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2016-09-08 | Evidence of Ballistic Thermal Transport in Lithium Niobate at Room Temperature | In ballistic transport, heat carriers such as phonons travel through the
solid without any scattering or interaction. Therefore, there is no temperature
gradient in the solid, which seems to transport the heat without getting heated
itself. Ballistic transport is typically seen in high purity crystals at either
temperatures below ~10 K, or physical size below ~100 nm, where the mean free
path of the carrier is larger than the solid itself. In this letter, we show
evidence of ballistic transport at room temperature in lithium niobate wafers
in the in-plane and cross-plane directions under both steady state and high
frequency heating that are monitored using both infrared and resistance
thermometry. We report phonon mean free path in lithium niobate around 425
microns, which is about 50 times higher than the largest phonon mean free path
in the literature at room temperature. Above this length-scale, temperature
gradient gradually emerges and the material shows completely diffusive, bulk
transport at about 4 mm length. Our observations will impact phonon-based
electronics such as thermal transistor, thermal logic gate and memory currently
impossible at room temperature. If 1 micron electron mean free path in graphene
gives the highest-mobility, the 425 microns mean free path of phonons in this
research may realize phononics without any need for nanoscale size or
ultra-cold temperatures. | 1609.02585v1 |
2018-07-03 | ssDNA sequencing by rectification | Fast, reliable and inexpensive DNA sequencing is an important pursuit in
healthcare, especially in personalized medicine with possible deep societal
impacts. Despite significant progress of various nanopore-based sequencing
configurations, challenges remain in resolution (due to thermal fluctuations or
to sensitivity to molecular orientation) and speed, which are calling for new
approaches. Here we propose a sequencing protocol for DNA translocation through
a nanopore with side-embedded N-terminated carbon nanotube electrodes.
Employing DFT and Non-Equilibrium Green's Function formalism, we show that the
rectification ratio (response to square pulses of alternating bias) bears high
nucleobase specificity. The rectification arises due to bias-dependent
resistance asymmetry on the deoxyribonucleotide-electrode interfaces. The
asymmetry induces molecular charging and HOMO pinning to the electrochemical
potential of one of the electrodes, assisted by an in-gap electric field effect
caused by dipoles at the terminated electrode ends. This sequencing protocol is
sensitive, selective with orders of magnitude, has high resolution, and it is
robust to molecular orientation. | 1807.01215v4 |
2018-12-16 | Quantum dots formed in three-dimensional Dirac semimetal Cd$_3$As$_2$ nanowires | We demonstrate quantum dot (QD) formation in three-dimensional Dirac
semimetal Cd$_{3}$As$_{2}$ nanowires using two electrostatically tuned p$-$n
junctions with a gate and magnetic fields. The linear conductance measured as a
function of gate voltage under high magnetic fields is strongly suppressed at
the Dirac point close to zero conductance, showing strong conductance
oscillations. Remarkably, in this regime, the Cd$_{3}$As$_{2}$ nanowire device
exhibits Coulomb diamond features, indicating that a clean single QD forms in
the Dirac semimetal nanowire. Our results show that a p$-$type QD can be formed
between two n$-$type leads underneath metal contacts in the nanowire by
applying gate voltages under strong magnetic fields. Analysis of the quantum
confinement in the gapless band structure confirms that p$-$n junctions formed
between the p$-$type QD and two neighboring n$-$type leads under high magnetic
fields behave as resistive tunnel barriers due to cyclotron motion, resulting
in the suppression of Klein tunneling. The p$-$type QD with magnetic
field-induced confinement shows a single hole filling. Our results will open up
a route to quantum devices such as QDs or quantum point contacts based on Dirac
and Weyl semimetals. | 1812.06416v1 |
2019-01-17 | Compositional dependence of epitaxial Tin+1SiCn MAX-phase thin films grown from a Ti3SiC2 compound target | We investigate sputtering of a Ti3SiC2 compound target at temperatures
ranging from RT (no applied external heating) to 970 oC as well as the
influence of the sputtering power at 850 oC for the deposition of Ti3SiC2 films
on Al2O3(0001) substrates. Elemental composition obtained from time-of-flight
energy elastic recoil detection analysis shows an excess of carbon in all
films, which is explained by differences in angular distribution between C, Si
and Ti, where C scatters the least during sputtering. The oxygen content is 2.6
at.% in the film deposited at RT and decreases with increasing deposition
temperature, showing that higher temperatures favor high purity films. Chemical
bonding analysis by X-ray photoelectron spectroscopy shows C-Ti and Si-C
bonding in the Ti3SiC2 films and Si-Si bonding in the Ti3SiC2 compound target.
X-ray diffraction reveals that the phases Ti3SiC2, Ti4SiC3, and Ti7Si2C5 can be
deposited from a Ti3SiC2 compound target at substrate temperatures above 850 oC
and with growth of TiC and the Nowotny phase Ti5Si3Cx at lower temperatures.
High-resolution scanning transmission electron microscopy shows epitaxial
growth of Ti3SiC2, Ti4SiC3, and Ti7Si2C5 on TiC at 970 oC. Four-point probe
resistivity measurements give values in the range 120 to 450 micro-Ohm-cm and
with the lowest values obtained for films containing Ti3SiC2, Ti4SiC3, and
Ti7Si2C5. | 1901.05904v1 |
2019-06-10 | Anomalous electron transport in epitaxial NdNiO$_3$ films | The origin of simultaneous electronic, structural and magnetic transitions in
bulk rare-earth nickelates ($RE$NiO$_3$) remains puzzling with multiple
conflicting reports on the nature of these entangled phase transitions.
Heterostructure engineering of these materials offers unique opportunity to
decouple metal-insulator transition (MIT) from the magnetic transition.
However, the evolution of underlying electronic properties across these
decoupled transitions remains largely unexplored. In order to address this, we
have measured Hall effect on a series of epitaxial NdNiO$_3$ films, spanning a
variety of electronic and magnetic phases. We find that the MIT results in only
partially gapped Fermi surface, whereas full insulating phase forms below the
magnetic transition. In addition, we also find a systematic reduction of the
Hall coefficient ($R_H$) in the metallic phase of these films with epitaxial
strain and also a surprising transition to negative value at large compressive
strain. Partially gapped weakly insulating, paramagnetic phase is reminiscence
of pseudogap behavior of high $T_c$ cuprates. The precursor metallic phase,
which undergoes transition to insulating phase is a non-Fermi liquid with the
temperature exponent ($n$) of resistivity of 1, whereas the exponent increases
to 4/3 in the non-insulating samples. Such nickelate phase diagram with
sign-reversal of $R_H$, pseudo-gap phase and non Fermi liquid behavior are
intriguingly similar to high $T_c$ cuprates, giving important guideline to
engineer unconventional superconductivity in oxide heterostructure. | 1906.03809v1 |
2019-10-22 | Magnetic topological insulator MnBi6Te10 with zero-field ferromagnetic state and gapped Dirac surface states | Magnetic topological insulators (TIs) with nontrivial topological electronic
structure and broken time-reversal symmetry exhibit various exotic topological
quantum phenomena. The realization of such exotic phenomena at high temperature
is one of central topics in this area. We reveal that MnBi6Te10 is a magnetic
TI with an antiferromagnetic ground state below 10.8 K whose nontrivial
topology is manifested by Dirac-like surface states. The ferromagnetic axion
insulator state with Z4 = 2 emerges once spins polarized at field as low as 0.1
T, accompanied with saturated anomalous Hall resistivity up to 10 K. Such a
ferromagnetic state is preserved even external field down to zero at 2 K.
Theoretical calculations indicate that the few-layer ferromagnetic MnBi6Te10 is
also topologically nontrivial with a non-zero Chern number. Angle-resolved
photoemission spectroscopy experiments further reveal three types of Dirac
surface states arising from different terminations on the cleavage surfaces,
one of which has insulating behavior with an energy gap of ~ 28 meV at the
Dirac point. These outstanding features suggest that MnBi6Te10 is a promising
system to realize various topological quantum effects at zero field and high
temperature. | 1910.10101v2 |
2020-01-03 | Coulomb Blockade Effects in a Topological Insulator Grown on a High-Tc Cuprate Superconductor | The evidence for proximity-induced superconductivity in heterostructures of
topological insulators and high-Tc cuprates has been intensely debated. We use
molecular beam epitaxy to grow thin films of topological insulator Bi2Te3 on a
cuprate Bi2Sr2CaCu2O8+x, and study the surface of Bi2Te3 using low-temperature
scanning tunneling microscopy and spectroscopy. In few unit-cell thick Bi2Te3
films, we find a V-shaped gap-like feature at the Fermi energy in dI/dV
spectra. By reducing the coverage of Bi2Te3 films to create nanoscale islands,
we discover that this spectral feature dramatically evolves into a much larger
hard gap, which can be understood as a Coulomb blockade gap. This conclusion is
supported by the evolution of dI/dV spectra with the lateral size of Bi2Te3
islands, as well as by topographic measurements that show an additional barrier
separating Bi2Te3 and Bi2Sr2CaCu2O8+x. We conclude that the prominent gap-like
feature in dI/dV spectra in Bi2Te3 films is not a proximity-induced
superconducting gap. Instead, it can be explained by Coulomb blockade effects,
which take into account additional resistive and capacitive coupling at the
interface. Our experiments provide a fresh insight into the tunneling
measurements of complex heterostructures with buried interfaces. | 2001.00906v2 |
2020-07-14 | Cyclic plasticity and fatigue damage of CrMnFeCoNi high entropy alloy fabricated by laser powder-bed fusion | The CrMnFeCoNi high-entropy alloy is highly printable and holds great
potential for structural applications. However, no significant discussions on
cyclic plasticity and fatigue damage in previous studies. This study provides
significant insights into the link between print processes, solidification
microstructure, cyclic plasticity and fatigue damage evolution in the alloy
fabricated by laser powder bed fusion. Thermodynamics-based predictions
(validated by scanning transmission electron microscopy (STEM) energy
dispersive X-ray spectroscopy (EDX)) showed that Cr, Co and Fe partition to the
core of the solidification cells, whilst Mn and Ni to the cell boundaries in
all considered print parameters. Both dislocation slip and deformation twinning
were found to be responsible for plastic deformation under monotonic loading.
However, the former was found to be the single dominant mechanism for cyclic
plasticity. The surface finish helped to substantially delay the crack
initiation and cause lack-of-fusion porosity to be the main source of crack
initiation. Most significantly, the scan strategies significantly affect grain
arrangements and grain dimensions, leading to noticeable effects on fatigue
crack propagation; in particular, the highest resistance crack propagation was
seen in the meander scan strategy with 0{\deg} rotation thanks to the most
columnar grains and the smallest spacing of grain boundaries along the crack
propagation path. | 2007.07043v1 |
2020-07-24 | Integration of multi-layer black phosphorus into photoconductive antennas for THz emission | We report the fabrication, characterization, and modeling of photoconductive
antennas using 40 nm thin-film flakes of black phosphorus (BP) as the
photoconductor and hexagonal boron nitride (hBN) as a capping layer to prevent
oxidation of BP. Dipole antennas were fabricated on oxidized high-resistivity
Si substrates, and BP and hBN flakes were picked up and transferred onto the
antenna inside a nitrogen glovebox. The transfer matrix technique was used to
optimize the thickness of BP and hBN for maximum absorption. BP flakes were
aligned with the armchair axis along the anode-cathode gap of the antenna, with
crystal orientation measured using reflection anisotropy. Photocurrent imaging
under illumination with 100 fs pulses at 780 and 1560 nm showed a
bias-dependent maximum photocurrent localized to the antenna gap with a peak
photoconductivity of 1 (2) S/cm in the linear regime of bias for excitation at
780 (1560) nm. Photocurrent saturation in bias (pump fluence) occurred at
approximately 1 V (0.25 mJ/cm$^2$). Device performance was modeled numerically
by solving Maxwell's equations and the drift-diffusion equation to obtain the
photocurrent density in response to pulsed laser excitation, which was largely
in qualitative agreement with the experimental observations. THz output
computed from surface current density suggests that BP THz PCA performance is
at least comparable to more traditional devices based on low-temperature-grown
GaAs. These devices represent a step toward high-performance THz
photoconductive antennas using BP. | 2007.12775v1 |
2020-08-06 | Constraining the ellipticity of millisecond pulsars with observed spin-down rates | A spinning neutron star (NS) that is asymmetric with respect to its spin axis
can emit continuous gravitational wave (GW) signals. The spin frequencies and
their distribution of radio millisecond pulsars (MSPs) and accreting MSPs
provide some evidences of GW radiation, and MSPs are ideal probes detecting
high frequency GW signals. It is generally thought that MSPs originate from the
recycled process, in which the NS accretes the material and angular momentum
from the donor star. The accreted matter would be confined at the polar cap
zone by an equatorial belt of compressed magnetic field fixed in the deep crust
of the NS, and yields "magnetic mountain". Based on an assumption that the
spin-down rates of three transitional MSPs including PSR J1023+0038 are the
combinational contribution of the accretion torque, the propeller torque, and
the GW radiation torque, in this work we attempt to constrain the ellipticities
of MSPs with observed spin-down rates. Assuming some canonical parameters of
NSs, the ellipticities of three transitional MSPs and ten redbacks are
estimated to be $\epsilon=(0.9-23.4)\times 10^{-9}$. The electrical
resistivities of three transitional MSPs are also derived to be in the range
$\eta=(1.2-15.3)\times 10^{-31}~\rm s$, which display an ideal power law
relation with the accretion rate. The characteristic strains ($h_{\rm
c}=(0.6-2.5)\times10^{-27}$) of GW signals emitting by these sources are
obviously beyond the sensitivity scope of the aLIGO. We expect that the
third-generation GW detectors like the Einstein Telescope can seize the GW
signals from these sources in the future. | 2008.02444v2 |
2020-08-22 | Large Transport Gap Modulation in Graphene via Electric Field Controlled Reversible Hydrogenation | Graphene is of interest in the development of next-generation electronics due
to its high electron mobility, flexibility and stability. However, graphene
transistors have poor on/off current ratios because of the absence of a
bandgap. One approach to introduce an energy gap is to use hydrogenation
reaction, which changes graphene into insulating graphane with sp3 bonding.
Here we show that an electric field can be used to control
conductor-to-insulator transitions in microscale graphene via a reversible
electrochemical hydrogenation in an organic liquid electrolyte containing
dissociative hydrogen ions. The fully hydrogenated graphene exhibits a lower
limit sheet resistance of 200 Gohm/sq, resulting in graphene field-effect
transistors with on/off current ratios of 10^8 at room temperature. The devices
also exhibit high endurance, with up to one million switching cycles. Similar
insulating behaviours are also observed in bilayer graphene, while trilayer
graphene remains highly conductive after the hydrogenation. Changes in the
graphene lattice, and the transformation from sp2 to sp3 hybridization, is
confirmed by in-situ Raman spectroscopy, supported by first-principles
calculations. | 2008.09749v2 |
2020-10-04 | Ballistic-hydrodynamic phase transition in flow of two-dimensional electrons | Phase transitions are characterized by a sharp change in the type of dynamics
of microparticles, and their description usually requires quantum mechanics.
