[Show abstract][Hide abstract] ABSTRACT: The crossover between an impurity band (IB) and a valence band (VB) regime as a function of the magnetic impurity concentration in a model for diluted magnetic semiconductors (DMSs) is studied systematically by taking into consideration the Coulomb attraction between the carriers and the magnetic impurities. The density of states and the ferromagnetic transition temperature of a spin-fermion model applied to DMSs are evaluated using dynamical mean-field theory and Monte Carlo (MC) calculations. It is shown that the addition of a square-well-like attractive potential can generate an IB at small enough Mn doping x for values of the p-d exchange J that are not strong enough to generate one by themselves. We observe that the IB merges with the VB when x⩾xc where xc is a function of J and the Coulomb strength V. Using MC simulations, we demonstrate that the range of the Coulomb attraction plays an important role. While the on-site attraction, which has been used in previous numerical simulations, effectively renormalizes J for all values of x, an unphysical result, a nearest-neighbor range attraction renormalizes J only at very low dopings, i.e., until the bound holes wave functions start to overlap. Thus, our results indicate that the Coulomb attraction can be neglected to study Mn-doped GaSb, GaAs, and GaP in the relevant doping regimes, but it should be included in the case of Mn-doped GaN, which is expected to be in the IB regime.
[Show abstract][Hide abstract] ABSTRACT: Recent progress in the numerical study of various strongly correlated electronic systems is reviewed. The study of transport in single molecule conductors and quantum dots is addressed with a recently proposed adaptive time-dependent density-matrix-renormalization group (DMRG). Experiments involving non-local spin control and their numerical simulation are also discussed. A section is devoted to recent efforts in the study of spin-fermion models for colossal magnetoresistive manganites, where we present insights on the effect of disorder and electron–phonon coupling. Finally, using a dynamical mean field approach, we review calculations in the area of diluted magnetic semiconductors that provides guidelines on how the Curie temperature could be increased in these itinerant ferromagnetic systems.
[Show abstract][Hide abstract] ABSTRACT: Several multiband models for Colossal Manetoresistance Materials and Diluted Magnetic Semiconductors are investigated within Dynamical Mean-Field Theory. The theoretical analysis of such models uses extensively the parametrization method for the bare Green's function to study the critical transition temperatures in a wide range of model's parameters; hoping amplitudes, couplings, and carrier concentrations. For both classes of materials, it is found within a two-band model that the transition temperature can be twice larger than the one predicted by the one band models and that its maximum is reached at a twice larger carrier concentration. It is also revealed that the off-diagonal hoppings, when considered, lead to a substantial boost of the ferromagnetic transition temperature. By accounting for the attractive Coulomb potential by acceptors in diluted magnetic semiconductors, the impurity to valence band crossover is studied as a function of doping and the critical values of doping, at which the crossover occurs, are estimated for various diluted magnetic semiconductors.
[Show abstract][Hide abstract] ABSTRACT: Within dynamical-mean field theory we investigate the ferromagnetic transition temperature (Tc) of a two-band model for diluted magnetic semiconductors in a large range of coupling constants, hopping parameters, and carrier densities [1]. We reveal that Tc is optimized at all fillings when both impurity bands fully overlap in the same energy interval, namely when the exchange couplings and bandwidths are equal. The optimal Tc is found to be about twice larger than the maximal value obtained in the one-band model. Within a one-band model we also discuss the influence of the Coulomb attractive potential by acceptors on the critical ferromagnetic temperature [2]. [1]. F. Popescu, Y. Yildirim, G. Alvarez, A. Moreo, and E. Dagotto, Phys. Rev. B, 73 (2006), 075206. [2]. F. Popescu, C. Sen, E. Dagotto, and A. Moreo, in preparation.
[Show abstract][Hide abstract] ABSTRACT: The ferromagnetic transition temperature (T{sub c}) of a two-band double-exchange (DE) model for colossal magnetoresistance materials is studied using dynamical mean-field theory in wide ranges of coupling constants, hopping parameters, and carrier densities. The results are shown to be in good agreement with Monte Carlo simulations. When the bands overlap, the value of T{sub c} is found to be much larger than in the one-band case, for all values of the chemical potential within the energy overlap interval. A nonzero off-diagonal hopping produces an additional boost of T{sub c}, showing the importance of these terms, as well as the concomitant use of multiband models, to increase the critical temperatures in DE-based theories.
