arXiv subjects

Georges Bouzerar

Publications and source records attributed to Georges Bouzerar.

44 records · Page 3Linked to original sources

arXiv2006

Non-perturbative $J_{pd}$ model and ferromagnetism in dilute magnets

We calculate magnetic couplings in the $J_{pd}$ model for dilute magnets, in order both to identify the relevant parameters which control ferromagnetism and also to bridge the gap between first principle calculations and model approaches. The magnetic exchange interactions are calculated non-perturbatively and disorder in the configuration of impurities is treated exacly, allowing us to test the validity of effective medium theories. Results differ qualitatively from those of weak coupling. In contrast to mean field theory, increasing $J_{pd}$ may not favor high Curie temperatures: $T_C$ scales primarily with the bandwidth. High temperature ferromagnetism at small dilutions is associated with resonant structure in the p-band. Comparison to diluted magnetic semiconductors indicate that Ga(Mn)As has such a resonant structure and thus this material is already close to optimality.

cond-mat.str-el↗

arXiv2006

Model for vacancy-induced d$^0$ ferromagnetism in oxide compounds

We propose a model with few parameters, for vacancy-induced ferromagnetism based on a correlated model for anions (oxygen orbitals) with random potentials that represent cation vacancies. There is a range of potential strength for which moments appear on oxygen sites neighboring the vacancies. By using techniques which allow us to treat the randomness exactly, we calculate the magnetic couplings between the total moment around each vacancy, the Curie temperature and dynamical correlations as a function of the effective stren gth of the vacancy potential and doping properties, the density of vacancies, as well as the correlation and band width of the host. For physically reasonable parameters this predicts Curie temperatures well above room temperature for small concentrations of vacancies and appropriate parameters. We disc uss our results in relation to questions of stability and reproducibility raised in experiments. To circumvent the difficulties of controlling intrinsic defects, we propose specific non-magnetic host doping that could be, for example, substituted for cations in HfO$_2$ or ZrO$_2$.

cond-mat.str-el↗

arXiv2005

Why RKKY exchange integrals are inappropriate to describe ferromagnetism in diluted magnetic semiconductors

We calculate Curie temperatures and study the stability of ferromagnetism in diluted magnetic materials, taking as a model for the exchange between magnetic impurities a damped Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction and a shor t range term representing the effects of superexchange. To properly include effects of spin and thermal fluctuations as well as geometric disorder, we solve the effective Heisenberg Hamiltonian by means of a recently developed semi-analytical approach. This approach, ``self-consistent local Random Phase Approximation (SC-L RPA)'', is explained. We show that previous mean-field treatments, which have been widely used in the literature, largely overestimate both the Curie temperatures and the stability of ferromagnetism as a function of carrier density. The discr epancy when compared to the current approach was that effects of frustration in RKKY oscillations had been strongly underestimated by such simple mea n-field theories. We argue that the use, as is frequent, of a weakly-disordered RKKY exchange to model ferromagnetism in diluted III-V systems is inconsistent with the observation of ferromagnetism over a wide region of itinerant carrier densities. This may be puzzling when compared to the apparent success of calculations based on {\it ab-initio} estimates of the coupling; we propose a resolution to this issue by taking RKKY-like interactions between resonant states close to the Fermi level.

cond-mat.str-el↗

arXiv2004

Boosting the Curie temperature with correlations in diluted magnetic semiconductors

We present a quantitative theory for the effects of correlated doping on the ferromagnetism of diluted magnetic semiconductors. It predicts that room temperature ferromagnetism should be possible in homogeneous, but correlated, samples of Mn$_x$Ga$_{1-x}$As. The theory predicts lower critical temperatures for Mn$_x$Ga$_{1-x}$N.

cond-mat.dis-nn↗

arXiv2004

Calculating the Curie Temperature reliably in diluted III-V ferromagnetic semiconductors

We present a semi-analytic theory for the Curie temperature in diluted magnetic semi-conductors that treats disorder effects exactly in the effective Heisenberg Hamiltonian, and spin fluctuations within a local RPA. The exchange couplings are taken from concentration dependent {\it ab initio} estimates. The theory gives very good agreement with published data for well-annealed samples of Mn$_x$Ga$_{1-x}$As. We predict the critical temperatures for Mn$_x$Ga$_{1-x}$N lower than in doped GaAs, despite the stronger nearest-neighbour ferromagnetic coupling. We also predict the dependence on the hole concentration.

cond-mat.dis-nn↗

arXiv1996

Persistent currents in interacting electronic systems

Persistent currents in disordered mesoscopic rings threaded by a magnetic flux are calculated using exact diagonalization methods in the one-dimensional (1D) case and self-consistent Hartree-Fock treatments for two dimensional (2D) systems. For multichannel systems, a comparative study between models of spinless or spinfull (Hubbard) fermions has been done. First, it is shown that a purely one-dimensional model can not reproduce the correct order of magnitude of the observed currents. For 2D systems, going beyond first order pertubative calculations, we show that the second harmonic of the current is {\it strongly suppressed} in the case of spinless fermion models but {\it significantly enhanced} for the Hubbard model. This reduction (resp. increase) of the second harmonic is related to a strong increase (resp. reduction) of the spacial charge density fluctuations. Our work underlines the important role of the spin degrees of freedom in the persistent currents.

cond-mat↗

arXiv1996

$η$-superconductivity in the one-dimensionnal pair hopping model

Using exact Lanczos diagonalizations we have shown that the pair-hopping model for {\it negative} $W$ exhibits a phase transition into $η$-{\it superconducting state}. The transition occurs at {\it any band filling} and the critical value $W_{c}$ varies between $W_{c} \simeq -1.75t$ at half-filling to $W_{c} \simeq -2t$ for two particles on the lattice.

cond-mat↗

arXiv1995

Persistent Currents in Interacting Systems: Role of the Spin

Persistent currents flowing through disordered mesoscopic rings threaded by a magnetic flux are investigated. Models of fermions with on-site interactions (Hubbard model) or models of spinless fermions with nearest neighbor interactions are considered on 2D cylinders with twisted boundary conditions in one direction to account for a magnetic flux. Self-consistent Hartree-Fock methods are used to treat the electron-electron interaction beyond first order. We show that the second harmonic of the current (which is relevant in the diffusive regime) is {\it strongly suppressed} by the interaction in the case of spinless fermions while it is {\it significantly enhanced} in the Hubbard model. Our data also strongly suggest that the reduction (increase) of this harmonic is related to a strong increase (reduction) of the spacial fluctuations of the charge density.

cond-mat↗