Recently, a peculiar type of conductors was discovered in which two-dimensional
(2D) electrons form a viscous fluid. In this work we reveal that such electron
fluid in high-quality samples can be formed from ballistic electrons via a
phase transition. For this purpose, we theoretically study the evolution of a
ballistic flow of 2D weakly interacting electrons with an increase of magnetic
field and trace an emergence of a fluid fraction at a certain critical field.
Such restructuring of the flow manifests itself in a kink in magnetic-field
dependencies of the longitudinal and the Hall resistances. It is remarkable
that the studied phase transition has a classical-mechanical origin and is
determined by both the ballistic size effects and the electron-electron
scattering. Our analysis shows that this effect was apparently observed in the
recent transport experiments on 2D electrons in graphene and high-mobility GaAs
quantum wells. | 2010.01642v4 |
2020-10-22 | Li-ion Battery Fault Detection in Large Packs Using Force and Gas Sensors | Internal short circuits are a leading cause of battery thermal runaway, and
hence a major safety issue for electric vehicles. An internal short circuit
with low resistance is called a hard internal short, which causes a high
internal current flow that leads to an extremely fast temperature rise, gas
generation, cell swelling, and ultimately battery rupture and failure. Thus it
is crucial to detect these faults immediately after they get triggered. In
large battery packs with many cells in parallel, detecting an internal short
circuit event using voltage is difficult due to suppression of the voltage
signal from the faulty cell by the other healthy cells connected in parallel.
In contrast, analyzing the gas composition in the pack enclosure can provide a
robust single cell failure detection method. At elevated temperature,
decomposition of the battery materials results in gas generation and cell
swelling. The cell structure is designed to rupture at a critical gas pressure
and vent the accumulated $CO_2$ gas, in order to prevent explosive forces. In
this paper, we extend our previous work by combining the models of cell thermal
dynamics, swelling, and $CO_2$ gas generation. In particular, we developed a
fast and high confidence level detection method of hard internal short circuit
events for a battery pack by measuring cell expansion force and monitoring
$CO_2$ concentrations in a pack enclosure. | 2010.13519v1 |
2020-11-01 | Experimental demonstration of particle acceleration with normal conducting accelerating structure at cryogenic temperature | Reducing the operating temperature of normal conducting particle accelerators
substantially increases their efficiency. Low-temperature operation increases
the yield strength of the accelerator material and reduces surface resistance,
hence a great reduction in cyclic fatigue could be achieved resulting in a
large reduction in breakdown rates compared to room-temperature operation.
Furthermore, temperature reduction increases the intrinsic quality factor of
the accelerating cavities, and consequently, the shunt impedance leading to
increased system efficiency and beam loading capabilities. In this paper, we
present an experimental demonstration of the high-gradient operation of an
X-band, 11.424 GHz, 20-cells linear accelerator (linac) operating at a liquid
nitrogen temperature of 77 K. The tested linac was previously processed and
tested at room temperature. We verified the enhanced accelerating parameters of
the tested accelerator at cryogenic temperature using different measurements
including electron beam acceleration up to a gradient of 150 MV/m,
corresponding to a peak surface electric field of 375 MV/m. We also measured
the breakdown rates in the tested structure showing a reduction of two orders
of magnitude, x100, compared to their values at room temperature for the same
accelerating gradient. | 2011.00391v1 |
2020-12-13 | Microfluidic device coupled with total internal reflection microscopy for in situ observation of precipitation | In situ observation of precipitation or phase separation induced by solvent
addition is important in studying its dynamics. Combined with optical and
fluorescence microscopy, microfluidic devices have been leveraged in studying
the phase separation in various materials including biominerals, nanoparticles,
and inorganic crystals. However, strong scattering from the subphases in the
mixture is problematic for in situ study of phase separation with high temporal
and spatial resolution. In this work, we present a quasi-2D microfluidic device
combined with total internal reflection microscopy as an approach for in situ
observation of phase separation. The quasi-2D microfluidic device comprises of
a shallow main channel and a deep side channel. Mixing between a solution in
the main channel (solution A) and another solution (solution B) in the side
channel is predominantly driven by diffusion due to high fluid resistance from
the shallow height of the main channel, which is confirmed using fluorescence
microscopy. Moreover, relying on diffusive mixing, we can control the
composition of the mixture in the main channel by tuning the composition of
solution B. We demonstrate the application of our method for in situ
observation of asphaltene precipitation and beta-alanine crystallization. | 2012.06962v1 |
2021-01-31 | Monoclinic EuSn$_2$As$_2$: A Novel High-Pressure Network Structure | The layered crystal of EuSn$_2$As$_2$ has a Bi$_2$Te$_3$-type structure in
rhombohedral ($R\bar{3}m$) symmetry and has been confirmed to be an intrinsic
magnetic topological insulator at ambient conditions. Combining {\it ab initio}
calculations and \emph{in-situ} x-ray diffraction measurements, we identify a
new monoclinic EuSn$_2$As$_2$ structure in $C2/m$ symmetry above $\sim$14 GPa.
It has a three-dimensional network made up of honeycomb-like Sn sheets and
zigzag As chains, transformed from the layered EuSn$_2$As$_2$ via a two-stage
reconstruction mechanism with the connecting of Sn-Sn and As-As atoms
successively between the buckled SnAs layers. Its dynamic structural stability
has been verified by phonon mode analysis. Electrical resistance measurements
reveal an insulator-metal-superconductor transition at low temperature around 5
and 15 GPa, respectively, according to the structural conversion, and the
superconductivity with a \textit{T}${\rm {_C}}$ value of $\sim 4$ K is observed
up to 30.8 GPa. These results establish a high-pressure EuSn$_2$As$_2$ phase
with intriguing structural and electronic properties and expand our
understandings about the layered magnetic topological insulators. | 2102.00437v2 |
2021-02-18 | Crystal orientation-dependent oxidation of epitaxial TiN films with tunable plasmonics | Titanium nitride (TiN) is a paradigm of refractory transition metal nitrides
with great potential in vast applications. Generally, the plasmonic performance
of TiN can be tuned by oxidation, which was thought to be only temperature-,
oxygen partial pressure-, and time-dependent. Regarding the role of
crystallographic orientation in the oxidation and resultant optical properties
of TiN films, little is known thus far. Here we reveal that both the oxidation
resistance behavior and the plasmonic performance of epitaxial TiN films follow
the order of (001) < (110) < (111). The effects of crystallographic orientation
on the lattice constants, optical properties, and oxidation levels of epitaxial
TiN films have been systematically studied by combined high-resolution X-ray
diffraction, spectroscopic ellipsometry, X-ray absorption spectroscopy, and
X-ray photoemission spectroscopy. To further understand the role of
crystallographic orientation in the initial oxidation process of TiN films,
density-functional-theory calculations are carried out, indicating the energy
cost of oxidation is (001) < (110) < (111), consistent with the experiments.
The superior endurance of the (111) orientation against mild oxidation can
largely alleviate the previously stringent technical requirements for the
growth of TiN films with high plasmonic performance. The crystallographic
orientation can also offer an effective controlling parameter to design
TiN-based plasmonic devices with desired peculiarity, e.g., superior chemical
stability against mild oxidation or large optical tunability upon oxidation. | 2102.09126v1 |
2021-03-15 | Thermal Visualization of Buried Interfaces by Transient and Steady-State Responses of Time-Domain Thermoreflectance | Thermal resistances from interfaces impede heat dissipation in
micro/nanoscale electronics, especially for high-power electronics. Despite the
growing importance of understanding interfacial thermal transport, advanced
thermal characterization techniques which can visualize thermal conductance
across buried interfaces, especially for nonmetal-nonmetal interfaces, are
still under development. This work reports a dual-modulation-frequency TDTR
mapping technique to visualize the thermal conduction across buried
semiconductor interfaces for beta-Ga2O3-SiC samples. Both the beta-Ga2O3
thermal conductivity and the buried beta-Ga2O3-SiC thermal boundary conductance
(TBC) are visualized for an area of 200 um x 200 um. Areas with low TBC values
( smaller than 20 MW/m2-K) are successfully identified on the TBC map, which
correspond to weakly bonded interfaces caused by high-temperature annealing.
The steady-state temperature rise (detector voltage), usually ignored in TDTR
measurements, is found to be able to probe TBC variations of the buried
interfaces without the limit of thermal penetration depth. This technique can
be applied to detect defects/voids in deeply buried heterogeneous interfaces
non-destructively, and also opens a door for the visualization of thermal
conductance in nanoscale nonhomogeneous structures. | 2103.08084v1 |
2021-03-24 | Quantum transport properties of beta-Bi4I4 near and well beyond the extreme quantum limit | We have investigated the magneto-transport properties of beta-Bi4I4 bulk
crystal, which was recently theoretically proposed and experimentally
demonstrated to be a topological insulator. At low temperature T and magnetic
field B, a series of Shubnikov-De Haas(SdH) oscillations are observed on the
magnetoresistivity (MR). The detailed analysis reveals a light cyclotron mass
of 0.1 me, and the field angle dependence of MR reveals that the SdH
oscillations originate from a convex Fermi surface. In the extreme quantum
limit (EQL) region, there is a metal-insulator transition occurring soon after
the EQL. We perform the scaling analysis, and all the isotherms fall onto a
universal scaling with a fitted critical exponent of 6.5. The enormous value of
critical exponent implies this insulating quantum phase originated from strong
electron-electron interactions in high fields. However, in the far end of EQL,
both the longitudinal and Hall resistivity increase exponentially with B, and
the temperature dependence of the MR reveals an energy gap induced by the high
magnetic field, signifying a magnetic freeze-out effect. Our findings indicate
that bulk beta-Bi4I4 is an excellent candidate for a 3D topological system for
exploring EQL physics and relevant exotic quantum phases. | 2103.13079v2 |
2021-04-15 | Negligible oxygen vacancies, low critical current density, electric-field modulation, in-plane anisotropic and high-field transport of a superconducting Nd0.8Sr0.2NiO2/SrTiO3 heterostructure | The emerging Ni-based superconducting oxide thin films are rather intriguing
to the entire condensed matter physics. Here we report some brief experimental
results on transport measurements for a 14-nm-thick superconducting
Nd0.8Sr0.2NiO2/SrTiO3 thin-film heterostructure with an onset transition
temperature of ~9.5 K. Photoluminescence measurements reveal that there is
negligible oxygen vacancy creation in the SrTiO3 substrate during thin-film
deposition and post chemical reduction for the Nd0.8Sr0.2NiO2/SrTiO3
heterostructure. It was found that the critical current density of the
Nd0.8Sr0.2NiO2/SrTiO3 thin-film heterostructure is relatively small, ~4x10^3
A/cm2. Although the surface steps of SrTiO3 substrates lead to an anisotropy
for in-plane resistivity, the superconducting transition temperatures are
almost the same. The out-of-plane magnetotransport measurements yield an upper
critical field of ~11.4 T and an estimated in-plane Ginzburg-Landau coherence
length of ~5.4 nm. High-field magnetotransport measurements up to 50 T reveal
anisotropic critical fields at 1.8 K for three different measurement geometries
and a complicated Hall effect. An electric field applied via the SrTiO3
substrate slightly varies the superconducting transition temperature. These
experimental results could be useful for this rapidly developing field. | 2104.07316v2 |
2021-04-26 | Impact of the nucleation of charge clusters on the retention of memristors: a self-consistent phase field computational study | In recent years, resistive RAM often referred to as memristor is actively
pursued as a replacement for nonvolatile-flash memory due to its superior
characteristics such as high density, scalability, low power operation, high
endurance, and fast operating speed. However, one of the challenges that need
to be overcome is the loss of retention for both ON- and OFF-states; the
retention loss. While various models are proposed to explain the retention loss
in memristors consisting of a switching layer, in this paper, we propose that
the nucleation of clusters made of electrical charges, charge-clusters, in the
switching layer acts as a potential root cause for the retention loss. The
nucleation results from localized electric-field produced intermittently during
cyclic switching operations. We use the phase-field method to illustrate how
the nucleation of charge-clusters gives rise to the retention loss. Our results
suggest that the degree at which the retention loss arises is linked to the
number of cyclic switching operations since the probability at which nucleation
centers form increases with the number of cycle switching operations, which is
consistent with a range of experimental findings previously reported. | 2104.12829v1 |
2021-06-02 | Multiband effects on the upper critical field angular dependence of 122-family iron pnictide superconductors | Detailed measurements of the in-plane resistivity were performed in a
high-quality Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ ($x=0.065$) single crystal, in
magnetic fields up to 9 T and with different orientations $\theta$ relative to
the crystal $c$ axis. A significant $\rho(T)_{H,\theta}$ rounding is observed
just above the superconducting critical temperature $T_c$ due to Cooper pairs
created by superconducting fluctuations. These data are analyzed in terms of a
generalization of the Aslamazov-Larkin approach, that extends its applicability
to high reduced-temperatures and magnetic fields. This method allows us to
carry out a criterion-independent determination of the angular dependence of
the upper critical field, $H_{c2}(\theta)$. In spite of the relatively small
anisotropy of this compound, it is found that $H_{c2}(\theta)$ presents a
significant deviation from the single-band 3D anisotropic Ginzburg-Landau
(3D-aGL) approach, particularly for large $\theta$ (typically above
$\sim60^o$). These results are interpreted in terms of the multiband nature of
these materials, in contrast with other proposals for similar $H_{c2}(\theta)$
anomalies. Our results are also consistent with an effective anisotropy factor
almost temperature independent near $T_c$, a result that differs from the ones
obtained by using a single-band model. | 2106.01307v1 |
2021-07-07 | Magnetization-tuned topological quantum phase transition in MnBi2Te4 devices | Recently, the intrinsic magnetic topological insulator MnBi2Te4 has attracted
enormous research interest due to the great success in realizing exotic
topological quantum states, such as the quantum anomalous Hall effect (QAHE),
axion insulator state, high-Chern-number and high-temperature Chern insulator
states. One key issue in this field is to effectively manipulate these states
and control topological phase transitions. Here, by systematic angle-dependent
transport measurements, we reveal a magnetization-tuned topological quantum
phase transition from Chern insulator to magnetic insulator with gapped Dirac
surface states in MnBi2Te4 devices. Specifically, as the magnetic field is
tilted away from the out-of-plane direction by around 40-60 degrees, the Hall
resistance deviates from the quantization value and a colossal, anisotropic
magnetoresistance is detected. The theoretical analyses based on modified
Landauer-Buttiker formalism show that the field-tilt-driven switching from
ferromagnetic state to canted antiferromagnetic state induces a topological
quantum phase transition from Chern insulator to magnetic insulator with gapped
Dirac surface states in MnBi2Te4 devices. Our work provides an efficient means
for modulating topological quantum states and topological quantum phase
transitions. | 2107.03224v1 |
2021-07-22 | Anomalous Transport in High-Mobility Superconducting SrTiO$_3$ Thin Films | The study of subtle effects on transport in semiconductors requires
high-quality epitaxial structures with low defect density. Using hybrid
molecular beam epitaxy (MBE), SrTiO$_3$ films with low-temperature mobility
exceeding 42,000 cm$^2$V$^{-1}$s$^{-1}$ at low carrier density of 3 x 10$^{17}$
cm$^{-3}$ were achieved. A sudden and sharp decrease in residual resistivity
accompanied by an enhancement in the superconducting transition temperature
were observed across the second Lifshitz transition (LT) where the third band
becomes occupied, revealing dominant intra-band scattering. These films further
revealed an anomalous behavior in the Hall carrier density as a consequence of
the antiferrodistortive (AFD) transition and the temperature-dependence of the
Hall scattering factor. Using hybrid MBE growth, phenomenological modeling,
temperature-dependent transport measurements, and scanning superconducting
quantum interference device imaging, we provide critical insights into the
important role of inter- vs intra-band scattering and of AFD domain walls on
normal-state and superconducting properties of SrTiO$_3$. | 2107.10904v1 |
2021-09-27 | Pressure-induced monotonic enhancement of Tc to over 30 K in the superconducting Pr0.82Sr0.18NiO2 thin films | The successful synthesis of superconducting infinite-layer nickelate thin
films with the highest Tc ~ 15 K has reignited great enthusiasms on this family
of potential analogue to high-Tc cuprates. Pursuing a higher Tc is always an
imperative task in studying a new superconducting material system. Here we
report high-quality Pr0.82Sr0.18NiO2 thin films with Tconset ~ 17 K synthesized
by carefully tuning the amount of CaH2 in the topological chemical reduction
and the effect of pressure on its superconducting properties by measuring
electrical resistivity under various pressures in a cubic anvil cell apparatus.
