
but also reduces the electrode resistance significantly by eliminating the
potential barrier at the nanowire/nanosheet junction, which is usually an issue
in other reported heterogeneous architectures. We further evaluate the
performances of the SOWAs by constructing an asymmetric hybrid supercapacitor
(ASC) with the SOWAs and activated carbon (AC). The optimized ASC shows
excellent electrochemical performances with 47.2 Wh/kg in energy density at
1.38 kW/kg at 0-1.2 V. Moreover, the specific capacity retention can be as high
as 91.4% after 4000 cycles, illustrating the remarkable cycling stability of
the NiMoO4@NiMoO4//AC ASC device. Our results show that this unique
NiMoO4@NiMoO4 SOWAs display great prospect for future energy storage
applications | 1903.11513v1 |
2019-04-29 | Type-II Ising superconductivity and anomalous metallic state in macro-size ambient-stable ultrathin crystalline films | Recent emergence of two-dimensional (2D) crystalline superconductors has
provided a promising platform to investigate novel quantum physics and
potential applications. To reveal essential quantum phenomena therein, ultralow
temperature transport investigation on high quality ultrathin superconducting
films is critically required, although it has been quite challenging
experimentally. Here we report a systematic transport study on the ultrathin
crystalline PdTe2 films grown by molecular beam epitaxy (MBE). Interestingly, a
new type of Ising superconductivity in 2D centrosymmetric materials is revealed
by the detection of large in-plane critical field more than 7 times Pauli
limit. Remarkably, in perpendicular magnetic field, we provide solid evidence
of anomalous metallic state characterized by the resistance saturation at low
temperatures with high quality filters. The robust superconductivity with
intriguing quantum phenomena in the macro-size ambient-stable ultrathin PdTe2
films remains almost the same for 20 months, showing great potentials in
electronic and spintronic applications. | 1904.12719v2 |
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