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Georges Bouzerar

Publications and source records attributed to Georges Bouzerar.

At least 37 records · Page 2Linked to original sources
arXiv2012

Effective Heisenberg exchange integrals of diluted magnetic semiconductors determined within realistic multi-band tight-binding models

Diluted magnetic semiconductors (DMS) like Ga$_{1-x}$Mn$_{x}$As are described by a realistic tight-binding model (TBM) for the (valence) bands of GaAs, by a Zener (J-)term modeling the coupling of the localized Mn-spins to the spins of the valence band electrons, and by an additional potential scattering (V-) term due to the Mn-impurities. We calculate the effective (Heisenberg) exchange interaction between two Mn-moments mediated by the valence electrons. The influence of the number of bands taken into account (6-band or 8-band TBM) and of the potential (impurity) scattering V-term is investigated. We find that for realistic values of the parameters Ð the indirect exchange integrals show a long-range, oscillating (RKKY-like) behavior, if the V-term is neglected, probably leading to spin-glass behavior rather than magnetic order. But by including a V-term of a realistic magnitude the exchange couplings become short ranged and mainly positive allowing for the possibility of ferromagnetic order. Our results are in good agreement with available results of ab-initio treatments.

cond-mat.str-el↗
arXiv2012

Spin Hall Conductivity on the Anisotropic Triangular Lattice

We present a detailed study of the spin Hall conductivity on a two-dimensional triangular lattice in the presence of Rashba spin-orbit coupling. In particular, we focus part of our attention on the effect of the anisotropy of the nearest neighbor hopping amplitude. It is found that the presence of anisotropy has drastic effects on the spin Hall conductivity, especially in the hole doped regime where a significant increase or/and reversed sign of the spin Hall conductivity has been obtained. We also provide a systematic analysis of the numerical results in terms of Berry phases. The changes of signs observed at particular density of carriers appear to be a consequence of both Fermi surface topology and change of sign of electron velocity. In addition, in contrast to the two-dimensional square lattice, it is shown that the tight binding spin-orbit Hamiltonian should be derived carefully from the continuous model on the triangular lattice.

cond-mat.mes-hall↗
arXiv2012

RKKY Interactions in Graphene: Dependence on Disorder and Gate Voltage

We report the dependence of Ruderman-Kittel-Kasuya-Yoshida\,(RKKY) interaction on nonmagmetic disorder and gate voltage in grapheme. First the semiclassical method is employed to reserve the expression for RKKY interaction in clean graphene. Due to the pseudogap at Dirac point, the RKKY coupling in undoped grapheme is found to be proportional to $1/R^3$. Next, we investigate how the RKKY interaction depends on nonmagnetic disorder strength and gate voltage by studying numerically the Anderson tight-binding model on a honeycomb lattice. We observe that the RKKY interaction along the armchair direction is more robust to nonmagnetic disorder than in other directions. This effect can be explained semiclassically: The presence of multiple shortest paths between two lattice sites in the armchair directions is found to be responsible for the reduceddisorder sensitivity. We also present the distribution of the RKKY interaction for the zigzag and armchair directions. We identify three different shapes of the distributions which are repeated periodically along the zigzag direction, while only one kind, and more narrow distribution, is observed along the armchair direction. Moreover, we find that the distribution of amplitudes of the RKKY interaction crosses over from a non-Gaussian shape with very long tails to a completely log-normal distribution when increasing the nonmagnetic disorder strength. The width of the log-normal distribution is found to linearly increase with the strength of disorder, in agreement with analytical predictions. At finite gate voltage near the Dirac point, Friedel oscillation appears in addition to the oscillation from the interference between two Dirac points. This results in a beating pattern. We study how these beating patterns are effected by the nonmagnetic disorder in doped graphene.

cond-mat.mes-hall↗
arXiv2012

Nanoscale inhomogeneities: A new path toward high Curie temperature ferromagnetism in diluted materials

