pinecone/core-2020-05-10-deduplication huggingface dataset:
Nearest-neighbour antiferromagnetic interaction as a limiting factor for
  critical temperature in model DMS system

In numerous diluted magnetic semiconductor (DMS) systems, the competition
takes place between the short-range antiferromagnetic (AF) superexchange
interactions and the long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling
mediated by the charge carriers. Such a situation strongly influences the
critical temperature, the maximization of which constitutes a challenging task
in DMS physics and technology. The aim of the paper is to discuss theoretically
the limiting effect of AF interactions between nearest-neighbour magnetic ions
on the stability of inhomogeneous ferromagnetic state in a model diluted
magnetic system reflecting some crucial features of DMS.
  The modified molecular field-based model is constructed to account for the
magnetic inhomogeneity. The behavior of the system is studied as a function of
the ratio of superexchange integral to effective ferromagnetic coupling
integral, including the possibility of clustering/anticlustering tendency for
the magnetic ions. The ground state of the system is analysed. The critical
temperature is found to change non-monotonically with the concentration of
magnetic ions and decrease severely for larger concentrations. The behavior of
the system significantly differs from the predictions of the usual homogeneous
mean-field model. Brief comparison with selected experimental results for
(Zn,Mn)Te is provided
Nanocontact size dependence of the properties of vortex-based spin torque oscillators

We study the frequency, linewidth, and power of spin torque driven vortex oscillators, based on a nanocontacted spin-valve (SV). The oscillation frequency strongly decreases with the contact size, and increases with the current. The power delivered by the oscillator is not quadratic with the current, in contrast with the behavior expected from the rigid vortex model (RVM). The linewidth is almost independent of the current at low current and does not strongly depend on the nanocontact size. We compare our findings with the outcomes of the RVM. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimstatus: publishe
Role of non-collinear magnetization: from ferromagnetic nano wires to
  quantum rings

We consider interaction effects related to a nonuniform magnetization in
ferromagnetic nanowires and their possible generalization to nanorings. First
we show that the electric current in a ferromagnetic nanowire with more than
one domain wall induces an exchange coupling between the walls mediated by the
spin-dependent interference of scattered carriers. This interaction reveals a
complex behavior as a function of mutual orientations and separation of the
domain walls, thus affecting the domain wall dynamics. Then we consider how the
theory should be modified in the case of a magnetized quantum ring. In this
case the situation is more complicated because the vortex and onion
magnetization states in the ring are not truly ferromagnetic.Comment: 7 pages, published in pss(b
A slave-boson mean-field theory for general multi-band Hubbard models

We introduce a new slave-boson mean-field theory which allows the
investigation of general multi-band Hubbard models. Unlike earlier attempts of
such a generalisation, in our approach the quantum-mechanical problem is
exactly reformulated in an extended Hilbert space of Fermions and Bosons before
a mean-field approximation is applied. Systems with superconducting order
parameters are naturally included in our formalism. Our ground-state energy
functional agrees with the corresponding quantity derived within the Gutzwiller
theory.Comment: 8 pages, accepted for publication in Phys. Status Solidi
Fidelity of Optically Controlled Single- and Two-Qubit Operations on
  Coulomb-Coupled Quantum Dots

We investigate the effect of the Coulomb interaction on the applicability of
quantum gates on a system of two Coulomb-coupled quantum dots. We calculate the
fidelity for a single- and a two-qubit gate and the creation of Bell states in
the system. The influence of radiative damping is also studied. We find that
the application of quantum gates based on the Coulomb interaction leads to
significant input state-dependent errors which strongly depend on the Coulomb
coupling strength. By optimizing the Coulomb matrix elements via the material
and the external field parameters, error rates in the range of $10^{-3}$ can be
reached. Radiative dephasing is a more serious problem and typically leads to
larger errors on the order of $10^{-2}$ for the considered gates. In the
specific case of the generation of a maximally entangled Bell state, error
rates in the range of $10^{-3}$ can be achieved even in the presence of
radiative dephasing.Comment: 8 pages, 10 figures; final versio
Grain boundary effects on electronic transport in metals

