pinecone/core-2020-05-10-deduplication huggingface dataset:
Transmission of charge and spin in a topological-insulator-based
  magnetic structure

We discuss the effect of a magnetic thin-film ribbon at the surface of a
topological insulator on the charge and spin transport due to surface
electrons.\\ If the magnetization in the magnetic ribbon is perpendicular to
the surface of a topological insulator, it leads to a gap in the energy
spectrum of surface electrons. As a result, the ribbon is a barrier for
electrons, which leads to electrical resistance.\\ We have calculated
conductance of such a structure. The conductance reveal some oscillations with
the length of the magnetized region due to the interference of transmitted and
reflected waves. We have also calculated the Seebeck coefficient when electron
flux is due to a temperature gradient.Comment: 7 pages, 7 figur
Effect of Ionized Impurity Screening on Spin Decoherence at Low and
  Intermediate Temperatures in GaAs

We study the effect of charged impurity screening on spin decoherence in bulk
{\it n}-type GaAs, and analyse in details the effect of the use of different
Born approximations applied to a linearized Thomas-Fermi screening theory. The
spin relaxation times are calculated by ensemble Monte Carlo techniques,
including electron-electron, electron-impurities, and electron-phonons
scattering. We carefully choose a parameter region so that all the physical
approximations hold, and, in particular, a Yukawa-type potential can be used to
describe the screened Coulomb interaction and the Born series converges. Our
results show that including the second order Born approximation yields much
shorter spin relaxation times compared to the commonly implemented first Born
approximation: spin relaxation times may be reduced by hundreds of picoseconds,
with the first Born approximation overestimating results by 30% or more for a
large region of parameters. Though our ensemble Monte Carlo simulations include
electron-electron and electron-phonon interactions, when considering low to
intermediate carrier densities and $T > 50 $ $\mathrm{K}$, but $T$ smaller than
the Fermi temperature, our results are in good agreement with Dyakonov-Perel
theory when this includes electron-impurity interactions only, which supports
this to be the most relevant scattering mechanism for bulk GaAs in this
low-intermediate temperature regime
Non-interacting two-impurity Anderson model on a lattice at
  particle-hole symmetry

We study the non-interacting two-impurity Anderson model on a lattice using
the Green function equation-of-motion method. A case of particular interest is
the RKKY limit that is characterized by a small hybridization between
impurities and host electrons and the absence of a direct coupling between the
impurities. In contrast to the low-density case, at half band-filling and
particle-hole symmetry, the RKKY interaction decays as the inverse square of
the impurity distance along the axis of a simple cubic lattice. In the RKKY
limit, for the spectral function we generically observe a small splitting of
the single-impurity resonance into two peaks. For a vanishing density-density
correlation function of the host electrons, we find only a broadened single
peak in the local density of states.Comment: 14 pages, 3 figure
Constraints on the total coupling strength to bosons in iron based
  superconductors

At present, there is still no consistent interpretation of the normal and
superconducting properties of Fe-based superconductors (FeSCs). The strength of
the el-el interaction and the role of correlation effects are under debate.
Here, we examine several common materials and illustrate various problems and
concepts that are generic for all FeSCs. Based on empirical observations and
qualitative insight from density functional theory, we show that the
superconducting and low-energy thermodynamic properties of the FeSCs can be
described semi-quantitively within multiband Eliashberg theory. We account for
an important high-energy mass renormalization phenomenologically,and in
agreement with constraints provided by thermodynamic, optical, and
angle-resolved photoemission data. When seen in this way, all FeSCs with
$T_\mathrm{c} <$ 40~K studied so far are found to belong to an {\it
intermediate} coupling regime. This finding is in contrast to the strong
coupling scenarios proposed in the early period of the FeSC history.We also
discuss several related issues, including the role of band shifts as measured
by the positions of van Hove singularities, and the nature of a recently
suggested quantum critical point in the strongly hole-doped systems
AFe$_2$As$_2$ (A = K, Rb, Cs). Using high-precision full relativistic GGA-band
structure calculations, we arrive at a somewhat milder mass renormalization in
comparison with previous studies. From the calculated mass anisotropies of all
Fermi surface sheets, only the $\varepsilon$-pocket near the corner of the BZ
is compatible with the experimentally observed anisotropy of the upper critical
field. pointing to its dominant role in the superconductivity of these three
compounds.Comment: 19 pages, 9 figure
Electronic structure of graphene: (nearly) free electrons bands vs.
  tight-binding bands