Physical Review B 05/2006; 73(18). DOI:10.1103/PhysRevB.73.180404 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Dynamical mean-field theory is used to study the magnetic instabilities and phase diagram of the double-exchange (DE) model with Hund's coupling J_H >0 in infinite dimensions. In addition to ferromagnetic (FM) and antiferromagnetic (AF) phases, the DE model supports a broad class of short-range ordered (SRO) states with extensive entropy and short-range magnetic order. For any site on the Bethe lattice, the correlation parameter q of a SRO state is given by the average q=, where theta_i is the angle between any spin and its neighbors. Unlike the FM (q=0) and AF (q=1) transitions, the transition temperature of a SRO state (T_{SRO}) with 0<q<1 cannot be obtained from the magnetic susceptibility. But a solution of the coupled Green's functions in the weak-coupling limit indicates that a SRO state always has a higher transition temperature than the AF for all fillings p<1 and even than the FM for 0.26\le p \le 0.39. For 0.390 but appears for J_H\neq 0. For p near 1, PS occurs between an AF with p=1 and either a SRO or a FM phase. The stability of a SRO state at T=0 can be understood by examining the interacting DOS,which is gapped for any nonzero J_H in an AF but only when J_H exceeds a critical value in a SRO state. Comment: 38 pages, 11 figures, submitted to New Journal of Physics
New Journal of Physics 04/2006; DOI:10.1088/1367-2630/8/7/116 · 3.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using dynamical mean-field theory, we have evaluated the magnetic
instabilities and T=0 phase diagram of the double-exchange model on a
Bethe lattice in infinite dimensions. In addition to ferromagnetic (FM)
and antiferromagnetic (AF) phases, we also study a broad class of
spin-glass (SG) solutions with extensive entropy and short-range
magnetic order. In the weak-coupling limit, a SG has a higher transition
temperature than the AF phase for all fillings p below 1 and can even
have a higher transition temperature than the FM phase. At T=0 and for
small Hund's coupling, a SG has lower energy than either the FM or AF
phases for 0.26 < p < 1. Phase separation is absent as the Hund's
coupling vanishes but appears for any non-zero value. Our T=0 phase
diagram agrees remarkably well with Monte-Carlo results in two and three
dimensions. The stability of a SG at T=0 can be understood by examining
the interacting density-of-states, which is gapped for any nonzero
Hund's coupling in an AF but only when the Hund's coupling exceeds a
critical value in a SG.
[Show abstract][Hide abstract] ABSTRACT: Using dynamical mean field theory and Monte Carlo simulations, we study the ferromagnetic transition temperature (T{sub c}) of a two-band model for diluted magnetic semiconductors (DMS), varying coupling constants, hopping parameters, and carrier densities. We found that T{sub c} is optimized at all fillings p when both impurity bands fully overlap in the same energy range, namely when the exchange couplings J and bandwidths are identical. The optimal T{sub c} is found to be about twice larger than the maximum value obtained in the one-band model, showing the importance of multiband descriptions of DMS at intermediate J's.
Physical Review B 02/2006; 73(7). DOI:10.1103/PhysRevB.73.075206 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using dynamical mean-field theory, we have evaluated the magnetic instabilities and T=0 phase diagram of the double-exchange model on a Bethe lattice in infinite dimensions. In addition to ferromagnetic (FM) and antiferromagnetic (AF) phases, we also study a class of disordered phases with magnetic short-range order (SRO). In the weak-coupling limit, a SRO phase has a higher transition temperature than the AF phase for all fillings p below 1 and can even have a higher transition temperature than the FM phase. At T=0 and for small Hund's coupling J_H, a SRO state has lower energy than either the FM or AF phases for 0.26\le p < 1. Phase separation is absent in the J_H --> 0 limit but appears for any non-zero value of J_H.
Physical Review B 10/2005; 73(14). DOI:10.1103/PhysRevB.73.140405 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using Dynamical Mean Field Theory (DMFT) we derive general expressions for the Curie Temperature (Tc) of a spin-fermion model for any coupling constant J and any concentration of localized spins x. In the case of manganites, we compare these results with those obtained previously for the case of an infinite Hund's coupling JH. In the case of diluted magnetic semiconductors (DMS) we discuss the dependence of Tc on model parameters and the effect of the inclusion of a more realistic band structure. We show that DMFT is a powerful tool to study spin-fermion models for DMS in the weak coupling regime.