publicationDate stringlengths 10 10 | title stringlengths 17 233 | abstract stringlengths 20 3.22k | id stringlengths 9 12 |
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2012-05-15 | Field-induced high coercive ferromagnetic state and magnetoresistance in the antiferromagnetically ordered compound Fe0.5TiS2 | The measurements of the magnetic susceptibility, magnetization, electrical
resistivity and magnetoresistance have been performed for the Fe intercalated
compound Fe0.5TiS2. According to X-ray diffraction measurements the Fe0.5TiS2
compound synthesized in the present work has a monoclinic crystal structure
(space group I12/m1) which results from the ordering of Fe ions and vacancies
between S-Ti-S tri-layres. The changes in the heat-treatment conditions at
temperatures below 1100 Celsius degrees do not lead to an order-disorder
transition within the subsystem of intercalated Fe ions. It has been shown that
this compound exhibits an antiferromagnetic (AF) ground state below the Neel
temperature TN = 140 K. Application of the magnetic field at T < TN induces a
metamagnetic phase transition to the ferromagnetic (F) state, which is
accompanied by the large magnetoresistance effect (up to 27 %). The
field-induced AF-F transition is found to be irreversible below ~ 100 K. The
magnetization reversal in the metastable F state at low temperatures is
accompanied by substantial hysteresis (~ 100 kOe) which is associated with the
Ising character of Fe ions. | 1205.3242v1 |
2012-05-16 | A new approach for improving global critical current density in Fe(Se0.5Te0.5) polycrystalline materials | A novel method to prepare bulk Fe(Se0.5Te0.5) samples is presented, based on
a melting process and a subsequent annealing treatment. With respect to the
standard sintering technique, it produces much more homogeneous and denser
samples, characterized by large and well interconnected grains. The resulting
samples exhibit optimal critical temperature values, sharp resistive and
magnetic transitions, large magnetic hysteresis loops and high upper critical
fields are observed. Interestingly, the global critical current density is much
enhanced as compared to the values reported in literature for bulk samples of
the same 11 family, reaching about 103 A/cm2 at zero field at 4.2 K as assessed
by magnetic, transport and magneto-optical techniques. Even more importantly,
its field dependence turns out to be very weak, such that at \mu_{0}H = 7 T it
is suppressed only by a factor \sim2. | 1205.3618v2 |
2012-05-29 | Transformation Electronics: Tailoring Electron's Effective Mass | The speed of integrated circuits is ultimately limited by the mobility of
electrons or holes, which depend on the effective mass in a semiconductor.
Here, building on an analogy with electromagnetic metamaterials and
transformation optics, we describe a new transport regime in a semiconductor
superlattice characterized by extreme anisotropy of the effective mass and a
low intrinsic resistance to movement - with zero effective mass - along some
preferred direction of electron motion. We theoretically demonstrate that such
regime may permit an ultra fast, extremely strong electron response, and
significantly high conductivity, which, notably may be weakly dependent on the
temperature at low temperatures. These ideas may pave the way for faster
electronic devices and detectors and new functional materials with a strong
electrical response in the infrared regime. | 1205.6325v2 |
2012-07-13 | Association of indigo with zeolites for improved colour stabilization | The durability of an organic colour and its resistance against external
chemical agents and exposure to light can be significantly enhanced by
hybridizing the natural dye with a mineral. In search for stable natural
pigments, the present work focuses on the association of indigo blue with
several zeolitic matrices (LTA zeolite, mordenite, MFI zeolite). The
manufacturing of the hybrid pigment is tested under varying oxidising
conditions, using Raman and UV-visible spectrometric techniques. Blending
indigo with MFI is shown to yield the most stable composite in all of our
artificial indigo pigments. In absence of defects and substituted cations such
as aluminum in the framework of the MFI zeolite matrix, we show that matching
the pore size with the dimensions of the guest indigo molecule is the key
factor. The evidence for the high colour stability of indigo@MFI opens a new
path for modeling the stability of indigo in various alumino-silicate
substrates such as in the historical Maya Blue pigment. | 1207.3236v1 |
2012-08-09 | The role of charge traps in inducing hysteresis: capacitance - voltage measurements on top gated bilayer graphene | Understanding the origin of hysteresis in the channel resistance from top
gated graphene transistors is important for transistor applications.
Capacitance - voltage measurements across the gate oxide on top gated bilayer
graphene show hysteresis with a charging and discharging time constant of ~100
{\mu}s. However, the measured capacitance across the graphene channel does not
show any hysteresis, but shows an abrupt jump at a high channel voltage due to
the emergence of an order, indicating that the origin of hysteresis between
gate and source is due to charge traps present in the gate oxide and graphene
interface. | 1208.1831v1 |
2012-08-15 | A GEANT4 Simulation Studyof BESIII endcap TOF Upgrade | A GEANT4-based Monte-Carlo model is developed to study the performance of
Endcap Time-Of-Flight (ETOF) at BESIII. It's found that the multiple scattering
effects, mainly from the materials at the MDC endcap, can cause multi-hit on
the ETOF's readout cell and significantly influence the timing property of
ETOF. Multi-gap Resistive Plate Chamber (MRPC) with a smaller readout cell
structure is more suitable for ETOF detector due to significantly reduced
multi-hit rate, from 71.5% for currently-used scintillator-based ETOF to 21.8%
or 16.7% for MRPC-based ETOF, depending on the readout pad size used. The
timing performance of a MRPC ETOF is also improved. These simulation results
suggest and guide an ETOF upgrade effort at BESIII. | 1208.3049v1 |
2012-09-03 | High Efficiency Graphene Solar Cells by Chemical Doping | We demonstrate single layer graphene/n-Si Schottky junction solar cells that
under AM1.5 illumination exhibit a power conversion efficiency (PCE) of 8.6%.
This performance, achieved by doping the graphene with
bis(trifluoromethanesulfonyl)amide, exceeds the native(undoped) device
performance by a factor of 4.5 and the best previously reported PCE in similar
devices by a factor of nearly 6. Current-voltage, capacitance-voltage and
external quantum efficiency measurements show the enhancement to be due to the
doping induced shift in the graphene chemical potential which increases the
graphene carrier density (decreasing the cell series resistance) and increases
the built-in potential. | 1209.0432v1 |
2012-09-12 | Channel Length Scaling of MoS2 MOSFETs | In this article, we investigate electrical transport properties in ultrathin
body (UTB) MoS2 two-dimensional (2D) crystals with channel lengths ranging from
2 {\mu}m down to 50 nm. We compare the short channel behavior of sets of
MOSFETs with various channel thickness, and reveal the superior immunity to
short channel effects of MoS2 transistors. We observe no obvious short channel
effects on the device with 100 nm channel length (Lch) fabricated on a 5 nm
thick MoS2 2D crystal even when using 300 nm thick SiO2 as gate dielectric, and
has a current on/off ratio up to ~109. We also observe the on-current
saturation at short channel devices with continuous scaling due to the carrier
velocity saturation. Also, we reveal the performance limit of short channel
MoS2 transistors is dominated by the large contact resistance from the Schottky
barrier between Ni and MoS2 interface, where a fully transparent contact is
needed to achieve a high-performance short channel device. | 1209.2525v1 |
2012-09-24 | Ba1-xNaxTi2Sb2O (0.0 <= x <= 0.33): A Layered Titanium-based Pnictide Oxide Superconductor | A new layered Ti-based pnictide oxide superconductor, Ba1-xNaxTi2Sb2O (0.0 <=
x <= 0.33), is reported. X-ray studies reveal it crystallizes in the tetragonal
CeCr2Si2C structure. The undoped parent compound, BaTi2Sb2O (P4/mmm;
a=4.1196(1){\AA}; c=8.0951(2){\AA}), exhibits a CDW/SDW transition at 54K. Upon
chemical doping with Na, the CDW/SDW transition is systematically suppressed
and super-conductivity arises with the critical temperatures, Tc, increasing to
5.5 K. Bulk superconductivity is confirmed by resistivity, magnetic and heat
capacity measurements. Like the high-Tc cuprates and the iron pnictides,
superconductivity in BaTi2Sb2O arises from an ordered state. Similarities and
differences to the cuprate and iron pnictide supercon-ductors are discussed. | 1209.5447v2 |
2012-09-27 | Fabrication and characterization of semiconducting half Heusler YPtSb thin films | The semiconducting half Heusler compound YPtSb was predicted theoretically to
be capable of changing into topological insulator under proper strain. In this
work, p type semiconducting half-Heusler YPtSb thin films were prepared by
magnetron co-sputtering method from a specially designed target for the first
time. Textured structure with (111) plane paralleling with (001) of MgO
substrate was observed when YPtSb thin films were grown on MgO (100) substrate
at 600{\deg}C.Electrical measurements show that the resistivity of YPtSb films
decreases with increasing temperature, indicating a semiconductor-like
behavior. The carrier density is as high as 1.15 X 10^21 cm-3 at 300 K. The
band gap of YPtSb thin films obtained by infrared spectroscopy is around 0.1 -
0.15 eV, which is well in agreement with the theoretical prediction and the
value measured in bulk YPtSb. | 1209.6288v2 |
2012-10-08 | Phonon thermal transport outside of local equilibrium in nanowires via molecular dynamics | We study thermal transport through Pt nanowires that bridge planar contacts
as a function of wire length and vibrational frequency of the contacts. When
phonons in the contacts have lower average frequencies than those in the wires
thermal transport occurs under conditions away from local equilibrium with
low-frequency phonons experiencing a higher thermal gradient than
high-frequency ones. This results in a size-dependent increase in the effective
thermal conductivity of the wire with decreasing vibrational frequencies of the
contacts. The interfacial resistivity when heat flows from the wire to the
contact is also size-dependent and has the same physical origin in the lack of
full equilibration in short nanowires. We develop a model based on a 1D atomic
chain that captures the salient physics of the MD results. | 1210.2321v1 |
2012-10-19 | Self-aligned graphene field-effect transistors with polyethyleneimine doped source/drain access regions | We report a method of fabricating self-aligned, top-gated graphene
field-effect transistors (GFETs) employing polyethyleneimine spin-on-doped
source/drain access regions, resulting in a 2X reduction of access resistance
and a 2.5X improvement in device electrical characteristics, over undoped
devices. The GFETs on Si/SiO$_2$ substrates have high carrier mobilities of up
to 6,300 cm$^2$/Vs. Self-aligned spin-on-doping is applicable to GFETs on
arbitrary substrates, as demonstrated by a 3X enhancement in performance for
GFETs on insulating quartz substrates, which are better suited for radio
frequency applications. | 1210.5535v2 |
2012-10-22 | Fabrication and characterization of the gapless half-Heusler YPtSb thin films | Half-Heusler YPtSb thin films were fabricated by magnetron co-sputtering
method on MgO-buffered SiO2/Si(001) substrates. X-ray diffraction pattern and
Energy dispersive X-ray spectroscopy confirmed the high-quality growth and
stoichiometry. The temperature dependence of the resistivity shows a
semiconducting-type behavior down to low temperature. The Hall mobility was
determined to be 450 cm2/Vs at 300K, which is much higher than the bulk value
(300 cm2/Vs). In-plane magnetoresistance (MR) measurements with fields applied
along and perpendicular to the current direction show opposite MR signs, which
suggests the possible existence of the topological surface states. | 1210.5808v1 |
2012-10-29 | Impact of graphene quantum capacitance on transport spectroscopy | We demonstrate experimentally that graphene quantum capacitance
$C_{\mathrm{q}}$ can have a strong impact on transport spectroscopy through the
interplay with nearby charge reservoirs. The effect is elucidated in a
field-effect-gated epitaxial graphene device, in which interface states serve
as charge reservoirs. The Fermi-level dependence of $C_{\mathrm{q}}$ is
manifested as an unusual parabolic gate voltage ($V_{\mathrm{g}}$) dependence
of the carrier density, centered on the Dirac point. Consequently, in high
magnetic fields $B$, the spectroscopy of longitudinal resistance ($R_{xx}$) vs.