We present quantum-based simulations of single grain boundary reflectivity of
electrons in metals, Cu and Ag. We examine twin and non-twin grain boundaries
using non-equilibrium Green's function and first principles methods. We also
investigate the mechanism of reflectivity in grain boundaries by modeling
atomic vacancies, disorder, and orientation and find that the change in grain
orientation and disorder in the boundary itself both contribute significantly
to reflectivity. We find that grain boundary reflectivity may vary widely
depending on the grain boundary structure consistent with experimental results.
Finally, we examine the reflectivity from multiple grain boundaries and find
that grain boundary reflectivity may depend on neighboring grain boundaries.
This study based on detailed numerical techniques reveals some potential
limitations in the independent grain boundary assumptions of the
Mayadas-Shatzkes model.Comment: To appear in Physica Status Solid
Accuracy of Quantum Monte Carlo Methods for Point Defects in Solids

Quantum Monte Carlo approaches such as the diffusion Monte Carlo (DMC) method
are among the most accurate many-body methods for extended systems. Their
scaling makes them well suited for defect calculations in solids. We review the
various approximations needed for DMC calculations of solids and the results of
previous DMC calculations for point defects in solids. Finally, we present
estimates of how approximations affect the accuracy of calculations for
self-interstitial formation energies in silicon and predict DMC values of
4.4(1), 5.1(1) and 4.7(1) eV for the X, T and H interstitial defects,
respectively, in a 16(+1)-atom supercell
Spin selective transport through Aharonov-Bohm and Aharonov-Casher
  triple quantum dot systems

We calculate the conductance through a system of three quantum dots under two
different sets of conditions that lead to spin filtering effects under an
applied magnetic field. In one of them, a spin is localized in one quantum dot,
as proposed by Delgado et al. [Phys. Rev. Lett. 101, 226810 (2008)]. In the
other one, all dots are equivalent by symmetry and the system is subject to a
Rashba spin-orbit coupling. We solve the problem using a simple effective
Hamiltonian for the low-energy subspace, improving the accuracy of previous
results. We obtain that correlation effects related to the Kondo physics play a
minor role for parameters estimated previously and high enough magnetic field.
Both systems lead to a magnetic field tunable "spin valve".Comment: 9 pages, 8 figures, improved presentation and references added,
  accepted for publication in Physica Status Solidi
Fabrication and characterization of graphene-based electronic devices

Graphene, a two-dimensional hexagonal carbon lattice, is a promising material for future electronics. High carrier mobility is viable through the two-dimensional plane and the true atomic thick layer enables to be transparency and flexiblility. The property is unique and never found before in other materials. A sp2-hybridized bonding in a lattice leads graphene to have physical strength that is about 100 times higher than steel. Its physical property is sustained while graphene is deformed. This is the reason why the graphene has been most attractive for electronics since it discovered in 2004. 
In this thesis, several different sources of graphene are introduced and investigated towards device applications. Among the sources ever known, graphene prepared on transition metal by chemical vapor deposition (CVD) is most popular since the method yields a uniform singlelayer without a size limit. For fast and cost-effective synthesis of graphene, photo-thermal CVD (PTCVD) was further developed by investigating the process conditions and parameters, such as, the flow rate of precursor gases, pressure, time, and temperature. Particularly, influence of growth temperature on the graphene quality was further examined. As a result, synthesis of high quality single-layer graphene was achieved on copper at 935-950 °C in about 60 s. The quality of graphene was preliminarily determined by scanning electron microscopy and Raman spectroscopy.
Employing the CVD graphene, field-effect devices were fabricated and characterized at room temperature. With the control of the gate, highly tunable and switchable devices performing as a rectifier and an inverter were demonstrated. Remarkably, the device exhibiting full-wave rectification for 100 kHz of the AC input was presented utilizing three-terminal T-branch junction (TBJ). By applying the same CVD graphene layer to the gate electrode, transparent functionality through the device structure was additionally achieved. The experimental results are comparable to the previously reported TBJs having efficiency of 5-12% as the CVD graphene based TBJs shown here exhibits rectification with efficiency of 18%. As an inverter in the TBJ device, the highest voltage gain was observed to 2.4 at VD= 4 V. Finally, a cascaded two TBJ device structure where the output of the first TBJ was utilized as a gate input for thesecond TBJ was demonstrated. The output of the cascaded structure was displayed as clear rectification without any external gate. This is a significant step to realize the possibility of layer-by-layer device architecture for graphene-based monolithic integrated circuit, overcoming a zero-bandgap limit
On the way to meet the experimental observation of persistent current in
  a mesoscopic cylinder: A mean field study