We find that the onset temperature of the superconductivity, Tconset, can be
enhanced monotonically from ~ 17 K at ambient pressure to ~ 31 K at 12.1 GPa
without showing signatures of saturation upon increasing pressure. This
encouraging result indicates that the Tc of infinite-layer nickelates
superconductors still has room to go higher and it can be further boosted by
applying higher pressures or strain engineering in the heterostructure films. | 2109.12811v2 |
2021-10-26 | Novel Lithium-Sulfur Polymer Battery Operating at Moderate Temperature | A safe lithium-sulfur (Li-S) battery employs a composite polymer electrolyte
based on a poly(ethylene glycol) dimethyl ether (PEGDME) solid at room
temperature. The electrolyte membrane enables a stable and reversible Li-S
electrochemical process already at 50{\deg}C, with low resistance at the
electrode/electrolyte interphase and fast Li+ transport. The relatively low
molecular weight of the PEGDME and the optimal membrane composition in terms of
salts and ceramic allow a liquid-like Li-S conversion reaction by heating at
moderately high temperature, still holding the solid-like polymer state of the
cell. Therefore, the electrochemical reaction of the polymer Li-S cell is
characterized by the typical dissolution of lithium polysulfides into the
electrolyte medium during discharge and the subsequent deposition of sulfur at
the electrode/electrolyte interphase during charge. On the other hand, the
remarkable thermal stability of the composite polymer electrolyte (up to
300{\deg}C) suggests a lithium-metal battery with safety content significantly
higher than that using the common, flammable liquid solutions. Hence, the Li-S
polymer battery delivers at 50{\deg}C and 2 V a stable capacity approaching 700
mAhgS-1, with a steady-state coulombic efficiency of 98%. These results suggest
a novel, alternative approach to achieve safe, high energy batteries with solid
polymer configuration. | 2110.13727v1 |
2021-11-30 | Bosonic metal states in crystalline iron-based superconductors at the two-dimensional limit | The nature of the anomalous metal, one of the quantum ground states of
two-dimensional (2D) bosonic systems, remains a major puzzle even after several
decades of study. Here, we report a systematic investigation on the transport
properties of ultrathin crystalline FeSe films grown on SrTiO3 (STO) as well as
the nanopatterned FeSe/STO, where the 2D high-temperature superconductivity is
confined at the interface. Remarkably, the bosonic anomalous metal state
emerges around 20 K, an exceptionally high temperature compared to all previous
observations. Furthermore, a linear-in-temperature (T-linear) resistance with
suppressed Hall coefficient below onset temperature for superconductivity is
observed, indicating a bosonic strange metal. We give quantitative analysis for
the bosonic anomalous metal state, based on the quantum dynamical property of
vortices influenced by ohmic dissipation. This microscopic model pins down the
origin of the intriguing anomalous state to the superconducting phase dynamics
in both spatial and temporal domain. Our findings shed new light on the bosonic
metal states in crystalline superconductors at the 2D limit. | 2111.15488v2 |
2022-03-14 | Large enhancement in thermal conductivity of solvent cast expanded-graphite/polyetherimide composites | We demonstrate in this work, that expanded graphite (EG) can lead to a very
large enhancement in thermal conductivity of polyetherimide-graphene and
epoxy-graphene nanocomposites prepared via solvent casting technique. A k value
of 6.56 Wm-1K-1 is achieved for 10 weight % composition sample, representing an
enhancement of ~2770% over pristine polyetherimide (k ~ 0.23 Wm-1K-1). This
extraordinary enhancement in thermal conductivity is shown to be due to a
network of continuous graphene sheets over long length scales, resulting in low
thermal contact resistance at bends/turns due to the graphene sheets being
covalently bonded at such junctions. Solvent casting offers the advantage of
preserving the porous structure of expanded graphite in the composite,
resulting in the above highly thermally conductive interpenetrating network of
graphene and polymer. Solvent casting also does not break down the expanded
graphite particles, due to minimal forces involved, allowing for efficient heat
transfer over long length scales, further enhancing overall composite thermal
conductivity. Comparisons with a recently introduced effective medium model
shows a very high value of predicted particle-particle interfacial conductance,
providing evidence for efficient interfacial thermal transport in expanded
graphite composites. Field Emission Environmental Scanning Electron Microscopy
(FE-ESEM) is used to provide detailed understanding of interpenetrating
graphene-polymer structure in the expanded graphite composite. These results
open up novel avenues for achieving high thermal conductivity polymer
composites. | 2203.06828v1 |
2022-03-18 | Thermoelectric transport properties of gapless pinned charge density waves | Quantum strongly correlated matter exhibits properties which are not easily
explainable in the conventional framework of Fermi liquids. Universal effective
field theory tools are applicable in these cases regardless of the microscopic
details of the quantum system, since they are based on symmetries. It is
necessary, however, to construct these effective tools in full generality,
avoiding restrictions coming from particular microscopic descriptions which may
inadequately constrain the coefficients that enter in the effective theory.
In this work we demonstrate on explicit examples how the novel hydrodynamic
coefficients which have been recently reinstated in the effective theory of
pinned charge density waves (CDW) can affect the phenomenology of the
thermo-electric transport in strongly correlated quantum matter. Our examples,
based on two classes of holographic models with pinned CDW, have microscopics
which are conceptually different from Fermi liquids. Therefore, the above novel
transport coefficients are nonzero, contrary to the conventional approach. We
show how these coefficients allow to take into account the change of sign of
the Seebeck coefficient and the low resistivity of the CDW phase of the cuprate
high temperature superconductors, without referring to the effects of Fermi
surface reconstruction. | 2203.10038v2 |
2022-05-11 | An mCherry biolaser based on microbubble cavity with ultra-low threshold | Biolasers show considerable potential in the biomedical field. Fluorescent
protein (FP) is a type of biomaterial with good luminescence efficiency that
can be used as the luminescent gain medium in biolasers. Due to the higher
cell/tissue permeability, lower cell phototoxicity, and relatively less
background fluorescence than other fluorescent proteins, the red fluorescent
protein is more suitable in biological applications. MCherry is the most
extensively used high-quality red fluorescent protein because of its short
maturation time and stable luminescence properties. In this study, using
mCherry and microbubble cavity, we realize a highly stable mCherry fluorescent
protein laser. The laser resonator achieves a quality factor of 10^8, which is
the highest Q factor among the currently available FP lasers. Moreover, this
laser exhibits low threshold of 286 fJ, which can effectively protect the
luminescent material from being damaged by pump light. Such a threshold is the
lowest in the FP lasers as per our knowledge. The prepared laser shows
excellent stability in a wide pH range with good photobleaching resistance and
can be stored at 4 degree for nearly a month. Also, the laser can serve as a
high-sensitivity molecular concentration detector with mCherry as biomarker,
owing to its lasing threshold behavior. | 2205.05220v2 |
2022-07-01 | Efficient and Scalable GaInAs Thermophotovoltaic Devices | Thermophotovoltaics are promising solid-state energy converters for a variety
of applications such as grid-scale energy storage, concentrating solar-thermal
power, and waste heat recovery. Here, we report the design, fabrication, and
testing of large area (0.8 cm$^2$), scalable, single junction 0.74-eV GaInAs
thermophotovoltaic devices reaching an efficiency of 38.8$\pm$2.0% and an
electrical power density of 3.78 W/cm$^2$ at an emitter temperature of
1850{\deg}C. Reaching such a high emitter temperature and power density without
sacrificing efficiency is a direct result of combining good spectral management
with a highly optimized cell architecture, excellent material quality, and very
low series resistance. Importantly, fabrication of 12 high-performing devices
on a two-inch wafer is shown to be repeatable, and the cell design can be
readily transferred to commercial epitaxy on even larger wafers. Further
improvements in efficiency can be obtained by using a multijunction
architecture, and early results for a two-junction 0.84-eV GaInPAs / 0.74-eV
GaInAs device illustrate this promise. | 2207.00565v1 |
2022-07-12 | Non-destructive Depth-Resolved Characterization of Residual Strain Fields in High Electron Mobility Transistors using Differential Aperture X-ray Microscopy | Localized residual stress and elastic strain concentrations in
microelectronic devices often affect the electronic performance, resistance to
thermomechanical damage, and, likely, radiation tolerance. A primary challenge
for characterization of these concentrations is that they exist over sub-$\mu$m
length-scales, precluding their characterization by more traditional residual
stress measurement techniques. Here we demonstrate the use of synchrotron X-ray
-based differential aperture X-ray microscopy (DAXM) as a viable,
non-destructive means to characterize these stress and strain concentrations in
a depth-resolved manner. DAXM is used to map two-dimensional strain fields
between source and drain in a gallium nitride (GaN) layer within high electron
mobility transistors (HEMTs) with sub-$\mu$m spatial resolution. Strain fields
at various positions in both pristine and irradiated HEMT specimens are
presented in addition to a preliminary stress analysis to estimate the
distribution of various stress components within the GaN layer.
$\gamma$-irradiation is found to significantly reduce the lattice plane spacing
in the GaN along the sample normal direction which is attributed to radiation
damage in transistor components bonded to the GaN during irradiation. | 2207.05789v2 |
2022-08-10 | Synthesis of Superconducting Phase of La$_{0.5}$Ce$_{0.5}$H$_{10}$ at High Pressures | Clathrate hydride \emph{Fm}\={3}\emph{m}-LaH$_{10}$ has been proven as the
most extraordinary superconductor with the critical temperature $T_c$ above 250
K upon compression of hundreds of GPa in recent years. A general hope is to
reduce the stabilization pressure and maintain the high $T_c$ value of the
specific phase in LaH$_{10}$. However, strong structural instability distorts
\emph{Fm}\={3}\emph{m} structure and leads to a rapid decrease of $T_c$ at low
pressures. Here, we investigate the phase stability and superconducting
behaviors of \emph{Fm}\={3}\emph{m}-LaH$_{10}$ with enhanced chemical
pre-compression through partly replacing La by Ce atoms from both experiments
and calculations. For explicitly characterizing the synthesized hydride, we
choose lanthanum-cerium alloy with stoichiometry composition of 1:1. X-ray
diffraction and Raman scattering measurements reveal the stabilization of
\emph{Fm}\={3}\emph{m}-La$_{0.5}$Ce$_{0.5}$H$_{10}$ in the pressure range of
140-160 GPa. Superconductivity with $T_c$ of 175$\pm$2 K at 155 GPa is
confirmed with the observation of the zero-resistivity state and supported by
the theoretical calculations. These findings provide applicability in the
future explorations for a large variety of hydrogen-rich hydrides. | 2208.05199v1 |
2022-09-12 | Emergent magnetic states and tunable exchange bias at all 3d nitride heterointerfaces | Interfacial magnetism stimulates the discovery of giant magnetoresistance and
spin-orbital coupling across the heterointerfaces, facilitating the intimate
correlation between spin transport and complex magnetic structures. Over
decades, functional heterointerfaces composed of nitrides are seldomly explored
due to the difficulty in synthesizing high-quality and correct composition
nitride films. Here we report the fabrication of single-crystalline
ferromagnetic Fe3N thin films with precisely controlled thickness. As film
thickness decreasing, the magnetization deteriorates dramatically, and
electronic state transits from metallic to insulating. Strikingly, the
high-temperature ferromagnetism maintains in a Fe3N layer with a thickness down
to 2 u. c. (~ 8 {\AA}). The magnetoresistance exhibits a strong in-plane
anisotropy and meanwhile the anomalous Hall resistance reserves its sign when
Fe3N layer thickness exceeds 5 u. c. Furthermore, we observe a sizable exchange
bias at the interfaces between a ferromagnetic Fe3N and an antiferromagnetic
CrN. The exchange bias field and saturation moment strongly depend on the
controllable bending curvature using cylinder diameter engineering (CDE)
technique, implying the tunable magnetic states under lattice deformation. This
work provides a guideline for exploring functional nitride films and applying
their interfacial phenomena for innovative perspectives towards the practical
applications. | 2209.05209v1 |
2022-11-02 | Tunable Rapid Electron Transport in Titanium Oxide Thin Films | Rapid electron transport in the quantum well triggers many novel physical
phenomena and becomes a critical point for the high-speed electronics. Here, we
found electrical properties of the titanium oxide changed from semiconducting
to metallic as the degree of oxidation decreased and Schottky quantum well was
formed at the interface. We take the asymmetry interface electron scattering
effect into consideration when studying the electrical transport properties of
the multilayer thin films. A novel physical conductivity model for the
multilayer thin films was developed. We found electron would be transferred
from the low-mobility semiconducting and metallic conductive channels to the
high-mobility Schottky quantum well conductive channel with an in-plane applied
electric field. Electron concentration and mobility of the forming 2DEG in the
Schottky quantum well could be tuned thus the nano-devices exhibited non-linear
voltage-current curves. The differential resistivity of the nano-devices could
decrease by two orders with increasing electric field at room temperature. Weak
electron localization of electrons has been experimentally observed in our
nano-devices at low temperature, which further demonstrated the existence of
2DEG in the Schottky quantum well. Our work will provide us new physics about
the rapid electron transport in the multilayer thin films, and bring novel
functional devices for the modern microelectronic industry. | 2211.01162v1 |
2023-03-06 | A Miniaturised Camera-based Multi-Modal Tactile Sensor | In conjunction with huge recent progress in camera and computer vision
technology, camera-based sensors have increasingly shown considerable promise
in relation to tactile sensing. In comparison to competing technologies (be
they resistive, capacitive or magnetic based), they offer
super-high-resolution, while suffering from fewer wiring problems. The human
tactile system is composed of various types of mechanoreceptors, each able to
perceive and process distinct information such as force, pressure, texture,
etc. Camera-based tactile sensors such as GelSight mainly focus on
high-resolution geometric sensing on a flat surface, and their force
measurement capabilities are limited by the hysteresis and non-linearity of the
silicone material. In this paper, we present a miniaturised dome-shaped
camera-based tactile sensor that allows accurate force and tactile sensing in a
single coherent system. The key novelty of the sensor design is as follows.