$V_{\mathrm{g}}$ represents the structure of the unequally spaced relativistic
graphene Landau levels (LLs). $R_{xx}$ mapping vs. $V_{\mathrm{g}}$ and $B$
thus reveals the vital role of the zero-energy LL on the development of the
anomalously wide $\nu=2$ quantum Hall state. | 1210.7601v1 |
2012-11-02 | Interface characterization of Co2MnGe/Rh2CuSn Heusler multilayers | All-Heusler multilayer structures have been investigated by means of high
kinetic x-ray photoelectron spectroscopy and x-ray magnetic circular dichroism,
aiming to address the amount of disorder and interface diffusion induced by
annealing of the multilayer structure. The studied multilayers consist of
ferromagnetic Co$_2$MnGe and non-magnetic Rh$_2$CuSn layers with varying
thicknesses. We find that diffusion begins already at comparably low
temperatures between 200 $^{\circ}$C and 250 $^{\circ}$C, where Mn appears to
be most prone to diffusion. We also find evidence for a 4 {\AA} thick
magnetically dead layer that, together with the identified interlayer
diffusion, are likely reasons for the small magnetoresistance found for
current-perpendicular-to-plane giant magneto-resistance devices based on this
all-Heusler system. | 1211.0489v2 |
2012-11-13 | The giant plasticity of a quantum crystal | When submitted to large stresses at high temperature, usual crystals may
irreversibly deform. This phenomenon is known as plasticity and it is due to
the motion of crystal defects such as dislocations. We have discovered that, in
the absence of impurities and in the zero temperature limit, helium 4 crystals
present a giant plasticity that is anisotropic and reversible. Direct
measurements on oriented single crystals show that their resistance to shear
nearly vanishes in one particular direction because dislocations glide freely
parallel to the basal planes of the hexagonal structure. This plasticity
disappears as soon as traces of helium 3 impurities bind to the dislocations or
if their motion is damped by collisions with thermal phonons. | 1211.2976v2 |
2012-11-14 | High-temperature structural phase transition in multiferroic LiCu_2O_2 | LiCu_2O_2 single crystals were studied in the temperature range 300-1100 K by
means of heating-cooling curves of differential thermal analysis (DTA),
thermogravimetry (TG), X-ray powder diffraction and electrical measurements. A
reversible first-order phase transition between orthorhombic and tetragonal
phases was found to take place at 993 K. At the transition, a peak is observed
in the DTA curves, as well as jumps of the unit cell parameters and electrical
resistivity. Considering the crystal structure of LiCu_2O_2 and the entropy
change associated with the phase transition, it is concluded that the phase
transition is related to processes of order-disorder of the Cu2+ and Li+
cations onto their crystallographic positions. | 1211.3275v1 |
2012-11-14 | Anisotropy and directional pinning in YBaCuO with BaZrO3 nanorods | Measurements of anisotropic transport properties (dc and high-frequency
regime) of driven vortex matter in YBa$_2$Cu$_3$O$_{7-x}$ with elongated
strong-pinning sites (c-axis aligned, self-assembled BaZrO$_3$ nanorods) are
used to demonstrate that the effective-mass angular scaling takes place only in
intrinsic physical quantities (flux-flow resistivity), and not in
pinning-related Labusch parameter and critical currents. Comparison of the
dynamics at different time scales shows evidence for a transition of the vortex
matter toward a Mott phase, driven by the presence of nanorods. The strong
pinning in dc arises partially from a dynamic effect. | 1211.3311v2 |
2012-11-23 | 1D to 3D Dimensional Crossover in the Superconducting Transition of the Quasi-One-Dimensional Carbide Superconductor Sc3CoC4 | The transition metal carbide superconductor Sc3CoC4 may represent a new
benchmark system of quasi-1D superconducting behavior. We investigate the
superconducting transition of a high-quality single crystalline sample by
electrical transport experiments. Our data show that the superconductor goes
through a complex dimensional crossover below the onset Tc of 4.5 K. First, a
quasi-1D fluctuating superconducting state with finite resistance forms in the
CoC4 ribbons which are embedded in a Sc matrix in this material. At lower
temperature, the transversal Josephson or proximity coupling of neighboring
ribbons establishes a 3D bulk superconducting state. This dimensional crossover
is very similar to Tl2Mo6Se6, which for a long time has been regarded as the
most appropriate model system of a quasi-1D superconductor. Sc3CoC4 appears to
be even more in the 1D limit than Tl2Mo6Se6. | 1211.5430v4 |
2012-11-26 | Local transport measurements at mesoscopic length scales using scanning tunneling potentiometry | Under mesoscopic conditions, the transport potential on a thin film with
current is theoretically expected to bear spatial variation due to quantum
interference. Scanning tunneling potentiometry is the ideal tool to investigate
such variation, by virtue of its high spatial resolution. We report in this
{\it Letter} the first detailed measurement of transport potential under
mesoscopic conditions. Epitaxial graphene at a temperature of 17K was chosen as
the initial system for study because the characteristic transport length scales
in this material are relatively large. Tip jumping artifacts are a major
possible contribution to systematic errors; and we mitigate such problems by
using custom-made slender and sharp tips manufactured by focussed ion beam. In
our data, we observe residual resistivity dipoles associated with
topoographical defects, and local peaks and dips in the potential that are not
associated with topographical defects. | 1211.6088v2 |
2012-12-06 | Effects of spin density wave quantization on the electrical transport in epitaxial Cr thin films | We present measurements of the electrical resistivity, $\rho$, in epitaxial
Cr films of different thicknesses grown on MgO (100) substrates, as a function
of temperature, $T$. The $\rho(T)$ curves display hysteretic behavior in
certain temperature range, which is film thickness dependent. The hysteresis
are related to the confinement of quantized incommensurate spin density waves
(ISDW) in the film thickness. Our important finding is to experimentally show
that the temperature $T_{mid}$ where the ISDW changes from $N$ to $N$\,+\,1
nodes {\it decreases} as the film thickness {\it increases}. Identifying
$T_{mid}$ with a first order transition between ISDW states with $N$ and
$N$\,+\,1 nodes, and using a Landau approach to the free energy of the ISDW
together with Monte Carlo simulations, we show that the system at high
temperatures explores all available modes for the ISDW, freezing out in one
particular mode at a transition temperature that indeed decreases with film
thickness, $L$. The detailed dependence of $T_{mid}(L)$ seems to depend rather
strongly on the boundary conditions at the Cr film interfaces. | 1212.1408v1 |
2013-01-09 | Hall effect measurements on epitaxial SmNiO3 thin films and implications for antiferromagnetism | The rare-earth nickelates (RNiO3) exhibit interesting phenomena such as
unusual antiferromagnetic order at wavevector q = (1/2, 0, 1/2) and a tunable
insulator-metal transition that are subjects of active research. Here we
present temperature-dependent transport measurements of the resistivity,
magnetoresistance, Seebeck coefficient, and Hall coefficient (RH) of epitaxial
SmNiO3 thin films with varying oxygen stoichiometry. We find that from room
temperature through the high temperature insulator-metal transition, the Hall
coefficient is hole-like and the Seebeck coefficient is electron-like. At low
temperature the N\'eel transition induces a crossover in the sign of RH to
electron-like, similar to the effects of spin density wave formation in
metallic systems but here arising in an insulating phase ~200 K below the
insulator-metal transition. We propose that antiferromagnetism can be
stabilized by bandstructure even in insulating phases of correlated oxides,
such as RNiO3, that fall between the limits of strong and weak electron
correlation. | 1301.1968v3 |
2013-01-11 | Batch-fabricated cantilever probes with electrical shielding for nanoscale dielectric and conductivity imaging | This paper presents the design and fabrication of batch-processed cantilever
probes with electrical shielding for scanning microwave impedance microscopy.
The diameter of the tip apex, which defines the electrical resolution, is less
than 50 nm. The width of the stripline and the thicknesses of the insulation
dielectrics are optimized for a small series resistance (< 5 W) and a small
background capacitance (~ 1 pF), both critical for high sensitivity imaging on
various samples. The coaxial shielding ensures that only the probe tip
interacts with the sample. The structure of the cantilever is designed to be
symmetric to balance the stresses and thermal expansions of different layers so
that the cantilever remains straight under variable temperatures. Such shielded
cantilever probes produced in the wafer scale will facilitate enormous
applications on nanoscale dielectric and conductivity imaging. | 1301.2402v1 |
2013-01-14 | Pressure-Induced Superconductivity and Its Scaling with Doping-Induced Superconductivity in the Iron Pnictide with Skutterudite Intermediary Layers | The Ca10(PtnAs8)(Fe2As2)5 (n=3,4) compounds are a new type of iron pnictide
superconductors whose structures consist of stacking Ca-PtnAs8-Ca-Fe2As2 layers
in a unit cell. When n=3 (the 10-3-8 phase), the undoped compound is an
antiferromagnetic (AFM) semiconductor, while, when n=4 (the 10-4-8 phase), the
undoped compound is a superconductor with the transition temperature of 26K.
Here we report the results of high-pressure studies on the 10-3-8 compound
obtained through a combination of in-situ resistance, magnetic susceptibility,
and Hall coefficient measurements. We find that its AFM order can be suppressed
completely at 3.5 GPa and then superconducting state appears in the pressure
range of 3.5-7 GPa. The pressure dependence of superconducting transition
temperature displays a dome-like shape. | 1301.2863v2 |
2013-02-08 | Electrical transport across Au/Nb:SrTiO3 Schottky interface with different Nb doping | We have investigated electron transport in Nb doped SrTiO$_3$ single crystals
for two doping densities. We find that the resistivity and mobility are
temperature dependent in both whereas the carrier concentration is almost
temperature invariant. We rationalize this using the hydrogenic theory for
shallow donors. Further, we probe electrical transport across Schottky
interfaces of Au on TiO$_2$ terminated n-type SrTiO$_3$. Quantitative analysis
of macroscopic I-V measurements reveal thermionic emission dominated transport
for the low doped substrate whereas it deviates from such behavior for the high
doped substrate. This work is relevant for designing devices to study
electronic transport using oxide-semiconductors. | 1302.2096v1 |
2013-02-14 | Transport studies of dual-gated ABC and ABA trilayer graphene: band gap opening and band structure tuning in very large perpendicular electric field | We report on the transport properties of ABC and ABA stacked trilayer
graphene using dual, locally gated field effect devices. The high efficiency
and large breakdown voltage of the HfO2 top and bottom gates enables
independent tuning of the perpendicular electric field and the Fermi level over
an unprecedentedly large range. We observe a resistance change of six orders of
magnitude in the ABC trilayer, which demonstrates the opening of a band gap.
Our data suggest that the gap saturates at a large displacement field of D ~ 3
V/nm, in agreement with self-consistent Hartree calculations. In contrast, the
ABA trilayer remains metallic even under a large perpendicular electric field.