In our previous paper (Phys. Rev. B {\bf 89}, 165430 (2014)) we have found
that in graphene, in distinction to the four occupied bands, which can be
described by the simple tight-binding model (TBM) with four atomic orbitals per
atom, the two lowest lying at the $\Gamma$-point unoccupied bands (one of them
of a $\sigma$ type and the other of a $\pi$ type) can not be described by such
model. In the present work we suggest a minimalistic model for these two bands,
based on (nearly) free electrons model (FEM), which correctly describes the
symmetry of these bands, their dispersion law and their localization with
respect to the graphene plane.Comment: LaTeX, 8 pages, 18 eps figures. As accepted for publication in PSS
Clustering resonance effects in the electronic energy spectrum of tridiagonal Fibonacci quasicrystals

In this work, we show that the fundamental structure of the electronic energy spectrum of binary Fibonacci quasicrystals can be decomposed in terms of two main contributions, stemming from two related characteristic symmetries. The algebraic approach, we introduce allows us for a unified and systematic description of the energy spectrum finer structure details in terms of block matrices commutators properties, within the framework of a renormalization approach based on transfer matrices. Close analytical expressions can be explicitly obtained by exploiting some algebraic properties of these blocks commutators. In particular, the overall main features of the electronic energy spectrum structure are related to the roots of a series of polynomials of the form _[PF_(j)](E), where the subscript denotes the degree of the polynomial, which is given by a Fibonacci number F_j. These polynomials, in turn, are derived in a systematic way from commutators involving progressively longer palindromic fundamental blocks containing two types of basic renormalized matrices. In this way, we can classify the resonance energies defining the different fragmentation patterns of the energy spectrum on the basis of purely algebraic criteria. The transmission coefficient of these resonant states is always bounded below, although their related Landauer conductance values may range from highly conductive to highly resistive ones, depending on the system length. The obtained results significantly contribute to gain a better understanding of the symmetry principles governing the fragmentation process leading to the characteristic nested structure of the energy spectrum
Competing Structures in (In,Ga)Se and (In,Ga)2Se3 Semiconductors

The electronic structure of four polytypes (beta, gamma, delta and epsilon)
of hexagonal GaSe and InSe is calculated from first principles, using the
WIEN2k and VASP codes and PBEsol prescription for the exchange-correlation
potential, aiming specifically at elucidating the crystallographic parameters
and comparing the energy placement of corresponding competing structures.
Further on, the compounds with different composition of the same constituents,
namely the ordered-vacancies systems Ga2Se3 and In2Se3, were subject to a
similar study, of which the relaxed crystal structure data for three different
phases is reported. Comparison is done with the nomi- nal wurtzite structure
over which the cation vacancies are introduced, and the relaxation pattern
discussed.Comment: 4 pages, 3 figures, to be published in Phys.Stat.Solidi C in the
  proceedings of ICTMC-2
The granularity effect in amorphous InGaZnO$_4$ films prepared by rf
  sputtering method

We systematically investigated the temperature behaviors of the electrical
conductivity and Hall coefficient of two series of amorphous indium gallium
zinc oxides (a-IGZO) films prepared by rf sputtering method. The two series of
films are $\sim$700\,nm and $\sim$25\,nm thick, respectively. For each film,
the conductivity increases with decreasing temperature from 300\,K to $T_{\rm
max}$, where $T_{\rm max}$ is the temperature at which the conductivity reaches
its maximum. Below $T_{\rm max}$, the conductivity decreases with decreasing
temperature. Both the conductivity and Hall coefficient vary linearly with $\ln
T$ at low temperature regime. The $\ln T$ behaviors of conductivity and Hall
coefficient cannot be explained by the traditional electron-electron
interaction theory, but can be quantitatively described by the current
electron-electron theory due to the presence of granularity. Combining with the
scanning electron microscopy images of the films, we propose that the
boundaries between the neighboring a-IGZO particles could make the film
inhomogeneous and play an important role in the electron transport processes.Comment: 4 pages and 4 figure
Topological suppression of magnetoconductance oscillations in NS
  junctions

