publicationDate stringlengths 10 10 | title stringlengths 17 233 | abstract stringlengths 20 3.22k | id stringlengths 9 12 |
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2005-06-17 | Theory of spin-polarized transport in ferromagnet-semiconductor structures: Unified description of ballistic and diffusive transport | A theory of spin-polarized electron transport in ferromagnet-semiconductor
heterostructures, based on a unified semiclassical description of ballistic and
diffusive transport in semiconductors, is outlined. The aim is to provide a
framework for studying the interplay of spin relaxation and transport mechanism
in spintronic devices. Transport inside the (nondegenerate) semiconductor is
described in terms of a thermoballistic current, in which electrons move
ballistically in the electric field arising from internal and external
electrostatic potentials, and are thermalized at randomly distributed
equilibration points. Spin relaxation is allowed to take place during the
ballistic motion. For arbitrary potential profile and arbitrary values of the
momentum and spin relaxation lengths, an integral equation for a spin transport
function determining the spin polarization in the semiconductor is derived. For
field-driven transport in a homogeneous semiconductor, the integral equation
can be converted into a second-order differential equation that generalizes the
spin drift-diffusion equation. The spin-polarization in ferromagnet
semiconductor structures is obtained by matching the spin-resolved chemical
potentials at the interfaces, with allowance for spin-selective interface
resistances. Illustrative examples are considered. | 0506441v1 |
2006-02-06 | Non-collinear Magnetoelectronics | The electron transport properties of hybrid ferromagnetic|normal metal
structures such as multilayers and spin valves depend on the relative
orientation of the magnetization direction of the ferromagnetic elements.
Whereas the contrast in the resistance for parallel and antiparallel
magnetizations, the so-called Giant Magnetoresistance, is relatively well
understood for quite some time, a coherent picture for non-collinear
magnetoelectronic circuits and devices has evolved only recently. We review
here such a theory for electron charge and spin transport with general
magnetization directions that is based on the semiclassical concept of a vector
spin accumulation. In conjunction with first-principles calculations of
scattering matrices many phenomena, e.g. the current-induced spin-transfer
torque, can be understood and predicted quantitatively for different material
combinations. | 0602151v1 |
2006-02-27 | Relating supercooling and glass-like arrest of kinetics for phase separated systems: studies on doped CeFe$_2$ and (La,Pr,Ca)MnO$_3$ | Coexisting ferromagnetic and antiferromagnetic phases over a range of
temperature as well as magnetic field have been reported in many materials of
current interest, showing disorder-broadened 1st order transitions. Anomalous
history effects observed in magnetization and resistivity are being explained
invoking the concepts of kinetic arrest akin to glass transitions. From
magnetization measurements traversing novel paths in field-temperature space,
we obtain the intriguing result that the regions of the sample which can be
supercooled to lower temperatures undergo kinetic-arrest at higher
temperatures, and vice versa. Our results are for two diverse systems viz. the
inter-metallic doped CeFe$_2$ which has an antiferromagnetic ground state, and
the oxide La-Pr-Ca-Mn-O which has a ferromagnetic ground state, indicating the
possible universality of this effect of disorder on the widely encountered
phenomenon of glass-like arrest of kinetics. | 0602627v1 |
2006-06-09 | The magnetotransport properties of La0.7Sr0.3MnO3/BaTiO3 superlattices grown by pulsed laser deposition technique | We have investigated the magnetotransport properties of La0.7Sr0.3MnO3/BaTiO3
superlattices, grown on SrTiO3 substrate by pulsed laser deposition technique,
both with current-in-plane and current-perpendicular-to-the-plane directions.
Several features indicate the presence of magnetic inhomogeneities at the
interfaces which is independent of BaTiO3 layer thickness variation. First, the
magnetic property in the superlattices decreases. Second, a hysteresis in
magnetoresistance due to the relaxation of the resistive state is observed.
Third, a threshold under an applied magnetic field in the magnetoresistance is
seen. Such behaviors are in agreement with the phase separation scenario which
could be the possible reason for these magnetic inhomogeneities at the
interfaces. On the contrary, the magnetoresistance with the
current-perpendicular-to-the-plane direction is mostly attributed to the
tunneling effect along with the ordering of the spin at the interface. This
study confirms the importance of the interfaces in superlattices that can be
used to control novel physical properties in oxide materials. | 0606232v1 |
2006-06-30 | Synthesis of VO_2 Nanowire and Observation of the Metal-Insulator Transition | We have fabricated crystalline nanowires of VO_2 using a new synthetic
method. A nanowire synthesized at 650^oC shows the semiconducting behavior and
a nanowire at 670^oC exhibits the first-order metal-insulator transition which
is not the one-dimensional property. The temperature coefficient of resistance
in the semiconducting nanowire is 7.06 %/K at 300 K, which is higher than that
of commercial bolometer. | 0606793v1 |
2006-11-17 | Evolution in Materio: Exploiting the Physics of Materials for Computation | We describe several techniques for using bulk matter for special purpose
computation. In each case it is necessary to use an evolutionary algorithm to
program the substrate on which the computation is to take place. In addition,
the computation comes about as a result of nearest neighbour interactions at
the nano- micro- and meso-scale. In our first example we describe evolving a
saw-tooth oscillator in a CMOS substrate. In the second example we demonstrate
the evolution of a tone discriminator by exploiting the physics of liquid
crystals. In the third example we outline using a simulated magnetic quantum
dot array and an evolutionary algorithm to develop a pattern matching circuit.
Another example we describe exploits the micro-scale physics of charge density
waves in crystal lattices. We show that vastly different resistance values can
be achieved and controlled in local regions to essentially construct a
programmable array of coupled micro-scale quasiperiodic oscillators. Lastly we
show an example where evolutionary algorithms could be used to control density
modulations, and therefore refractive index modulations, in a fluid for optical
computing. | 0611462v1 |
2006-12-23 | Magnetic phase diagram of Ce2Fe17 | Rare-earth-based permanent-magnet materials rich in iron have relatively low
ferromagnetic ordering temperatures. This is believed to be due to the presence
of antiferromagnetic exchange interactions, besides the ferromagnetic
interactions responsible for the magnetic order. The magnetic properties of
Ce2Fe17 are anomalous. Instead of ferromagnetic, it is antiferromagnetic, and
instead of one ordering temperature, it shows two, at the Neel temperature TN ~
208 K and at TT ~ 124 K. Ce2Fe17, doped by 0.5% Ta, also shows two ordering
temperatures, one to an antiferromagnetic phase, at TN ~ 214 K, and one to a
ferromagnetic phase, at T0 ~ 75 K. In order to clarify this behavior,
single-crystalline samples were prepared by solution growth, and characterized
by electron microscopy, single crystal x-ray diffraction, temperature-dependent
specific heat, and magnetic field and temperature-dependent electrical
resistivity and magnetization. From these measurements, magnetic H-T phase
diagrams were determined for both Ta-doped Ce2Fe17 and undoped Ce2Fe17. These
phase diagrams can be very well described in terms of a theory that gives
magnetic phase diagrams of systems with competing antiferro- and
ferromagnetism. | 0612603v1 |
2006-05-06 | Radiation enhancement and radiation suppression by a left-handed metamaterial | The perfect lens property of a dispersive and lossy left-handed metamaterial
(LHM) disk is exploited to superimpose a source of electromagnetic radiation
onto its mirror image, formed as a result of reflection from a perfect electric
conductor (PEC) or a perfect magnetic conductor (PMC). The superposition of a
vertical wire-dipole antenna with its PEC-image results in an increase of the
radiation resistance of the antenna compared to that of an antenna emitting in
free space. On the other hand, if the same antenna is coupled to a PMC-image it
is shown that the result is the formation of a non-radiating configuration. The
finite-difference time-domain (FDTD) analysis is performed and this allows a
detailed characterization of the systems. It is shown that the non-radiating
system allows relatively large amounts of electromagnetic energy to be stored
in the LHM-disk and that is indicative of strong electromagnetic fields inside
the material. This property is employed in a second-harmonic generation (SHG)
process and the potential of a non-radiating configuration as an efficient
nonlinear device is demonstrated. | 0605055v1 |
2006-07-25 | Superconductivity: coherent "tunnelling" by a dielectric array of charge-carriers | Superconduction manifests when a steady-state current flows through a
material without an electric field being present. It is argued here that the
absence of scattering of the charge-carriers, although absolutely necessary, is
not sufficient to explain why an electric field is zero when a current flows
between two contacts to a superconducting material. It is concluded that an
electric field, and thus a resistance, must manifest unless (i) the
charge-carriers form part of an array of dielectric charge centres, and (ii)
the charge-carriers can increase their velocities without increasing their
kinetic energies. A model is propoased which allows these requirements to
manifest. The model is fitted to selected experimental results which have been
published for low temperature metals, YBCO, and highly-doped p-type diamond. In
each case a satisfactory description of the experimental results is
demonstrated. | 0607227v1 |
2005-04-05 | Reduction of Magnetic Noise in Atom Chips by Material Optimization | We discuss the contribution of the material type in metal wires to the
electromagnetic fluctuations in magnetic microtraps close to the surface of an
atom chip. We show that significant reduction of the magnetic noise can be
achieved by replacing the pure noble metal wires with their dilute alloys. The
alloy composition provides an additional degree of freedom which enables a
controlled reduction of both magnetic noise and resistivity if the atom chip is
cooled. In addition, we provide a careful re-analysis of the magnetically
induced trap loss observed by Yu-Ju Lin et al. [Phys. Rev. Lett. 92, 050404
(2004)] and find good agreement with an improved theory. | 0504027v2 |
2007-04-20 | Transport measurements across a tunable potential barrier in graphene | The peculiar nature of electron scattering in graphene is among many exciting
theoretical predictions for the physical properties of this material. To
investigate electron scattering properties in a graphene plane, we have created
a gate-tunable potential barrier within a single-layer graphene sheet. We
report measurements of electrical transport across this structure as the
tunable barrier potential is swept through a range of heights. When the barrier
is sufficiently strong to form a bipolar junctions (npn or pnp) within the
graphene sheet, the resistance across the barrier sharply increases. We compare
these results to predictions for both diffusive and ballistic transport, as the
barrier rises on a length scale comparable to the mean free path. Finally, we
show how a magnetic field modifies transport across the barrier. | 0704.2626v2 |
2007-05-16 | Ab initio estimate of temperature dependence of electrical conductivity in a model amorphous material: hydrogenated amorphous silicon | We present an ab initio calculation of the DC conductivity of amorphous
silicon and hydrogenated amorphous silicon. The Kubo-Greenwood formula is used
to obtain the DC conductivity, by thermal averaging over extended dynamical
simulation. Its application to disordered solids is discussed. The conductivity
is computed for a wide range of temperatures and doping is explored in a naive
way by shifting the Fermi level. We observed the Meyer-Neldel rule for the
electrical conductivity with E_MNR = 0.06 eV and a temperature coefficient of
resistance, TCR ~ -2.0% K^-1 for a-Si:H. In general, experimental trends are
reproduced by these calculations, and this suggests the possible utility of the
approach for modeling carrier transport in other disordered systems. | 0705.2384v3 |
2007-07-08 | Tensoresistive Effect in Single Crystal Microwires of Pbte Doped with Tl | Results of room temperature measurements of tensoresistive effect of thin
single crystal microwires of Pb1-xTlxTe (x=0.0000 - 0.0025, d = 5 - 20
micrometers) obtained from the melted compound of corresponding composition by
the filling of quartz capillary with the following crystallization of material
are presented. For the samples corresponding to chemical composition with
concentration of thallium x ~0,0025 an essential increase of tensoresistive
effect (resistance changes for elastic elongations per unit length of a
crystal) in comparison with nondoped samples is observed. Various mechanisms
which can lead to observable anomalies, including resonance scattering are
discussed. Obtained experimental results allow us to suppose that the observed
peculiarities can be interpreted on the basis of model of an impurity band of
Tl in PbTe. | 0707.1124v1 |
2007-08-13 | Orbital Ordering Structures in (Nd,Pr)0.5Sr0.5MnO3 Manganite Thin Films on Perovskite (011) Substrates | Structural study of orbital-ordered manganite thin films has been conducted
using synchrotron radiation, and a ground state electronic phase diagram is
made. The lattice parameters of four manganite thin films, Nd0.5Sr0.5MnO3
(NSMO) or Pr0.5Sr0.5MnO3 (PSMO) on (011) surfaces of SrTiO3 (STO) or
[(LaAlO3){0.3}(SrAl0.5Ta0.5O3){0.7}] (LSAT), were measured as a function of
temperature. The result shows, as expected based on previous knowledge of bulk
materials, that the films' resistivity is closely related to their structures.