Despite the absence of a band gap, the band structure of the ABA trilayer
continues to evolve with increasing D. We observe signatures of two-band
conduction at large D fields. Our self-consistent Hartree calculation
reproduces many aspects of the experimental data, but also points to the need
for more sophisticated theory. | 1302.3432v1 |
2013-02-21 | Superconductivity induced by U-doping in the SmFeAsO system | Through partial substitution of Sm by U in SmFeAsO, a different member of the
family of iron-based superconductors was successfully synthesized. X-ray
diffraction measurements show that the lattice constants along the a and c axes
are both squeezed through U doping, indicating a successful substitution of U
at the Sm site. The parent compound shows a strong resistivity anomaly near 150
K, associated with spin-density-wave instability.U doping suppresses this
instability and leads to a transition to the superconducting state at
temperatures up to 49 K. Magnetic measurements confirm the bulk
superconductivity in this system. For the sample with a doping level of x =
0.2, the external magnetic field suppresses the onset temperature very slowly,
indicating a rather high upper critical field. In addition, the Hall effect
measurements show that U clearly dopes electrons into the material. | 1302.5155v1 |
2013-04-10 | Pinning in a Porous Bi2223 | The current-voltage characteristics of a porous superconductor Bi2Sr2Ca2Cu3Ox
(Bi2223) have been measured at temperature range from 10 to 90 K. The
experimental dependences have been analyzed within the model allowing for
pinning by clusters of a normal phase with fractal boundaries, as well as the
model taking into account phase transformations of vortex matter. It has been
found that the electrical resistance of the superconductor material
significantly increases at temperatures of 60-70 K over the entire range of
magnetic fields under consideration without changing in the sign of the
curvature of the R(I) dependence. The melting of the vortex structure occurs at
these temperatures. It has been assumed that this behavior is associated with
the specific feature of the pinning in a highly porous high-temperature
superconductor, which lies in the fractal distribution of pinning centers in a
wide range of self-similarity scales. | 1304.2844v1 |
2013-04-10 | Multi-band superconductivity and nanoscale inhomogeneity at oxide interfaces | The two-dimensional electron gas at the LaTiO3/SrTiO3 or LaAlO3/SrTiO3 oxide
interfaces becomes superconducting when the carrier density is tuned by gating.
The measured resistance and superfluid density reveal an inhomogeneous
superconductivity resulting from percolation of filamentary structures of
superconducting "puddles" with randomly distributed critical temperatures,
embedded in a non-superconducting matrix. Following the evidence that
superconductivity is related to the appearance of high-mobility carriers, we
model intra-puddle superconductivity by a multi-band system within a weak
coupling BCS scheme. The microscopic parameters, extracted by fitting the
transport data with a percolative model, yield a consistent description of the
dependence of the average intra-puddle critical temperature and superfluid
density on the carrier density. | 1304.2970v1 |
2013-04-22 | Electrical transport in C-doped GaAs nanowires: surface effects | The resistivity and the mobility of Carbon doped GaAs nanowires have been
studied for different doping concentrations. Surface effects have been
evaluated by comparing upassivated with passivated nanowires. We directly see
the influence of the surface: the pinning of the Fermi level and the consequent
existence of a depletion region lead to an increase of the mobility up to 30
cm^2/(V*s) for doping concentrations lower than 3*10^18 cm^-3. Electron beam
induced current measurements show that the minority carrier diffusion path can
be as high as 190 nm for passivated nanowires. | 1304.5891v1 |
2013-05-23 | Electronic origin of the orthorhombic Cmca structure in compressed elements and binary alloys | Formation of the complex structure with 16 atoms in the orthorhombic cell,
space group Cmca (Pearson symbol oC16) was experimentally found under high
pressure in the alkali elements (K, Rb, Cs) and polyvalent elements of groups
IV (Si, Ge) and V (Bi). Intermetallic phases with this structure form under
pressure in binary Bi-based alloys (Bi-Sn, Bi-In, Bi-Pb). Stability of the Cmca
- oC16 structure is analyzed within the nearly free-electron model in the frame
of Fermi sphere - Brillouin zone interaction. A Brillouin-Jones zone formed by
a group of strong diffraction reflections close to the Fermi sphere is the
reason for reduction of crystal energy and stabilization of the structure. This
zone corresponds well to the 4 valence electrons in Si and Ge and leads to
assume a spd-hybridization for Bi. To explain the stabilization of this
structure within the same model in alkali metals, that are monovalent at
ambient conditions, a possibility of an overlap of the core and valence band
electrons at strong compression is considered. The assumption of the increase
in the number of valence electrons helps to understand sequences of complex
structures in compressed alkali elements and unusual changes in their physical
properties such as electrical resistance and superconductivity. | 1305.5341v1 |
2013-05-29 | Intrinsic and extrinsic origins of low-frequency noise in GaAs/AlGaAs Schottky-gated nanostructures | We study low-frequency noise in current passing through quantum point
contacts fabricated from several GaAs/AlGaAs heterostructures with different
layer structures and fabrication processes. In contrast to previous reports,
there is no gate-dependent random telegraph noise (RTN) originating from
tunneling through a Schottky barrier in devices fabricated using the standard
low-damage process. Gate-dependent RTN appears only in devices fabricated with
a high-damage process that induces charge trap sites. We show that the
insertion of AlAs/GaAs superlattices in the AlGaAs barrier helps to suppress
trap formation. Our results enable the fabrication of damage-resistant and thus
low-noise devices. | 1305.6701v1 |
2013-06-29 | Coexistence of Half-Metallic Itinerant Ferromagnetism with Local-Moment Antiferromagnetism in Ba{0.60}K{0.40}Mn2As2 | Magnetization, nuclear magnetic resonance, high-resolution x-ray diffraction
and magnetic field-dependent neutron diffraction measurements reveal a novel
magnetic ground state of Ba{0.60}K{0.40}Mn2As2 in which itinerant
ferromagnetism (FM) below a Curie temperature TC = 100 K arising from the doped
conduction holes coexists with collinear antiferromagnetism (AFM) of the Mn
local moments that order below a Neel temperature TN = 480 K. The FM ordered
moments are aligned in the tetragonal ab-plane and are orthogonal to the
AFM-ordered Mn moments that are aligned along the c-axis. The magnitude and
nature of the low-T FM ordered moment correspond to complete polarization of
the doped-hole spins (half-metallic itinerant FM) as deduced from magnetization
and ab-plane electrical resistivity measurements. | 1307.0091v2 |
2013-07-22 | Dimensional-Crossover-Driven Mott Insulators in SrVO3 Ultrathin Films | High-quality epitaxial SrVO3 (SVO) thin films of various thicknesses were
grown on (001)-oriented LSAT substrates by pulsed electron-beam deposition
technique. Thick SVO films (~25 nm) exhibited metallic behavior with the
electrical resistivity following the T2 law corresponding to a Fermi liquid
system. We observed a temperature driven metal-insulator transition (MIT) in
SVO ultrathin films with thicknesses below 6.5 nm, the transition temperature
TMIT was found to be at 50 K for the 6.5 nm film, 120 K for the 5.7 nm film and
205 K for the 3 nm film. The emergence of the observed MIT can be attributed to
the dimensional crossover from a three-dimensional metal to a two-dimensional
Mott insulator, as the resulting reduction in the effective bandwidth W opens a
band gap at the Fermi level. The magneto-transport study of the SVO ultrathin
films also confirmed the observed MIT is due to the electron-electron
interactions other than localization. | 1307.5819v2 |
2013-07-26 | Electron-phonon coupling in cuprate and iron-based superconductors revealed by Raman scattering | Electron-phonon coupling (EPC) is one of the most common and fundamental
interactions in solids. It not only dominates many basic dynamic processes like
resistivity, thermal conductivity etc, but also provides the pairing glue in
conventional superconductors. But in high-temperature superconductors (HTSC),
it is still controversial whether or not EPC is in favor of paring. Despite the
controversies, many experiments have provided clear evidence for EPC in HTSC.
In this paper, we briefly review EPC in cuprate and iron-based superconducting
systems revealed by Raman scattering. We introduce how to extract the coupling
information through phonon lineshape. Then we discuss the strength of EPC in
different HTSC systems and possible factors affecting the strength. The
comparative study between Raman phonon theories and experiments allows us to
gain insight into some crucial electronic properties, especially
superconductivity. Finally we summarize and compare EPC in the two existing
HTSC systems, and discuss what role it may play in HTSC. | 1307.6972v1 |
2014-02-10 | Evolution and defect analysis of vertical graphene nanosheets | We report catalyst-free direct synthesis of vertical graphene nanosheets
(VGNs) on SiO2/Si and quartz substrates using microwave electron cyclotron
resonance - plasma enhanced chemical vapor deposition. The evolution of VGNs is
studied systematically at different growth stages. Raman analysis as a function
of growth time reveals that two different disorder-induced competing mechanisms
contributing to the defect band intensity. The VGNs grown on SiO2/Si substrates
predominantly consists of both vacancy-like and hopping defects. On the other
hand, the VGNs grown on quartz substrates contain mainly boundary-like defects.
XPS studies also corroborate Raman analysis in terms of defect density and
vacancy-like defects for the VGNs grown on SiO2/Si substrates. Moreover, the
grown VGNs exhibit a high optical transmittance from 95 to 78 % at 550 nm and
the sheet resistance varies from 30 to 2.17 kohms/square depending on growth
time. | 1402.2074v3 |
2014-02-19 | Phase Diagram of Micron-Size Bridges of SrTiO$_3/$LaAlO$_3$ Interface: Link Between Multiple Band Structure and Superconductivity | The rich phase diagram of the two dimensional electron gas (2DEG) at the
\STO/\LAO interface is probed using Hall and longitudinal resistivity. Thanks
to a special bridge design we are able to tune through the superconducting
transition temperature T$_c$ and to mute superconductivity by either adding or
removing carriers in a gate bias range of a few volts. Hall signal measurements
pinpoint the onset of population of a second mobile band right at the carrier
concentration where maximum superconducting T$_c$ and critical field H$_c$
occur. These results emphasize the advantages of our design, which can be
applied to many other two dimensional systems assembled on top of a dielectric
substrate with high permittivity. | 1402.4646v1 |
2014-03-04 | Effect of modulations of doping and strain on the electron transport in monolayer MoS_2 | The doping and strain effects on the electron transport of monolayer MoS_2
are systematically investigated using the first-principles calculations with
Boltzmann transport theory. We estimate the mobility has a maximum 275
cm^2/(Vs) in the low doping level under the strain-free condition. The applying
a small strain (3%) can improve the maximum mobility to 1150 cm^2/(Vs) and the
strain effect is more significant in the high doping level. We demonstrate that
the electric resistance mainly due to the electron transition between K and Q
valleys scattered by the M momentum phonons. However, the strain can
effectively suppress this type of electron-phonon coupling by changing the
energy difference between the K and Q valleys. This sensitivity of mobility to
the external strain may direct the improving electron transport of MoS_2. | 1403.0695v2 |
2014-03-29 | Two components for one resistivity in LaVO3/SrTiO3 heterostructures | A series of 100 nm LaVO3 thin films have been synthesized on (001)-oriented
SrTiO3 substrates using the pulsed laser deposition technique, and the effects
of growth temperature are analyzed. Transport properties reveal a large
electronic mobility and a non-linear Hall effect at low temperature. In
addition, a cross-over from a semiconducting state at high-temperature to a
metallic state at low-temperature is observed, with a clear enhancement of the
metallic character as the growth temperature increases. Optical absorption
measurements combined with the two-bands analysis of the Hall effect show that
the metallicity is induced by the diffusion of oxygen vacancies in the SrTiO3
substrate. These results allow to understand that the film/substrate
heterostructure behaves as an original semiconducting-metallic parallel
resistor, and electronic transport properties are consistently explained. | 1403.7648v2 |
2014-04-07 | Directional pinning and anisotropy in YBa2Cu3O7-x with BaZrO3 nanorods: intrinsic and nanorods-induced anisotropy | We present a study of the anisotropic vortex parameters as obtained from
measurements of the microwave complex resistivity in the vortex state with a
tilted applied magnetic field in YBa2Cu3O7-x thin films with BaZrO3 nanorods.