The behavior of persistent current in a mesoscopic cylinder threaded by an
Aharonov-Bohm flux $\phi$ is carefully investigated within a Hartree-Fock mean
field approach. We examine the combined effect of second-neighbor hopping
integral and Hubbard correlation on the enhancement of persistent current in
presence of disorder. A significant change in current amplitude is observed
compared to the traditional nearest-neighbor hopping model and the current
amplitude becomes quite comparable to experimental realizations. Our analysis
is found to exhibit several interesting results which have so far remained
unaddressed.Comment: 9 pages, 8 figure
Direct Graphene Growth on Insulator

Fabrication of graphene devices is often hindered by incompatibility between the silicon technology and the methods of graphene growth. Exfoliation from graphite yields excellent films but is good mainly for research. Graphene grown on metal has a technological potential but requires mechanical transfer. Growth by SiC decomposition requires a temperature budget exceeding the technological limits. These issues could be circumvented by growing graphene directly on insulator, implying Van der Waals growth. During growth, the insulator acts as a support defining the growth plane. In the device, it insulates graphene from the Si substrate. We demonstrate planar growth of graphene on mica surface. This was achieved by molecular beam deposition above 600 °C. High resolution Raman scans illustrate the effect of growth parameters and substrate topography on the film perfection. Ab initio calculations suggest a growth model. Data analysis highlights the competition between nucleation at surface steps and flat surface. As a proof of concept, we show the evidence of electric field effect in a transistor with a directly grown channel.

Energetic carbon atoms impinge onto the mica. Few-layer graphene nucleates at step edges and at chemisorbed carbon atoms (black circles). At temperatures above 600 °C, nano-sized graphene flakes can slide freely to coalesce into a graphene film.Physic
Optical parametric oscillation in 1D semiconductor microcavities

International audienceWe present an experimental investigation of parametric scattering processes and optical parametric oscillations in wire shaped, one-dimensional semiconductor microcavities. Far field emission patterns and corresponding band dispersion are studied by polarization resolved measurements and power dependence measurements under resonant and non-resonant excitation. The multiplicity of the photonic bands allows for an efficient engineering of interbranch parametric scattering processes. We demonstrate the onset of optical parametric oscillation of perfectly balanced twin beams, degenerate in energy and split in momentum space
Curie temperatures of the concentrated and diluted Kondo-lattice model
  as a possible candidate to describe magnetic semiconductors and metals

We present a theory to model carrier mediated ferromagnetism in concentrated
or diluted local moment systems. The electronic subsystem of the Kondo lattice
model is described by a combined equation of motion / coherent potential
approximation method. Doing this we can calculate the free energy of the system
and its minimum according to the magnetization of the local moments. Thus also
the Curie temperature can be determined and its dependence on important model
parameters. We get qualitative agreement with the Curie temperatures'
experimental values of Ga$_{1-x}$Mn$_x$As for a proper hole density.Comment: 11 pages, 10 figures, refereed version of Physica Status Solidi b
  (http://dx.doi.org/10.1002/pssb.201147059
Interference of coherent spin waves in micron-sized ferromagnetic
  waveguides