Observed superlattice reflections indicate that NSMO thin films have an
antiferro-orbital-ordered phase as their low-temperature phase while PSMO film
on LSAT has a ferro-orbital-ordered phase, and that on STO has no
orbital-ordered phase. A metallic ground state was observed only in films
having a narrow region of A-site ion radius, while larger ions favor
ferro-orbital-ordered structure and smaller ions stabilize
antiferro-orbital-ordered structure. The key to the orbital-ordering transition
in (011) film is found to be the in-plane displacement along [0-1 1] direction. | 0708.1693v1 |
2007-08-16 | The nanostructural origin of the ac conductance in dielectric granular metals: the case study of Co_20(ZrO_2)_80 | We show which is the nanostructure required in granular Co20(ZrO2)80 thin
films to produce an ac response such as the one that is universally observed in
a very wide variety of dielectric materials. A bimodal size distribution of Co
particles yields randomly competing conductance channels which allow both
thermally assisted tunneling through small particles and capacitive conductance
among larger particles that are further apart. A model consisting on a simple
cubic random resistance-capacitor network describes quantitatively the
experimental results as functions of temperature and frequency, and enables the
determination of the microscopic parameters controlling the ac response of the
samples. | 0708.2140v1 |
2007-08-16 | Spin-transfer torques in anti-ferromagnetic metals from first principles | In spite of the absence of a macroscopic magnetic moment, an anti-ferromagnet
is spin-polarized on an atomic scale. The electric current passing through a
conducting anti-ferromagnet is polarized as well, leading to spin-transfer
torques when the order parameter is textured, such as in anti-ferromagnetic
non-collinear spin valves and domain walls. We report a first principles study
on the electronic transport properties of anti-ferromagnetic systems. The
current-induced spin torques acting on the magnetic moments are comparable with
those in conventional ferromagnetic materials, leading to measurable angular
resistances and current-induced magnetization dynamics. In contrast to
ferromagnets, spin torques in anti-ferromagnets are very nonlocal. The torques
acting far away from the center of an anti-ferromagnetic domain wall should
facilitate current-induced domain wall motion. | 0708.2143v2 |
2007-12-11 | Frequency- and electric-field-dependent conductivity of single-walled carbon nanotube networks of varying density | We present measurements of the frequency and electric field dependent
conductivity of single walled carbon nanotube(SWCNT) networks of various
densities. The ac conductivity as a function of frequency is consistent with
the extended pair approximation model and increases with frequency above an
onset frequency $\omega_0$ which varies over seven decades with a range of film
thickness from sub-monolayer to 200 nm. The nonlinear electric field-dependent
DC conductivity shows strong dependence on film thickness as well. Measurement
of the electric field dependence of the resistance R(E) allows for the
determination of a length scale $L_{E}$ possibly characterizing the distance
between tube contacts, which is found to systematically decrease with
increasing film thickness. The onset frequency $\omega_0$ of ac conductivity
and the length scale $L_{E}$ of SWCNT networks are found to be correlated, and
a physically reasonable empirical formula relating them has been proposed. Such
studies will help the understanding of transport properties and benefit the
applications of this material system. | 0712.1733v3 |
2007-12-21 | Kinks in the electronic specific heat | We find that the heat capacity of a strongly correlated metal presents
striking changes with respect to Landau Fermi liquid theory. In contrast with
normal metals, where the electronic specific heat is linear at low temperature
(with a T^3 term as a leading correction), a dynamical mean-field study of the
correlated Hubbard model reveals a clear kink in the temperature dependence,
marking a rapid change from a low-temperature linear behavior and a second
linear regime with a reduced slope. Experiments on LiV2O4 support our findings,
implying that correlated materials are more resistive to cooling at low T than
expected from the intermediate temperature behavior. | 0712.3723v2 |
2008-01-07 | Utility of transient testing to characterize thermal interface materials | This paper analyzes a transient method for the characterization of
low-resistance thermal interfaces of microelectronic packages. The transient
method can yield additional information about the package not available with
traditional static methods at the cost of greater numerical complexity,
hardware requirements, and sensitivity to noise. While the method is
established for package-level thermal analysis of mounted and assembled parts,
its ability to measure the relatively minor thermal impedance of thin thermal
interface material (TIM) layers has not yet been fully studied. We combine the
transient thermal test with displacement measurements of the bond line
thickness to fully characterize the interface. | 0801.1009v1 |
2008-03-05 | Giant room temperature piezoresistance in a metal/silicon hybrid | Metal/semiconductor hybrids are artificially created structures presenting
novel properties not exhibited by either of the component materials alone. Here
we present a giant piezoresistance effect in a hybrid formed from silicon and
aluminum. The maximum piezoresistive gage factor (GF) of 843, measured at room
temperature, compares with a GF of -93 measured in the bulk homogeneous
silicon. This piezoresistance boost is not due to the silicon/aluminum
interface, but results from a stress induced anisotropy in the silicon
conductivity that acts to switch current away from the highly conductive
aluminum for uniaxial tensile strains. Its magnitude is shown, via the
calculation of hybrid resistivity weighting functions, to depend only on the
geometrical arrangement of the component parts of the hybrid. | 0803.0655v1 |
2008-04-02 | Physical properties of the new Uranium ternary compounds U3Bi4M3 (M=Ni, Rh) | We report the properties of two new isostructural compounds, U3Bi4Ni3 and
U3Bi4Rh3. The first of these compounds is non-metallic, and the second is a
nearly ferromagnetic metal, both as anticipated from their electron count
relative to other U-based members of the larger 3-4-3 family. For U3Bi4Rh3, a
logarithmic increase of C/T below 3 K, a resistivity proportional to T^4/3, and
the recovery of Fermi-liquid behavior in both properties with applied fields
greater than 3T, suggest that U3Bi4Rh3 may be a new example of a material
displaying ferromagnetic quantum criticality. | 0804.0393v1 |
2008-11-19 | Intrinsic Response of Graphene Vapor Sensors | Graphene is a purely two-dimensional material that has extremely favorable
chemical sensor properties. It is known, however, that conventional
nanolithographic processing typically leaves a resist residue on the graphene
surface, whose impact on the sensor characteristics of the system has not yet
been determined. Here we show that the contamination layer both degrades the
electronic properties of the graphene and masks graphene s intrinsic sensor
responses. The contamination layer chemically dopes the graphene, enhances
carrier scattering, and acts as an absorbent layer that concentrates analyte
molecules at the graphene surface, thereby enhancing the sensor response. We
demonstrate a cleaning process that verifiably removes the contamination on the
device structure and allows the intrinsic chemical responses of graphene to be
measured. | 0811.3091v1 |
2009-03-19 | Superconductivity at 17 K in (Fe2P2)(Sr4Sc2O6): a new superconducting layered pnictide oxide with a thick perovskite oxide layer | A new layered oxypnictide (Fe2P2)(Sr4Sc2O6) have been synthesized by
solid-state reaction. This material has an alternating layer stacking structure
of anti-fluorite Fe2P2 and perovskite-based Sr4Sc2O6 oxide layers. Space group
of the material is P4/nmm and lattice constants a and c are 4.016 A and 15.543
A, respectively. The interlayer Fe-Fe distance corresponding to the c-axis
length is the longest ever reported in the iron-based oxypnictide systems. In
both magnetization and resistivity measurements, the present compound exhibited
superconductivity below 17 K, which is much higher than that of LaFePO and the
highest in arsenic-free iron-based oxypnictide systems under ambient pressure. | 0903.3314v3 |
2009-05-27 | Precision Cutting and Patterning of Graphene with Helium Ions | We report nanoscale patterning of graphene using a helium ion microscope
configured for lithography. Helium ion lithography is a direct-write
lithography process, comparable to conventional focused ion beam patterning,
with no resist or other material contacting the sample surface. In the present
application, graphene samples on Si/SiO2 substrates are cut using helium ions,
with computer controlled alignment, patterning, and exposure. Once suitable
beam doses are determined, sharp edge profiles and clean etching are obtained,
with little evident damage or doping to the sample. This technique provides
fast lithography compatible with graphene, with ~15 nm feature sizes. | 0905.4407v2 |
2009-06-08 | Tunneling electroresistance effect in ferroelectric tunnel junctions at the nanoscale | Stable and switchable polarization of ferroelectric materials opens a
possibility to electrically control their functional behavior. A particularly
promising approach is to employ ferroelectric tunnel junctions where the
polarization reversal in a ferroelectric barrier changes the tunneling current
across the junction. Here, we demonstrate the reproducible tunneling
electroresistance effect using a combination of Piezoresponse Force Microscopy
(PFM) and Conducting Atomic Force Microscopy (C-AFM) techniques on
nanometer-thick epitaxial BaTiO3 single crystal thin films on SrRuO3 bottom
electrodes. Correlation between ferroelectric and electronic transport
properties is established by the direct nanoscale visualization and control of
polarization and tunneling current in BaTiO3 films. The obtained results show a
change in resistance by about two orders of magnitude upon polarization
reversal on a lateral scale of 20 nm at room temperature. These results are
promising for employing ferroelectric tunnel junctions in non-volatile memory
and logic devices, not involving charge as a state variable. | 0906.1521v1 |
2009-06-30 | Electric field effects, Mott insulator, Surface patterning, Scanning tunneling microscopy, Transition metal chalcogenides | We report the first experimental evidence for a strong electromechanical
coupling in the Mott insulator GaTa4Se8 allowing a highly reproducible
nano-writing with a Scanning Tunneling Microscope (STM). The local electric
field across the STM junction is observed to have a threshold value above which
the clean (100) surface of GaTa4Se8 becomes mechanically instable: At voltage
biases V > 1.1V the surface suddenly inflates and comes in contact with the STM
tip, resulting in nanometer size craters. The formed pattern can be
indestructibly "read" by STM at lower voltage bias, thus allowing a 5
Tdots/inch2 dense writing/reading at room temperature. The discovery of the
electromechanical coupling in GaTa4Se8 might give new clues in the
understanding of the Electric Pulse Induced Resistive Switching recently
observed in this stoechiometric Mott insulator. | 0906.5473v1 |
2009-11-30 | Kondo-like behaviors in magnetic and thermal properties of single crystal Tm5Si2Ge2 | We grew the single crystal of stoichiometric Tm5Si2.0Ge2.0 using a Bridgeman
method and performed XRD, EDS, magnetization, ac and dc magnetic
susceptibilities, specific heat, electrical resistivity and XPS experiments. It
crystallizes in orthorhombic Sm5Ge4-type structure. The mean valence of Tm ions
in Tm5Si2.0Ge2.0 is almost trivalent. The 4f states is split by the crystalline
electric field. The ground state exhibits the long range antiferromagnetic
order with the ferromagnetically coupled magnetic moments in the ac plane below
8.01 K, while the exited states exhibit the reduction of magnetic moment and
magnetic entropy and -log T-behaviors observed in Kondo materials. | 0911.5640v1 |
2009-12-05 | Theory and Simulation of Spin Transport in Antiferromagnetic Semiconductors: Application to MnTe | We study in this paper the parallel spin current in an antiferromagnetic
semiconductor thin film where we take into account the interaction between
itinerant spins and lattice spins. The spin model is an anisotropic Heisenberg
model. We use here the Boltzmann's equation with numerical data on cluster
distribution obtained by Monte Carlo simulations and cluster-construction
algorithms. We study the cases of degenerate and non-degenerate semiconductors.