We show that the magnetoconductance oscillations of laterally-confined 2D NS
junctions are completely suppressed when the superconductor side enters a
topological phase. This suppression can be attributed to the modification of
the vortex structure of local currents at the junction caused by the
topological transition of the superconductor. The two regimes (with and without
oscillations) could be seen in a semiconductor 2D junction with a cleaved-edge
geometry, one of the junction arms having proximitized superconductivity. We
predict similar oscillations and suppression as a function of the Rashba
coupling. The oscillation suppression is robust against differences in chemical
potential and phases of lateral superconductors.Comment: 7 pages, 7 figure
Structure and energetics of embedded Si patterns in graphene

Recent experiments have revealed the possibility of precise electron beam
manipulation of silicon impurities in graphene. Motivated by these findings and
studies on metal surface quantum corrals, the question arises what kind of
embedded Si structures are possible within the hexagonal lattice, and how these
are limited by the distortion caused by the preference of Si for $sp^{3}$
hybridization. In this work, we study the geometry and stability of elementary
Si patterns in graphene, including lines, hexagons, triangles, circles and
squares. Due to the size of the required unit cells, to obtain the relaxed
geometries we use an empirical bond-order potential as a starting point for
density functional theory. Despite some interesting discrepancies, the
classical geometries provide an effective route for the simulation of large
structures.Comment: 6 pages, 5 figure
Spin and optical properties of silicon vacancies in silicon carbide (a
  review)

We discuss the fine structure and spin dynamics of spin-3/2 centers
associated with silicon vacancies in silicon carbide. The centers have
optically addressable spin states which makes them highly promising for quantum
technologies. The fine structure of the spin centers turns out to be highly
sensitive to mechanical pressure, external magnetic and electric fields,
temperature variation, etc., which can be utilized for efficient
room-temperature sensing, particularly by purely optical means or through the
optically detected magnetic resonance. We discuss the experimental achievements
in magnetometry and thermometry based on the spin state mixing at level
anticrossings in an external magnetic field and the underlying microscopic
mechanisms. We also discuss spin fluctuations in an ensemble of vacancies
caused by interaction with environment.Comment: 10 pages, for a special issue of Physica Status Solidi
Effect of Hydrogen Fluoride Addition and Synthesis Temperature on the Structure of Double-Walled Carbon Nanotubes Fluorinated by Molecular Fluorine

Double‐walled carbon nanotubes (DWCNTs) have been fluorinated by pure molecular fluorine (F2) at room temperature or 200 °C and a mixture of F2 with hydrogen fluoride (HF) at 200 °C that resulted in products with compositions of CF0.12, CF0.39, and CF0.53 as determined by X‐ray photoelectron spectroscopy. The differences in the structures of three kinds of fluorinated DWCNTs were revealed using transmission electron microscopy, Raman scattering, and near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy. Quantum‐chemical modeling of the NEXAFS F K‐edge spectra detected a change in the fluorine pattern with the increase of the F2 treatment temperature. The presence of HF in fluorine gas was found to accelerate the fluorination process and cause a partial destruction of outer shells of the DWCNT
Simulations on the influence of spatially-varying spin transport
  parameters on the measured spin lifetime in graphene non-local spin valves

Spin transport properties of graphene non-local spin valve devices are
typically determined from Hanle spin precession measurements by using a
simplified solution of the one-dimensional Bloch-Torrey equation which assumes
infinitely long transport channels and uniform spin transport parameter. We
investigate the effects of a finite graphene size and explore the influence of
spatially-varying transport parameters on the measured Hanle curves by finite
element simulations. We assume enhanced spin dephasing in the contact-covered
graphene areas with additional Fermi level pinning and explore the influence of
non-magnetic reference electrodes which are not properly decoupled from
graphene. In experiments, it is typically observed that the spin lifetime
increases with increasing charge carrier density. None of our simulations can
reproduce this trend indicating that this dependency originates from spin
transport through graphene areas which are not covered by contacts. We find
that the extracted spin lifetime might be overestimated in flakes which are
shorter than the spin diffusion length. Moreover, contact-induced spin
dephasing leads to an overall reduction of the extracted spin lifetime.
Additionally, we show that non-magnetic reference electrodes may also influence
the measured spin lifetime even though they are not part of the transport area
under investigation.Comment: 7 pages, 3 figure
The impact of hysteresis on the electrocaloric effect at first-order
  phase transitions