We present the angular dependence of the vortex viscosity $\eta$, the pinning
constant k_p and the upper limit for the creep factor \chi_M. We show that the
directional effect of the nanorods is absent in \eta, which is dictated by the
mass anisotropy \gamma. By contrast, pinning-mediated properties are strongly
affected by the nanorods. It is significant that the pinning and creep affected
by the nanorods is detectable also at our very high operating frequency, which
implies very short-range displacements of the vortices from their equilibrium
position. | 1404.1670v1 |
2014-05-07 | Anisotropic giant magnetoresistance in NbSb2 | The extremely large transverse magnetoreistance (the magnetoresistant ratio
$\sim 1.3\times10^5\%$ in 2 K and 9 T field, and $4.3\times 10^6\%$ in 0.4 K
and 32 T field, without saturation), and the metal-semiconductor crossover
induced by magnetic field, are reported in NbSb$_2$ single crystal with
electric current parallel to the $b$-axis. The metal-semiconductor crossover is
preserved when the current is along the $ac$-plane but the magnetoresistant
ratio is significantly suppressed. The sign reversal of the Hall resistivity in
the field close to the crossover point, and the electronic structure
calculation reveals the coexistence of a small number of holes with very high
mobility and a large number of electrons with low mobility. These effects are
attributed to the change of the Fermi surface induced by the magnetic field. | 1405.1719v2 |
2014-06-05 | Current transport and thermoelectric properties of very high power factor Fe3O4 / SiO2 / p-type Si (001) devices | The current transport and thermoelectric properties of Fe3O4 / SiO2 / p-type
Si(001) heterostructures with Fe3O4 thicknesses of 150, 200, and 350 nm have
been investigated between 100 and 300 K. We observe a sharp drop of the
in-plane resistivity at 200K due to the onset of conduction along the Si / SiO2
interface related to tunneling of electrons from the Fe3O4 into the
accumulation layer of holes at the Si / SiO2 interface, whose existence was
confirmed by capacitance-voltage measurements and a two band analysis of the
Hall effect. This is accompanied by a large increase of the Seebeck coefficient
reaching +1000 {\mu}V/K at 300K that is related to holes in the p-type Si(001)
and gives a power factor of 70 mW/K2m when the Fe3O4 layer thickness is reduced
down to 150 nm. We show that most of the current flows in the Fe3O4 layer at
300 K, while the Fe3O4 / SiO2 / p-type Si(001) heterostructures behave like
tunneling p-n junctions in the transverse direction. | 1406.1282v1 |
2014-06-20 | Anomalous Hall Effect in Ge(1-x-y)Pb(x)Mn(y)Te Composite System | The purpose of this study was to investigate the magnetotransport properties
of the Ge(0.743)Pb(0.183)Mn(0.074)Te mixed crystal. The results of
magnetization measurements indicated that the compound is a spin-glass-like
diluted magnetic semiconductor with critical temperature TSG = 97.5 K.
Nanoclusters in the sample are observed. Both, matrix and clusters are
magnetically active. Resistivity as a function of temperature has a minimum at
30 K. Below the minimum a variable-range hopping is observed, while above the
minimum a metallic-like behavior occurs. The crystal has high hole
concentration, p = 6.6E20 cm-3, temperature-independent. Magnetoresistance
amplitude changes from -0.78 to 1.18% with increase of temperature. In the
magnetotransport measurements we observed the anomalous Hall effect (AHE) with
hysteresis loops. Calculated AHE coefficient, RS = 2.0E6 m3/C, is temperature
independent. The analysis indicates the extrinsic skew scattering mechanism to
be the main physical mechanism responsible for AHE in
Ge(0.743)Pb(0.183)Mn(0.074)Te alloy. | 1406.5294v1 |
2014-07-22 | Universal scaling of the critical temperature for thin films near the superconducting-to-insulating transition | Thin superconducting films form a unique platform for geometrically-confined,
strongly-interacting electrons. They allow an inherent competition between
disorder and superconductivity, which in turn enables the intriguing
superconducting-to-insulator transition and believed to facilitate the
comprehension of high-Tc superconductivity. Furthermore, understanding thin
film superconductivity is technologically essential e.g. for photo-detectors,
and quantum-computers. Consequently, the absence of an established universal
relationships between critical temperature ($T_c$), film thickness ($d$) and
sheet resistance ($R_s$) hinders both our understanding of the onset of the
superconductivity and the development of miniaturised superconducting devices.
We report that in thin films, superconductivity scales as $d^.$$T_c(R_s)$. We
demonstrated this scaling by analysing the data published over the past 46
years for different materials (and facilitated this database for further
analysis). Moreover, we experimentally confirmed the discovered scaling for NbN
films, quantified it with a power law, explored its possible origin and
demonstrated its usefulness for superconducting film-based devices. | 1407.5945v2 |
2014-09-18 | Electrical and optical properties of ITO thin films prepared by DC magnetron sputtering for low-emitting coatings | Optimized DC magnetron sputtering system for the deposition of transparent
conductive oxides (TCOs), such indium tin oxide (ITO) on glass substrate has
been applied in order to achieve low-emitting (low-e) transparent coatings. To
obtain the concerned electrical resistance and high infrared reflection, first
the effect of applied sputtering power then oxygen flow on the properties of
films have been investigated. The other depositions parameters are kept
constant. Film deposition at at temperature 400 degree of Celsius in oxygen
flow of 3 Standard Cubic Centimeters per Minute results in transparent and
infrared reflecting coatings. Under this condition the highest attained average
reflectance in the infrared is ({\lambda}=3-25 micron) 89.5% (lowest emittance
equals to less than 11%), whereas transparency in the visible is 85%
approximately. Plasma wavelength and carrier concentration was measured. | 1409.5293v1 |
2014-09-25 | Characterisation of Glass Electrodes and RPC Detectors for $INO-ICAL$ Experiment | India-based Neutrino Observatory (INO) is a planned neutrino experiment to be
build up in southern part of India.The INO observatory will host a 51 kton Iron
Calorimeter (ICAL) detector to detect atmospheric neutrinos. Resistive Plate
Chamber (RPC) has been chosen as the active detector element for the ICAL
experiment. The ICAL experiment will consist of about 28,000 RPC's of dimension
$2~m\times 2~m$, divided into three modules. The experiment is planned to take
data at least for 20 years from its start date. Due to the large number of RPC
needed for ICAL experiment and the long lifetime of the experiment, it is
necessary to carry out detailed $R\&D$ to optimise each and every parameter of
the detector performance. We report on the performance studies carried out on
the RPC's made with these electrodes, and finally compare the detector
performance with that of the material properties to optimise the detector
parameters. | 1409.7184v1 |
2014-11-24 | Thermoelectric power of bulk black-phosphorus | The potential of bulk black-phosphorus for thermoelectric applications has
been experimentally studied. The Seebeck Coefficient (S) has been measured in
the temperature range from 300 K to 385 K, finding a value of S = +335 +- 10
uV/K at room temperature (indicating a naturally occurring p-type
conductivity). S increases with temperature, as expected for p-type
semiconductors, which can be attributed to an increase of the charge carrier
density. The electrical resistance drops up to a 40 % while heating in the
studied temperature range. As a consequence, the power factor at 385 K is 2.7
times higher than that at room temperature. This work demonstrates the
feasibility of black-phosphorus in thermoelectric applications, such as thermal
energy scavenging, which typically require devices with high performance at
temperatures above room temperature. | 1411.6468v2 |
2014-11-26 | Towards high mobility InSb nanowire devices | We study the low-temperature electron mobility of InSb nanowires. We extract
the mobility at 4.2 Kelvin by means of field effect transport measurements
using a model consisting of a nanowire-transistor with contact resistances.
This model enables an accurate extraction of device parameters, thereby
allowing for a systematic study of the nanowire mobility. We identify factors
affecting the mobility, and after optimization obtain a field effect mobility
of $\sim2.5\mathbin{\times}10^4$ cm$^2$/Vs. We further demonstrate the
reproducibility of these mobility values which are among the highest reported
for nanowires. Our investigations indicate that the mobility is currently
limited by adsorption of molecules to the nanowire surface and/or the
substrate. | 1411.7285v2 |
2014-12-22 | Large anisotropic thermal conductivity of intrinsically two-dimensional metallic oxide PdCoO$_2$ | The highly conductive layered metallic oxide \pdcoo{} is a near-perfect
analogue to an alkali metal in two dimensions. It is distinguished from other
two-dimensional electron systems where the Fermi surface does not reach the
Brillouin zone boundary by a high planar electron density exceeding $10^{15}$
cm$^{-2}$. The simple single-band quasi-2D electronic structure results in
strongly anisotropic transport properties and limits the effectiveness of
electron-phonon scattering. Measurements on single crystals in the temperature
range from 10-300K show that the thermal conductivity is much more weakly
anisotropic than the electrical resistivity, as a result of significant phonon
heat transport. The in-plane thermoelectric power is linear in temperature at
300\,K and displays a purity-dependent peak around 50K. Given the extreme
simplicity of the band-structure, it is possible to identify this peak with
phonon drag driven by normal electron-phonon scattering processes. | 1412.6919v1 |
2015-02-06 | Anomalous Pressure Dependence of magnetic Ordering Temperature in Tb to 141 GPa: Comparison with Gd and Dy | In previous studies the pressure dependence of the magnetic ordering
temperature $T_{\text{o}}$ of Dy was found to exhibit a sharp increase above
its volume collapse pressure of 73 GPa, appearing to reach temperatures well
above ambient at 157 GPa. In a search for a second such lanthanide, electrical
resistivity measurements were carried out on neighboring Tb to 141 GPa over the
temperature range 3.8 - 295 K. Below Tb's volume collapse pressure of 53 GPa,
the pressure dependence $T_{\text{o}}(P)$ mirrors that of both Dy and Gd.
However, at higher pressures $T_{\text{o}}(P)$ for Tb becomes highly anomalous.
This result, together with the very strong suppression of superconductivity by
dilute Tb ions in Y, suggests that extreme pressure transports Tb into an
unconventional magnetic state with an anomalously high magnetic ordering
temperature. | 1502.01785v1 |
2015-02-07 | Interband Tunneling for Hole Injection in III-Nitride Ultra-violet Emitters | Ultra-violet emitters have several applications in the areas of sensing,
water purification, and data storage. While the III-Nitride semiconductor
system has the band gap region necessary for ultraviolet emission, achieving
efficient ultraviolet solid state emitters remains a challenge due to the low
p-type conductivity and high contact resistance in wide band gap AlGaN-based
ultra-violet light emitters. In this work, we show that efficient interband
tunneling can be used for non-equilibrium injection of holes into ultraviolet
emitters. Polarization-engineered tunnel junctions were used to enhance
tunneling probability by several orders of magnitude over a PN homojunction,
leading to highly efficient tunnel injection of holes to ultraviolet light
emitters. This demonstration of efficient interband tunneling introduces a new
paradigm for design of ultra-violet light emitting diodes and diode lasers, and
enables higher efficiency and lower cost ultra-violet emitters. | 1502.02080v1 |
2015-02-27 | Quantum oscillations, thermoelectric coefficients and the Fermi surface of semi-metallic WTe2 | We present a study of angle-resolved quantum oscillations of electric and
thermoelectric transport coefficients in semi-metallic WTe$_{2}$, which has the
particularity of displaying a large B$^{2}$ magneto-resistance. The Fermi
surface consists of two pairs of electron-like and hole-like pockets of equal
volumes in a "Russian doll" structure. Carrier density, Fermi energy, mobility
and the mean-free-path of the system are quantified. An additional frequency is
observed above a threshold field and attributed to magnetic breakdown across
two orbits. In contrast to all other dilute metals, the Nernst signal remains
linear in magnetic field even in the high-field ($\omega_c\tau \gg 1$) regime.
Surprisingly, none of the pockets extend across the c-axis of the first
Brillouin zone, making the system a three-dimensional metal with moderate
anisotropy in Fermi velocity yet a large anisotropy in mean-free-path. | 1502.07797v2 |
2015-05-19 | Energy Exchange between Phononic and Electronic Subsystem Governing The Nonlinear Conduction in DCNQI$_2$Cu | We present a dynamical study on the nonlinear conduction behaviour in the
commensurate charge-density-wave phase of the quasi-one-dimensional conductor
DCNQI$_2$Cu below 75 K. We can accurately simulate magnitude and
time-dependence of the measured conductivity in response to large voltage
pulses by accounting for the energy exchange between the phononic and
electronic subsystems by means of an electrothermal model. Our simulations
reveal a distinct non-equilibrium population of optical phonon states with an
average energy of E$_{ph}$ = 19 meV being half the activation energy of about
$\Delta$E$_a$ = 39 meV observed in DC resistivity measurements. By inelastic
scattering, this hot optical phonon bath generates additional charge-carrying
excitations thus providing a multiplication effect while energy transferred to
the acoustic phonons is dissipated out of the system via heat conduction.