The spin resistivity in both cases is shown to depend on the temperature with a
broad maximum at the transition temperature of the lattice spin system. The
shape of the maximum depends on the spin anisotropy and on the magnetic field.
It shows however no sharp peak in contrast to ferromagnetic materials. Our
method is applied to MnTe. Comparison to experimental data is given. | 0912.0989v4 |
2010-05-14 | Effect of phonon dispersion on thermal conduction across Si/Ge interfaces | We report finite-volume simulations of the phonon Boltzmann transport
equation (BTE) for heat conduction across the heterogeneous interfaces in SiGe
superlattices. The diffuse mismatch model incorporating phonon dispersion and
polarization is implemented over a wide range of Knudsen numbers. The results
indicate that the thermal conductivity of a Si/Ge superlattice is much lower
than that of the constitutive bulk materials for superlattice periods in the
submicron regime. We report results for effective thermal conductivity of
various material volume fractions and superlattice periods. Details of the
non-equilibrium energy exchange between optical and acoustic phonons that
originate from the mismatch of phonon spectra in silicon and germanium are
delineated for the first time. Conditions are identified for which this effect
can produce significantly more thermal resistance than that due to boundary
scattering of phonons. | 1005.2578v1 |
2010-07-22 | Anisotropic weakly localized transport in nitrogen-doped ultrananocrystalline diamond films | We establish the dominant effect of anisotropic weak localization (WL) in
three dimensions associated with a propagative Fermi surface, on the
conductivity correction in heavily nitrogen doped ultrananocrystalline diamond
(UNCD) films based on magneto-resistance studies at low temperatures. Also, low
temperature electrical conductivity can show weakly localized transport in 3D
combined with the effect of electron-electron interactions in these materials,
which is remarkably different from the conductivity in 2DWL or strong
localization regime. The corresponding dephasing time of electronic
wavefunctions in these systems described as ~ T^-p with p < 1, follows a
relatively weak temperature dependence compared to the generally expected
nature for bulk dirty metals having $p \geq 1$. The temperature dependence of
Hall (electron) mobility together with an enhanced electron density has been
used to interpret the unusual magneto-transport features and show delocalized
electronic transport in these n-type UNCD films, which can be described as
low-dimensional superlattice structures. | 1007.3918v2 |
2010-09-01 | Anomalous scattering in superconducting indium-doped tin telluride | Results of resistivity, Hall effect, magnetoresistance, susceptibility and
heat capacity measurements are presented for single crystals of indium-doped
tin telluride with compositions Sn$_{.988-x}$In$_x$Te where $0 \leq x \leq 8.4
%$, along with microstructural analysis based on transmission electron
microscopy. For small indium concentrations, $x \leq 0.9 %$ the material does
not superconduct above 0.3 K, and the transport properties are consistent with
simple metallic behavior. For $x \geq 2.7 %$ the material exhibits anomalous
low temperature scattering and for $x \geq 6.1 %$ bulk superconductivity is
observed with critical temperatures close to 2 K. Intermediate indium
concentrations $2.7% \leq x \leq 3.8%$ do not exhibit bulk superconductivity
above 0.7 K. Susceptibility data indicate the absence of magnetic impurities,
while magnetoresistance data are inconsistent with localization effects,
leading to the conclusion that indium-doped SnTe is a candidate charge Kondo
system, similar to thallium-doped PbTe. | 1009.0090v1 |
2010-09-06 | Understanding adhesion at as-deposited interfaces from ab initio thermodynamics of deposition growth: thin-film alumina on titanium carbide | We investigate the chemical composition and adhesion of chemical vapour
deposited thin-film alumina on TiC using and extending a recently proposed
nonequilibrium method of ab initio thermodynamics of deposition growth (AIT-DG)
[Rohrer J and Hyldgaard P 2010 Phys. Rev. B 82 045415]. A previous study of
this system [Rohrer J, Ruberto C and Hyldgaard P 2010 J. Phys.: Condens. Matter
22 015004] found that use of equilibrium thermodynamics leads to predictions of
a non-binding TiC/alumina interface, despite the industrial use as a
wear-resistant coating. This discrepancy between equilibrium theory and
experiment is resolved by the AIT-DG method which predicts interfaces with
strong adhesion. The AIT-DG method combines density functional theory
calculations, rate-equation modelling of the pressure evolution of the
deposition environment and thermochemical data. The AIT-DG method was
previously used to predict prevalent terminations of growing or as-deposited
surfaces of binary materials. Here we extent the method to predict surface and
interface compositions of growing or as-deposited thin films on a substrate and
find that inclusion of the nonequilibrium deposition environment has important
implications for the nature of buried interfaces. | 1009.1027v1 |
2010-09-07 | Phase coherent transport in SrTiO3/LaAlO3 interfaces | The two dimensional electron gas formed between the two band insulators
SrTiO3 and LaAlO3 exhibits a variety of interesting physical properties which
make it an appealing material for use in future spintronics and/or quantum
computing devices. For this kind of applications electrons have to retain their
phase memory for sufficiently long times or length. Using a mesoscopic size
device we were able to extract the phase coherence length, and its temperature
variation. We find the dephasing rate to have a power law dependence on
temperature. The power depends on the temperature range studied and sheet
resistance as expected from dephasing due to strong electron-electron
interactions. | 1009.1273v1 |
2010-10-11 | Restoring the electrical conductivity of graphene oxide films by UV light induced oxygen desorption | We report a chemical free method for the reduction of graphene oxide (GO)
thin films. It was observed how for GO films annealed in air the oxygen species
desorb from the GO surface upon exposure to UV light and how the surface
resistivity of such deoxygenated GO films decreases with increasing the
exposure to the UV light. The obtained results open a clean as well as easy
route for the integration of GO based materials into optoelectronic devices. | 1010.2108v4 |
2010-11-12 | Memory effects in complex materials and nanoscale systems | Memory effects are ubiquitous in nature and are particularly relevant at the
nanoscale where the dynamical properties of electrons and ions strongly depend
on the history of the system, at least within certain time scales. We review
here the memory properties of various materials and systems which appear most
strikingly in their non-trivial time-dependent resistive, capacitative and
inductive characteristics. We describe these characteristics within the
framework of memristors, memcapacitors and meminductors, namely memory circuit
elements whose properties depend on the history and state of the system. We
examine basic issues related to such systems and critically report on both
theoretical and experimental progress in understanding their functionalities.
We also discuss possible applications of memory effects in various areas of
science and technology ranging from digital to analog electronics,
biologically-inspired circuits, and learning. We finally discuss future
research opportunities in the field. | 1011.3053v1 |
2010-12-13 | The split-ring Josephson resonator as an artificial atom | Using the resistive-shunted-junction model we show that a split-ring
Josephson oscillator or radio-frequency SQUID in the hysteretic regime is
similar to an atomic system. It has a number of stationary states that we
characterize. Applying a short magnetic pulse we switch the system from one
state to another. These states can be detected via the reflection of a small
amplitude signal forming the base of a new spectroscopy. | 1012.2833v1 |
2010-12-23 | Memristive Systems Analysis of 3-Terminal Devices | Memristive systems were proposed in 1976 by Leon Chua and Sung Mo Kang as a
model for 2-terminal passive nonlinear dynamical systems which exhibit memory
effects. Such systems were originally shown to be relevant to the modeling of
action potentials in neurons in regards to the Hodgkin-Huxley model and, more
recently, to the modeling of thin film materials such as TiO2-x proposed for
non-volatile resistive memory. However, over the past 50 years a variety of
3-terminal non-passive dynamical devices have also been shown to exhibit memory
effects similar to that predicted by the memristive system model. This article
extends the original memristive systems framework to incorporate 3-terminal,
non-passive devices and explains the applicability of such dynamic systems
models to 1) the Widrow-Hoff memistor, 2) floating gate memory cells, and 3)
nano-ionic FETs.
Keywords-memristive systems, memistor, transconductance, synaptic transistor,
non-linear dynamic systems | 1012.5124v1 |
2011-01-14 | Ni-Mn-Ga films in the austenite and the martensite structures at room temperature: Uniaxial texturation and epitaxial growth | Ni-Mn-Ga films in the austenite and the martensite structures at room
temperature have been obtained using the DC magnetron sputtering technique. Two
elaboration processes were studied. A first batch of samples was deposited
using a resist sacrificial layer in order to release the film from the
substrate before vacuum annealing. This process leads to polycrystalline films
with a strong (022) fiber texture. The martensitic phase transformation of such
polycrystalline freestanding films has been studied by optical and scanning
electron microscopy. A second batch of samples was grown epitaxially on
(100)MgO substrates using different deposition temperatures. The texture has
been analyzed with four-circle X-ray diffraction. Epitaxial films crystallized
both in the austenite and the martensite structures at room temperature have
been studied. | 1101.2811v1 |
2011-01-31 | Yielding and irreversible deformation below the microscale: Surface effects and non-mean-field plastic avalanches | Nanoindentation techniques recently developed to measure the mechanical
response of crystals under external loading conditions reveal new phenomena
upon decreasing sample size below the microscale. At small length scales,
material resistance to irreversible deformation depends on sample morphology.