First, we demonstrate how to build a smooth silicone hemispheric sensing medium
with uniform markers on its curved surface. Second, we enhance the illumination
of the rounded silicone with diffused LEDs. Third, we construct a
force-sensitive mechanical structure in a compact form factor with usage of
springs to accurately perceive forces. Our multi-modal sensor is able to
acquire tactile information from multi-axis forces, local force distribution,
and contact geometry, all in real-time. We apply an end-to-end deep learning
method to process all the information. | 2303.03093v1 |
2023-03-15 | Rare observation of spin-gapless semiconducting characteristics and related band topology of quaternary Heusler alloy CoFeMnSn | In this paper, we report the theoretical investigation and experimental
realization of a new spin-gapless semiconductor (SGSs) compound CoFeMnSn
belonging to the family of quaternary Heusler alloys. Through the use of
several ground-state energy calculations, the most stable structure has been
identified. Calculations of the spin-polarized band structure in optimized
structure's reveals the SGS nature of the compound. The compound form in an
ordered crystal structure and exhibit a high ferromagnetic transition
temperature (T$_{\rm C}$ = 560 K), making the material excellent for room
temperature applications. Adherence of saturation magnetization to the
Slater-Pauling rule, together with the nearly temperature-independent
resistivity, conductivity, and carrier concentration of the compound in the
temperature regime 5$-$300 K along with the low value of anomalous Hall
conductivity (AHC) further confirms the SGS nature. Theoretical calculations
also reveal the robustness of the SGS state due to lattice contraction and one
can obtain a high value of intrinsic AHC using hole doping. Combined SGS and
topological properties of the compound make CoFeMnSn suitable for spintronics
and magneto-electronics devices. | 2303.08589v2 |
2023-04-11 | Effects of equivalent composition on superconducting properties of high-entropy REOBiS$_2$ (RE = La, Ce, Pr, Nd, Sm, Gd) single crystals | Superconductors are influenced by high-entropy alloys (HEAs); these have been
investigated in various functional materials. REOBiS$_2$ (RE = La, Ce, Pr, Nd,
Sm, and Gd in different combinations) single crystals with HEAs at the RE-site
were successfully grown using the flux method. The obtained crystals were
plate-shaped (1 mm$^2$) with a well-developed c-plane. Ce was present in both
trivalent (Ce$^{3+}$) and tetravalent (Ce$^{4+}$) electronic configurations;
the concentration of Ce$^{4+}$ at the RE-site was approximately 10 at% in all
single crystals. The single crystals showed superconducting transition
temperature with zero resistivity within 1.2-4.2 K. The superconducting
transition temperature, superconducting anisotropy, electronic specific heat
coefficient, and Debye temperature of the crystals were not correlated with the
mixed entropy at the RE-site. Except for the electronic specific heat
coefficient, the variation of these parameters as a function of mixed entropy
showed different trends for equivalent and non-equivalent RE element
compositions. Thus, the configuration of RE elements influences the
superconducting properties of REOBiS$_2$ single crystals, alluding to a method
of modulating transition temperatures. | 2304.04993v1 |
2023-06-21 | An Analytical Model to Quantify the Local Lattice Distortion of Refractory High Entropy Alloys | Local lattice distortion (LLD) of high entropy alloys (HEAs) especially
refractory HEAs, which is different from one lattice site to another,
determines the mechanical properties of HEAs such as yield strength and
radiation resistance, and is crucial to modulating catalytic activity of HEAs
via the atomic strain. In particular, this site-to-site LLD is strongly coupled
with the short-range order (SRO) of HEAs. Therefore it is essential to reveal
the physical picture of LLD. However, the random distribution of
multi-principal constituents of HEAs prohibits the understanding of LLD,
including the determinants of LLD and their coupling rules. Herein, we build
the first analytical model to realize the site-to-site prediction of LLD in
refractory HEAs, by using the neighbor number ratio of central atoms, the
central-atom radii, the standard deviation of constituent radii and the
constituent number, which demonstrates that LLD surprisingly exhibits a similar
mechanism as the relaxation of metal surfaces. The involved parameters depend
only on the radii of constituents and are readily accessible. Moreover, our
scheme determines not only LLD but also the average lattice distortion, which
enables us to predict the phase stability and yield strength of HEAs. These
results build a novel physical picture of LLD, in particular the quantitative
relationship between LLD and SRO, which lay a solid foundation for the further
target-oriented design of HEAs. | 2306.11959v1 |
2023-06-28 | High-Q trenched aluminum coplanar resonators with an ultrasonic edge microcutting for superconducting quantum devices | Dielectric losses are one of the key factors limiting the coherence of
superconducting qubits. The impact of materials and fabrication steps on
dielectric losses can be evaluated using coplanar waveguide (CPW) microwave
resonators. Here, we report on superconducting CPW microwave resonators with
internal quality factors systematically exceeding 5x106 at high powers and
2x106 (with the best value of 4.4x106) at low power. Such performance is
demonstrated for 100-nm-thick aluminum resonators with 7-10.5 um center trace
on high-resistivity silicon substrates commonly used in quantum Josephson
junction circuits. We investigate internal quality factors of the resonators
with both dry and wet aluminum etching, as well as deep and isotropic reactive
ion etching of silicon substrate. Josephson junction compatible CPW resonators
fabrication process with both airbridges and silicon substrate etching is
proposed. Finally, we demonstrate the effect of airbridges positions and extra
process steps on the overall dielectric losses. The best quality fa ctors are
obtained for the wet etched aluminum resonators and isotropically removed
substrate with the proposed ultrasonic metal edge microcutting. | 2306.16301v1 |
2023-10-05 | Liquid Cooling System for a High Power, Medium Frequency, and Medium Voltage Isolated Power Converter | Power electronics systems, widely used in various applications such as
industrial automation, electric cars, and renewable energy, have the primary
function of converting and controlling electrical power to the desired type of
load. Despite their reliability and efficiency, power losses in these systems
generate significant heat that must be dissipated to maintain performance and
prevent damage. Cooling systems play a crucial role in ensuring safe operating
temperatures for system components. Air and liquid cooling are the leading
technologies used in the power electronics world. Air cooling is simple and
cost-effective but is limited by ambient temperature and component thermal
resistance. While more efficient, liquid cooling requires more maintenance and
has higher upfront costs. Water-cooling systems have become famous for
regulating thermal loads as they can effectively remove heat from localized
high-temperature areas, such as the challenging hotspots in power electronics
systems. In addition to designing a cooling system for a power electronic
system, this study investigated the impact of three major parameters; cold
plate material, channel shape/size, and coolant inlet velocity. The research
examined and analyzed these factors and their trade-off analysis to obtain
cooling system design and optimization insights. This study might improve power
electronics system performance, reliability, and durability by improving heat
dissipation and thermal management. | 2310.03577v1 |
2023-10-19 | Detailed and high-throughput measurement of composition dependence of magnetoresistance and spin-transfer torque using a composition-gradient film: application to Co$_{x}$Fe$_{1-x}$ (0 $\le$ $\textit{x}$ $\le$ 1) system | We develop a high-throughput method for measuring the composition dependence
of magnetoresistance (MR) and spin-transfer-torque (STT) effects in
current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices and
report its application to the CoFe system. The method is based on the use of
composition-gradient films deposited by combinatorial sputtering. This
structure allows the fabrication of devices with different compositions on a
single substrate, drastically enhancing the throughput in investigating
composition dependence. We fabricated CPP-GMR devices on a single GMR film
consisting of a Co$_{x}$Fe$_{1-x}$ (0 $\le$ $\textit{x}$ $\le$ 1)
composition-gradient layer, a Cu spacer layer, and a NiFe layer. The MR ratio
obtained from resistance-field measurements exhibited the maximum in the broad
Co concentration range of 0.3 $\le$ $\textit{x}$ $\le$ 0.65. In addition, the
STT efficiency was estimated from the current to induce magnetization reversal
of the NiFe layer by spin injection from the Co$_{x}$Fe$_{1-x}$ layer. The STT
efficiency was also the highest around the same Co concentration range as for
the MR ratio, and this correlation was theoretically explained by the change in
the spin polarization of the Co$_{x}$Fe$_{1-x}$ layer. The results revealed the
Co$_{x}$Fe$_{1-x}$ composition range suitable for spintronic applications,
demonstrating the advantages of the developed method. | 2310.12434v1 |
2023-11-25 | Our Dark Matter Stopping in the Earth | We have worked for some time on a model for dark matter, in which dark matter
consists of small bubbles of a new speculated type of vacuum, which are pumped
up by some ordinary matter such as diamond, so as to resist the pressure of the
domain wall separating the two vacua. Here we put forward thoughts on, how such
macroscopic pearls would have their surrounding dust cleaned off passing
through the atmosphere and the Earth, and what their distribution would be as a
function of the depth of their stopping point and the distribution of the
radiation emitted from them. In our model we assume that they radiate 3.5 keV
electrons and photons, after having been excited during their passage into the
Earth. The purpose of such an estimation of the radiation distribution is to
explain the truly mysterious fact that, among all the underground experiments
seeking dark matter colliding with the Earth material, only the DAMA-LIBRA
experiment has seen any evidence of dark matter. This is an experiment based on
solid NaI scintillators and is rather deep at 1400 m. It is our point that we
can arrange the main radiation to appear in the relatively deep DAMA- LIBRA
site, and explain that the dark matter pearls cannot stop in a fluid, such as
xenon in the xenon based experiments. | 2311.14996v1 |
2023-11-27 | Tailoring the Opto-Electronic Properties of Oxide-Metal-Oxide Transparent Electrode Using Cu Seed Layer | The oxide-metal-oxide architecture is a promising approach for the
development of the high-performance indium-free transparent electrode (TE),
which is a key component of various optoelectronic applications such as solar
cells, organic LEDs, and touchscreen panels. Here in this work, we have shown
high-performance TE consisting of TiO2/Ag/TiO2 (TAT), with the incorporation of
a copper seed layer. The seed layer increases the wettability and improves the
adhesion of deposited Ag film on the bottom TiO2 layer. Before the experimental
realization, optical modeling is performed by using MATLAB code based on the
transfer matrix method. The optimum thickness obtained from the simulation is
30 nm for both undercoat and overcoat TiO2 with the average transmittance in
the visible region >85% with the Ag thickness of 9nm. With inputs from the
optical modeling, TEs were experimentally realized with and without the Cu seed
layer. It has been found that the TE with an additional sputtered Cu (1 nm)
seed layer is essential for the smooth growth of silver film and shows better
electro-optical performance (sheet resistance < 10 and average transmittance in
the visible spectral range > 80%) than TAT-TE without any seed layer. The
electro-optical and morphological properties of the TiO2/Cu/Ag/TiO2 structure
make it suitable for optoelectronic applications. | 2311.15535v2 |
2023-11-30 | Vanishing of the anomalous Hall effect and enhanced carrier mobility in the spin-gapless ferromagnetic Mn2CoGa1-xAlx alloys | Spin gapless semiconductor (SGS) has attracted long attention since its
theoretical prediction, while concrete experimental hints are still lack in the
relevant Heusler alloys. Here in this work, by preparing the series alloys of
Mn2CoGa1-xAlx (x=0, 0.25, 0.5, 0.75 and 1), we identified the vanishing of
anomalous Hall effect in the ferromagnetic Mn2CoGa (or x=0.25) alloy in a wide
temperature interval, accompanying with growing contribution from the ordinary
Hall effect. As a result, comparatively low carrier density (1020 cm-3) and
high carrier mobility (150 cm2/Vs) are obtained in Mn2CoGa (or x=0.25) alloy in
the temperature range of 10-200K. These also lead to a large dip in the related
magnetoresistance at low fields. While in high Al content, despite the
magnetization behavior is not altered significantly, the Hall resistivity is
instead dominated by the anomalous one, just analogous to that widely reported
in Mn2CoAl. The distinct electrical transport behavior of x=0 and x=0.75 (or 1)
is presently understood by their possible different scattering mechanism of the
anomalous Hall effect due to the differences in atomic order and conductivity.
Our work can expand the existing understanding of the SGS properties and offer
a better SGS candidate with higher carrier mobility that can facilitate the
application in the spin-injected related devices. | 2311.18335v1 |
2024-01-31 | Electrical 180o switching of Néel vector in spin-splitting antiferromagnet | Antiferromagnetic spintronics have attracted wide attention due to its great
potential in constructing ultra-dense and ultra-fast antiferromagnetic memory
that suits modern high-performance information technology. The electrical 180o
switching of N\'eel vector is a long-term goal for developing
electrical-controllable antiferromagnetic memory with opposite N\'eel vectors
as binary "0" and "1". However, the state-of-art antiferromagnetic switching
mechanisms have long been limited for 90o or 120o switching of N\'eel vector,
which unavoidably require multiple writing channels that contradicts
ultra-dense integration. Here, we propose a deterministic switching mechanism
based on spin-orbit torque with asymmetric energy barrier, and experimentally
achieve electrical 180o switching of spin-splitting antiferromagnet Mn5Si3.