Therefore, in high electric fields a preferred interaction of charge-carrying
excitations with optical phonons compared to acoustic phonon modes is
considered to be responsible for the nonlinear conduction effects observed in
DCNQI$_2$Cu. | 1505.04907v2 |
2015-08-19 | Argument on superconductivity pairing mechanism from cobalt impurity doping in FeSe: spin ($s_{\pm}$) or orbital ($s_{++}$) fluctuation | In high-superconducting transition temperature ($T_{\rm c}$) iron-based
superconductors, interband sign reversal ($s_{\rm \pm}$) and sign preserving
($s_{\rm ++}$) $s$-wave superconducting states have been primarily discussed as
the plausible superconducting mechanism. We study Co impurity scattering
effects on the superconductivity in order to achieve an important clue on the
pairing mechanism using single crystal Fe$_{1-x}$Co$_x$Se and depict a phase
diagram of a FeSe system. Both superconductivity and structural transition /
orbital order are suppressed by the Co replacement on the Fe sites and
disappear above $x$ = 0.036. These correlated suppressions represent a common
background physics behind these physical phenomena in the multiband Fermi
surfaces of FeSe. By comparing experimental data and theories so far proposed,
the suppression of $T_{\rm c}$ against the residual resistivity is shown to be
much weaker than that predicted in the case of a general sign reversal and a
full gap $s_{\pm}$ models. The origin of the superconducting paring in FeSe is
discussed in terms of its multiband electronic structure. | 1508.04605v2 |
2015-08-19 | Electric field control of spin lifetimes in Nb-SrTiO$_3$ by spin-orbit fields | We show electric field control of the spin accumulation at the interface of
the oxide semiconductor Nb-SrTiO$_{3}$ with Co/AlO$_{x}$ spin injection
contacts at room temperature. The in-plane spin lifetime $\tau_\parallel$ as
well as the ratio of the out-of-plane to in-plane spin lifetime
$\tau_\perp/\tau_\parallel$ is manipulated by the built-in electric field at
the semiconductor surface, without any additional gate contact. The origin of
this manipulation is attributed to Rashba Spin-Orbit Fields (SOFs) at the
Nb-SrTiO$_3$ surface and shown to be consistent with theoretical model
calculations based on SOF spin flip scattering. Additionally, the junction can
be set in a high or low resistance state, leading to a non-volatile control of
$\tau_\perp/\tau_\parallel$, consistent with the manipulation of the Rashba SOF
strength. Such room temperature electric field control over the spin state is
essential for developing energy-efficient spintronic devices and shows promise
for complex oxide based (spin)electronics | 1508.04649v1 |
2015-09-13 | Effects of Cr substitution on the magnetic and transport properties and electronic states of SrRuO3 epitaxial thin films | The effect of Cr substitution in a SrRuO3 epitaxial thin film on SrTiO3
substrate was investigated by measuring the magnetic and transport properties
and the electronic states. The ferromagnetic transition temperature of the
SrRu0.9Cr0.1O3 film (166 K) was higher than that of the SrRuO3 film (147 K).
Resonant photoemission spectroscopy experimentally revealed that the Cr 3dt2g
orbital is hybridized with the Ru 4dt2g orbital in the SrRu0.9Cr0.1O3 film,
supporting the assumption that the enhancement of the ferromagnetic transition
temperature through Cr substitution stems from the widening of energy bands due
to the hybridization of Cr 3dt2g and Ru 4dt2g orbitals. Furthermore, we found
that the Hall resistivity of the SrRu0.9Cr0.1O3 film at low temperature is not
a linear function of magnetic field in the high-field region where the
out-of-plane magnetization was saturated; this result suggests that the
SrRu0.9Cr0.1O3 film undergoes a structural transition at low temperature
accompanied with the modulation of the Fermi surface. | 1509.03804v1 |
2015-10-22 | Anomalous transport and thermoelectric performances of CuAgSe compounds | The copper silver selenide has two phases: the low-temperature semimetal
phase ({\alpha}-CuAgSe) and high-temperature phonon-glass superionic phase
(\b{eta}-CuAgSe). In this work, the electric transport and thermoelectric
properties of the two phases are investigated. It is revealed that the
\b{eta}-CuAgSe is a p-type semiconductor and exhibits low thermal conductivity
while the {\alpha}-CuAgSe shows metallic conduction with dominant n-type
carriers and low electrical resistivity. The thermoelectric figure of merit zT
of the polycrystalline \b{eta}-CuAgSe at 623 K is ~0.95, suggesting that
superionic CuAgSe can be a promising thermoelectric candidate in the
intermediate temperature range. | 1510.06616v1 |
2015-11-12 | Experimental and numerical analysis of tribological behavior of CrAl(Si)N films during Scratch | Scratch sliding tests with a ZrO2 ball and CrAlSiN films with different Si
content were conducted due to CrAlSiN films having high hardness and good wear
resistance. After up to 6000 cycles the specimens were analyzed by Scanning
Electron Microscopy. The friction coefficient of CrAlSiN was lower than that of
CrAlN film.A corresponding three-dimensional finite element model was
constructed with the help of the ABAQUS to describe the mechanical response
during scratch. A comparison of experimental and computational results revealed
that the small elastic deformation took place in the films and substrates;the
deformation friction coefficient was negligible in comparison with the Coulomb
friction coefficient;and with increasing Young's modulus, the stress
concentration was more obvious in CrAlSiN than in CrAlN. | 1511.04092v2 |
2015-12-27 | Room-temperature paramagnetoelectric effect in magnetoelectric multiferroics Pb(Fe1/2Nb1/2)O3 and its solid solution with PbTiO3 | We have observed the magnetoelectric response at room temperature and above
in high-resistive ceramics made of multiferroic Pb(Fe1/2Nb1/2)O3 (PFN) and
PFN-based solid solution 0.91PFN-0.09PbTiO3 (PFN-PT). The value of the
paramagnetoelectric (PME) coefficient shows a pronounced maximum near the
ferroelectric-to-paraelectric phase transition temperature, T_C, and then
decreases sharply to zero for T>T_C. The maximal PME coefficient in PFN is
about 4x10(-18) s/A. The theoretical description of the PME effect, within the
framework of a Landau theory of phase transitions allowing for realistic
temperature dependences of spontaneous polarization, dielectric and magnetic
susceptibilities, qualitatively reproduces well the temperature dependence of
the PME coefficient. In particular, the Landau theory predicts the significant
increase of the PME effect at low temperatures and near the temperature of the
paraelectric-to-ferroelectric phase transition, since the PME coefficient is
equal to the product of the spontaneous polarization, dielectric permittivity,
square of magnetic susceptibility and the coefficient quantifying the strength
of the biquadratic magnetoelectric coupling | 1512.08217v1 |
2015-12-31 | Intrinsic Nanotwin Effect on Thermal Boundary Conductance in Bulk and Single-Nanowire Twinning Superlattices | Coherent twin boundaries form periodic lamellar twinning in a wide variety of
semiconductor nanowires, and are often viewed as near-perfect interfaces with
reduced phonon and electron scattering behaviors. Such unique characteristics
are of practical interest for high-performance thermoelectrics and
optoelectronics; however, insufficient understanding of twin-size effects on
thermal boundary resistance poses significant limitations for potential
applications. Here, using atomistic simulations and ab-initio calculations, we
report direct computational observations showing a crossover from diffuse
interface scattering to superlattice-like behavior for thermal transport across
nanoscale twin boundaries present in prototypical bulk and nanowire Si
examples. Intrinsic interface scattering is identified for twin periods larger
than or equal to 22.6 nm, but also vanishes below this size to be replaced by
ultrahigh Kapitza thermal conductances. Detailed analysis of vibrational modes
shows that modeling twin boundaries as atomically-thin 6H-Si layers, rather
than phonon scattering interfaces, provides an accurate description of
effective cross-plane and in-plane thermal conductivities in twinning
superlattices, as a function of the twin period thickness. | 1512.09357v2 |
2016-02-20 | Investigation of the electron-phonon interaction in $N{{b}_{3}}Sn$ with the aid of microcontacts | The method of microcontact spectroscopy in the superconducting state was used
to investigate weak nonlinearities of the current-voltage characteristics of
point contacts made of $N{{b}_{3}}Sn$ single crystals. The nature of the
spectrum of the electron-phonon interaction was found to vary considerably from
contact to contact, indicating considerable deviations of the composition of
the surface of $N{{b}_{3}}Sn$ from stoichiometry. A correlation was established
between the nature of the spectrum and the magnitude of the gap singularities
of the current-voltage characteristics. In the case of "dirty" high-resistance
contacts with strong gap singularities the microcontact spectra were reasonably
reproducible, which made it possible to relate them sufficiently closely to the
microcontact function of the electron-phonon interaction in the bulk material.
It was found that microcontact spectroscopy of this interaction was possible in
the superconducting state not only in dirty $S-c-S$ contacts, but also in dirty
$S-c-N$ contacts. | 1602.06398v1 |
2016-04-11 | Single crystal growth of CeTAl$_3$ (T = Cu, Ag, Au, Pd and Pt) | We report single crystal growth of the series of CeTAl$_3$ compounds with T =
Cu, Ag, Au, Pd and Pt by means of optical float zoning. High crystalline
quality was confirmed in a thorough characterization process. With the
exception of CeAgAl$_3$, all compounds crystallize in the non-centrosymmetric
tetragonal BaNiSn$_{3}$ structure (space group: I4mm, No. 107), whereas
CeAgAl$_3$ adopts the related orthorhombic PbSbO$_2$Cl structure (Cmcm, No.
63). An attempt to grow CeNiAl$_3$ resulted in the composition CeNi$_2$Al$_5$.
Low temperature resistivity measurements down to $\sim$0.1K did not reveal
evidence suggestive of magnetic order in CePtAl$_3$ and CePdAl$_3$. In
contrast, CeAuAl$_3$, CeCuAl$_3$ and CeAgAl$_3$ display signatures of magnetic
transitions at 1.3K, 2.1K and 3.2K, respectively. This is consistent with
previous reports of antiferromagnetic order in CeAuAl$_3$, and CeCuAl$_3$ as
well as ferromagnetism in CeAgAl$_3$, respectively. | 1604.03146v1 |
2016-04-14 | Large magnetothermopower and Fermi surface reconstruction in Sb$_2$Te$_2$Se | We report the magnetoresistance, magnetothermopower and quantum oscillation
study of Sb$_2$Te$_2$Se single crystal. The in-plane transverse
magnetoresistance exhibits a crossover at a critical field $B^*$ from
semiclassical weak-field $B^2$ dependence to the high-field unsaturated linear
magnetoresistance which persists up to the room temperature. The
low-temperature Seebeck coefficient is negative in zero field contrary to the
positive Hall resistivity, indicating the multiband effect. The magnetic field
induced the sign reversion of the Seebeck coefficient between 2 K and 150 K, .