Here we study the mechanisms of yield and plastic flow in inherently small
crystals under uniaxial compression. Discrete structural rearrangements emerge
as series of abrupt discontinuities in stress-strain curves. We obtain the
theoretical dependence of the yield stress on system size and geometry and
elucidate the statistical properties of plastic deformation at such scales. Our
results show that the absence of dislocation storage leads to crucial effects
on the statistics of plastic events, ultimately affecting the universal scaling
behavior observed at larger scales. | 1102.0019v2 |
2011-05-11 | Linear magnetoresistance in commercial n-type silicon due to inhomogeneous doping | Free electron theory tells us that resistivity is independent of magnetic
field. In fact, most observations match the semiclassical prediction of a
magnetoresistance that is quadratic at low fields before saturating. However, a
non-saturating linear magnetoresistance has been observed in exotic
semiconductors such as silver chalcogenides, lightly-doped InSb, N-doped InAs,
MnAs-GaAs composites, PrFeAsO, and epitaxial graphene. Here we report the
observation of a large linear magnetoresistance in the ohmic regime in
commonplace commercial n-type silicon wafer. It is well-described by a
classical model of spatially fluctuating donor densities, and may be amplified
by altering the aspect ratio of the sample to enhance current-jetting:
increasing the width tenfold increased the magnetoresistance at 8 T from 445 %
to 4707 % at 35 K. This physical picture may well offer insights into the large
magnetoresistances recently observed in n-type and p-type Si in the non-ohmic
regime. | 1105.2174v1 |
2011-05-27 | Development of correlated quasiparticle conductance peak as molecule-linked gold nanoparticle films transition from Mott-insulator to metal phases | We have studied conductance ($\it{g}$) of butanedithiol-linked gold
nanoparticle films across a percolation insulator-to-metal transition. As the
transition proceeds, electrons become itinerant (i.e. Coulomb charging and
kinetic effects are both significant), and films exhibit a previously
unobserved zero-bias conductance peak (ZBCP). The peak is much more pronounced
and easily observed using electromigration-induced break junction (BJ) contacts
rather than macroscopic 4-probe electrodes. We attribute this ZBCP to quantum
correlations amongst electrons, in view of other temperature- ($\it{T}$-) and
magnetic ($\it{B}$-) dependent measurements as well as predictions of the
Hubbard model and dynamic mean field theory in this transition regime. Metallic
film resistances ($\it{R}$'s) increase linearly with $\it{T}$, but with
suggested scattering lengths that, anomalously, are shorter than inter-atomic
distances. Similar so-called "bad-metallic" behaviour has been observed in
several studies of correlated systems, and is still being understood. We find
here that the anomalous $\it{R}$ behaviours are associated with the ZBCP. This
system can serve as a new test bed for studying correlated electrons and points
to a nano building-block strategy for fashioning novel correlated materials. | 1105.5647v1 |
2011-07-31 | Observation of Quantized Hall Effect and Shubnikov-de Hass Oscillations in Highly Doped Bi2Se3: Evidence for Layered Transport of Bulk Carriers | Bi2Se3 is an important semiconductor thermoelectric material and a prototype
topological insulator. Here we report observation of Shubnikov-de Hass (SdH)
oscillations accompanied by quantized Hall resistances (Rxy) in highly-doped
n-type Bi2Se3 with bulk carrier concentrations of few 10^19 cm^-3. Measurements
under tilted magnetic fields show that the magnetotransport is 2D-like, where
only the c-axis component of the magnetic field controls the Landau level
formation. The quantized step size in 1/Rxy is found to scale with the sample
thickness, and average ~e2/h per quintuple layer (QL). We show that the
observed magnetotransport features do not come from the sample surface, but
arise from the bulk of the sample acting as many parallel 2D electron systems
to give a multilayered quantum Hall effect. Besides revealing a new electronic
property of Bi2Se3, our finding also has important implications for electronic
transport studies of topological insulator materials. | 1108.0204v2 |
2011-08-30 | Linear magnetoresistivity in the ternary AM2B2 and A3Rh8B6 phases (A = Ca, Sr; M = Rh, Ir) | We studied the magnetoresistivity of the AM2B2 and A3Rh8B6 (A = Ca, Sr; M =
Rh, Ir) compounds within the ranges 1.8<=T<=300 K and 0<=H<=50 kOe. The
zero-field resistivity {\rho}0(T) is metallic and follows closely the
Bloch-Gr\"uneisen description. A positive, nonsaturating, and dominantly
linear-in-H magnetoresistivity was observed in all samples, including the ones
with a superconducting ground state. Such {\Delta}{\rho}T(H)/{\rho}T(0),
reaching 1200% in favorable cases, was found to be much stronger for the AM2B2
compounds and to decrease with temperature as well as when Ca is replaced by
Sr, or Rh is replaced by Ir. Finally, the general features of the observed
magnetoresistivity will be discussed in terms of the Abrikosov model for the
linear magnetoresistivity in inhomogeneous materials. | 1108.5803v1 |
2011-09-22 | Microscopic model for the ferroelectric field effect in oxide heterostructures | A microscopic model Hamiltonian for the ferroelectric field effect is
introduced for the study of oxide heterostructures with ferroelectric
components. The long-range Coulomb interaction is incorporated as an
electrostatic potential, solved self-consistently together with the charge
distribution. A generic double-exchange system is used as the conducting
channel, epitaxially attached to the ferroelectric gate. The observed
ferroelectric screening effect, namely the charge accumulation/depletion near
the interface, is shown to drive interfacial phase transitions that give rise
to robust magnetoelectric responses and bipolar resistive switching, in
qualitative agreement with previous density functional theory calculations. The
model can be easily adapted to other materials by modifying the Hamiltonian of
the conducting channel, and it is useful in simulating ferroelectric field
effect devices particularly those involving strongly correlated electronic
components where ab-initio techniques are difficult to apply. | 1109.4791v1 |
2011-10-09 | Optimizing the Bi_(2-x)Sb_(x)Te_(3-y)Se_(y) solid solutions to approach the intrinsic topological insulator regime | To optimize the bulk-insulating behavior in the topological insulator
materials having the tetradymite structure, we have synthesized and
characterized single-crystal samples of Bi_(2-x)Sb_(x)Te_(3-y)Se_(y) (BSTS)
solid solution at various compositions. We have elucidated that there are a
series of "intrinsic" compositions where the acceptors and donors compensate
each other and present a maximally bulk-insulating behavior. At such
compositions, the resistivity can become as large as several Ohmcm at low
temperature and one can infer the role of the surface-transport channel in the
non-linear Hall effect. In particular, the composition of Bi1.5Sb0.5Te1.7Se1.3
achieves the lowest bulk carrier density and appears to be best suited for
surface transport studies. | 1110.1788v1 |
2011-10-24 | Ni(111)|Graphene|h-BN Junctions as Ideal Spin Injectors | Deposition of graphene on top of hexagonal boron nitride (h-BN) was very
recently demonstrated while graphene is now routinely grown on Ni. Because the
in-plane lattice constants of graphite, h-BN, graphite-like BC2N and of the
close-packed surfaces of Co, Ni and Cu match almost perfectly, it should be
possible to prepare ideal interfaces between these materials which are
respectively, a semimetal, insulator, semiconductor, ferromagnetic and
nonmagnetic metals. Using parameter-free energy minimization and electronic
transport calculations, we show how h-BN can be combined with the perfect spin
filtering property of Ni|graphite and Co|graphite interfaces to make perfect
tunnel junctions or ideal spin injectors (SI) with any desired resistance-area
product. | 1110.5291v1 |
2011-11-30 | Synthesis, Characterization, and Finite Size Effects on Electrical Transport of Nanoribbons of the Charge-Density Wave Conductor NbSe3 | NbSe3 exhibits remarkable anisotropy in most of its physical properties and
has been a model system for studies of quasi-one-dimensional
charge-density-wave (CDW) phenomena. Herein, we report the synthesis,
characterization, and electrical transport of single-crystalline NbSe3
nanoribbons by a facile one-step vapour transport process involving the
transport of selenium powder onto a niobium foil substrate. Our investigations
aid the understanding of the CDW nature of NbSe3 and the growth process of the
material. They also indicate that NbSe3 nanoribbons have enhanced CDW
properties compared to those of the bulk phase due to size confinement effects,
thus expanding the search for new mesoscopic phenomena at the nanoscale level.
Single nanoribbon measurements on the electrical resistance as a function of
temperature show charge-density wave transitions at 59 K and 141 K. We also
demonstrate significant enhancement in the depinning effect and sliding regimes
mainly attributed to finite size effects. | 1112.0039v1 |
2012-04-02 | Phase Stability, Structures and Properties of the (Bi2)m(Bi2Te3)n Natural Superlattices | The phase stability of the (Bi2)m(Bi2Te3)n natural superlattices has been
investigated through the low temperature solid state synthesis of a number of
new binary BixTe1-x compositions. Powder X-ray diffraction revealed that an
infinitely adaptive series forms for 0.44 < x < 0.70, while an unusual 2-phase
region with continuously changing compositions is observed for 0.41 < x < 0.43.
For x > 0.70, mixtures of elemental Bi and an almost constant composition
(Bi2)m(Bi2Te3)n phase are observed. Rietveld analysis of synchrotron X-ray
powder diffraction data collected on Bi2Te (m = 2, n = 1) revealed substantial
interchange of Bi and Te between the Bi2 and Bi2Te3 blocks, demonstrating that
the block compositions are variable. All investigated phase pure compositions
are degenerate semiconductors with low residual resistivity ratios and moderate
positive magnetoresistances (R/R0 = 1.05 in 9 T). The maximum Seebeck
coefficient is +80 muV K-1 for x = 0.63, leading to an estimated thermoelectric
figure of merit, zT = 0.2 at 250 K. | 1204.0356v1 |
2012-05-16 | Spin transport and spin dephasing in zinc oxide | The wide bandgap semiconductor ZnO is interesting for spintronic applications
because of its small spin-orbit coupling implying a large spin coherence
length. Utilizing vertical spin valve devices with ferromagnetic electrodes
(TiN/Co/ZnO/Ni/Au), we study the spin-polarized transport across ZnO in
all-electrical experiments. The measured magnetoresistance agrees well with the
prediction of a two spin channel model with spin-dependent interface
resistance. Fitting the data yields spin diffusion lengths of 10.8nm (2K),
10.7nm (10K), and 6.2nm (200K) in ZnO, corresponding to spin lifetimes of 2.6ns
(2K), 2.0ns (10K), and 31ps (200K). | 1205.3666v3 |
2012-05-28 | Visualizing Landau levels of Dirac electrons in a one dimensional potential | Using scanning tunneling spectroscopy we have measured the response of Dirac
electrons in a magnetic field to the presence of a well-defined smoothly
varying 1D periodic potential. We find that the lower index Landau level
energies reliably trace the potential variations, while the higher index levels
appear surprisingly homogeneous. Modeling the effects of the periodic potential
on the Landau level spectra, we show that the Landau level behavior encodes
information on the spatial extent of the wavefunctions. The lower index maps
reveal Landau level stripes, which would act as traps for chiral
one-dimensional modes. Our findings have important implications for transport
and magneto-resistance measurements in Dirac materials with engineered
potential landscapes. | 1205.6230v2 |
2012-07-14 | Topological phase transition induced by random substitution | The transition from topologically nontrivial to a trivial state is studied by
first-principles calculations on bulk zinc-blende type
(Hg$_{1-x}$Zn$_x$)(Te$_{1-x}$S$_x$) disordered alloy series. The random
chemical disorder was treated by means of the Coherent Potential Approximation.