Such a 180o switching is read out by the N\'eel vector-induced anomalous Hall
effect. Based on our writing and readout methods, we fabricate an
antiferromagnet device with electrical-controllable high and low resistance
states that accomplishes robust write and read cycles. Besides fundamental
advance, our work promotes practical spin-splitting antiferromagnetic devices
based on spin-splitting antiferromagnet. | 2401.17608v1 |
2024-02-29 | Controllable suppression of the unconventional superconductivity in bulk and thin-film Sr$_{2}$RuO$_{4}$ via high-energy electron irradiation | In bulk Sr$_{2}$RuO$_{4}$, the strong sensitivity of the superconducting
transition temperature $T_{\text{c}}$ to nonmagnetic impurities provides robust
evidence for a superconducting order parameter that changes sign around the
Fermi surface. In superconducting epitaxial thin-film Sr$_{2}$RuO$_{4}$, the
relationship between $T_{\text{c}}$ and the residual resistivity $\rho_0$,
which in bulk samples is taken to be a proxy for the low-temperature elastic
scattering rate, is far less clear. Using high-energy electron irradiation to
controllably introduce point disorder into bulk single-crystal and thin-film
Sr$_{2}$RuO$_{4}$, we show that $T_{\text{c}}$ is suppressed in both systems at
nearly identical rates. This suggests that part of $\rho_0$ in films comes from
defects that do not contribute to superconducting pairbreaking, and establishes
a quantitative link between the superconductivity of bulk and thin-film
samples. | 2402.19454v1 |
2017-07-09 | Ultrafast Epitaxial Growth of Metre-Sized Single-Crystal Graphene on Industrial Cu Foil | A foundation of the modern technology that uses single-crystal silicon has
been the growth of high-quality single-crystal Si ingots with diameters up to
12 inches or larger. For many applications of graphene, large-area high-quality
(ideally of single-crystal) material will be enabling. Since the first growth
on copper foil a decade ago, inch-sized single-crystal graphene has been
achieved. We present here the growth, in 20 minutes, of a graphene film of 5 x
50 cm2 dimension with > 99% ultra-highly oriented grains. This growth was
achieved by: (i) synthesis of sub-metre-sized single-crystal Cu(111) foil as
substrate; (ii) epitaxial growth of graphene islands on the Cu(111) surface;
(iii) seamless merging of such graphene islands into a graphene film with high
single crystallinity and (iv) the ultrafast growth of graphene film. These
achievements were realized by a temperature-driven annealing technique to
produce single-crystal Cu(111) from industrial polycrystalline Cu foil and the
marvellous effects of a continuous oxygen supply from an adjacent oxide. The
as-synthesized graphene film, with very few misoriented grains (if any), has a
mobility up to ~ 23,000 cm2V-1s-1 at 4 K and room temperature sheet resistance
of ~ 230 ohm/square. It is very likely that this approach can be scaled up to
achieve exceptionally large and high-quality graphene films with single
crystallinity, and thus realize various industrial-level applications at a low
cost. | 1707.02512v1 |
2020-06-04 | Thermal conductivity of amorphous and crystalline GeTe thin film at high temperature: Experimental and theoretical study | Thermal transport properties bear a pivotal role in influencing the
performance of phase change memory (PCM) devices, in which the PCM operation
involves fast and reversible phase change between amorphous and crystalline
phases. In this paper, we present a systematic experimental and theoretical
study on the thermal conductivity of GeTe at high temperatures involving fast
change from amorphous to crystalline phase upon heating. Modulated photothermal
radiometry (MPTR) is used to experimentally determine thermal conductivity of
GeTe at high temperatures in both amorphous and crystalline phases. Thermal
boundary resistances are accurately taken into account for experimental
consideration. To develop a concrete understanding of the underlying physical
mechanism, rigorous and in-depth theoretical exercises are carried out. For
this, first-principles density functional methods and linearized Boltzmann
transport equations (LBTE) are employed using both direct and relaxation time
based approach (RTA) and compared with that of the phenomenological Slack
model. The amorphous phase experimental data has been described using the
minimal thermal conductivity model with sufficient precision. The theoretical
estimation involving direct solution and RTA method are found to retrieve well
the trend of the experimental thermal conductivity for crystalline GeTe at high
temperatures despite being slightly overestimated and underestimated,
respectively, compared to the experimental data. A rough estimate of vacancy
contribution has been found to modify the direct solution in such a way that it
agrees excellently with the experiment. Umklapp scattering has been determined
as the significant phonon-phonon scattering process. Umklapp scattering
parameter has been identified for GeTe for the whole temperature range which
can uniquely determine and compare Umklapp scattering processes for different
materials | 2006.02625v1 |
2021-08-31 | Toward 100% Spin-Orbit Torque Efficiency with High Spin-Orbital Hall Conductivity Pt-Cr Alloys | 5d transition metal Pt is the canonical spin Hall material for efficient
generation of spin-orbit torques (SOTs) in Pt/ferromagnetic layer (FM)
heterostructures. However, for a long while with tremendous engineering
endeavors, the damping-like SOT efficiencies (${\xi}_{DL}$) of Pt and Pt alloys
have still been limited to ${\xi}_{DL}$<0.5. Here we present that with proper
alloying elements, particularly 3d transition metals V and Cr, a high
spin-orbital Hall conductivity
(${\sigma}_{SH}{\sim}6.5{\times}10^{5}({\hbar}/2e){\Omega}^{-1}{\cdot} m^{-1}$)
can be developed. Especially for the Cr-doped case, an extremely high
${\xi}_{DL}{\sim}0.9$ in a Pt$_{0.69}$Cr$_{0.31}$/Co device can be achieved
with a moderate Pt$_{0.69}$Cr$_{0.31}$ resistivity of ${\rho}_{xx}{\sim}133
{\mu}{\Omega}{\cdot}cm$. A low critical SOT-driven switching current density of
$J_{c}{\sim}3.2{\times}10^{6} A{\cdot}cm^{-2}$ is also demonstrated. The
damping constant (${\alpha}$) of Pt$_{0.69}$Cr$_{0.31}$/FM structure is also
found to be reduced to 0.052 from the pure Pt/FM case of 0.078. The overall
high ${\sigma}_{SH}$, giant ${\xi}_{DL}$, moderate ${\rho}_{xx}$, and reduced
${\alpha}$ of such a Pt-Cr/FM heterostructure makes it promising for versatile
extremely low power consumption SOT memory applications. | 2108.13857v3 |
2024-01-05 | Signatures of room-temperature superconductivity emerging in two-dimensional domains within the new Bi/Pb-based ceramic cuprate superconductors at ambient pressure | We predict the possibility of realizing room-temperature superconductivity in
different 2D domains within the ceramic high-Tc cuprates at ambient pressure
and experimentally confirm this prediction of 2D room-temperature
superconductivity in the newly derived Bi/Pb-based ceramic cuprates containing
many grain boundaries, interfaces and multiplate blocks. We argue that, in
these high-Tc materials, besides bulk superconductivity in 3D domains there is
also strongly enhanced 2D superconductivity emerging in the 3D-2D crossover
region well above the superconducting transition temperature Tc. We study the
possibility of the existence of distinct 3D and 2D superconducting phases in
high-Tc ceramic cuprates, in which the unconventional Cooper pairs behave like
bosons and condense below certain critical temperatures into 3D and 2D Bose
superfluids in 3D and 2D domains. We show that the superconducting transition
temperature in 2D domains is much higher than in 3D domains and can reach up to
room temperature. We report signatures of room-temperature superconductivity
occurring at different grain boundaries and 3D/2D interfaces and in multiplate
blocks within the ceramic superconductors, synthesized by using the new melt
technology in a large solar furnace. The samples of these materials synthesized
under the influence of concentrated solar energy have the bulk Tc values
ranging from 100 K to about 140 K and the more higher superconducting
transition temperatures, possibly even as high as room temperature in the 3D-2D
crossover region. The remnant 2D superconductivity in newly derived Bi/Pb-based
ceramic cuprate superconductors is observed at temperatures 200-300 K well
above the bulk Tc and the onset of room-temperature superconductivity is
evidenced by the observations of a sharp step-like drop in the resistance and a
well-detectable partial Meissner effect at around 300 K and ambient pressure. | 2401.02642v1 |
2024-04-17 | On-liquid-gallium surface synthesis of ultra-smooth conductive metal-organic framework thin films | Conductive metal-organic frameworks (MOFs) are emerging electroactive
materials for (opto-)electronics. However, it remains a great challenge to
achieve reliable MOF-based devices via the existing synthesis methods that are
compatible with the complementary metal-oxide-semiconductor technology, as the
surface roughness of thus-far synthetic MOF films or pellets is rather high for
efficient electrode contact. Here, we develop an on-liquid-gallium surface
synthesis (OLGSS) strategy under chemical vapor deposition (CVD) conditions for
the controlled growth of two-dimensional conjugated MOF (2D c-MOF) thin films
with ten-fold improvement of surface flatness (surface roughness can reach as
low as ~2 {\AA}) compared with MOF films grown by the traditional methods.
Supported by theoretical modeling, we unveil a layer-by-layer CVD growth mode
for constructing flattening surfaces, that is triggered by the high adhesion
energy between gallium (Ga) and planar aromatic ligands. We further demonstrate
the generality of the as-proposed OLGSS strategy by reproducing such a flat
surface over nine different 2D c-MOF films with variable thicknesses (~2 to 208
nm) and large lateral sizes (over 1 cm2). The resultant ultra-smooth 2D c-MOF
films enable the formation of high-quality electrical contacts with gold (Au)
electrodes, leading to a reduction of contact resistance by over ten orders of
magnitude compared to the traditional uneven MOF films. Furthermore, due to the
efficient interfacial interaction benifited from the high-quality contacts, the
prepared van der Waals heterostructure (vdWH) of OLGSS c-MOF and MoS2 exhibits
intriguing photoluminescence (PL) enhancement, PL peak shift and large work
function modulation. The establishment of the reliable OLGSS method provides
the chances to push the development of MOF electronics and the construction of
multicomponent MOF-based heterostructure materials. | 2404.15357v1 |
2009-01-08 | Transport properties of Layer-Antiferromagnet CuCrS2: A possible thermoelectric material | The electrical, thermal conductivity and Seebeck coefficient of the quenched,
annealed and slowly cooled phases of the layer compound CuCrS2 have been
reported between 15K to 300K. We also confirm the antiferromagnetic transition
at 40K in them by our magnetic measurements between 2K and 300K. The crystal
flakes show a minimum around 100K in their in-plane resistance behavior. For
the polycrystalline pellets the resistivity depends on their flaky texture and
it attains at most 10 to 20 times of the room temperature value at the lowest
temperature of measurement. The temperature dependence is complex and no
definite activation energy of electronic conduction can be discerned. We find
that the Seebeck coefficient is between 200-450 microV/K and is unusually large
for the observed resistivity values of between 5-100 mOhm-cm at room
temperature. The figure of merit ZT for the thermoelectric application is 2.3
for our quenched phases, which is much larger than 1 for useful materials. The
thermal conductivity K is mostly due to lattice conduction and is reduced by
the disorder in Cu- occupancy in our quenched phase. A dramatic reduction of
electrical and thermal conductivity is found as the antiferromagnetic
transition is approached from the paramagnetic region, and K subsequently rises
in the ordered phase. We discuss the transport properties as being similar to a
doped Kondo-insulator. | 0901.0977v2 |
2009-06-29 | A novel wear-resistant magnetic thin film material based on a $Ti_{1-x}Fe_xC_{1-y}$ nanocomposite alloy | In this study we report on the film growth and characterization of thin
(approximately 50 nm thick) Ti-Fe-C films deposited on amorphous quartz. The
experimental studies have been complemented by first principles density
functional theory (DFT) calculations. Upon annealing of as-prepared films, the
composition of the metastable Ti-Fe-C film changes. An iron-rich phase is first
formed close to the film surface, but with increasing annealing time this phase
is gradually displaced toward the film-substrate interface where its position
stabilizes. Both the magnetic ordering temperature and the saturation
magnetization changes significantly upon annealing. The DFT calculations show
that the critical temperature and the magnetic moment both increase with
increasing Fe and C-vacancy concentration. The formation of the metastable
iron-rich Ti-Fe-C compound is reflected in the strong increase of the magnetic
ordering temperature. Eventually, after enough annealing time ($\geq 10$
minutes), nano-crystalline $\alpha$-Fe starts to precipitate and the amount and
size of these precipitates can be controlled by the annealing procedure; after
20 minutes of annealing, the experimental results indicate a nano-crystalline
iron-film embedded in a wear resistant TiC compound. This conclusion is further
supported by transmission electron microscopy studies on epitaxial Ti-Fe-C
films deposited on single crystalline MgO substrates where, upon annealing, an
iron film embedded in TiC is formed. Our results suggest that annealing of
metastable Ti-Fe-C films can be used as an efficient way of creating a
wear-resistant magnetic thin film material. | 0906.5386v1 |
2010-11-25 | Resistive transition in disordered superconductors with varying intergrain coupling | The effect of disorder is investigated in granular superconductive materials
with strong and weak links. The transition is controlled by the interplay of
the \emph{tunneling} $g$ and \emph{intragrain} $g_{intr}$ conductances, which
depend on the strength of the intergrain coupling. For $g \ll g_{intr}$, the
transition involves first the grain boundary, while for $g \sim g_{intr}$ the
transition occurs into the whole grain. The different intergrain coupling is
considered by modelling the superconducting material as a disordered network of
Josephson junctions. Numerical simulations show that on increasing the
disorder, the resistive transition occurs for lower temperatures and the curve
broadens. These features are enhanced in disordered superconductors with strong
links. The different behaviour is further checked by estimating the average
network resistance for weak and strong links in the framework of the effective
medium approximation theory. These results may be relevant to shed light on
long standing puzzles as: (i) enhancement of the superconducting transition
temperature of many metals in the granular states; (ii) suppression of
superconductivity in homogeneously disordered films compared to standard
granular systems close to the metal-insulator transition; (iii) enhanced
degradation of superconductivity by doping and impurities in strongly linked
materials, such as magnesium diboride, compared to weakly-linked
superconductors, such as cuprates. | 1011.5607v1 |
2015-10-23 | Consequences of breaking time reversal symmetry in LaSb: a resistivity plateau and extreme magnetoresistance | Time reversal symmetry (TRS) protects the metallic surface modes of
topological insulators (TIs). The transport signature of robust metallic
surface modes of TIs is a plateau that arrests the exponential divergence of
the insulating bulk with decreasing temperature. This universal behavior is
observed in all TI candidates ranging from Bi2Te2Se to SmB6. Recently, several
topological semimetals (TSMs) have been found that exhibit extreme
magnetoresistance (XMR) and TI universal resistivity behavior revealed only
when breaking TRS, a regime where TIs theoretically cease to exist. Among these
new materials, TaAs and NbP are nominated for Weyl semimetal due to their lack
of inversion symmetry, Cd3As2 is nominated for Dirac semimetal due to its
linear band crossing at the Fermi level, and WTe2 is nominated for resonant
compensated semimetal due to its perfect electron-hole symmetry. Here we
introduce LaSb, a simple rock-salt structure material without broken inversion
symmetry, without perfect linear band crossing, and without perfect
electron-hole symmetry. Yet LaSb portrays all the exotic field induced
behaviors of the aforementioned semimetals in an archetypal fashion. It shows
(a) the universal TI resistivity with a plateau at 15 K, revealed by a magnetic
field, (b) ultrahigh mobility of carriers in the plateau region, (c) quantum
oscillations with a non-trivial Berry phase, and (d) XMR of about one million
percent at 9 tesla rivaled only by WTe2 and NbP. Due to its dramatic
simplicity, LaSb is the ideal model system to formulate a theoretical
understanding of the exotic consequences of breaking TRS in TSMs. | 1510.06931v1 |
2015-11-30 | Electrical transport in nano-thick ZrTe$_5$ sheets: from three to two dimensions | ZrTe$_5$ is a newly discovered topological material. Shortly after a single
layer ZrTe$_5$ had been predicted to be a two-dimensional topological
insulator, a handful of experiments have been carried out on bulk ZrTe$_5$
crystals, which however suggest that its bulk form may be a three-dimensional
topological Dirac semimetal. We report the first transport study on ultra thin
ZrTe$_5$ flakes down to 10 nm. A significant modulation of the characteristic
resistivity maximum in the temperature dependence by thickness has been
observed. Remarkably, the metallic behavior, occurring only below about 150 K
in bulk, persists to over 320 K for flakes less than 20 nm thick. Furthermore,
the resistivity maximum can be greatly tuned by ionic gating. Combined with the
Hall resistance, we identify contributions from a semiconducting and a
semimetallic bands. The enhancement of the metallic state in thin flakes are
consequence of shifting of the energy bands. Our results suggest that the band
structure sensitively depends on the film thickness, which may explain the
divergent experimental observations on bulk materials. | 1511.09315v2 |
2016-06-08 | Multi-orbital physics in lithium-molybdenum purple-bronze: going beyond paradigm | We investigate the role of inter-orbital fluctuations in the low energy
physics of a quasi-1D material - lithium molybdenum purple bronze (LMO). It is
an exceptional material that may provide us a long sought realization of a
Tomonaga-Luttinger liquid (TLL) physics, but its behaviour at temperatures of
the order of $T^*\approx 30$K remains puzzling despite numerous efforts. Here
we make a conjecture that the physics around $T^*$ is dominated by
multi-orbital excitations. Their properties can be captured using an excitonic
picture. Using this relatively simple model we compute fermionic Green's
function in the presence of excitons. We find that the spectral function is
broadened with a Gaussian and its temperature dependence acquires an extra
$T^1$ factor. Both effects are in perfect agreement with experimental findings.