The quantum oscillation of crystals reveals the quasi-two-dimensional
(quasi-2D) Fermi surface. These effects are possibly attributed to the large
Fermi surface which touches Brillouin zone boundary to becomes quasi-2D and the
variation in the chemical potential induced by the magnetic field. | 1604.04281v1 |
2016-04-15 | Electronic Structure Descriptor for Discovery of Narrow-Band Red-Emitting Phosphors | Narrow-band red-emitting phosphors are a critical component in
phosphor-converted light-emitting diodes for highly efficient
illumination-grade lighting. In this work, we report the discovery of a
quantitative descriptor for narrow-band Eu2+-activated emission identified
through a comparison of the electronic structure of known narrow-band and
broad-band phosphors. We find that a narrow emission bandwidth is characterized
by a large splitting of more than 0.1 eV between the two highest Eu2+ 4f7
bands. By incorporating this descriptor in a high throughput first principles
screening of 2,259 nitride compounds, we identify five promising new nitride
hosts for Eu2+-activated red-emitting phosphors that are predicted to exhibit
good chemical stability, thermal quenching resistance and quantum efficiency,
as well as narrow-band emission. Our findings provide important insights into
the emission characteristics of rare-earth activators in phosphor hosts, and a
general strategy to the discovery of phosphors with a desired emission peak and
bandwidth. | 1604.04581v1 |
2016-04-18 | Superconductivity in undoped CaFe2As2 single crystals | Single crystals of undoped CaFe2As2 were grown by a FeAs self-flux method,
and the crystals were quenched in ice-water rapidly after high temperature
growth. The quenched crystal undergoes a collapsed tetragonal structural phase
transition around 80 K revealed by the temperature dependent X-ray diffraction
measurements. Superconductivity below 25 K was observed in the collapsed phase
by resistivity and magnetization measurements. The isothermal magnetization
curve measured at 2 K indicates that this is a typical type-II superconductor.
For comparison, we systematically characterized the properties of the furnace
cooled, quenched, and post-annealed single crystals, and found strong internal
crystallographic strain existing in the quenched samples, which is the key for
the occurrence of superconductivity in the undoped CaFe2As2 single crystals. | 1604.04964v1 |
2016-04-22 | Multiband behavior and non-metallic low-temperature state of K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$ | We report evidence for multiband transport and an insulating low-temperature
normal state in superconducting K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$ with
$T_{c}\approx 20$ K. The temperature-dependent upper critical field, $H_{c2}$,
is well described by a two-band BCS model. The normal-state resistance,
accessible at low temperatures only in pulsed magnetic fields, shows an
insulating logarithmic temperature dependence as $T \rightarrow 0$ after
superconductivity is suppressed. This is similar as for high-$T_{c}$ copper
oxides and granular type-I superconductors, suggesting that the
superconductor-insulator transition observed in high magnetic fields is related
to intrinsic nanoscale phase separation. | 1604.06793v1 |
2016-05-30 | High-field electron transport in bulk ZnO | Current-voltage dependence is measured in (Ga,Sb)-doped ZnO up to 150 kV/cm
electric fields. A channel temperature is controlled by applying relatively
short (few ns) voltage pulses to two-terminal samples. The dependence of
electron drift velocity on electron density ranging from 1.42$\times$10$^{17}$
cm$^{-3}$ to 1.3$\times$10$^{20}$ cm$^{-3}$ at a given electric field is
deduced after estimation of the sample contact resistance and the Hall electron
mobility. Manifestation of the highest electron drift velocity up to
$\sim$1.5$\times$10$^{7}$ cm/s is estimated for electron density of
1.42$\times$10$^{17}$ cm$^{-3}$ and is in agreement with Monte Carlo simulation
when hot-phonon lifetime is below 1 ps. A local drift velocity maximum is
observed at $\sim$1.1$\times$10$^{19}$ cm$^{-3}$ and is in agreement with
ultra-fast hot phonon decay. | 1605.09117v1 |
2016-06-27 | Hafnium carbide formation in oxygen deficient hafnium oxide thin films | On highly oxygen deficient thin films of hafnium oxide (hafnia, HfO$_{2-x}$)
contaminated with adsorbates of carbon oxides, the formation of hafnium carbide
(HfC$_x$) at the surface during vacuum annealing at temperatures as low as 600
{\deg}C is reported. Using X-ray photoelectron spectroscopy the evolution of
the HfC$_x$ surface layer related to a transformation from insulating into
metallic state is monitored in situ. In contrast, for fully stoichiometric
HfO$_2$ thin films prepared and measured under identical conditions, the
formation of HfC$_x$ was not detectable suggesting that the enhanced adsorption
of carbon oxides on oxygen deficient films provides a carbon source for the
carbide formation. This shows that a high concentration of oxygen vacancies in
carbon contaminated hafnia lowers considerably the formation energy of hafnium
carbide. Thus, the presence of a sufficient amount of residual carbon in
resistive random access memory devices might lead to a similar carbide
formation within the conducting filaments due to Joule heating. | 1606.08227v1 |
2016-06-29 | Improved model for the thermal conductivity of binary metallic systems | We extended and corrected Mott's two-band model for the
composition-dependence of thermal and electrical conductivity in binary metal
alloys based on high-throughput time-domain thermoreflectance (TDTR)
measurements on diffusion multiples and scatterer-density calculations from
first principles. Examining PdAg, PtRh, AuAg, AuCu, PdCu, PdPt, and NiRh binary
alloys, we found that the nature of the two dominant scatterer-bands considered
in the Mott model changes with the alloys, and should be interpreted as a
combination of the dominant element-specific s- and/or d-orbitals. Using
calculated orbital and element-resolved density-of-states values calculated
with density functional theory as input, we determined the correct orbital mix
that dominates electron scattering for all examined alloys and find excellent
agreement between fitted models and experiments. The proposed description of
the composition dependence of the resistivity can be readily implemented into
the CALPHAD framework. | 1606.09287v2 |
2016-06-30 | Superconductivity at 5.5 K in Nb2PdSe5 compound | We report superconductivity in as synthesized Nb2PdSe5, which is similar to
recently discovered Nb2PdS5 compound having very high upper critical field,
clearly above the Pauli paramagnetic limit [Sci. Rep. 3, 1446 (2013)]. A bulk
polycrystalline Nb2PdSe5 sample is synthesized by solid state reaction route in
phase pure structure. The structural characterization has been done by X ray
diffraction, followed by Rietveld refinements, which revealed that Nb2PdSe5
sample is crystallized in monoclinic structure with in space group C2/m.
Structural analysis revealed the formation of sharing of one dimensional PdSe2
chains. Electrical and magnetic measurements confirmed superconductivity in
Nb2PdSe5 compound at 5.5K. Detailed magneto-resistance results, exhibited the
value of upper critical field to be around 8.2Tesla. The estimated Hc2(0) is
within Pauli Paramagnetic limit, which is unlike the Nb2PdS5. | 1606.09369v2 |
2016-08-01 | Phase Transition in IrTe$_2$ induced by spin-orbit coupling | IrTe$_2$ has been renewed as an interesting system showing competing
phenomenon between a questionable density-wave transition near 270 K followed
by superconductivity with doping of high atomic number materials. Higher atomic
numbers of Te and Ir supports strong spin-orbital coupling in this system.
Using dynamical mean field theory with LDA band structure I have introduced
Rashba spin orbit coupling in this system to get the interpretation for
anomalous resistivity and related transition in this system. While no
considerable changes are observed in DMFT results of Ir-5d band other than
orbital selective pseudogap pinned to Fermi level, Te-p band shows a van Hove
singularity at the Fermi level except low temperature. Finally I discuss the
implications of these results in theoretical understanding of ordering in
IrTe$_2$. | 1608.00467v1 |
2016-08-03 | Edge-induced Schottky barrier modulation at metal contacts to exfoliated molybdenum disulfide flakes | Ultrathin two-dimensional semiconductors obtained from layered
transition-metal dichalcogenides such as molybdenum disulfide (MoS2) are
promising for ultimately scaled transistors beyond Si. Although the shortening
of the semiconductor channel is widely studied, the narrowing of the channel,
which should also be important for scaling down the transistor, has been
examined to a lesser degree thus far. In this study, the impact of narrowing on
mechanically exfoliated MoS2 flakes was investigated according to the
channel-width-dependent Schottky barrier heights at Cr/Au contacts. Narrower
channels were found to possess a higher Schottky barrier height, which is
ascribed to the edge-induced band bending in MoS2. The higher barrier heights
degrade the transistor performance as a higher electrode-contact resistance.
Theoretical analyses based on Poisson's equation showed that the edge-induced
effect can be alleviated by a high dopant impurity concentration, but this
strategy should be limited to channel widths of roughly 0.7 micrometers because
of the impurity-induced charge-carrier mobility degradation. Therefore, proper
termination of the dangling bonds at the edges should be necessary for
aggressive scaling with layered semiconductors. | 1608.01061v1 |
2016-08-23 | Chiral anomaly and longitudinal magnetotransport in type-II Weyl semimetals | In the presence of parallel electric and magnetic fields, the violation of
separate number conservation laws for the three dimensional left and right
handed Weyl fermions is known as the chiral anomaly. The recent discovery of
Weyl and Dirac semimetals has paved the way for experimentally testing the
effects of chiral anomaly via longitudinal magneto-transport measurements. More
recently, a type-II Weyl semimetal (WSM) phase has been proposed, where the
nodal points possess a finite density of states due to the touching between
electron- and hole- pockets. It has been suggested that the main difference
between the two types of WSMs (type-I and type-II) is that in the latter,
chiral anomaly and the associated longitudinal magneto-resistance are strongly
anisotropic, vanishing when the applied magnetic field is perpendicular to the
direction of tilt of Weyl fermion cones in a type-II WSM. We analyze chiral
anomaly in a type-II WSM in quasiclassical Boltzmann framework, and find that
the chiral anomaly induced positive longitudinal magneto-conductivity is
present along any arbitrary direction. | 1608.06625v2 |
2016-10-19 | Anomalous Thermal Diffusivity in Underdoped YBa$_2$Cu$_3$O$_{6+x}$ | We present local optical measurements of thermal diffusivity in the $ab$
plane of underdoped YBCO crystals. We find that the diffusivity anisotropy is
comparable to reported values of the electrical resistivity anisotropy,
suggesting that the anisotropies have the same origin. The anisotropy drops
sharply below the charge order transition. We interpret our results through a
strong electron-phonon scattering picture and find that both electronic and
phononic contributions to the diffusivity saturate a proposed bound. Our
results suggest that neither well-defined electron nor phonon quasiparticles
are present in this material. | 1610.05845v2 |
2016-10-22 | High performance THz emitters based on ferromagnetic/nonmagnetic heterostructures | We report a THz emitter with excellent performances based on nonmagnetic (NM)
and ferromagnetic (FM) heterostructures. The spin currents are first excited by
the femtosecond laser beam in the NM/FM bilayer, and then transient charge
currents are generated by inverse spin Hall effect, leading to THz emission out
of the structure. The broadband THz waves emitted from our film stacks have a
peak intensity exceeding 500 um thick ZnTe crystals (standard THz emitters).
Our device is insensitive to the polarization of an incident laser beam which
indicates the noise resistive feature. In contrast, the polarization of THz
waves is fully controllable by an external magnetic field. We have also
fabricated the devices on flexible substrates with a great performance, and
demonstrated that the devices can be driven by low power lasers. Together with
the low cost and mass productive sputtering growth method for the film stacks,
the proposed THz emitters can be readily applied to a wide range of THz
equipment. Our study also points towards an alternative approach to
characterize spintronic devices with NM/FM bilayers. | 1610.07020v1 |
2016-10-26 | S2DS: Physics-Based Compact Model for Circuit Simulation of Two-Dimensional Semiconductor Devices Including Non-Idealities | We present a physics-based compact model for two-dimensional (2D)
field-effect transistors (FETs) based on monolayer semiconductors such as MoS2.