We found that although the phase transition occurs at the strongest disorder
regime (${x\approx 0.5}$), it is still manifested by well-defined Bloch states
forming a clear Dirac cone at the Fermi energy of the bulk disordered material.
The computed residual resistivity tensor confirm the topologically-nontrivial
state of the HgTe-rich (${x<0.5}$), and the trivial state of the ZnS-rich alloy
series (${x>0.5}$) by exhibiting the quantized behavior of the off-diagonal
spin-projected component, independently on the concentration $x$. | 1207.3463v1 |
2012-07-23 | Superconducting and thermoelectric properties of new layered Superconductor Bi4O4S3 | Polycrystalline sample of the new layered superconductor Bi4O4S3 is
successfully synthesized by solid-state reaction method by using Bi, S and
Bi2O3 powders with one step reaction. The superconducting transition
temperature (Tconset=4.5 K), the zero resistance transition temperature
(Tc0=4.07 K) and the diamagnetic transition temperature (4.02 K at H=10 Oe)
were confirmed by electrical transport and magnetic measurements. Also, our
results indicate a typical type II-superconductor behavior. In addition, a
large thermoelectric effect was observed with a dimensionless thermoelectric
figure of merit (ZT) of about 0.03 at 300K, indicating Bi4O4S3 can be a
potential thermoelectric material. | 1207.5395v1 |
2012-09-13 | Topological-Metal to Band-Insulator Transition in (Bi1-xInx)2Se3 Thin Films | By combining transport and photo emission measurements on (Bi1-xInx)2Se3 thin
films, we report that this system transforms from a topologically non-trivial
metal into a topologically trivial band insulator through three quantum phase
transitions. At x = 3-7%, there is a transition from a topologically
non-trivial metal to a trivial metal. At x = 15%, the metal becomes a
variable-range-hopping insulator. Finally, above x = 25%, the system becomes a
true band insulator with its resistance immeasurably large even at room
temperature. This material provides a new venue to investigate topologically
tunable physics and devices with seamless gating/tunneling insulators. | 1209.2840v1 |
2012-11-01 | Spin Hall Magnetoresistance Induced by a Non-Equilibrium Proximity Effect | We report anisotropic magnetoresistance in Pt|Y3Fe5O12 bilayers. In spite of
Y3Fe5O12 being a very good electrical insulator, the resistance of the Pt layer
reflects its magnetization direction. The effect persists even when a Cu layer
is inserted between Pt and Y3Fe5O12, excluding the contribution of induced
equilibrium magnetization at the interface. Instead, we show that the effect
originates from concerted actions of the direct and inverse spin Hall effects
and therefore call it "spin Hall magnetoresistance." | 1211.0098v1 |
2012-11-07 | Tuning the thermoelectric properties of SrTiO3 by controlled oxygen doping | We report the thermoelectric properties (Seebeck coefficient, thermal
conductivity, and electrical resistivity) of lightly doped single crystals of
(001)-oriented SrTiO3 (STO). Hall effect measurements show that electron doping
around 10^-5 carriers per unit cell can be achieved by vacuum annealing of the
crystals under carefully controlled conditions. The steep density of states
near the Fermi energy of STO at this doping level (confirmed by ab initio
calculations) retains an unusually large Seebeck coefficient, in spite of an
increase in the electronic conductivity by several orders of magnitude. This
effect, combined with a decrease in thermal conductivity due to vacancy
disorder scattering makes intrinsic doping in STO (and other materials) an
alternative strategy to optimize its thermoelectric figure of merit. | 1211.1615v1 |
2012-12-21 | Multigap RPC for PET: development and optimisation of the detector design | Transforming the resistive plate chambers from charged-particle into
gamma-quanta detectors opens the way towards their application as a basic
element of a hybrid imaging system, which combines positron emission tomography
(PET) with magnetic resonance imaging (MRI) in a single device and provides
non- and minimally- invasive quantitative methods for diagnostics. To this end,
we performed detailed investigations encompassing the whole chain from the
annihilation of the positron in the body, through the conversion of the created
photons into electrons and to the optimization of the electron yield in the
gas. GEANT4 based simulations of the efficiency of the RPC photon detectors
with different converter materials and geometry were conducted for optimization
of the detector design. The results justify the selection of a sandwich-type
gas-insulator-converter design, with Bi or Pb as converter materials. | 1212.5551v1 |
2013-02-18 | Comparative Measurements of Inverse Spin Hall and Magnetoresistance in YIG|Pt and YIG|Ta | We report on a comparative study of spin Hall related effects and
magnetoresistance in YIG|Pt and YIG|Ta bilayers. These combined measurements
allow to estimate the characteristic transport parameters of both Pt and Ta
layers juxtaposed to YIG: the spin mixing conductance $G_{\uparrow \downarrow}$
at the YIG$|$normal metal interface, the spin Hall angle $\Theta_{SH}$, and the
spin diffusion length $\lambda_{sd}$ in the normal metal. The inverse spin Hall
voltages generated in Pt and Ta by the pure spin current pumped from YIG
excited at resonance confirm the opposite signs of spin Hall angles in these
two materials. Moreover, from the dependence of the inverse spin Hall voltage
on the Ta thickness, we extract the spin diffusion length in Ta, found to be
$\lambda_{sd}^\text{Ta}=1.8\pm0.7$ nm. Both the YIG|Pt and YIG|Ta systems
display a similar variation of resistance upon magnetic field orientation,
which can be explained in the recently developed framework of spin Hall
magnetoresistance. | 1302.4416v1 |
2013-02-20 | Anomalous metallic state above the upper critical field of the conventional three-dimensional superconductor AgSnSe2 with strong intrinsic disorder | We report superconducting properties of AgSnSe2 which is a conventional
type-II superconductor in the very dirty limit due to intrinsically strong
electron scatterings. While this material is an isotropic three-dimensional
(3D) superconductor with a not-so-short coherence length where strong vortex
fluctuations are NOT expected, we found that the magnetic-field-induced
resistive transition at fixed temperatures becomes increasingly broader toward
zero temperature and, surprisingly, that this broadened transition is taking
place largely ABOVE the upper critical field determined thermodynamically from
the specific heat. This result points to the existence of an anomalous metallic
state possibly caused by quantum phase fluctuations in a strongly-disordered 3D
superconductor. | 1302.4787v1 |
2013-03-06 | Li$_2$RhO$_3$: A spin-glassy relativistic Mott insulator | Motivated by the rich interplay among electronic correlation, spin-orbit
coupling (SOC), crystal-field splitting, and geometric frustrations in the
honeycomb-like lattice, we systematically investigated the electronic and
magnetic properties of Li$_2$RhO$_3$. The material is semiconducting with a
narrow band gap of $\Delta\sim$78 meV, and its temperature dependence of
resistivity conforms to 3D variable range hopping mechanism. No long-range
magnetic ordering was found down to 0.5 K, due to the geometric frustrations.
Instead, single atomic spin-glass behavior below the spin-freezing temperature
($\sim$6 K) was observed and its spin dynamics obeys the universal critical
slowing down scaling law. First principle calculations suggested it to be a
relativistic Mott insulator mediated by both electronic correlation and SOC.
With moderate strength of electronic correlation and SOC, our results shed new
light to the research of Heisenberg-Kitaev model in realistic materials. | 1303.1235v2 |
2013-06-13 | Combinatorial search of superconductivity in Fe-B composition spreads | We have fabricated Fe-B thin film composition spreads in search of possible
superconducting phases following a theoretical prediction by Kolmogorov et
al.^1 Co-sputtering was used to deposit spreads covering a large compositional
region of the Fe-B binary phase diagram. A trace of superconducting phase was
found in the nanocrystalline part of the spread, where the film undergoes a
metal to insulator transition as a function of composition in a region with the
average composition of FeB_2. The resistance drop occurs at 4K, and a
diamagnetic signal has also been detected at the same temperature. The
superconductivity is suppressible in the magnetic field up to 2 Tesla. | 1306.3024v1 |
2013-07-16 | Correlating toughness and roughness in ductile fracture | Three dimensional calculations of ductile crack growth under mode I plane
strain, small scale yielding conditions are carried out using an
elastic-viscoplastic constitutive relation for a progres- sively cavitating
plastic solid with two populations of void nucleating second phase particles.
Full field solutions are obtained for three dimensional material
microstructures characterized by ran- dom distributions of void nucleating
particles. Crack growth resistance curves and fracture surface roughness
statistics are calculated using standard procedures. The range of void
nucleating particle volume fractions considered give rise to values of
toughness, JIC, that vary by a factor of four. For all volume fractions
considered, the computed fracture surfaces are self-affine over a size range of
about two orders of magnitude with a roughness exponent of 0.54 $\pm$ 0.03. For
small void nucleating particle volume fractions, the mean large particle
spacing serves as a single dominant length scale. In this regime, the
correlation length of the fracture surface corresponding to the cut-off of the
self-affine behavior is found to be linearly related to JIC thus quantitatively
correlating toughness and fracture surface roughness. | 1307.4413v1 |
2013-08-30 | A linear nonequilibrium thermodynamics approach to optimization of thermoelectric devices | Improvement of thermoelectric systems in terms of performance and range of
applications relies on progress in materials science and optimization of device
operation. In this chapter, we focuse on optimization by taking into account
the interaction of the system with its environment. For this purpose, we
consider the illustrative case of a thermoelectric generator coupled to two
temperature baths via heat exchangers characterized by a thermal resistance,
and we analyze its working conditions. Our main message is that both electrical
and thermal impedance matching conditions must be met for optimal device
performance. Our analysis is fundamentally based on linear nonequilibrium
thermodynamics using the force-flux formalism. An outlook on mesoscopic systems
is also given. | 1308.6735v1 |
2013-09-24 | Efficient Gate-tunable light-emitting device made of defective boron nitride nanotubes: from ultraviolet to the visible | Boron nitride is a promising material for nanotechnology applications due to
its two-dimensional graphene-like, insulating, and highly-resistant structure.