We also compute the resistivity for temperatures above and below critical
temperature $T_o$. We explain an upturn of the resistivity at 28K and interpret
the suppression of this extra component of resistivity when a magnetic field is
applied along the conducting axis. Furthermore, in the framework of our model,
we qualitatively discuss and consistently explain other experimentally detected
peculiarities of purple bronze: the breaking of Wiedmann-Franz law and the
magnetochromatic behaviour. | 1606.02687v3 |
2016-10-08 | Enhancement of Impedance by Chromium Substitution and Correlation with DC Resistivity in Cobalt Ferrite | Chromium substituted cobalt ferrite with grain size less than the single
domain (approx. 70 nm) has been prepared by the sol-gel method. XRD analysis
reveals that the samples crystallize to cubic symmetry with spacegroup number
227. Two transition temperatures (TD (approx. 450 K) and TM (approx. 600 K)
have been observed from the impedance verses temperature measurement. TD
increases with the increase in frequency due to dipole response to the
frequency. TM is comparable with the para-ferrimagnetic transition temperature
of cobalt ferrite, which is independent of frequency. This result is well
supported by the temperature dependent DC conductivity measurement. The
modified Debye relaxation could be explained the impedance spectra of
CoFe2-xCrxO4. The grain and grain boundary effect on impedance spectroscopy has
been observed from Cole-Cole analysis. The ac conductivity follows Arrhenius
behavior at different frequencies. All the samples exhibit the negative
temperature coefficient of resistance behavior which reveals the semiconducting
behavior of the material. The Mott VRH model could explain the DC electrical
conductivity. Both ac impedance and DC resistivity are well co-related each
other to explain the electron transport properties in Cr substituted cobalt
ferrite. The electrical transport properties could be explained by the electron
hopping between different metal ions via oxygen in the material. | 1610.02489v1 |
2018-08-08 | The viscosities of partially molten materials undergoing diffusion creep | Partially molten materials resist shearing and compaction. This resistance is
described by a fourth-rank effective viscosity tensor. When the tensor is
isotropic, two scalars determine the resistance: an effective shear and an
effective bulk viscosity. Here, calculations are presented of the effective
viscosity tensor during diffusion creep for a 2D tiling of hexagonal unit cells
and a 3D tessellation of tetrakaidecahedrons (truncated octahedrons). The
geometry of the melt is determined by assuming textural equilibrium. The
viscosity tensor for the 2D tiling is isotropic, but that for the 3D
tessellation is anisotropic. Two parameters control the effect of melt on the
viscosity tensor: the porosity and the dihedral angle. Calculations for both
Nabarro-Herring (volume diffusion) and Coble (surface diffusion) creep are
presented. For Nabarro-Herring creep the bulk viscosity becomes singular as the
porosity vanishes. This singularity is logarithmic, a weaker singularity than
typically assumed in geodynamic models. The presence of a small amount of melt
(0.1% porosity) causes the effective shear viscosity to approximately halve.
For Coble creep, previous modelling work has argued that a very small amount of
melt may lead to a substantial, factor of 5, drop in the shear viscosity. Here,
a much smaller, factor of 1.4, drop is obtained for tetrakaidecahedrons. Owing
to a Cauchy relation symmetry, the Coble creep bulk viscosity is a constant
multiple of the shear viscosity when melt is present. | 1808.02734v2 |
2019-01-14 | Gate tunable giant anisotropic resistance in ultra-thin GaTe | In crystals, the duplication of atoms often follows different periodicity
along different directions. It thus gives rise to the so called anisotropy,
which is usually even more pronounced in two dimensional (2D) materials due to
the absence of $\textbf{z}$ dimension. Indeed, in the emerging 2D materials,
electrical anisotropy has been one of the focuses in recent experimental
efforts. However, key understandings of the in-plane anisotropic resistance in
low-symmetry 2D materials, as well as demonstrations of model devices taking
advantage of it, have proven difficult. Here, we show that, in few-layered
semiconducting GaTe, electrical conductivity along $\textbf{x}$ and
$\textbf{y}$ directions of the 2D crystal can be gate tuned from a ratio of
less than one order to as large as 10$^{3}$. This effect is further
demonstrated to yield an anisotropic memory resistor behaviour in ultra-thin
GaTe, when equipped with an architecture of van der Waals floating gate. Our
findings of gate tunable giant anisotropic resistance (GAR) effect pave the way
for potential applications in nano-electronics such as multifunctional
directional memories in the 2D limit. | 1901.04262v1 |
2020-01-27 | Graphene-based Nanoscale version of da Vinci's Reciprocal Structures | A reciprocal structure (RS) is a mechanical resistant structure formed by a
set of self-supporting elements satisfying certain conditions of structural
reciprocity (SR) . The first condition is that each element of the structure
has to support and be supported by the others. The second condition is that
these functions cannot occur in the same part of the element. These two
properties make beams and two-dimensional materials very much appropriate to
build RSs. Commonly seen in floors or roofs, SR is also present in art,
religious symbols and decorative objects. Da Vinci has drawn several examples
of such RSs. Here, we propose a simple nano version of a da Vinci's RS based on
graphene nanoribbons. The stability and resistance against mechanical impacts
(ballistic projectile) were investigated through fully atomistic molecular
dynamics (MD) simulations. We considered structures with three and four joins
with and without RS topologies. Our MD results showed that structures with RS
topologies are more impact resistant than those without SR, despite the fact
that the used graphene nanoribbons are highly pliable. We discuss these results
in terms of the number of joins, energy absorption and stress on the
structures. We discuss possible applications in nanoengineering. | 2001.10027v2 |
2021-05-21 | Vanadium Dioxide Thin Films Synthesized Using Low Thermal Budget Atmospheric Oxidation | Vanadium dioxide is a complex oxide material, which shows large resistivity
and optical reflectance change while transitioning from the insulator to metal
phase at ~68 {\deg}C. In this work, we use a modified atmospheric thermal
oxidation method to oxidize RF-sputtered Vanadium films. Structural,
surface-morphology and phase-transition properties of the oxidized films as a
function of oxidation duration are presented. Phase-pure VO2 films are obtained
by oxidizing ~130 nm Vanadium films in short oxidation duration of ~30 seconds.
Compared to previous reports on VO2 synthesis using atmospheric oxidation of
Vanadium films of similar thickness, we obtain a reduction in oxidation
duration by more than one order. Synthesized VO2 thin film shows resistance
switching of ~3 orders of magnitude. We demonstrate optical reflectance
switching in long-wave infrared wavelengths in VO2 films synthesized using
atmospheric oxidation of Vanadium. The extracted refractive index of VO2 in the
insulating and in the metallic phase is in good agreement with VO2 synthesized
using other methods. The considerable reduction in oxidation time of VO2
synthesis while retaining good resistance and optical switching properties will
help in integration of VO2 in limited thermal budget processes, enabling
further applications of this phase-transition material. | 2105.10264v1 |
2023-08-18 | Magnetoresistance anomaly during the electrical triggering of a metal-insulator transition | Phase separation naturally occurs in a variety of magnetic materials and it
often has a major impact on both electric and magnetotransport properties. In
resistive switching systems, phase separation can be created on demand by
inducing local switching, which provides an opportunity to tune the electronic
and magnetic state of the device by applying voltage. Here we explore the
magnetotransport properties in the ferromagnetic oxide (La,Sr)MnO3 (LSMO)
during the electrical triggering of an intrinsic metal-insulator transition
(MIT) that produces volatile resistive switching. This switching occurs in a
characteristic spatial pattern, i.e., the formation of an insulating barrier
perpendicular to the current flow, enabling an electrically actuated
ferromagnetic-paramagnetic-ferromagnetic phase separation. At the threshold
voltage of the MIT triggering, both anisotropic and colossal magnetoresistances
exhibit anomalies including a large increase in magnitude and a sign flip.
Computational analysis revealed that these anomalies originate from the
coupling between the switching-induced phase separation state and the intrinsic
magnetoresistance of LSMO. This work demonstrates that driving the MIT material
into an out-of-equilibrium resistive switching state provides the means to
electrically control of the magnetotransport phenomena. | 2308.09260v2 |
2024-01-30 | Picosecond transfer from short-term to long-term memory in analog antiferromagnetic memory device | Experiments in materials with a compensated ordering of magnetic moments have
demonstrated a potential for approaching the thermodynamic limit of the fastest
and least-dissipative operation of a digital memory bit. In addition, these
materials are very promising for a construction of energy-efficient analog
devices with neuromorphic functionalities, which are inspired by
computing-in-memory capabilities of the human brain. In this paper, we report
on experimental separation of switching-related and heat-related resistance
signal dynamics in memory devices microfabricated from CuMnAs antiferromagnetic
metal. We show that the memory variable multilevel resistance can be used as a
long-term memory (LTM), lasting up to minutes at room temperature. In addition,
ultrafast reflectivity change and heat dissipation from nanoscale-thickness
CuMnAs films, taking place on picosecond to hundreds of nanoseconds time
scales, can be used as a short-term memory (STM). Information about input
stimuli, represented by femtosecond laser pulses, can be transferred from STM
to LTM after rehearsals at picosecond to nanosecond times in these memory
devices, where information can be retrieved at times up to 10^15 longer than
the input pulse duration. Our results open a route towards ultra-fast low-power
implementations of spiking neuron and synapse functionalities using a resistive
analog antiferromagnetic memory. | 2401.17370v1 |
2021-04-21 | Machine-Learning Assisted Optimization Strategies for Phase Change Materials Embedded within Electronic Packages | Leveraging the latent heat of phase change materials (PCMs) can reduce the
peak temperatures and transient variations in temperature in electronic
devices. But as the power levels increase, the thermal conduction pathway from
the heat source to the heat sink limits the effectiveness of these systems. In
this work, we evaluate embedding the PCM within the silicon device layer of an
electronic device to minimize the thermal resistance between the source and the
PCM to minimize this thermal resistance and enhance the thermal performance of
the device. The geometry and material properties of the embedded PCM regions
are optimized using a combination of parametric and machine learning
algorithms. For a fixed geometry, considering commercially available materials,
Solder 174 significantly outperforms other organic and metallic PCMs. Also with
a fixed geometry, the optimal melting points to minimize the peak temperature
is higher than the optimal melting point to minimize the amplitude of the
transient temperature oscillation, and both optima increase with increasing
heater power. Extending beyond conventional optimization strategies, genetic
algorithms and particle swarm optimization with and without neural network
surrogate models are used to enable optimization of many geometric and material
properties. For the test case evaluated, the optimized geometries and
properties are similar between all ML-assisted algorithms, but the
computational time depends on the technique. Ultimately, the optimized design
with embedded phase change materials reduces the maximum temperature rise by
19% and the fluctuations by up to 88% compared to devices without PCM. | 2104.14433v1 |
2023-10-26 | A Critical Assessment of Electronic Structure Descriptors for Predicting Perovskite Catalytic Properties | The discovery and design of new materials which can efficiently catalyze the
oxygen reduction and evolution reactions at reduced temperatures is important
for facilitating the widespread adoption of fuel cell and electrolyzer
technologies. Numerous studies have produced correlations between catalytic
properties, such as oxygen surface exchange or electrode area specific
resistance (ASR), and properties of the catalyst material. However,
correlations have historically been limited in scope (e.g., using only a few
materials or at a single temperature) and it has been difficult to provide
detailed assessments of their robustness. Here, we assess the ability of the O
p-band center electronic structure descriptor, obtained from density functional
theory (DFT) calculations, to correlate with oxygen surface exchange rates,
diffusivities, and area specific resistances for a large database of perovskite
oxide catalytic properties. By data mining the literature, we obtain 747
catalytic property value data points spanning 299 unique perovskite
compositions from 313 studies. We assess linear correlations of each property
with the O p-band center and find generally modest correlations that are
qualitatively useful (prediction mean absolute errors of about 0.5 log units
are typical), where the correlations are improved at higher temperatures (e.g.,
800 {\deg}C vs. 500 {\deg}C) and significantly improve when considering fits to
the subset of materials which have multiple independent measurements. These
findings suggest that the spread of property data is significantly influenced
by experimental uncertainty, and subsequent measurements of additional
materials will likely improve the O p-band center correlations. | 2310.17744v1 |
1998-11-11 | Superconducting Material Diagnostics using a Scanning Near-Field Microwave Microscope | We have developed scanning near-field microwave microscopes which can image
electrodynamic properties of superconducting materials on length scales down to
about 2 $\mu$m. The microscopes are capable of quantitative imaging of sheet
resistance of thin films, and surface topography. We demonstrate the utility of
the microscopes through images of the sheet resistance of a YBa2Cu3O7-d thin
film wafer, images of bulk Nb surfaces, and spatially resolved measurements of
Tc of a YBa2Cu3O7-d thin film. We also discuss some of the limitations of the
microscope and conclude with a summary of its present capabilities. | 9811158v1 |
2001-08-21 | Infrared and optical properties of pure and cobalt-doped LuNi_2B_2C | We present optical conductivity data for Lu(Ni$_{1-x}$Co$_x$)$_2$B$_2$C over
a wide range of frequencies and temperatures for x=0 and x=0.09. Both materials
show evidence of being good Drude metals with the infrared data in reasonable
agreement with dc resistivity measurements at low frequencies. An absorption
threshold is seen at approximately 700 cm-1. In the cobalt-doped material we
see a superconducting gap in the conductivity spectrum with an absorption onset
at 24 +/- 2 cm-1 = 3.9$ +/- 0.4 k_BT_c suggestive of weak to moderately strong
coupling. The pure material is in the clean limit and no gap can be seen. We
discuss the data in terms of the electron-phonon interaction and find that it
can be fit below 600 cm-1 with a plasma frequency of 3.3 eV and an
electron-phonon coupling constant lambda_{tr}=0.33 using an alpha^{2}F(omega)
spectrum fit to the resistivity. | 0108333v1 |
2005-04-22 | Low field magneto-transport in La_0.7Ca_0.3MnO_3-PMMA composites synthesized by polymeric precursor route | A detailed investigation of the effect of PMMA on the structure,
microstructure and magneto-transport properties of manganite La_0.7Ca_0.3MnO_3
(LCMO) is presented. LCMO-PMMA nanostructured composites have been synthesized
by a unique polymeric sol-gel route, which leads to improved solubility of PMMA
in the LCMO matrix. The LCMO phase is grown in the presence of varying PMMA
concentration at ~500 ^0 C. This route yields single phase material and the
grain size is observed to decrease slightly with increasing PMMA concentration.