We study the impact of thermal hysteresis at the first-order
structural/ferroelectric phase transitions on the electrocaloric response in
bulk BaTiO$_3$ by performing molecular dynamics simulations for a
first-principles-based effective Hamiltonian. We demonstrate that the
electrocaloric response can conceptually be separated in two contributions: a
transitional part, stemming from the discontinuous jump in entropy at the first
order phase transition, and a configurational part, due to the continuous
change of polarization and entropy within each phase. This latter part
increases with the strength of the applied field, but for small fields it is
very small. In contrast, we find a large temperature change of $\sim 1$ K
resulting from the transition entropy, which is essentially independent of the
field strength. However, due to the coexistence region close to the first order
phase transition, this large electrocaloric response depends on the thermal
history of the sample and is generally not reversible. We show that this
irreversibility can be overcome by using larger fields.Comment: 7 pages, 5 figure
Moment-volume coupling in La(Fe$_{1-x}$Si$_x$)$_{13}$

We investigate the origin of the volume change and magnetoelastic interaction
observed at the magnetic first-order transition in the magnetocaloric system
La(Fe$_{1-x}$Si$_x$)$_{13}$ by means of first-principles calculations combined
with the fixed-spin moment approach. We find that the volume of the system
varies with the square of the average local Fe moment, which is significantly
smaller in the spin disordered configurations compared to the ferromagnetic
ground state. The vibrational density of states obtained for a hypothetical
ferromagnetic state with artificially reduced spin-moments compared to a
nuclear inelastic X-ray scattering measurement directly above the phase
transition reveals that the anomalous softening at the transition essentially
depends on the same moment-volume coupling mechanism. In the same spirit, the
dependence of average local Fe moment on the Si content can account for the
occurence of first- and second-order transitions in the system
Site occupancies and their effects on the physical properties of spinel
  $Co\left(Cr_{1-x}Fe_{x} \right)_{2}O_{4}$: an {\it ab initio} study

Recent experimental studies on Fe substituted spinel CoCr$_{2}$O$_{4}$ have
discovered multiple functional properties in the system such as temperature and
composition dependent magnetic compensation, tunable exchange bias and
magnetostriction. These properties are attributed to the renormalisation of the
inter-atomic magnetic exchange interactions arising due to the non-regular site
occupancies of the magnetic cations in the system. In this work, we perform
{\it ab initio} electronic structure calculations by DFT+U method and combine
with a generalised thermodynamic model to compute the site occupancy patterns
of the magnetic cations, the structural properties and the magnetic exchange
interactions of Co$\left(Cr_{1-x}Fe_{x} \right)_{2}$O$_{4}$ for the entire
composition range $0<x<1$. We find that the substituting Fe atoms prefer to
occupy the tetrahedral sites of the spinel structure for the entire range of
$x$, in agreement with the experimental inferences. Our results on the
variations of the structural parameters with compositions agree very well with
the experiments. By computing the variations of the various inter-atomic
magnetic exchange interactions, we provide a microscopic picture of the
evolution of a collinear structure from a non-collinear one due to substitution
of Fe in CoCr$_{2}$O$_{4}$. The computed results are analysed in terms of the
elements of the crystal field theory, and the features in the atoms and
orbital-projected densities of states. The results and analysis presented in
this work is the first comprehensive study on this system which would help
understanding the complexities associated with the site occupancies, the
electronic structures and the magnetic interactions in this multi-functional
material.Comment: Submitted in Journal of Physics
Using the Callaway model to deduce relevant phonon scattering processes:
  The importance of phonon dispersion