Recently it has received a lot of attention as a substrate to grow and isolate
graphene as well as for its intrinsic UV lasing response. Similar to carbon,
one-dimensional boron nitride nanotubes (BNNTs) have been theoretically
predicted and later synthesised. Here we use first principles simulations to
unambiguously demonstrate that i) BN nanotubes inherit the highly efficient UV
luminescence of hexagonal BN; ii) the application of an external perpendicular
field closes the electronic gap keeping the UV lasing with lower yield; iii)
defects in BNNTS are responsible for tunable light emission from the UV to the
visible controlled by a transverse electric field (TEF). Our present findings
pave the road towards optoelectronic applications of BN-nanotube-based devices
that are simple to implement because they do not require any special doping or
complex growth. | 1309.6117v1 |
2013-10-11 | Dropping the Ball: The effect of anisotropic granular materials on ejecta and impact crater shape | In this fluid dynamics video, we present an experimental investigation of the
shape of impact craters in granular materials. Complex crater shapes, including
polygons, have been observed in many terrestrial planets as well as moons and
asteroids. We release spherical projectiles from different heights above a
granular bed (sand). The experiments demonstrate two different techniques to
create non-circular impact craters, which we measure by digitizing the final
crater topography. In the first method, we create trenches in the sand to mimic
fault lines or valleys on a planetary target. During impact, ejecta move faster
in the direction of the trenches, creating nearly elliptical craters with the
major axis running parallel to the trench. Larger trenches lead to more oblong
craters. In the second method, a hose beneath the surface of the sand injects
nitrogen gas. The pressure of the gas counters the hydrostatic pressure of the
sand, greatly reducing static friction between grains above the injection
point, without disturbing the surface. The affected sand has lower resistance
to impact, creating a knob in the otherwise circular crater rim. | 1310.3290v1 |
2013-10-20 | Impurity effects in Cu$_2$O | The doping of wide gap semiconductors is an interesting problem both from the
scientific and technological point of view. A well known example of this
problem is the doping of Cu$_2$O. The only element which has produced an order
of magnitude increase in the conductivity of Cu$_2$O bulk samples is chlorine,
as previously reported by us and others. However the solar cells produced with
this material do not show any improvement in performances because of the
reduction in the minority carrier diffusion length. In this paper we
investigate the effect of other impurities in order to check their possible use
as dopants and to assess their effects on the minority carrier diffusion
length. Seven impurities have been introduced by evaporation on the starting
copper sheet before the oxidation used to produce Cu$_2$O: chromium (Cr), iron
(Fe), silver (Ag), silicon (Si), sodium (Na), sulfur (S) and phosphorus (P).
The experiments show that a 20 ppm of concentration of these dopants does not
give any relevant effect neither on the resistivity, nor on the mobility. The
effect on minority carrier diffusion length is also negligible except for
sodium which produces a slight degradation of the samples. | 1310.5341v1 |
2013-11-07 | Magnetic domain and magnetic resistance phase transition in strongly correlated electronic material of perovskites junction | The junction magnetoresistivity and domain phase transition were studied
between ZnO and La0.4Gd0.1Sr0.5CoO3 thin films grown on LaAlO3 (100) substrates
epitaxially by pulse laser deposit. The ferromagnetic transformation into
phase-separated (two phase) state was displayed below Tc~127 and has observed
that the lattice change discontinuously in the doped cobalt perovskites
La0.4Gd0.1Sr0.5CoO3. The Ginzburg-Landau phase field is introduced to deduce
antiferroelectric domain structure in LGSCO thin film. On the basis of the
domain structures, the phase boundary of thin film is strongly dependent on the
combination of electric-mechanical coupling. The phase transformation into
phase separated state occurs below Tc~127-128K, and have displayed that the
lattice constants change discontinuously at the transformation. The positive MR
of ZnO/LGSCO heterojunction exhibited the MIT behavior at 0.2 T is 4.86%, at
0.5 T is 6.05% for approximately 140K. | 1311.1650v1 |
2013-11-17 | First-principle study of octahedral tilting and Ferroelectric like transition in metallic LiOsO3 | The octahedral tilting and ferroelectric-like structural transition of LiOsO3
metallic perovskite [Nature Materials 12, 1024 (2013)] was examined using
first-principles density-functional theory. In LiOsO3, a-a-a- octahedral
titling mode is responsible for the cubic to rhombohedral structural
transition, which is stable phase at room temperature. At low temperatures, a
non-centrosymmetric transition to a rhombohedra phase was realized due to zone
center phonon softening. The phase transition behavior of LiOsO3 can be
explained fully by density functional calculations and phonon calculations. The
electronic structure and Fermi surface changes due to the electron lattice
coupling effect are also presented. The carrier density of state across the
phase transition is associated with the resistivity, heat capacity, and
susceptibility. | 1311.4139v1 |
2014-02-24 | Experimental realization of a semiconducting full Heusler compound: Fe2TiSi | Single-phase films of the full Heusler compound Fe2TiSi have been prepared by
magnetron sputtering. The compound is found to be a semiconductor with a gap of
0.4eV. The electrical resistivity has a logarithmic temperature dependence up
to room temperature due to Kondo scattering of a dilute free electron gas off
superparamagnetic impurities. The origin of the electron gas is extrinsic due
to disorder or off-stoichiometry. Density functional theory calculations of the
electronic structure are in excellent agreement with electron energy loss,
optical, and x-ray absorption experiments. Fe2TiSi may find applications as a
thermoelectric material. | 1402.5755v1 |
2014-03-12 | Magnetoresistance of double layer hybrid system in tilted magnetic field | Magnetoresistance and Hall coefficient of a graphene layer are investigated
in the presence of a tilted magnetic field. We consider the graphene layer is
assembled by either another graphene layer or a two-dimensional electron gas
(2DEG) and an interlayer electron-electron interaction is modeled within Random
Phase Approximation. Our calculated magnetoresistances show different
interlayer screening effects between decoupled graphene-graphene and
graphene-2DEG systems. We also analyze the dependence of dielectric materials
as well as the distance between layers on magnetoresistances. The angle
dependence of the Hall coefficient is studied and we show that a quite large
Hall resistivity occurs in the graphene layer. | 1403.2857v2 |
2014-03-28 | Doping nature of native defects in 1T-TiSe2 | The transition metal dichalcogenide 1T-TiSe2 is a quasi two-dimensional
layered material with a charge density wave (CDW) transition temperature of
TCDW 200 K. Self-doping effects for crystals grown at different temperatures
introduce structural defects, modify the temperature dependent resistivity and
strongly perturbate the CDW phase. Here we study the structural and doping
nature of such native defects combining scanning tunneling
microscopy/spectroscopy and ab initio calculations. The dominant native single
atom dopants we identify in our single crystals are intercalated Ti atoms, Se
vacancies and Se substitutions by residual iodine and oxygen. | 1403.7339v2 |
2014-04-08 | Modulation of pure spin currents with a ferromagnetic insulator | We propose and demonstrate spin manipulation by magnetically controlled
modulation of pure spin currents in cobalt/copper lateral spin valves,
fabricated on top of the magnetic insulator Y$_3$Fe$_5$O$_{12}$ (YIG). The
direction of the YIG magnetization can be controlled by a small magnetic field.
We observe a clear modulation of the non-local resistance as a function of the
orientation of the YIG magnetization with respect to the polarization of the
spin current. Such a modulation can only be explained by assuming a finite
spin-mixing conductance at the Cu/YIG interface, as it follows from the
solution of the spin-diffusion equation. These results open a new path towards
the development of spin logics. | 1404.2311v2 |
2014-04-10 | Tunable chiral spin texture in magnetic domain-walls | Magnetic domain-walls (DWs) with a preferred chirality exhibit very efficient
current-driven motion. Since structural inversion asymmetry (SIA) is required
for their stability, the observation of chiral domain walls in highly symmetric
Pt/Co/Pt is intriguing. Here, we tune the layer asymmetry in this system and
observe, by current-assisted DW depinning experiments, a small chiral field
which sensitively changes. Moreover, we convincingly link the observed
efficiency of DW motion to the DW texture, using DW resistance as a direct
probe for the internal orientation of the DW under the influence of in-plane
fields. The very delicate effect of capping layer thickness on the chiral field
allows for its accurate control, which is important in designing novel
materials for optimal spin-orbit-torque-driven DW motion. | 1404.2945v1 |
2014-04-17 | Extreme thermopower anisotropy and interchain transport in the quasi-one-dimensional metal Li(0.9)Mo(6)O(17) | Thermopower and electrical resistivity measurements transverse to the
conducting chains of the quasi-one-dimensional metal Li(0.9)Mo(6)O(17) are
reported in the temperature range 5 K <= T <= 500 K. For T>= 400 K the
interchain transport is determined by thermal excitation of charge carriers
from a valence band ~ 0.14 eV below the Fermi level, giving rise to a large,
p-type thermopower that coincides with a small, n-type thermopower along the
chains. This dichotomy -- semiconductor-like in one direction and metallic in a
mutually perpendicular direction -- gives rise to substantial transverse
thermoelectric (TE) effects and a transverse TE figure of merit among the
largest known for a single compound. | 1404.4576v3 |
2014-05-15 | Damage nucleation from repeated dislocation absorption at a grain boundary | Damage nucleation from repeated dislocation absorption at a grain boundary is
simulated with molecular dynamics. At the grain boundary-dislocation
intersection site, atomic shuffling events determine how the free volume
brought by the incoming dislocation is accommodated. This process in turn
determines the crack nucleation mechanism, as well as the critical strain and
number of dislocations that can be absorbed before cracking. Slower strain
rates promote earlier crack nucleation and a damage nucleation mode where
cracking is preceded by dislocation emission. The simulation methodology
presented here can be used to probe other types of boundaries as well, although
multiple thermodynamically equivalent starting configurations are required to
quantify the damage resistance of a given grain boundary. | 1405.3974v2 |
2014-06-04 | Magnetic-Field Induced Semimetal in Topological Crystalline Insulator Thin Films | We investigate electromagnetic properties of a topological crystalline
insulator (TCI) thin film under external electromagnetic fields. The TCI thin
film is a topological insulator indexed by the mirror-Chern number. It is
demonstrated that the gap closes together with the emergence of a pair of
gapless cones carrying opposite chirarities by applying in-plane magnetic
field. A pair of gapless points have opposite vortex numbers. This is a
reminiscence of a pair of Weyl cones in 3D Weyl semimetal. We thus present an a
magnetic-field induced semimetal-semiconductor transition in 2D material. This
is a giant-magnetoresistance, where resistivity is controlled by magnetic
field. Perpendicular electric field is found to shift the gapless points and
also renormalize the Fermi velocity in the direction of the in-plane magnetic
field. | 1406.1009v2 |
2014-06-05 | Magnetotransport in ferromagnetic Mn5Ge3, Mn5Ge3C0.8, and Mn5Si3C0.8 thin films | The electrical resistivity, anisotropic magnetoresistance (AMR), and
anomalous Hall effect of ferromagnetic Mn5Ge3, Mn5Ge3C0.8, and Mn5Si3C0.8 thin
films has been investigated. The data show a behavior characteristic for a
ferromagnetic metal, with a linear increase of the anomalous Hall coefficient
with Curie temperature. While for ferromagnetic Mn5Si3C0.8 the normal Hall
coefficient R0 and the AMR ratio are independent of temperature, these
parameters strongly increase with temperature for the germanide films. This
difference is attributed to the different hybridization of electronic states in
the materials due different lattice parameters and different atomic
configurations (Ge vs. Si metalloid). The concomitant sign change of R0 and the
AMR ratio with temperature observed for the germanide films is discussed in a
two-current model indicating an electron-like minority-spin transport at low
temperatures. | 1406.1442v1 |
2014-07-16 | Mesoscale Imperfections in MoS2 Atomic Layers Grown by Vapor Transport Technique | The success of isolating small flakes of atomically thin layers through
mechanical exfoliation has triggered enormous research interest in graphene and
other two-dimensional materials. For device applications, however, controlled
large-area synthesis of highly crystalline monolayers with a low density of
electronically active defects is imperative. Here, we demonstrate the
electrical imaging of dendritic ad-layers and grain boundaries in monolayer
molybdenum disulfide (MoS2) grown by vapor transport technique using microwave
impedance microscopy. The micrometer-sized precipitates in our films, which
appear as a second layer of MoS2 in conventional height and optical
measurements, show 2 orders of magnitude higher conductivity than that of the
single layer. The zigzag grain boundaries, on the other hand, are shown to be
more resistive than the crystalline grains, consistent with previous studies.