On increasing the PMMA concentration, T_C undergoes a small decrease,
resistivity is observed to increase by two orders of magnitude, with a
concomitant large decrease in T_IM, e.g., from 218 K for virgin LCMO to 108 K
for 50 wt% PMMA admixed LCMO. Low field magneto-resistance measured in the
temperature range 77-300 K shows considerable enhancement as a function of the
PMMA concentration. These phenomena are explained by taking into account the
increased intergranular disorder as a consequence of PMMA admixture. | 0504579v1 |
2007-03-01 | Magnetic effects at the interface between nonmagnetic oxides | The electronic reconstruction at the interface between two insulating oxides
can give rise to a highly-conductive interface. In analogy to this remarkable
interface-induced conductivity we show how, additionally, magnetism can be
induced at the interface between the otherwise nonmagnetic insulating
perovskites SrTiO3 and LaAlO3. A large negative magnetoresistance of the
interface is found, together with a logarithmic temperature dependence of the
sheet resistance. At low temperatures, the sheet resistance reveals magnetic
hysteresis. Magnetic ordering is a key issue in solid-state science and its
underlying mechanisms are still the subject of intense research. In particular,
the interplay between localized magnetic moments and the spin of itinerant
conduction electrons in a solid gives rise to intriguing many-body effects such
as Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, the Kondo effect, and
carrier-induced ferromagnetism in diluted magnetic semiconductors. The
conducting oxide interface now provides a versatile system to induce and
manipulate magnetic moments in otherwise nonmagnetic materials. | 0703028v2 |
2008-07-09 | Molecular Beam Epitaxy grown (Ga,Mn)(As,P) with perpendicular to plane magnetic easy axis | We present an experimental investigation of the magnetic, electrical and
structural properties of Ga0.94Mn0.06As1-yPy layers grown by molecular beam
epitaxy on GaAs substrates for y less than or equal to 0.3. X-ray diffraction
measurements reveal that the layers are under tensile strain which gives rise
to a magnetic easy axis perpendicular to the plane of the layers. The strength
of the magnetic anisotropy and the coercive field increase as the phosphorous
concentration is increased. The resistivity of all samples shows metallic
behaviour with the resistivity increasing as y increases. These materials will
be useful for studies of micromagnetic phenomena requiring metallic
ferromagnetic material with perpendicular magnetic anisotropy. | 0807.1469v1 |
2009-09-10 | Electric Pulse Induced Resistive Switching, Electronic Phase Separation, and Possible Superconductivity in a Mott insulator | Metal-insulator transitions (MIT) belong to a class of fascinating physical
phenomena, which includes superconductivity, and colossal magnetoresistance
(CMR), that are associated with drastic modifications of electrical resistance.
In transition metal compounds, MIT are often related to the presence of strong
electronic correlations that drive the system into a Mott insulator state. In
these systems the MIT is usually tuned by electron doping or by applying an
external pressure. However, it was noted recently that a Mott insulator should
also be sensitive to other external perturbations such as an electric field. We
report here the first experimental evidence of a non-volatile
electric-pulse-induced insulator-to-metal transition and possible
superconductivity in the Mott insulator GaTa4Se8. Our Scanning Tunneling
Microscopy experiments show that this unconventional response of the system to
short electric pulses arises from a nanometer scale Electronic Phase Separation
(EPS) generated in the bulk material. | 0909.1978v1 |
2010-03-13 | Ferromagnetic Clusters in the Brownmillerite Bilayered Compounds Ca2.5-xLaxSr0.5GaMn2O8: An Approach to Achieve Layered Spintronics Materials | We report the effect of La-substitution on the magnetic and magnetotransport
properties of Brownmillerite-like bilayered compounds Ca2.5-xLaxSr0.5GaMn2O8 (x
= 0, 0.05, 0.075, and 0.1) by using dc-magnetization, resistivity and
magnetoresistance techniques. The Rietveld analysis of the room temperature
x-ray diffraction patterns confirms no observable change of average crystal
structure with the La-substitution. Both magnetic and magnetotransport
properties are found to be very sensitive to the La-substitution.
Interestingly, the La-substituted compounds show ferromagnetic-like behavior
(due to the occurrence of a double exchange mechanism) whereas, the parent
compound is an antiferromagnet (TN 150 K). All compounds show an insulating
behavior, in the measured temperature range of 100 - 300 K, with an overall
decrease in the resistivity with the substitution. A higher value of
magnetoresistance has been successfully achieved by the La-substitution. We
have proposed an electronic phase separation model, considering the formation
of ferromagnetic clusters in the antiferromagnetic matrix, to interpret the
observed magnetization and magnetotransport results for the La-substituted
samples. The present study demonstrates an approach to achieve new functional
materials, based on naturally occurring layered system like
Ca2.5-xLaxSr0.5GaMn2O8, for possible spintronics applications. | 1003.2685v1 |
2013-07-24 | Large resistivity change and phase transition in LiMnAs | Antiferromagnetic semiconductors are new alternative materials for spintronic
applications and spin valves. In this work, we report a detailed investigation
of two antiferromagnetic semiconductors AMnAs (A = Li, LaO), which are
isostructural to the well-known LiFeAs and LaOFeAs superconductors. Here we
present a comparison between the structural, magnetic, and electronic
properties of LiMnAs, LaOMnAs and related materials. Interestingly, both LiMnAs
and LaOMnAs show a variation in resistivity with more than five orders of
magnitude, making them particularly suitable for use in future electronic
devices. From neutron and X-ray diffraction measurements on LiMnAs we have
observed a magnetic phase transition corresponding to the Neel temperature of
373.8 K, and a structural transition from the tetragonal to the cubic phase at
768 K. These experimental results are supported by density functional theory
(DFT) calculations. | 1307.6404v4 |
2014-03-31 | Investigation of Complex Impedance and Modulus Properties of Nd Doped 0.5BiFeO3-0.5PbTiO3 Multiferroic Composites | 0.5BiNdxFe1-xO3-0.5PbTiO3 (x=0.05, 0.10, 0.15, 0.20) composites were
successfully synthesized by a solid state reaction technique. At room
temperature X-ray diffraction shows tetragonal structure for all concentrations
of Nd doped 0.5BiFeO3-0.5PbTiO3 composites. The nature of Nyquist plot confirms
the presence of bulk effects only for all compositions of Nd-doped
0.5BiFeO3-0.5PbTiO3 composites. The bulk resistance is found to decreases with
the increasing in temperature as well as Nd concentration and exhibits a
typical negative temperature coefficient of resistance (NTCR) behavior. Both
the complex impedance and modulus studies have suggested the presence of
non-Debye type of relaxation in the materials. Conductivity spectra reveal the
presence of hopping mechanism in the electrical transport process of the
materials. The activation energy of the composite increases with increasing Nd
concentration and were found to be 0.28, 0.27, 0.31 and 0.32eV for x=0.05,
0.10, 0.15, 0.20 respectively at 200-275 oC for conduction process. | 1403.7981v1 |
2014-04-09 | A new first-principles calculation of field-dependent RF surface impedance of BCS superconductor and application to SRF cavities | There is a need to better understand the intrinsic limit of radiofrequency
(RF) surface impedance that determines the performance of superconducting RF
cavities in particle accelerators. Here we present a field-dependent derivation
of Mattis-Bardeen (M-B) theory of the RF surface impedance of BCS
superconductors based on the shifted Density of States (DoS) resulting from
coherently moving Cooper pairs [1].The surprising reduction in resistance with
increasing field is explained to be an intrinsic effect. Using this analysis
coded in MathematicaTM, survey calculations have been completed which examine
the sensitivities of this surface impedance to variation of the BCS material
parameters and temperature.Our theoretical prediction of the effective BCS RF
surface resistance (Rs) of niobium as a function of peak surface magnetic field
amplitude agrees well with recently reported record low loss resonant cavity
measurements from Jefferson Lab (JLab) and Fermi National Accelerator Lab
(FNAL) with carefully, yet differently, prepared niobium material. The results
present a refined description of the "best theoretical" performance available
to potential applications with corresponding materials. [1]Xiao, B.P., C.E.
Reece, and M.J. Kelley, Superconducting surface impedance under radiofrequency
field. Physica C: Superconductivity, 2013. 490(0): p. 26-31. | 1404.2523v1 |
2014-11-25 | Computing with spins and magnets | The possible use of spin and magnets in place of charge and capacitors to
store and process information is well known. Magnetic tunnel junctions are
being widely investigated and developed for magnetic random access memories.
These are two terminal devices that change their resistance based on switchable
magnetization of magnetic materials. They utilize the interaction between
electron spin and magnets to read information from the magnets and write onto
them. Such advances in memory devices could also translate into a new class of
logic devices that offer the advantage of nonvolatile and reconfigurable
information processing over transistors. Logic devices having a transistor-like
gain and directionality could be used to build integrated circuits without the
need for transistor-based amplifiers and clocks at every stage. We review
device characteristics and basic logic gates that compute with spins and
magnets from the mesoscopic to the atomic scale, as well as materials,
integration, and fabrication challenges and methods. | 1411.6960v1 |
2015-05-22 | Direct Method for Calculating Temperature-Dependent Transport Properties | We show how temperature-induced disorder can be combined in a direct way with
first-principles scattering theory to study diffusive transport in real
materials. Excellent (good) agreement with experiment is found for the
resistivity of Cu, Pd, Pt (and Fe) when lattice (and spin) disorder are
calculated from first principles. For Fe, the agreement with experiment is
limited by how well the magnetization (of itinerant ferromagnets) can be
calculated as a function of temperature. By introducing a simple Debye-like
model of spin disorder parameterized to reproduce the experimental
magnetization, the temperature dependence of the average resistivity, the
anisotropic magnetoresistance and the spin polarization of a Ni$_{80}$Fe$_{20}$
alloy are calculated and found to be in good agreement with existing data.
Extension of the method to complex, inhomogeneous materials as well as to the
calculation of other finite-temperature physical properties within the
adiabatic approximation is straightforward. | 1505.06231v1 |
2015-11-04 | Numerical Calculations of Wake Fields and Impedances of LHC Collimators' Real Structures | The LHC collimators have very complicated mechanical designs including
movable jaws made of higly resistive materials, ferrite materials, tiny RF
contacts. Since the jaws are moved very close to the circulating beams their
contribution in the overall LHC coupling impedance is dominant, with respect to
other machine components. For these reasons accurate simulation of collimators'
impedance becomes very important and challenging. Besides, several dedicated
tests have been performed to verify correct simulations of lossy dispersive
material properties, such as resistive wall and ferrites, benchmarking code
results with analytical, semi-analytical and other numerical codes outcomes.
Here we describe all the performed numerical tests and discuss the results of
LHC collimators' impedances and wake fields calculations. | 1511.01236v1 |
2016-04-19 | Metallicity and conductivity crossover in white light illuminated CH$_3$NH$_3$PbI$_3$ perovskite | The intrinsic d.c. electrical resistivity ($\rho$) - measurable on single
crystals only - is often the quantity first revealing the properties of a given
material. In the case of CH$_3$NH$_3$PbI$_3$ perovskite measuring $\rho$ under
white light illumination provides insight into the coexistence of extended and
shallow localized states (0.1 eV below the conduction band). The former ones
dominate the electrical conduction while the latter, coming from neutral
defects, serve as a long-lifetime charge carrier reservoir accessible for
charge transport by thermal excitation. Remarkably, in the best crystals the
electrical resistivity shows a metallic behaviour under illumination up to room
temperature, giving a new dimension to the material in basic physical studies. | 1604.05637v1 |
2016-05-13 | Quantifying structure-property relationships during resistance spot welding of an aluminum 6061-T6 joint | Microstructure-property relationships of resistance spot welded 6061-T6
aluminum alloy lap joints were investigated via mechanical testing and
microscopy techniques. Quasi-static tensile and novel shear punch tests were
employed to measure the mechanical properties of the distinct weld regions.
Quasi-static tensile and shear punch tests revealed constantly decreasing
strength and ductility as the weld center was approached. For instance, the
ultimate tensile strength of the fusion zone decreased by ~52% from the parent
material (341 MPa to 162 MPa) while the yield strength decreased by ~62% (312
MPa to 120 MPa). The process-induced microstructures were analyzed with
scanning electron microscopy and optical microscopy to elucidate the underlying
cause of the reduced mechanical properties. Fractography reveals void growth
from particles being the dominant damage mechanism in the parent material as
compared to void nucleation in the fusion zone. Overall, significant changes in
the mechanical behavior across the weld are the result of a change in
microstructure congruent with a loss of T6 condition (precipitate coarsening). | 1605.04251v1 |
2016-06-17 | Electronic ground state of MnB$_{4}$ | Recent studies have dealt with the electronic and magnetic ground state
properties of the tetraboride material MnB$_4$. So far, however, the ground
state properties could not be established unambiguously. Therefore, here we
present an experimental study on single-crystalline MnB$_4$ by means of
resistivity and magnetization measurements. For this, we have developed a
sample holder that allows four-point ac resistivity measurements on these very
small ($\sim$\,100\,$\mu$m) samples. With our data we establish that the
electronic ground state of MnB$_4$ is intrinsically that of a pseudo-gap
system, in agreement with recent band structure calculations. Furthermore, we
demonstrate that the material does neither show magnetic order nor a behavior
arising from the vicinity to a magnetically ordered state, this way disproving
previous claims. | 1606.05484v3 |
2016-12-15 | Superconductivity above 500 K in conductors made by bringing n-alkane into contact with graphite | In 1986, a cuprate superconductor (Ba-La-Cu-O system) having a critical
temperature which goes over the BCS limit (~30 K) was discovered and then a
cuprate superconductor (Y-Ba-Cu-O system) with a critical temperature higher
than 77 K was discovered. Furthermore, a Hg-based cuprate with a critical
temperature of 133 K was found. The 133 K is still the highest critical
temperature of conventional superconductors under atmospheric pressure.
We have shown that materials obtained by bringing n-alkanes into contact with
graphite are capable of conducting electricity with almost no energy loss at
room temperature. We here report that the sudden jump in resistance showing a
phase transition is observed in the materials during heating by two-probe
resistance measurement. The measured critical temperatures of the materials
consisting of pitch-based graphite fibers and n-alkanes having 7-16 carbon
atoms range from 363.08 to 504.24 K and the transition widths range between
0.15 and 3.01 K. We also demonstrate that superconductors with critical
temperatures beyond 504 K are obtained by alkanes with 16 or more carbon atoms. | 1612.05294v1 |
2017-02-20 | Gate Tunable Magneto-resistance of Ultra-Thin WTe2 Devices | In this work, the magneto-resistance (MR) of ultra-thin WTe2/BN
heterostructures far away from electron-hole equilibrium is measured. The
change of MR of such devices is found to be determined largely by a single
tunable parameter, i.e. the amount of imbalance between electrons and holes. We
also found that the magnetoresistive behavior of ultra-thin WTe2 devices is
well-captured by a two-fluid model. According to the model, the change of MR
could be as large as 400,000%, the largest potential change of MR among all
materials known, if the ultra-thin samples are tuned to neutrality when
preserving the mobility of 167,000 cm2V-1s-1 observed in bulk samples. Our
findings show the prospects of ultra-thin WTe2 as a variable magnetoresistance
material in future applications such as magnetic field sensors, information
storage and extraction devices, and galvanic isolators. The results also
provide important insight into the electronic structure and the origin of the
large MR in ultra-thin WTe2 samples. | 1702.05876v1 |
2017-10-05 | Multi-scale Modeling of Plasticity Nearby Precipitates in Nanostructured Materials | Precipitation strengthening is one of the most effective methods to design
alloys with the desired combination of strength and ductility. The main
mechanism of strengthening is generally known to be the interaction between
dislocations and precipitates. When a dislocation encounters a precipitate, it
bends and therefore the level of applied stress to the precipitate increases.