A semi-classical transport approach is appropriate for the 2D channel, enabling
simplified analytical expressions for the drain current. In addition to
intrinsic FET behavior, the model includes contact resistance, traps and
impurities, quantum capacitance, fringing fields, high-field velocity
saturation and self-heating, the latter being found to play a strong role. The
model is calibrated with state-of-the-art experimental data for n- and p-type
2D-FETs, and it can be used to analyze device properties for sub-100 nm gate
lengths. Using the experimental fit, we demonstrate feasibility of circuit
simulations using properly scaled devices. The complete model is implemented in
SPICE-compatible Verilog-A, and a downloadable version is freely available on
the nanoHUB.org. | 1610.08489v2 |
2016-11-14 | Nonlinear Transport of Graphene in the Quantum Hall Regime | We have studied the breakdown of the integer quantum Hall (QH) effect with
fully broken symmetry, in an ultra-high mobility graphene device sandwiched
between two single crystal hexagonal boron nitride substrates. The evolution
and stabilities of the QH states are studied quantitatively through the
nonlinear transport with dc Hall voltage bias. The mechanism of the QH
breakdown in graphene and the movement of the Fermi energy with the electrical
Hall field are discussed. This is the first study in which the stabilities of
fully symmetry broken QH states are probed all together. Our results raise the
possibility that the v=6 states might be a better target for the quantum
resistance standard. | 1611.04221v1 |
2016-11-15 | Encapsulated Nanowires: Boosting Electronic Transport in Carbon Nanotubes | The electrical conductivity of metallic carbon nanotubes (CNTs) quickly
saturates with respect to bias voltage due to scattering from a large
population of optical phonons. Decay of these dominant scatterers in pristine
CNTs is too slow to offset an increased generation rate at high voltage bias.
We demonstrate from first principles that encapsulation of 1D atomic chains
within a single-walled CNT can enhance decay of "hot" phonons by providing
additional channels for thermalisation. Pacification of the phonon population
growth reduces electrical resistivity of metallic CNTs by 51% for an example
system with encapsulated beryllium. | 1611.04867v2 |
2016-12-06 | Enhanced Structural Stability and Photo Responsiveness of CH3NH3SnI3 Perovskite via Pressure-Induced Amorphization and Recrystallization | An organic-inorganic halide perovskite of CH3NH3SnI3 with significantly
improved structural stability is obtained via pressure-induced amorphization
and recrystallization. In situ high-pressure resistance measurements reveal an
increased electrical conductivity by 300% in the pressure-treated perovskite.
Photocurrent measurements also reveal a substantial enhancement in
visible-light responsiveness. The mechanism underlying the enhanced properties
is demonstrated to be associated with the improved structural stability. | 1612.01649v1 |
2016-12-20 | Enhanced Superconductivity in TiO Epitaxial Thin Films | Titanium oxides have many fascinating optical and electrical properties, such
as the superconductivity at 2.0 K in cubic titanium monoxide TiO
polycrystalline bulk. However, the lack of TiO single crystals or epitaxial
films has prevented systematic investigations on its superconductivity. Here,
we report the basic superconductivity characterizations of cubic TiO films
epitaxially grown on (0001)-oriented Al2O3 substrates. The magnetic and
electronic transport measurements confirmed that TiO is a type-II
superconductor and the record high Tc is about 7.4 K. The lower critical field
(Hc1) at 1.9 K, the extrapolated upper critical field Hc2(0) and coherence
length are about 18 Oe, 13.7 T and 4.9 nm, respectively. With increasing
pressure, the value of Tc shifts to lower temperature while the normal state
resistivity increases. Our results on the superconducting TiO films confirm the
strategy to achieve higher Tc in epitaxial films, which may be helpful for
finding more superconducting materials in various related systems. | 1612.06506v1 |
2017-02-02 | Two-temperature equations of state for d-band metals irradiated by femtosecond laser pulses | The cold curves for energy and pressure of Copper, Iron, and Tantalum were
obtained using methods of the density functional theory. We consider
hydrostatic and uniaxial deformations in the range from double compression of
the initial volume per atom to double stretching. The presence of allotropic
transformation from $\alpha$ - phase of Iron to the hexaferrum with the growth
of pressure is observed. In the case of hydrostatic deformations we also have
obtained analogous cold curves, but with non-zero electronic temperatures in
the range up to 5 eV. The similar volume and electronic temperature ranges have
been considered recently. The behavior of electronic internal energy, pressure,
and density of states was investigated in the volume and temperature ranges
called above. The maximum hydrostatic strains and the types of lattice
instabilities were theoretically predicted for the considered metals. The
influence of high electronic temperature on the electronic heat conductivity
and electric resistivity has been provided for d-band metals by the approach
based on the solution of Boltzmann kinetic equation in $\tau$-approximation.
This data is compared with the results of quantum molecular dynamics for Gold. | 1702.00825v3 |
2017-02-15 | New bulk p-type skutterudites DD0.7Fe2.7Co1.3Sb12-xXx (X = Ge, Sn) reaching ZT>1.3 | The best p-type skutterudites so far are didymium filled, Fe/Co substituted,
Sb-based skutterudites. Substitution at the Sb-sites influences the electronic
structure, deforms the Sb4-rings, enhances the scattering of phonons on
electrons and impurities and this way reduces the lattice thermal conductivity.
In this paper we study structural and transport properties of p-type
skutterudites with the nominal composition DD0.7Fe2.7Co1.3Sb11.7{Ge/Sn}0.3,
which were prepared by a rather fast reaction-annealing-melting technique. The
Ge-doped sample showed impurities, which did not anneal out completely and even
with ZT > 1 the result was not satisfying. However, the single-phase Sn-doped
sample, DD0.7Fe2.7Co1.3Sb11.8Sn0.2, showed a lower thermal and lattice thermal
conductivity than the undoped skutterudite leading to a higher ZT=1.3, hitherto
the highest ZT for a p-type skutterudite. Annealing at 570 K for 3 days proved
the stability of the microstructure. After severe plastic deformation (SPD),
due to additionally introduced defects, an enhancement of the electrical
resistivity was compensated by a significantly lower thermal conductivity and
the net effect led to a record high figure of merit: ZT = 1.45 at 850 K for
DD0.7Fe2.7Co1.3Sb11.8Sn0.2. | 1702.04498v1 |
2017-02-24 | Out-of-plane easy-axis in thin films of diluted magnetic semiconductor Ba1-xKx(Zn1-yMny)2As2 | Single-phased, single-oriented thin films of Mn-doped ZnAs-based diluted
magnetic semiconductor (DMS) Ba1-xKx(Zn1-yMny)2As2 (x = 0.03, 0.08; y = 0.15)
have been deposited on Si, SrTiO3, LaAlO3, (La,Sr)(Al,Ta)O3, and MgAl2O4
substrates, respectively. Utilizing a combined synthesis and characterization
system excluding the air and further optimizing the deposition parameters,
high-quality thin films could be obtained and be measured showing that they can
keep inactive-in-air up to more than 90 hours characterized by electrical
transport measurements. In comparison with films of x = 0.03 which possess
relatively higher resistivity, weaker magnetic performances, and larger energy
gap, thin films of x = 0.08 show better electrical and magnetic performances.
Strong magnetic anisotropy was found in films of x = 0.08 grown on
(La,Sr)(Al,Ta)O3 substrate with their magnetic polarization aligned almost
solely on the film growth direction. | 1702.07506v1 |
2017-06-16 | An argon ion beam milling process for native $\text{AlO}_\text{x}$ layers enabling coherent superconducting contacts | We present an argon ion beam milling process to remove the native oxide layer
forming on aluminum thin films due to their exposure to atmosphere in between
lithographic steps. Our cleaning process is readily integrable with
conventional fabrication of Josephson junction quantum circuits. From
measurements of the internal quality factors of superconducting microwave
resonators with and without contacts, we place an upper bound on the residual
resistance of an ion beam milled contact of 50$\,\mathrm{m}\Omega \cdot \mu
\mathrm{m}^2$ at a frequency of 4.5 GHz. Resonators for which only $6\%$ of the
total foot-print was exposed to the ion beam milling, in areas of low electric
and high magnetic field, showed quality factors above $10^6$ in the single
photon regime, and no degradation compared to single layer samples. We believe
these results will enable the development of increasingly complex
superconducting circuits for quantum information processing. | 1706.06424v1 |
2017-06-29 | Magnetotransport properties of MoP$_2$ | We report magnetotransport and de Haas-van Alphen (dHvA) effect studies on
MoP$_2$ single crystals, predicted to be type-2 Weyl semimetal with four pairs
of robust Weyl points located below the Fermi level and long Fermi arcs. The
temperature dependence of resistivity shows a peak before saturation, which
does not move with magnetic field. Large nonsaturating magnetoresistance (MR)
was observed, and the field dependence of MR exhibits a crossover from
semicalssical weak-field $B^2$ dependence to the high-field linear-field
dependence, indicating the presence of Dirac linear energy dispersion. In
addition, systematic violation of Kohler's rule was observed, consistent with
multiband electronic transport. Strong spin-orbit coupling (SOC) splitting has
an effect on dHvA measurements whereas the angular-dependent dHvA orbit
frequencies agree well with the calculated Fermi surface. The cyclotron
effective mass $\sim$ 1.6$m_e$ indicates the bands might be trivial, possibly
since the Weyl points are located below the Fermi level. Interestingly,
quasi-two dimensional(2D) band structure is observed even though the crystal
structure of MoP$_2$ is not layered. | 1706.09830v1 |
2017-09-11 | Massive fermions with low mobility in antiferromagnet orthorhombic CuMnAs single crystals | We report the physical properties of orthorhombic o-CuMnAs single crystal,
which is predicted to be a topological Dirac semimetal with magnetic ground
state and inversion symmetry broken. o-CuMnAs exhibits an antiferromagnetic
transition with TN ~ 312 K. Further characterizations of magnetic properties
suggest that the AFM order may be canted with the spin orientation in the bc
plane. Small isotropic MR and linearly field-dependent Hall resistivity with
positive slope indicate that single hole-type carries with high density and low
mobility dominate the transport properties of o-CuMnAs. Furthermore, the result
of low-temperature heat capacity shows that the effective mass of carriers is
much larger than those in typical topological semimetals. These results imply
that the carriers in o-CuMnAs exhibit remarkably different features from those
of Dirac fermions predicted in theory. | 1709.03394v2 |
2017-09-13 | Ultra-broadband photodetectors based on epitaxial graphene quantum dots | Graphene is an ideal material for hot-electron bolometers, due to its low
heat capacity and weak electron-phonon coupling. Nanostructuring graphene with
quantum dot constrictions yields detectors with extraordinarily high intrinsic
responsivity, higher than 1x10^9 V/W at 3K. The sensing mechanism is bolometric
in nature: the quantum confinement gap causes a strong dependence of the
electrical resistance on the electron temperature. Here we show that this
quantum confinement gap does not impose a limitation on the photon energy for
light detection and these quantum dot bolometers work in a very broad spectral
range, from terahertz, through telecom to ultraviolet radiation, with
responsivity independent of wavelength. We also measure the power dependence of
the response. Although the responsivity decreases with increasing power, it
stays higher than 1x10^8 V/W in a wide range of absorbed power, from 1 pW to
0.4 nW. | 1709.04498v1 |
2017-09-27 | Pressure-induced Lifshitz transition in NbP: Raman, x-ray diffraction, electrical transport and density functional theory | We report high pressure Raman, synchrotron x-ray diffraction and electrical
transport studies on Weyl semimetals NbP and TaP along with first-principles
density functional theoretical (DFT) analysis. The frequencies of first-order
Raman modes of NbP harden with increasing pressure and exhibit a slope change
at P$_c$ $\sim$ 9 GPa, and its resistivity exhibits a minimum at P$_c$. The
pressure-dependent volume of NbP exhibits a change in its bulk modulus from 207
GPa to 243 GPa at P$_c$. Using DFT calculations, we show that these anomalies
are associated with pressure induced Lifshitz transition which involves
appearance of electron and hole pockets in its electronic structure. In
contrast, results of Raman and synchrotron x-ray diffraction experiments on TaP
and DFT calculations show that TaP is quite robust under pressure and does not
undergo any phase transition. | 1709.09368v1 |
2017-10-27 | Unusual behavior of cuprates explained by heterogeneous charge localization | The cuprate high-temperature superconductors are among the most intensively