Room temperature ferromagnetism has been one of the most sought after topics in today's emerging field of spintronics. It is strongly believed that defect- and inhomogeneity- free sample growth should be the optimal route for achieving room-temperature ferromagnetism and huge efforts are made in order to grow samples as "clean" as possible. However, until now, in the dilute regime it has been difficult to obtain Curie temperatures larger than that measured in well annealed samples of (Ga,Mn)As ($\sim$190 K for 12% doping). In the present work, we propose an innovative path to room-temperature ferromagnetism in diluted magnetic semiconductors. We theoretically show that even a very small concentration of nanoscale inhomogeneities can lead to a tremendous boost of the critical temperatures: up to a 1600% increase compared to the homogeneous case. In addition to a very detailed analysis, we also give a plausible explanation for the wide variation of the critical temperatures observed in (Ga,Mn)N and provide a better understanding of the likely origin of very high Curie temperatures measured occasionally in some cases. The colossal increase of the ordering temperatures by nanoscale cluster inclusions should open up a new direction toward the synthesis of materials relevant for spintronic functionalities.

cond-mat.mtrl-sci↗
arXiv2011

Magnetic spin excitations in Mn doped GaAs : A model study

We provide a quantitative theoretical model study of the dynamical magnetic properties of optimally annealed Ga$_{1-x}$Mn$_x$As. This model has already been shown to reproduce accurately the Curie temperatures for Ga$_{1-x}$Mn$_x$As. Here we show that the calculated spin stiffness are in excellent agreement with those which were obtained from ab-initio based studies. In addition, an overall good agreement is also found with available experimental data. We have also evaluated the magnon density of states and the typical density of states from which the "mobility edge", separating the extended from localized magnon states, was determined. The power of the model lies in its ability to be generalized for a broad class of diluted magnetic semiconductor materials, thus it bridges the gap between first principle calculations and model based studies.

cond-mat.mtrl-sci↗
arXiv2011

RKKY Interaction in Disordered Graphene

We investigate the effects of nonmagnetic disorder on the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction in graphene by studying numerically the Anderson model with on-site and hopping disorder on a honeycomb lattice at half filling. We evaluate the strength of the interaction as a function of the distance R between two magnetic ions, as well as their lattice positions and orientations. In the clean limit, we find that the strength of the interaction decays as 1/R^3, with its sign and oscillation amplitude showing strong anisotropy. With increasing on-site disorder, the mean amplitude decreases exponentially at distances exceeding the elastic mean free path. At smaller distances, however, the oscillation amplitude increases strongly and its sign changes on the same sublattice for all directions but the armchair direction. For random hopping disorder, no sign change is observed. No significant changes to the geometrical average values of the RKKY interaction are found at small distances, while exponential suppression is observed at distances exceeding the localization length.

cond-mat.mes-hall↗
arXiv2011

Origins of shortcomings in recent realistic multiband Monte-Carlo studies for GaMnAs

Magnetic properties of Mn-doped GaAs are re-investigated within a realistic multiband description of the host valence bands. We explicitely demonstrate that the recent Monte Carlo (MC) simulations performed on a large scale supercomputer suffer from severe shortcomings. Indeed, it is shown, using identical parameters that (i) the calculated Zeeman-splitting largely underestimates that obtained from first principle studies, (ii) the couplings exhibit strong RKKY oscillations, (iii) the stability region for ferromagnetism is much narrower than obtained previously and (iv) the calculated Curie temperatures appear to be at least one order of magnitudes smaller. We show that the proposed choice of physical parameters cannot describe the physics in (Ga,Mn)As.

cond-mat.str-el↗
arXiv2010

Optical conductivity of Mn doped GaAs

We study the optical conductivity in the III-V diluted magnetic semiconductor GaMnAs and compare our calculations to available experimental data. Our model study is able to reproduce both qualitatively and quantitatively the observed measurements. We show that compensation (low carrier density) leads, in agreement to the observed measurements to a red shift of the broad peak located at approximately 200 meV for the optimally annealed sample. The non perturbative treatment appears to be essential, otherwise a blueshift and an incorrect amplitude would be obtained. By calculating the Drude weight (order parameter) we establish the metal-insulator phase diagram. We indeed find that Mn doped GaAs is close to the metal-insulator transition and that for 5$%$ and 7$%$ doped samples, 20$%$ of the carriers only are delocalized. We have found that the optical mass is approximately 2 m$_{e}$. We have also interesting results for overdoped samples which could be experimentally realized by Zn codoping.