Once the applied stress reaches the precipitate resistance, it passes the
precipitate. Dislocations can bypass precipitates either by forming the Orowan
loops or by cutting them. In this research, the focus is set on a small domain
nearby precipitates to investigate their effects on the effective plastic
strain. Both penetrable and impenetrable precipitates are considered. Two
scales are coupled to model this phenomenon, the nano-micro scale where
plasticity is determined by explicit three-dimensional discrete dislocation
dynamics analysis and the continuum scale where the finite element method is
applied. With this hybrid approach, complex problems in plastic deformation of
nanostructured materials can be addressed. Finally, the relation between the
precipitate resistance and the effective plastic strain is investigated. | 1710.02075v3 |
2018-05-03 | Enhanced Electron Transport in Thin Copper Films via Atomic-Layer Materials Capping | Using first-principles calculations based on density functional theory and
non-equilibrium Green's functions, we characterized the effect of surface
termination on the electronic transport properties of nanoscale Cu slabs. With
ideal, clean (111) surfaces and oxidized ones as baselines we explore the
effect of capping the slabs with graphene, hexagonal boron nitrate, molybdenum
disulfide and stanene. Surface oxide suppresses balistic conductance by a
factor of 10 compared to the ideal surface. Capping the ideal copper surface
with graphene slightly increase conductance but MoS$_2$ and stanene have the
opposite effect due to stronger interactions at the interface. Interestingly,
we find that capping atomistically roughed copper surfaces with graphene or
MoS$_2$ decreases the resistance per unit length by 20 and 13%, respectively,
due to reduced scattering. The results presented in this work suggest that
two-dimensional materials can be used as an ultra-thin liner in metallic
interconnect technology without increasing the interconnect line resistivity
significantly. | 1805.01517v1 |
2018-06-07 | On the magnetic and electronic properties of NpPdSn | We have studied NpPdSn by means of the heat capacity, electrical resistivity,
Seebeck and Hall effect, $^{237}$Np M\"{o}ssbauer spectroscopy, and neutron
diffraction measurements in the temperature range 2-300 K and under magnetic
fields up to 14 T. NpPdSn orders antiferromagnetically below the N\'eel
temperature $T_N$ = 19 K and shows localized magnetism of Np$^{3+}$ ion with a
a doubly degenerate ground state. In the magnetic state the electrical
resistivity and heat capacity are characterized by electron-magnon scattering
with spin-waves spectrum typical of anisotropic antiferromagnets. An enhanced
Sommerfeld coefficient and typical behavior of magnetorestistivity, Seebeck and
Hall coefficients are all characteristic of systems with strong electronic
correlations. The low temperature antiferromagnetic state of NpPdSn is verified
by neutron diffraction and $^{237}$Np M\"{o}ssbauer spectroscopy and possible
magnetic structures are discussed. | 1806.02879v1 |
2019-09-06 | Unveiling multiferroic proximity effect in graphene | We demonstrate that electronic and magnetic properties of graphene can be
tuned via proximity of multiferroic substrate. Our first-principles
calculations performed both with and without spin-orbit coupling clearly show
that by contacting graphene with bismuth ferrite BiFeO$_3$ (BFO) film, the
spin-dependent electronic structure of graphene is strongly impacted both by
the magnetic order and by electric polarization in the underlying BFO. Based on
extracted Hamiltonian parameters obtained from the graphene band structure, we
propose a concept of six-resistance device based on exploring multiferroic
proximity effect giving rise to significant proximity electro- (PER), magneto-
(PMR), and multiferroic (PMER) resistance effects. This finding paves a way
towards multiferroic control of magnetic properties in two dimensional
materials. | 1909.02844v1 |
2017-01-27 | Strain-modulated Bandgap and Piezo-resistive Effect in Black Phosphorus Field-effect Transistors | Energy bandgap largely determines the optical and electronic properties of a
semiconductor. Variable bandgap therefore makes versatile functionality
possible in a single material. In layered material black phosphorus, the
bandgap can be modulated by the number of layers; as a result, few-layer black
phosphorus has discrete bandgap values that are relevant for opto-electronic
applications in the spectral range from red, in monolayer, to mid-infrared in
the bulk limit. Here, we further demonstrate continuous bandgap modulation by
mechanical strain applied through flexible substrates. The strain-modulated
bandgap significantly alters the charge transport in black phosphorus at room
temperature; we for the first time observe a large piezo-resistive effect in
black phosphorus field-effect transistors (FETs). The effect opens up
opportunities for future development of electro-mechanical transducers based on
black phosphorus, and we demonstrate strain gauges constructed from black
phosphorus thin crystals. | 1701.08041v1 |
2019-07-27 | Microwave derived monoclinic Ba1-xSnxNb2O6 materials as an alternative of ITO | Microwave synthesis was optimized for preparing novel monoclinic Tin-doped
Barium Niobate ceramics (Ba1-xSnxNb2O6; x= 0.0, 0.01, 0.05, 0.1, 0.2, 0.3) BSN.
The intensity of monoclinic phase formation was observed to decrease on
increasing tin as dopant indicating decreased crystallinity. Strained
crystalline phase was observed in undoped sample that became severe on doping
tin. Monoclinic metal alloy Sn2O3 formation was confirmed on increasing tin
doping beyond 5%. Electronic configuration of tin (II) oxide supports the local
site-wise monoclinic disorder in crystal structure due to sterically active
lone pair. Such a disorder arranges increased number of degenerate energy
states and reduce effective energy gap between conduction band minimum and
valence band maximum. All BSN compositions were investigated for monoclinic
phase stabilization, ultra-violet absorption, dielectric response, raman modes
and type of carrier concentration along with hall resistivity. All measurements
possessed inflexion corresponding to 5 atomic % tin doping indicating
successful site substitution and estimated Sn2O3 metal formation beyond this.
The values of reducing optical energy band gap, transparency to visible
spectrum and hall resistivity indicated utility of these materials as a
substitute transparent conducting oxide (TCO) for well-known indium tin oxide
(ITO). | 1907.11946v1 |
2019-04-24 | Progressive amorphization of GeSbTe phase-change material under electron beam irradiation | Fast and reversible phase transitions in chalcogenide phase-change materials
(PCMs), in particular, Ge-Sb-Te compounds, are not only of fundamental
interests, but also make PCMs based random access memory (PRAM) a leading
candidate for non-volatile memory and neuromorphic computing devices. To RESET
the memory cell, crystalline Ge-Sb-Te has to undergo phase transitions firstly
to a liquid state and then to an amorphous state, corresponding to an abrupt
change in electrical resistance. In this work, we demonstrate a progressive
amorphization process in GeSb2Te4 thin films under electron beam irradiation on
transmission electron microscope (TEM). Melting is shown to be completely
absent by the in situ TEM experiments. The progressive amorphization process
resembles closely the cumulative crystallization process that accompanies a
continuous change in electrical resistance. Our work suggests that if
displacement forces can be implemented properly, it should be possible to
emulate symmetric neuronal dynamics by using PCMs. | 1904.10601v2 |
2015-06-02 | Nonmonotonic fracture behavior of polymer nanocomposites | Polymer composite materials are widely used for their exceptional mechanical
properties, notably their ability to resist large deformations. Here we examine
the failure stress and strain of rubbers reinforced by varying amounts of
nano-sized silica particles. We find that small amounts of silica increase the
fracture stress and strain, but too much filler makes the material become
brittle and consequently fracture happens at small deformations. We thus find
that as a function of the amount of filler there is an optimum in the breaking
resistance at intermediate filler concentrations. We use a modified Griffith
theory to establish a direct relation between the material properties and the
fracture behavior that agrees with the experiment. | 1506.00832v1 |
2015-07-23 | Tuning bad metal and non-Fermi liquid behavior in a Mott material: rare earth nickelate thin films | Resistances that exceed the Mott-Ioffe-Regel limit, known as bad metal
behavior, and non-Fermi liquid behavior are ubiquitous features of the normal
state of many strongly correlated materials. Here we establish the conditions
that lead to bad metal and non-Fermi liquid phases in NdNiO3, which exhibits a
prototype, bandwidth-controlled metal-insulator transition. We show that
resistance saturation is determined by the magnitude of the Ni eg orbital
splitting, which can be tuned by strain in epitaxial films, causing the
appearance of bad metal behavior under certain conditions. The results shed
light on the nature of a crossover to non-Fermi liquid metal phase and provide
a predictive criterion for strong localization. They elucidate a seemingly
complex phase behavior as a function of film strain and confinement and provide
guidelines for orbital engineering and novel devices. | 1507.06619v1 |
2018-07-17 | Current jetting distorted planar Hall effect in a Weyl semimetal with ultrahigh mobility | A giant planar Hall effect (PHE) and anisotropic magnetoresistance (AMR) is
observed in TaP, a nonmagnetic Weyl semimetal with ultrahigh mobility. The
perpendicular resistivity (i.e., the planar magnetic field applied normal to
the current) far exceeds the zero-field resistivity, which thus rules out the
possible origin of negative longitudinal magnetoresistance. The giant PHE/AMR
is finally attributed to the large anisotropic orbital magnetoresistance that
stems from the ultrahigh mobility. Furthermore, the mobility-enhanced current
jetting effects are found to strongly deform the line shape of the curves, and
their evolution with the changing magnetic field and temperature is also
studied. Although the giant PHE/AMR suggests promising applications in
spintronics, the enhanced current jetting shows the other side of the coin,
which needs to be considered in the future device design. | 1807.06229v3 |
2019-01-03 | Realization of Kondo chain in CeCo$_2$Ga$_8$ | We revisited the anisotropy of the heavy-fermion material CeCo$_2$Ga$_8$ by
measuring the electrical resistivity and magnetic susceptibility along all the
principal $\mathbf{a}$-, $\mathbf{b}$- and $\mathbf{c}$-axes. Resistivity along
$\mathbf{c}$-axis ($\rho_c$) shows clear Kondo coherence below about 17 K,
while both $\rho_{a}$ and $\rho_{b}$ remain incoherent down to 2 K. The
magnetic anisotropy is well understood within the theoretical frame of
crystalline electric field effect in combination with magnetic exchange
interactions. We found the anisotropy ratio of these magnetic exchange
interactions, $|J_{ex}^c/J_{ex}^{a,b}|$, reaches a large value of 4-5. We,
therefore, firmly demonstrate that CeCo$_2$Ga$_8$ is a quasi-one-dimensional
heavy-fermion compound both electrically and magnetically, and thus provide a
realistic example of \textit{Kondo chain}. | 1901.00558v2 |
2019-01-04 | Experimental and Numerical Investigation of the Fracture Behavior of Particle Reinforced Alkali Activated Slag Mortars | This paper presents fracture response of alkali-activated slag (AAS) mortars
with up to 30% (by volume) of slag being replaced by waste iron powder which
contains a significant fraction of elongated particles. The elongated iron
particles act as micro-reinforcement and improve the crack resistance of AAS
mortars by increasing the area of fracture process zone (FPZ). Increased area
of FPZ signifies increased energy-dissipation which is reflected in the form of
significant increase in the crack growth resistance as determined from
R-curves. Fracture response of notched AAS mortar beams under three-point
bending is simulated using extended finite element method (XFEM) to develop a
tool for direct determination of fracture characteristics such as crack
extension and fracture toughness in particulate-reinforced AAS mortars.
Fracture response simulated using the XFEM based framework correlates well with
experimental observations. The comprehensive fracture studies reported here
provide an economical and sustainable means towards improving the ductility of
AAS systems which are generally more brittle than their conventional ordinary
portland cement counterparts. | 1901.01025v1 |
2019-10-17 | The grain-size effect on thermal conductivity of uranium dioxide | We have investigated the grain boundary scattering effect on the thermal
transport behavior of uranium dioxide (UO$_2$). The polycrystalline samples
having different grain-sizes (0.125, 1.8, and 7.2 $\mu$m) have been prepared by
spark plasma sintering technique and characterized by x-ray powder diffraction
(XRD), scanning electron microscope (SEM), and Raman spectroscopy. The thermal
transport properties (the thermal conductivity and thermoelectric power) have
been measured in the temperature range 2-300~K and the results were analyzed in
terms of various physical parameters contributing to the thermal conductivity
in these materials in relation to grain-size. We show that thermal conductivity
decreases systematically with lowering grain-size in the temperatures below 30
K, where the boundary scattering dominates the thermal transport. At higher
temperatures more scattering processes are involved in the heat transport in
these materials, making the analysis difficult. We determined the grain
boundary Kapitza resistance that would result in the observed increase in
thermal conductivity with grain size, and compared the value with Kapitza
resistances calculated for UO$_2$ using molecular dynamics from the literature. | 1910.08014v1 |
2020-01-06 | Transport characteristics of type II Weyl semimetal MoTe2 thin films grown by chemical vapor deposition | Theoretical calculations and experimental observations show MoTe2 is a type
II Weyl semimetal, along with many members of transition metal dichalcogenides
family. We have grown highly crystalline large-area MoTe2 thin films on Si/SiO2
substrates by chemical vapor deposition. Very uniform, continuous, and smooth
films were obtained as confirmed by scanning electron microscopy and atomic
force microscopy analyses. Measurements of the temperature dependence of
longitudinal resistivity and current-voltage characteristics at different
temperature are discussed. Unsaturated, positive quadratic magnetoresistance of
the as-grown thin films has been observed from 10 K to 200 K. Hall resistivity
measurements confirm the majority charge carriers are hole. | 2001.01703v1 |
2020-05-11 | Skyrmion phase in MnSi on sapphire grown by a conventional sputtering | Topologically protected chiral skyrmion is an intriguing spin texture, which
has attracted much attention because of fundamental research and future
spintronic applications. MnSi with the non-centrosymmetric structure is
well-known material hosting skyrmion phase. To date, preparation of MnSi
crystals has been investigated by using special instruments with ultrahigh
vacuum chamber. Here, we introduce a facile way to grow MnSi films on sapphire,
which is in relatively low vacuum environment of conventional magnetron
sputtering. Magnetotransport properties including Hall resistivity measurements
allow to confirm the existence of skyrmion phase in MnSi film. Because as-grown
MnSi films on sapphire has polycrystalline nature, the emergent features of
skyrmion phase are limited and complicated. However, we observed the stable
skyrmion phase in a broad range of temperatures and magnetic fields, which is
explained by phenomenological scaling analyses of Hall resistivities
contribution. Our findings provide not only a general way to prepare the
materials possessing skyrmion phase, but also insight into further research to
stimulate more degrees of freedom in our inquisitiveness. | 2005.04841v1 |
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