We present experimental observations of the interference of spin-wave modes
propagating in opposite directions in micron-sized NiFe-waveguides. To monitor
the local spin-wave intensity distribution and phase of the formed interference
pattern, we use Brillouin light scattering microscopy. The two-dimensional
spin-wave intensity map can be understood by considering the interference of
several waveguide eigenmodes with different wavevectors quantized across the
width of the stripe. The phase shows a transition from linear dependence on the
space coordinate near the antennas characteristic for propagating waves to
discrete values in the center region characteristic for standing waves
The effect of chemical disorder on the magnetic anisotropy of strained
  Fe-Co films

Strained Fe-Co films have recently been demonstrated to exhibit a large
magnetocrystalline anisotropy (MCA) and thus to be of potential interest as
magnetic storage material. Here, we show by means of density-functional (DF)
calculations, that chemical order can remarkably enhance the MCA. We also
investigate the effect of relaxation perpendicular to the applied strain and
evaluate the strain energy as a function of Co concentration and substrate
lattice parameter. On this basis, favourable preparation routes for films with
a large perpendicular anisotropy are suggested.Comment: - 7 pages, 7 figures - the paper is intended for a special issue on
  the occasion of M. Faehnles 60th birthda
Ultrafast magnetization dynamics of Gd(0001): Bulk vs. surface

Ultrafast laser-induced demagnetization of Gd(0001) has been investigated by
magneto-induced optical second harmonic generation and the magneto-optical Kerr
effect which facilitate a comparison of surface and bulk dynamics. We observe
pronounced differences in the transient changes of the surface and bulk
sensitive magneto-optical signals which we attribute to transfer of optically
excited, spin-polarized carriers between surface and bulk states of the
Gd(0001) film. A fluence dependent analysis of the bulk magnetization dynamics
results in a weak variation of the demagnetization time constant, which starts
at about 700 fs and increases by 10% within a fluence variation up to 1 mJ/cm2.
We compare these results with fluence dependent changes in the transient energy
density calculated by the two temperature model. The determined characteristic
times of excess energy transfer from the electron system to the lattice, which
is mediated by e-ph scattering, range from 0.2 - 0.6 ps. Such a more pronounced
fluence dependent change in the characteristic time compared to the observed
rather weakly varying demagnetization times suggests a more advanced
description of the optically excited state than by the two-temperature model.Comment: 15 pages, 4 figure
Impurity states on the honeycomb lattice using the Green's function
  method

Using the Green's function method, we study the effect of an impurity
potential on the electronic structure of the honeycomb lattice in the one-band
tight-binding model that contains both the nearest neighbor ($t$) and the
second neighbor ($t'$) interactions. The model is relevant to the case of the
substitutional vacancy in graphene. If the second neighbor interaction is large
enough ($t' > t /3$), then the linear Dirac bands no longer occur at the Fermi
energy and the electronic structure is therefore fundamentally changed. With
only the nearest neighbor interactions present, there is particle-hole
symmetry, as a result of which the vacancy induces a "zero-mode" state at the
band center with its wave function entirely on the majority sublattice, i. e.,
on the sublattice not containing the vacancy. With the introduction of the
second neighbor interaction, the zero-mode state broadens into a resonance peak
and its wave function spreads into both sublattices, as may be argued from the
Lippmann-Schwinger equation. The zero-mode state disappears entirely for the
triangular lattice and if $t'$ is large for the honeycomb lattice as well. In
case of graphene, $t'$ is relatively small, so that a well-defined zero-mode
state occurs in the vicinity of the band center.Comment: 8 pages, 9 figure
Isothermal magnetocaloric effect in the vicinity of the Lifshitz point
  in Mn_{0.9}Co_{0.1}P