studied materials, yet essential questions regarding their principal phases and
the transitions between them remain unanswered. Generally thought of as doped
charge-transfer insulators, these complex lamellar oxides exhibit pseudogap,
strange-metal, superconducting and Fermi-liquid behaviour with increasing
hole-dopant concentration. Here we propose a simple inhomogeneous Mott-like
(de)localization model wherein exactly one hole per copper-oxygen unit is
gradually delocalized with increasing doping and temperature. The model is
percolative in nature, with parameters that are experimentally constrained. It
comprehensively captures pivotal unconventional experimental results, including
the temperature and doping dependence of the pseudogap phenomenon, the
strange-metal linear temperature dependence of the planar resistivity, and the
doping dependence of the superfluid density. The success and simplicity of our
model greatly demystify the cuprate phase diagram and point to a local
superconducting pairing mechanism involving the (de)localized hole. | 1710.10221v1 |
2017-11-05 | Investigations of a Robotic Testbed with Viscoelastic Liquid Cooled Actuators | We design, build, and thoroughly test a new type of actuator dubbed
viscoelastic liquid cooled actuator (VLCA) for robotic applications. VLCAs
excel in the following five critical axes of performance: energy efficiency,
torque density, impact resistence, joint position and force controllability. We
first study the design objectives and choices of the VLCA to enhance the
performance on the needed criteria. We follow by an investigation on
viscoelastic materials in terms of their damping, viscous and hysteresis
properties as well as parameters related to the long- term performance. As part
of the actuator design, we configure a disturbance observer to provide
high-fidelity force control to enable a wide range of impedance control
capabilities. We proceed to design a robotic system capable to lift payloads of
32.5 kg, which is three times larger than its own weight. In addition, we
experiment with Cartesian trajectory control up to 2 Hz with a vertical range
of motion of 32 cm while carrying a payload of 10 kg. Finally, we perform
experiments on impedance control and mechanical robustness by studying the
response of the robotics testbed to hammering impacts and external force
interactions. | 1711.01649v2 |
2017-11-07 | Epitaxial stabilization of pulsed laser deposited Sr$_{n+1}$Ir$_n$O$_{3n+1}$ thin films: entangled effect of growth dynamics and strain | The subtle balance of electronic correlations, crystal field splitting and
spin--orbit coupling in layered Ir$^{4+}$ oxides can give rise to novel
electronic and magnetic phases. Experimental progress in this field relies on
the synthesis of epitaxial films of these oxides. However, the growth of
layered iridates with excellent structural quality is a great experimental
challenge. Here we selectively grow high quality single--phase films of
Sr$_2$IrO$_4$, Sr$_3$Ir$_2$O$_7$, and SrIrO$_3$ on various substrates from a
single Sr$_3$Ir$_2$O$_7$ target by tuning background oxygen pressure and
epitaxial strain. We demonstrate a complex interplay between growth dynamics
and strain during thin film deposition. Such interplay leads to the
stabilization of different phases in films grown on different substrates under
identical growth conditions, which cannot be explained by a simple kinetic
model. We further investigate the thermoelectric properties of the three phases
and propose that weak localization is responsible for the low temperature
activated resistivity observed in SrIrO$_3$ under compressive strain. | 1711.02767v3 |
2017-11-09 | Magnetic phase separation and strong AFM nature of hexagonal Gd$_5$Sb$_3$ | We report on the combined results of structural, magnetic, transport and
calorimetric properties of Mn$_5$Si$_3$-type hexagonal Gd$_5$Sb$_3$. With
decreasing temperature, it exhibits a ferromagnetic-like transition at 265 K,
N\'{e}el transition at 95.5 K and a spin-orientation transition at 62 K. The
system is found to be in AFM state down to 2 K in a field of 70 kOe. Magnetic
phase coexistence is not noticeable despite large positive Curie-Weiss
temperature. Instead low-temperature AFM and high-temperature FM-like phases
are separated in large temperature. Temperature-magnetic field ($H$-$T$) phase
diagram reveals field-driven complex magnetic phases. Within the AFM phase, the
system is observed to undergo field-driven spin-orientation transitions.
Field-induced tricritical and quantum critical points appear to be absent due
to strong AFM nature and by the intervention of FM-like state between PM and
AFM states. Electrical resistivity along with large Sommerfeld parameter
suggests metallic nature. | 1711.03263v1 |
2017-11-29 | Pressure-induced ferromagnetism due to an anisotropic electronic topological transition in Fe1.08Te | A rapid and anisotropic modification of the Fermi-surface shape can be
associated with abrupt changes in crystalline lattice geometry or in the
magnetic state of a material. In this study we show that such an electronic
topological transition is at the basis of the formation of an unusual
pressure-induced tetragonal ferromagnetic phase in Fe$_{1.08}$Te. Around 2 GPa,
the orthorhombic and incommensurate antiferromagnetic ground-state of
Fe$_{1.08}$Te is transformed upon increasing pressure into a tetragonal
ferromagnetic state via a conventional first-order transition. On the other
hand, an isostructural transition takes place from the paramagnetic
high-temperature state into the ferromagnetic phase as a rare case of a `type
0' transformation with anisotropic properties. Electronic-structure
calculations in combination with electrical resistivity, magnetization, and
x-ray diffraction experiments show that the electronic system of Fe$_{1.08}$Te
is instable with respect to profound topological transitions that can drive
fundamental changes of the lattice anisotropy and the associated magnetic
order. | 1711.10745v1 |
2018-01-15 | Improving Graphene-metal Contacts: Thermal Induced Polishing | Graphene is a very promising material for nanoelectronics applications due to
its unique and remarkable electronic and thermal properties. However, when
deposited on metallic electrodes the overall thermal conductivity is
significantly decreased. This phenomenon has been attributed to the mismatch
between the interfaces and contact thermal resistance. Experimentally, one way
to improve the graphene/metal contact is thorough high-temperature annealing,
but the detailed mechanisms behind these processes remain unclear. In order to
address these questions, we carried out fully atomistic reactive molecular
dynamics simulations using the ReaxFF force field to investigate the
interactions between multi-layer graphene and metallic electrodes (nickel)
under (thermal) annealing. Our results show that the annealing induces an
upward-downward movement of the graphene layers, causing a pile-driver-like
effect over the metallic surface. This graphene induced movements cause a
planarization (thermal polishing-like effect) of the metallic surface, which
results in the increase of the effective graphene/metal contact area. This can
also explain the experimentally observed improvements of the thermal and
electric conductivities. | 1801.04785v1 |
2018-01-31 | Commensurability Oscillations in One-Dimensional Graphene Superlattices | We report the experimental observation of commensurability oscillations (COs)
in 1D graphene superlattices. The widely tunable periodic potential modulation
in hBN encapsulated graphene is generated via the interplay of nanopatterned
few layer graphene acting as a local bottom gate and a global Si back gate. The
longitudinal magneto-resistance shows pronounced COs, when the sample is tuned
into the unipolar transport regime. We observe up to six CO minima, providing
evidence for a long mean free path despite the potential modulation. Comparison
to existing theories shows that small angle scattering is dominant in
hBN/graphene/hBN heterostructures. We observe robust COs persisting to
temperature exceeding $T=150$ K. At high temperatures, we find deviations from
the predicted $T$-dependence, which we ascribe to electron-electron scattering. | 1802.00016v2 |
2018-03-22 | High density carriers at a strongly coupled graphene-topological insulator interface | We report on a strongly coupled bilayer graphene (BLG) - \bise\ device with a
junction resistance of less than 1.5 k$\Omega\mu$m$^2$. This device exhibits
unique behavior at the interface, which cannot be attributed to either material
in absence of the other. We observe quantum oscillations in the
magnetoresistance of the junction, indicating the presence of well-resolved
Landau levels due to hole carriers of unknown origin with a very large Fermi
surface. These carriers, found only at the interface, could conceivably arise
due to significant hole doping of the bilayer graphene with charge transfer on
the order of 2$\times$10$^{13}$ cm$^{-2}$, or due to twist angle dependent
mini-band transport. | 1803.08260v1 |
2018-03-27 | Large-Scale Fabrication of RF MOSFETs on Liquid-Exfoliated MoS2 | For the first time, thousands of RF MOSFETs were batch-fabricated on
liquid-exfoliated MoS2 below 300 {\deg}C with nearly 100% yield. The
large-scale fabrication with high yield allowed the average performance instead
of the best performance to be reported. The DC performance of these devices
were typical, but the RF performance, enabled by buried gates and on the order
of 100 MHz, was reported for the first time for liquid-exfoliated MoS2. To
resolve the dilemma of thin vs. thick films, gate recess was used on 20-nm
thick films to improve the gate control while keeping the contact resistance
lower than that on 10-nm films. These innovations may enable thin-film
transistors to operate in the microwave range. | 1803.09906v1 |
2018-05-20 | Autonomous actuation of zero modes in mechanical networks far from equilibrium | A zero mode, or floppy mode, is a non-trivial coupling of mechanical
components yielding a degree of freedom with no resistance to deformation.
Engineered zero modes have the potential to act as microscopic motors or memory
devices, but this requires an internal actuation mechanism that can overcome
unwanted fluctuations in other modes and the dissipation inherent in real
systems. In this work, we show theoretically and experimentally that complex
zero modes in mechanical networks can be selectively mobilized by
non-equilibrium activity. We find that a correlated active bath actuates an
infinitesimal zero mode while simultaneously suppressing fluctuations in higher
modes compared to thermal fluctuations, which we experimentally mimic by high
frequency shaking of a physical network. Furthermore, self-propulsive dynamics
spontaneously mobilise finite mechanisms as exemplified by a self-propelled
topological soliton. Non-equilibrium activity thus enables autonomous actuation
of coordinated mechanisms engineered through network topology. | 1805.07728v2 |
2018-06-06 | Pressure Evolution of Magnetism in URhGa | In this paper, we report the results of an ambient and high pressure study of
a 5f-electron ferromagnet URhGa. The work is focused on measurements of
magnetic and thermodynamic properties of a single crystal sample and on the
construction of the p-T phase diagram. Diamond anvil cells were employed to
measure the magnetization and electrical resistivity pressures up to ~ 9 GPa.
At ambient pressure, URhGa exhibits collinear ferromagnetic ordering of uranium
magnetic moments {\mu}U ~ 1.1 {\mu}B (at 2 K) aligned along the c-axis of the
hexagonal crystal structure below the Curie temperature TC = 41K. With the
application of pressure up to 5GPa the ordering temperature TC initially
increases whereas the saturated moment slightly decreases. The rather
unexpected evolution is put in the context of the UTX family of compounds. | 1806.02686v1 |
2018-06-25 | Ferromagnetism above 1000 K in highly cation-ordered double-perovskite insulator Sr3OsO6 | Magnetic insulators have been intensively studied for over 100 years, and
they, in particular ferrites, are considered to be the cradle of magnetic
exchange interactions in solids. Their wide range of applications include
microwave devices and permanent magnets . They are also suitable for spintronic
devices owing to their high resistivity, low magnetic damping, and
spin-dependent tunneling probabilities. The Curie temperature is the crucial
factor determining the temperature range in which any ferri/ferromagnetic
system remains stable. However, the record Curie temperature has stood for over
eight decades in insulators and oxides (943 K for spinel ferrite LiFe5O8). Here
we show that a highly B-site ordered double-perovskite, Sr2(SrOs)O6 (Sr3OsO6),
surpasses this long standing Curie temperature record by more than 100 K. We
revealed this B-site ordering by atomic-resolution scanning transmission
electron microscopy. The density functional theory (DFT) calculations suggest
that the large spin-orbit coupling (SOC) of Os6+ 5d2 orbitals drives the system
toward a Jeff = 3/2 ferromagnetic (FM) insulating state. Moreover, the Sr3OsO6
is the first epitaxially grown osmate, which means it is highly compatible with
device fabrication processes and thus promising for spintronic applications. | 1806.09308v1 |
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