Our ability to map the local electrical properties in a rapid and
nondestructive manner is highly desirable for optimizing the growth process of
large-scale MoS2 atomic layers. | 1407.4188v1 |
2014-07-30 | Synthesis and transport properties of ternary type-I Si clathrate K8Al7Si39 | A ternary type-I Si clathrate, K8AlxSi46-x, which is a candidate functional
material composed of abundant non-toxic elements, was synthesized and its
transport properties were investigated at temperatures ranging from 10 to 320
K. The synthesized compound is confirmed to be the ternary type-I Si clathrate
K8Al7Si39 with a lattice parameter of a = 10.442 A using neutron powder
diffractometry and inductively coupled plasma optical emission spectrometry.
Electrical resistivity and Hall coefficient measurements revealed that
K8Al7Si39 is a metal with electrons as the dominant carriers at a density of
approximately 1x10^27 /m3. The value of Seebeck coefficient for K8Al7Si39 is
negative and its absolute value increases with the temperature. The temperature
dependence of the thermal conductivity is similar to that for a crystalline
solid. The dimensionless figure of merit is approximately 0.01 at 300 K, which
is comparable to that for other ternary Si clathrates. | 1407.7911v2 |
2014-08-30 | Chemical Pressure effect at the boundary of Mott insulator and itinerant electron limit of Spinel Vanadates | The chemical pressure effect on the structural, transport, magnetic and
electronic properties (by measuring X-ray photoemission spectroscopy) of ZnV2O4
has been investigated by doping Mn and Co on the Zinc site of ZnV2O4. With Mn
doping the V-V distance increases and with Co doping it decreases. The
resistivity and thermoelectric power data indicate that as the V-V distance
decreases the system moves towards Quantum Phase Transition. The transport data
also indicate that the conduction is due to the small polaron hopping. The
chemical pressure shows the non-monotonous behaviour of charge gap and
activation energy. The XPS study also supports the observation that with
decrease of the V-V separation the system moves towards Quantum Phase
Transition. On the other hand when Ti is doped on the V-site of ZnV2O4 the
metal-metal distance decreases and at the same time the TN also increases. | 1409.0106v1 |
2014-12-23 | Temperature controlled motion of an antiferromagnet-ferromagnet interface within a dopant-graded FeRh epilayer | Chemically ordered B2 FeRh exhibits a remarkable
antiferromagnetic-ferromagnetic phase transition that is first order. It thus
shows phase coexistence, usually by proceeding though nucleation at random
defect sites followed by propagation of phase boundary domain walls. The
transition occurs at a temperature that can be varied by doping other metals
onto the Rh site. We have taken advantage of this to yield control over the
transition process by preparing an epilayer with oppositely directed doping
gradients of Pd and Ir throughout its height, yielding a gradual transition
that occurs between 350~K and 500~K. As the sample is heated, a horizontal
antiferromagnetic-ferromagnetic phase boundary domain wall moves gradually up
through the layer, its position controlled by the temperature. This mobile
magnetic domain wall affects the magnetisation and resistivity of the layer in
a way that can be controlled, and hence exploited, for novel device
applications. | 1412.7346v1 |
2015-01-21 | Efficient Linear Scaling Approach for Computing the Kubo Hall Conductivity | We report an order-N approach to compute the Kubo Hall conductivity for
disorderd two-dimensional systems reaching tens of millions of orbitals, and
realistic values of the applied external magnetic fields (as low as a few
Tesla). A time-evolution scheme is employed to evaluate the Hall conductivity
$\sigma_{xy}$ using a wavepacket propagation method and a continued fraction
expansion for the computation of diagonal and off-diagonal matrix elements of
the Green functions. The validity of the method is demonstrated by comparison
of results with brute-force diagonalization of the Kubo formula, using
(disordered) graphene as system of study. This approach to mesoscopic system
sizes is opening an unprecedented perspective for so-called reverse engineering
in which the available experimental transport data are used to get a deeper
understanding of the microscopic structure of the samples. Besides, this will
not only allow addressing subtle issues in terms of resistance standardization
of large scale materials (such as wafer scale polycrystalline graphene), but
will also enable the discovery of new quantum transport phenomena in complex
two-dimensional materials, out of reach with classical methods. | 1501.05100v2 |
2015-01-26 | Raman scattering investigation of large positive magnetoresistance material WTe$_2$ | We have performed polarized Raman scattering measurements on WTe$_2$, for
which an extremely large positive magnetoresistance has been reported recently.
We observe 5 A$_1$ phonon modes and 2 A$_2$ phonon modes out of 33 Raman active
modes, with frequencies in good accordance with first-principles calculations.
The angular dependence of the intensity of the peaks observed is consistent
with the Raman tensors of the $C_{2v}$ point group symmetry attributed to
WTe$_2$. Although the phonon spectra suggest neither strong electron-phonon nor
spin-phonon coupling, the intensity of the A$_1$ phonon mode at 160.6 cm$^{-1}$
shows an unconventional decrease with temperature decreasing, for which the
origin remains unclear. | 1501.06321v3 |
2015-02-17 | Superconducting Dome and Crossover to an Insulating State in [Tl4]Tl1-xSnxTe3 | The structural, superconducting, and electronic phase diagram of
[Tl4]Tl1-xSnxTe3 is reported. Magnetization and specific heat measurements show
bulk superconductivity exists for 0 < x < 0.4. Resistivity measurements
indicate a crossover from a metallic state for low dopings to a doped insulator
at x = 1. Universally, there is a large non-Debye specific heat contribution,
characterized by an Einstein temperature of {\theta}E ~ 35 K. Density
functional theory calculations predict x = 0 to be a topological metal, while x
= 1 is a topological crystalline insulator. The disappearance of
superconductivity correlates with the transition between these distinct
topological states. | 1502.05059v2 |
2015-02-24 | Room temperature magnetodielectric studies on Mn doped LaGaO3 | The polycrystalline samples of LaGa1-xMnxO3 (0<x<0.3) has been prepared by
solid state reaction route. The phase purity of these samples has been
confirmed by powder x-ray diffraction experiments carried out on BL-12 at
Indus-2 synchrotron radiation source. The sample with x=0.2 shows significant
change in the value of capacitance with the application of magnetic field. The
observed results were understood by systematically analyzing
magneto-capacitance (MC), magneto resistance (MR) and dielectric loss as a
function of frequency. Our results and analysis suggests that the observed
magneto dielectric (MD) coupling may be due to the MR effect of Maxwell-Wagner
type and/or field induced dipolar relaxation. Further it is observed that the
oxygen stoichiometry plays a very crucial role in observed MD coupling. | 1502.06887v2 |
2015-03-18 | Atomically Thin Resonant Tunnel Diodes built from Synthetic van der Waals Heterostructures | Vertical integration of two-dimensional van der Waals materials is predicted
to lead to novel electronic and optical properties not found in the constituent
layers. Here, we present the direct synthesis of two unique, atomically thin,
multi-junction heterostructures by combining graphene with the monolayer
transition-metal dichalocogenides: MoS2, MoSe2, and WSe2.The realization of
MoS2-WSe2-Graphene and WSe2-MoSe2-Graphene heterostructures leads toresonant
tunneling in an atomically thin stack with spectrally narrow room temperature
negative differential resistance characteristics. | 1503.05592v1 |
2015-09-17 | Microwave Loss Reduction in Cryogenically Cooled Conductors | Measurements of microwave attenuation at room temperature and 4.2 K have been
performed on some conductors commonly used in receiver input circuits. The
reduction in loss on cooling is substantial, particularly for copper and plated
gold, both of which showed a factor of 3 loss reduction. Copper passivated with
benzotriazole shows the same loss as without passivation. The residual
resistivity ratio between room temperature and 4.2 K, deduced from the
measurements using the classical skin effect formula, was smaller than the
measured DC value to a degree consistent with conduction in the extreme
anomalous skin effect regime at cryogenic temperatures. The measurements were
made in the 5-10 GHz range. The materials tested were: aluminum alloys 1100-T6
and 6061-O, C101 copper, benzotriazole treated C101 copper, and brass plated
with electroformed copper, Pur-A-Gold 125-Au soft gold, and BDT200 bright gold. | 1509.05273v1 |
2015-09-17 | Symmetry constraints on the elastoresistivity tensor | The elastoresistivity tensor $m_{ij,kl}$ characterizes changes in a
material's resistivity due to strain. As a fourth-rank tensor,
elastoresistivity can be a uniquely useful probe of the symmetries and
character of the electronic state of a solid. We present a symmetry analysis of
$m_{ij,kl}$ (both in the presence and absence of a magnetic field) based on the
crystalline point group, focusing for pedagogic purposes on the $D_{4h}$ point
group (of relevance to several materials of current interest). We also discuss
the relation between $m_{ij,kl}$ and various thermodynamic susceptibilities,
particularly where they are sensitive to critical fluctuations proximate to a
critical point at which a point group symmetry is spontaneously broken. | 1509.05462v1 |
2015-10-19 | Structure, Energy, and Thermal Transport Properties of Si-SiO$_2$ Nanostructures using an Ab initio based Parameterization of a Charge-Optimized Many-Body Forcefield | In an effort to extend the reach of current ab initio calculations to
simulations requiring millions of configurations for complex systems such as
heterostructures, we have parameterized the third-generation Charge Optimized
Many-Body (COMB3) potential using solely ab initio total energies, forces, and
stress tensors as input. The quality and the predictive power of the new
forcefield is assessed by computing properties including the cohesive energy
and density of SiO$_2$ polymorphs, surface energies of alpha-quartz, and phonon
densities of states of crystalline and amorphous phases of SiO$_2$. Comparison
with data from experiments, ab initio calculations, and molecular dynamics
simulations using published forcefields including BKS (van Beest, Kramer, and
van Santen), ReaxFF, and COMB2 demonstrate an overall improvement of the new
parameterization. The computed temperature dependence of the thermal
conductivity of crystalline alpha-quartz and the Kapitza resistance of the
interface between crystalline Si(001) and amorphous silica are in excellent
agreement with experiment, setting the stage for simulations of complex
nanoscale heterostructures. | 1510.05416v1 |
2015-11-07 | Anomalous magneto-elastic and charge doping effects in thallium-doped BaFe2As2 | Within the BaFe2As2 crystal lattice, we partially substitute thallium for
barium and report the effects of interlayer coupling in Ba1-xTlxFe2As2
crystals. We demonstrate the unusual effects of magneto-elastic coupling and
charge doping in this iron-arsenide material, whereby Neel temperature rises
with small x, and then falls with additional x. Specifically, we find that Neel
and structural transitions in BaFe2As2 (TN =Ts= 133 K) increase for x=0.05 (TN
= 138 K, Ts = 140 K) from magnetization, heat capacity, resistivity, and
neutron diffraction measurements. Evidence from single crystal X-ray
diffraction and first principles calculations attributes the stronger magnetism
in x=0.05 to magneto-elastic coupling related to the shorter intraplanar Fe-Fe
bond distance. With further thallium substitution, the transition temperatures
decrease for x = 0.09 (TN = Ts = 131 K), and this is due to charge doping. We
illustrate that small changes related to 3d transition-metal state can have
profound effects on magnetism. | 1511.02400v1 |
2015-11-24 | Transport and thermodynamic properties under anharmonic motion in type-I Ba8Ga16Sn30 clathrate | Anharmonic guest atom oscillation has direct connection to the thermal
transport and thermo- electric behavior of type-I Ba8Ga16Sn30 clathrates. This
behavior can be observed through several physical properties, with for example
the heat capacity providing a measure of the overall excita- tion level
structure. Localized anharmonic excitations also influence the low-temperature
resistivity, as we show in this paper. By combining heat capacity, transport
measurements and our previous NMR relaxation results, we address the
distribution of local oscillators in this material, as well as the shape of the
confining potential and the excitation energies for Ba(2) ions in the cages. We
also compare to the soft-potential model and other models used for similar
systems. The results show good agreement between the previously deduced
anharmonic rattler potential and experimental data. | 1511.07594v1 |
2015-11-28 | Extremely large magnetoresistance in few-layer graphene/boron-nitride heterostructures | Understanding magnetoresistance, the change in electrical resistance upon an
external magnetic field, at the atomic level is of great interest both
fundamentally and technologically. Graphene and other two-dimensional layered
materials provide an unprecedented opportunity to explore magnetoresistance at
its nascent stage of structural formation. Here, we report an extremely large
local magnetoresistance of ~ 2,000% at 400 K and a non-local magnetoresistance
of > 90,000% in 9 T at 300 K in few-layer graphene/boron-nitride
heterostructures. The local magnetoresistance is understood to arise from large
differential transport parameters, such as the carrier mobility, across various
layers of few-layer graphene upon a normal magnetic field, whereas the
non-local magnetoresistance is due to the magnetic field induced
Ettingshausen-Nernst effect. Non-local magnetoresistance suggests the
possibility of a graphene based gate tunable thermal switch. In addition, our
results demonstrate that graphene heterostructures may be promising for
magnetic field sensing applications. | 1511.08867v1 |
2015-12-22 | Large-signal model of the bilayer graphene field-effect transistor targeting radio-frequency applications: theory versus experiment | Bilayer graphene is a promising material for radio-frequency transistors
because its energy gap might result in a better current saturation than the
monolayer graphene. Because the great deal of interest in this technology,
especially for flexible radio-frequency applications, gaining control of it
requires the formulation of appropriate models for the drain current, charge
and capacitance. In this work we have developed them for a dual-gated bilayer
graphene field-effect transistor. A drift-diffusion mechanism for the carrier
transport has been considered coupled with an appropriate field-effect model
taking into account the electronic properties of the bilayer graphene.