The magnetic field - temperature (B - T) phase diagram of the Mn0.9Co0.1P
single crystal is studied in the vicinity of the Lifshitz point by means of
isothermal magnetocaloric coefficient (Mt) and AC susceptibility measurements.
Results confirm previously reported shape of the B - T phase diagram and
locations of characteristic temperatures and fields. At the Curie temperature
(Tc) the critical exponent w, which describes a singularity of Mt as a function
of magnetic field (Mt \propto B^-w), is estimated for B parallel to the easy
axis to be equal to w \approx 0.35. Below Tc an evidence of a new enigmatic
phase, reported already for pure MnP, is found in susceptibility data also for
Mn0.9Co0.1P. However, the range of existence of this phase is significantly
larger here, than in MnP. At the Lifshitz point there is observed a sharp peak
in the imaginary part of the magnetic susceptibility. A phenomenological theory
is introduced to describe the field dependence of the critical lines from the
disordered phase (paramagnetic) to ordered phases (ferromagnetic and
modulated). The temperature and field dependences of the magnetocaloric
coefficient and susceptibility are also calculated within the same framework.Comment: 20 pages, 12 figure
Time evolution of resistance in response to magnetic field: Evidence of glassy transport in La0.85Sr0.15CoO3

We demonstrate the distinct glassy transport phenomena associated with the phase separated and spin-glass-like phases of La0.85Sr0.15CoO3, prepared under different heat-treatment conditions. The low-temperature annealed (phase-separated) sample, exhibits a small change in resistance, with evolution of time, as compared to the high-temperature annealed (spin glass) one. However, the resistance change as a function of time, in both cases, is well described by a stretched exponential fit, signifying the slow dynamics. Moreover, the ultraviolet spectroscopy study evidences a relatively higher density of states in the vicinity of EF for low-temperature annealed sample and this correctly points to its less semiconducting behavior
Molecular beam growth of graphene on mica

We demonstrate molecular beam growth of graphene on biotite mica substrates
at temperatures below 1000{\deg}C. As indicated by optical and atomic force
microscopy, evaporation of carbon from a high purity solid-state source onto
biotite surface results in the formation of single-, bi-, and multilayer
graphene with size in the micrometer regime. Graphene grown directly on mica
surface is of very high crystalline quality with the defect density below the
threshold detectable by Raman spectroscopy. The interaction between graphene
and the mica substrate is studied by comparison of the Raman spectroscopy and
atomic force microscopy data with the corresponding results obtained for
graphene flakes mechanically exfoliated onto biotite substrates. Experimental
insights are combined with density functional theory calculations to propose a
model for the initial stage of the van der Waals growth of graphene on mica
surfaces. This work provides important hints on how the direct growth of high
quality graphene on insulators can be realized in general without exceeding the
thermal budget limitations of Si technologies.Comment: 17 pages, 4 figure
DFT investigations of the piezoresistive effect of carbon nanotubes for
  sensor application

We investigate the piezoresistive effect of carbon nanotubes (CNTs) within
density functional theory (DFT) aiming at application-relevant CNTs. CNTs are
excellent candidates for the usage in nano-electromechanical sensors (NEMS) due
to their small band gap at zero strain leading to a finite resistivity at room
temperature. The application of strain induces a band gap-opening leading to a
tremendous change in the resistivity. DFT with the LDA approximation yields
reasonable results for pure carbon systems like CNTs and is applied to
calculate the electronic structure of experimentally relevant CNTs. For the
transport part, a simple ballistic transport model based on the band gap is
used. We compare our DFT results for the band gaps of strained CNTs to results
of tight binding (TB) models. By introducing a scaling factor of $\sqrt{2}$, an
excellent agreement of the the DFT data with TB model published by Yang et al.
[1] is obtained.Comment: 4 pages, 5 figure
Non-linear electro-elastic coupling in highly strained zinc-blende compounds: InGaP/GaP [111] quantum wells

International audienceA symmetry-based thermodynamical model of third-order electro-elastic coupling is applied to zinc-blende semiconductors. Density functional theory is used to calculate the complete set of linear and non-linear coefficients. The entanglement of non-linear piezoelectricity, electrostriction and other non-linear phenomena, is studied in details at the same level of theory. Numerical results for InxGa1−xP/GaP [111] quantum wells show the predominance of the non-linear piezoelectricity contribution. However, for a coupled non-linear model, the electrostriction has also to be taken into account. Finally, the non-linear elasticity gives a significant correction on the strain calculation