cond-mat.dis-nn↗
arXiv2010

Unified picture for diluted magnetic semiconductors

For already a decade the field of diluted magnetic semiconductors (DMS) is one of the hottest. In spite of the great success of material specific Density Functional Theory (DFT) to provide accurately critical Curie temperatures ($T_{C}$) in various III-V based materials, the ultimate search for a unifying model/theory was still an open issue. Many crucial questions were still without answer, as for example: Why, after one decade, does GaMnAs still exhibit the highest $T_{C}$? Is there any intrinsic limitations or any hope to reach room temperature? How to explain in a unique theory the proximity of GaMnAs to the metal-insulator transition, and the change from RKKY couplings in II-VI materials to the double exchange regime in GaMnN? The aim of the present work is to provide this missing theory. We will show that the key parameter is the position of the Mn level acceptor and that GaMnAs has the highest $T_{C}$ among III-V DMS. Our theory (i) provides an overall understanding, (ii) is quantitatively consistent with existing DFT based studies, (iii) able to explain both transport and magnetic properties in a broad variety of DMS and (iv) reproduces the $T_{C}$ obtained from first principle studies for many materials including both GaMnN and GaMnAs. The model also reproduces accurately recent experimental data of the optical conductivity of GaMnAs and predicts those of other materials.

cond-mat.dis-nn↗
arXiv2010

Dynamical properties of a three-dimensional diluted Heisenberg model

We study the magnetic excitation spectrum in three-dimensional diluted ferromagnetic nearest-neighbor systems down to the percolation threshold. The disorder effects resulting from the dilution are handled accurately within self-consistent local random phase approximation approach. The calculations are performed using relatively large systems containing typically 20 000 localized spins, a systematic average over many configurations of disorder is performed. We analyze in details the change in the magnon spectrum and magnon density of states as we increase the dilution. The zone of stability of the well-defined magnon modes is shown to shrink drastically as we approach the percolation threshold. We also calculate the spin stiffness which appears to vanish at the percolation threshold exactly. A comparison with available data, based on a different theoretical approach, is also provided. We hope that this study will motivate new experimental studies based on inelastic neutron-scattering measurements.

cond-mat.str-el↗
arXiv2009

On the reliability of recent Monte Carlo studies of dilute systems of localized spins interacting with itinerant carriers

In this paper, we discuss magnetic properties of dilute systems of localized spins interacting with itinerant carriers. More precisely, we compare recent available Monte Carlo results with our two step approach (TSA) calculations. The TSA consists first on the determination of the magnetic couplings and then on a proper treatment of the resulting effective dilute Heisenberg Hamiltonian. We show important disagreement between the Monte Carlo results (in principle exact) and our TSA calculations. We analyze the origins and shed light on the reasons of those dissensions. In contrast to one could expect, we demonstrate that the available MC calculations suffer from severe numerical shortcomings. More precisely, (i) the finite size effects appear to be huge in dilute systems, (ii) the statistical sampling (disorder configurations) was far too small, and (iii) the determination of the Curie temperature was too rough. In addition, we provide new arguments to explain a recent disagreement between the Monte Carlo simulations and TSA for the model study of the well known III-V diluted magnetic semiconductor Ga$_{1-x}$Mn$_{x}$As. We hope that this work will motivate new systematic large scale Monte Carlo calculations.

cond-mat.dis-nn↗
arXiv2007

Magnetism without magnetic impurities in oxides ZrO2 and TiO2

We perform a theoretical study of the magnetism induced in transition metal dioxides ZrO2 and TiO2 by substitution of the cation by a vacancy or an impurity from the groups 1A or 2A of the periodic table, where the impurity is either K or Ca. In the present study both supercell and embedded cluster methods are used. It is demonstrated that the vacancy and the K-impurity leads to a robust induced magnetic moment on the surrounding O-atoms for both the cubic ZrO2 and rutile TiO2 host crystals. On the other hand it is shown that Ca-impurity leads to a non magnetic state. The native O-vacancy does not induce a magnetic moment in the host dioxide crystal.