Extrinsic resistances have been included considering the formation of a
Schottky barrier at the metal-bilayer graphene interface. The proposed model
has been benchmarked against experimental prototype transistors, discussing the
main figures of merit targeting radio-frequency applications. | 1512.07159v1 |
2016-01-21 | Saturation of ion irradiation effects in Cr2AlC | Cr2AlC materials were irradiated with 7 MeV Xe26+ ions and 500 keV He2+ ions
at room temperature. A structural transition with an increased c lattice
parameter and a decreased a lattice parameter occurs after irradiation to doses
above 1 dpa. Nevertheless, the modified structure is stable up to the dose of
5.2 dpa without obvious lattice disorder. The three samples irradiated to doses
above 1 dpa have comparable lattice parameters and hardness values, suggesting
a saturation of irradiation effects in Cr2AlC. The structural transition and
irradiation effects saturation are ascribed to irradiation-induced antisite
defects (CrAl and AlCr) and C interstitials, which is supported by the
calculations of the formation energies of various defects in Cr2AlC. The
irradiation-induced antisite defects and C interstitials may be critical to
understand the excellent resistance to irradiation-induced amorphization of MAX
phases. | 1601.05514v1 |
2016-02-02 | Fast synthesis of Fe1.1Se1-xTex superconductors in a self-heating and furnace-free way | A fast and furnace-free method of combustion synthesis is employed for the
first time to synthesize iron-based superconductors. Using this method,
Fe1.1Se1-xTex (0<=x<=1) samples can be prepared from self-sustained reactions
of element powders in only tens of seconds. The obtained Fe1.1Se1-xTex samples
show clear zero resistivity and corresponding magnetic susceptibility drop at
around 10-14 K. The Fe1.1Se0.33Te0.67 sample shows the highest onset Tc of
about 14 K, and its upper critical field is estimated to be approximately 54 T.
Compared with conventional solid state reaction for preparing polycrystalline
FeSe samples, combustion synthesis exhibits much-reduced time and energy
consumption, but offers comparable superconducting properties. It is expected
that the combustion synthesis method is available for preparing plenty of
iron-based superconductors, and in this direction further related work is in
progress. | 1602.00880v1 |
2016-02-11 | A multiscale model of distributed fracture and permeability in solids in all-round compression | We present a microstructural model of permeability in fractured solids, where
the fractures are described in terms of recursive families of parallel,
equidistant cohesive faults. Faults originate upon the attainment of a tensile
or shear resistance in the undamaged material. Secondary faults may form in a
hierarchical orga- nization, creating a complex network of connected fractures
that modify the permeability of the solid. The undamaged solid may possess
initial porosity and permeability. The particular geometry of the superposed
micro-faults lends itself to an explicit analytical quantification of the
porosity and permeability of the dam- aged material. The approach is
particularly appealing as a means of modeling low permeability oil and gas
reservoirs stimulated by hydraulic fracturing. | 1602.03801v2 |
2016-04-04 | Universal scaling for the spin-electricity conversion on surface states of topological insulators | We have investigated spin-electricity conversion on surface states of
bulk-insulating topological insulator (TI) materials using a spin pumping
technique. The sample structure is Ni-Fe|Cu|TI trilayers, in which magnetic
proximity effects on the TI surfaces are negligibly small owing to the inserted
Cu layer. Voltage signals produced by the spin-electricity conversion are
clearly observed, and enhanced with decreasing temperature in line with the
dominated surface transport at lower temperatures. The efficiency of the
spin-electricity conversion is greater for TI samples with higher resistivity
of bulk states and longer mean free path of surface states, consistent with the
surface spin-electricity conversion. | 1604.00751v1 |
2016-04-06 | Enhanced Thermoelectric Power and Electronic Correlations in RuSe$_2$ | We report the electronic structure, electric and thermal transport properties
of Ru$_{1-x}$Ir$_{x}$Se$_2$ ($x \leq 0.2$). RuSe$_2$ is a semiconductor that
crystallizes in a cubic pyrite unit cell. The Seebeck coefficient of RuSe$_2$
exceeds -200 $\mu$V/K around 730 K. Ir substitution results in the suppression
of the resistivity and the Seebeck coefficient, suggesting the removal of the
peaks in density of states near the Fermi level. Ru$_{0.8}$Ir$_{0.2}$Se$_{2}$
shows a semiconductor-metal crossover at about 30 K. The magnetic field
restores the semiconducting behavior. Our results indicate the importance of
the electronic correlations in enhanced thermoelectricity of RuSb$_{2}$. | 1604.01755v1 |
2016-04-12 | EPW: Electron-phonon coupling, transport and superconducting properties using maximally localized Wannier functions | The EPW (Electron-Phonon coupling using Wannier functions) software is a
Fortran90 code that uses density-functional perturbation theory and maximally
localized Wannier functions for computing electron-phonon couplings and related
properties in solids accurately and efficiently. The EPW v4 program can be used
to compute electron and phonon self-energies, linewidths, electron-phonon
scattering rates, electron-phonon coupling strengths, transport spectral
functions, electronic velocities, resistivity, anisotropic superconducting gaps
and spectral functions within the Migdal-Eliashberg theory. The code now
supports spin-orbit coupling, time-reversal symmetry in non-centrosymmetric
crystals, polar materials, and $\mathbf{k}$ and $\mathbf{q}$-point
parallelization. Considerable effort was dedicated to optimization and
parallelization, achieving almost a ten times speedup with respect to previous
releases. A computer test farm was implemented to ensure stability and
portability of the code on the most popular compilers and architectures. Since
April 2016, version 4 of the EPW code is fully integrated in and distributed
with the Quantum ESPRESSO package, and can be downloaded through QE-forge at
http://qe-forge.org/gf/project/q-e. | 1604.03525v2 |
2016-05-09 | A non-volatile memory based on nonlinear magnetoelectric effects | The magnetoelectric effects in multiferroics have a great potential in
creating next-generation memory devices. We conceive a new concept of
non-volatile memories based on a type of nonlinear magnetoelectric effects
showing a butterfly-shaped hysteresis loop. The principle is to utilize the
states of the magnetoelectric coefficient, instead of magnetization, electric
polarization or resistance, to store binary information. Our experiments in a
device made of the PMN-PT/Terfenol-D multiferroic heterostructure clearly
demonstrate that the sign of the magnetoelectric coefficient can be repeatedly
switched between positive and negative by applying electric fields, confirming
the feasibility of this principle. This kind of non-volatile memory has
outstanding practical virtues such as simple structure, easy operations in
writing and reading, low power, fast speed, and diverse materials available. | 1605.02505v1 |
2016-05-17 | Pressure-induced gap closing and metallization of MoSe$_{2}$ and MoTe$_{2}$ | Layered molybdenum dichalchogenides are semiconductors whose gap is
controlled by delicate interlayer interactions. The gap tends to drop together
with the interlayer distance, suggesting collapse and metallization under
pressure. We predict, based on first principles calculations, that layered
semiconductors 2H$_c$-MoSe$_2$ and 2H$_c$-MoTe$_2$ should undergo metallization
at pressures between 28 and 40 GPa (MoSe$_2$) and 13 and 19 GPa (MoTe$_2$).
Unlike MoS$_2$ where a 2H$_c$ $\to$ 2H$_a$ layer sliding transition is known to
take place, these two materials appear to preserve the original 2H$_c$ layered
structure at least up to 100 GPa and to increasingly resist lubric layer
sliding under pressure. Similar to metallized MoS$_2$ they are predicted to
exhibit a low density of states at the Fermi level, and presumably very modest
superconducting temperatures if any. We also study the $\beta$-MoTe$_2$
structure, metastable with a higher enthalpy than 2H$_c$-MoTe$_2$. Despite its
ready semimetallic and (weakly) superconducting character already at zero
pressure, metallicity is not expected to increase dramatically with pressure. | 1605.05111v1 |
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