The thermal conductivity $\kappa$ of a material is an important parameter in
many different applications. Optimization strategies of $\kappa$ often require
insight into the dominant phonon scattering processes of the material under
study. The Callaway model is widely used as an experimentalist's tool to
analyze the lattice part of the thermal conductivity, $\kappa_l$. Here, we
investigate how deviations from the implicitly assumed linear phonon dispersion
relation affect $\kappa_l$ and in turn conclusions regarding the relevant
phonon scattering processes. As an example, we show for the half-Heusler system
(Hf,Zr,Ti)NiSn, that relying on the Callaway model in its simplest form has
earlier resulted in a misinterpretation of experimental values by assigning the
low measured $\kappa_l$ with unphysically strong phonon scattering in these
materials. Instead, we propose an implementation of more realistic phonon
dispersion curves, combined with empirical expressions for typical phonon
scattering processes, which leads to far better quantitative agreement with
both theoretical and experimental values. This method can easily be extended to
other materials with known phonon dispersion relations
Low lying resonant band modes in CsBr and CsI crystals doped with In+ and Tl+

Measurements of the far infrared absorption due to In+ and Tl+ impurities in CsBr and CsI single crystals yielded low frequency resonances with narrow linewidths at 11.0 cm−1 in CsBr:In+, 12.0 cm−1 in CsI:In+, 14.1 cm−1 in CsI:Tl+, and 16.95 cm−1 in CsBr:Tl+. The frequencies of the resonant band modes in the Tl+ doped crystals shift to higher values with increasing temperature. Using a model of the perturbed crystal which involves the mass defect and the change of the central force constants between the defect and its eight nearest neighbours, the force constant change is fitted to the various resonance frequencies. The resulting linewidths are compared with the experimentally determined values
Far-infrared absorption in amorphous III-V compound semiconductors

The infrared spectra of amorphous films of GaP, GaAs, GaSb, InAs, and Ge prepared by sputtering have been measured from 10 to 4000 cm−1. The absorption spectra in the region of the “optical” phonon frequencies show similarities to the phonon density of states as deduced from Raman scattering but the absorption becomes much smaller at low frequencies. It is shown that while the coupling constant for Raman scattering by an amorphous material varies as the square of the wave vector, it varies in the case of infrared absorption as its fourth power. The results are compared with those of Raman scattering and with the predictions of a simple model.\ud
\ud
Amorphe Filme aus Gap, GaAs, GaSb, InAs und Ge wurden durch Kathodenzerstäubung hergestellt und ihre Infrarotapektren von 10 bis 4000 cm−1 gemessen. Im Frequenzbereich der optischen Phononen zeigen die Absorptionsspektren Ähnlichkeiten mit der Zustandsdichte der Phononen, die aus der Ramanstreuung folgt, jedoch ist die Absorption bei niedrigen Frequenzen wesentlich geringer. Während die Kopplungskonstante der Ramanstreuung an einem amorphen Material sich wie das Quadrat des Wellenvektors verhält, wird gezeigt, daß sie im Falle der Infrarotabsorption proportional zu seiner vierten Potenz ist. Die Ergebnisse werden mit den Vorhersagen eines einfachen Modells verglichen
Magnetotransport Properties and Subband Structure of the Two-Dimensional Electron Gas in the Inversion Layer of Hg1-xCdxTe Bicrystals

The electronic and magnetotransport properties of conduction electrons in the grain boundary interface of p-type Hg1-xCdxTe bicrystals are investigated. The results clearly demonstrate the existence of a two-dimensional degenerate n-type inversion layer in the vicinity of the grain boundary. The observed quantum oscillations of the magnetoresistivity result from a superposition of the Shubnikov-de Haas effect in several occupied electric subbands. The occupation of higher subbands is presumable depending on the total carrier density ns of the inversion layer. Electron densities, subband energies, and effective masses of these electric subbands in samples with different total densities are determined. The effective masses of lower subbands are markedly different from the band edge values of the bulk material, their values decrease with decreasing electron density and converging to the bulk values at lower densities. This agrees with predictions of the triangular potential well model and a pronounced nonparabolicity of the energy bands in Hg1-xCdxTe. At high magnetic fields (B > 10 T) it is experimentally verified that the Hall resistivity xy is quantized into integer multiplies of h/e2.