cond-mat.mtrl-sci↗
arXiv2007

Ordering effects in diluted magnetic semiconductors

We review recently developed two-step approach for description of electronic and magnetic properties of a new class of materials, the diluted magnetic semiconductors. In the first step we construct, on the basis of the state-of-the-art first-principles electronic structure calculations, the effective Ising and Heisenberg Hamiltonians which describe, respectively, the alloy phase stability and the magnetic excitations in the system. In the second step, we analyze properties of these effective Hamiltonians by various methods of statistical physics. As a case study, the prototypical diluted magnetic semiconductor Ga_{1-x}Mn_{x}As is studied in detail. We determine, among others, a possibility for clustering in this system, formation energies of various compensating defects, and estimate short-range order parameters describing ordering tendencies in a system. On the other hand, by using recently developed local random-phase approximation approach, we evaluate the system Curie temperature and demonstrate its strong dependence on the sample preparation. We also emphasize the relevance of proper inclusion of the randomness in positions of magnetic impurities for a reliable estimate of the system critical temperature. Finally, we compare calculated Curie temperatures with available experimental data and briefly mention relation to other theoretical approaches.

cond-mat.mtrl-sci↗
arXiv2007

Magnetic spin excitations in diluted ferromagnetic systems: the case of $Ga_{1-x}Mn_{x}As$

We propose a theory which allow to calculate the magnetic excitation spectrum in diluted ferromagnetic systems. The approach is rather general and based on the Self-Consistent local Random Phase Approximation in which disorder (dilution) and thermal fluctuations are properly treated. To illustrate its reliability and accuracy we calculate the magnetic excitation in the diluted III-V magnetic semiconductor $Ga_{1-x}Mn_{x}As$. It is shown that dilution has a drastic effect on the excitation spectrum, indeed well defined magnon excitations exist only in a small region of the Brillouin zone centered around the $Γ$ point. We also calculate the spin stiffness in optimally annealed sample as a function of $Mn^{2+}$ concentration. A comparison to available measurements is done. We find a very good agreement for both the Curie temperature and the spin stiffness measured in well annealed samples and provide a plausible explanation for the very small values measured in as grown samples.

cond-mat.dis-nn↗
arXiv2007

Superexchange induced canted ferromagnetism in dilute magnets

We argue, in contrast to recent studies, that the antiferromagnetic superexchange coupling between nearest neighbour spins does not fully destroy the ferromagnetism in dilute magnets with long-ranged ferromagnetic couplings. Above a critical coupling, we find a \textit{canted} ferromagnetic phase with unsaturated moment. We have calculated the transition temperature using a simplified local Random Phase Approximation procedure which accounts for the canting. For the dilute magnetic semiconductors, such as GaMnAs, using \textit{ab-initio} couplings allows us to predict the existence of a canted phase and provide an explanation to the apparent contradictions observed in experimental measurements. Finally, we have compared with previous studies that used RKKY couplings and reported non-ferromagnetic state when the superexchange is too strong. Even in this case the ferromagnetism should remain essentially stable in the form of a canted phase.

cond-mat.dis-nn↗
arXiv2007

Effects of correlated disorder on the magnetism of double exchange systems

We study the effects of short-range correlated disorder arising from chemical dopants or local lattice distortions, on the ferromagnetism of 3d double exchange systems. For this, we integrate out the carriers and treat the resulting disordered spin Hamiltonian within local random phase approximation, whose reliability is shown by direct comparison with Monte Carlo simulations. We find large scale inhomogeneities in the charge, couplings and spin densities. Compared with the homogeneous case, we obtain larger Curie temperatures ($T_{C}$) and very small spin stiffnesses ($D$). As a result, large variations of $\frac{D}{T_{C}}$ measured in manganites may be explained by correlated disorder. This work also provides a microscopic model for Griffiths phases in double exchange systems.

cond-mat.str-el↗