Metal-insulator transition and the character of the hole impurity bands in ferromagnetic GaMnAs disordered dilute magnet
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1118-K05-08
Metal-insulator transition and the character of the hole impurity bands in ferromagnetic GaMnAs disordered dilute magnetic semiconductor R. da Silva Neves1, A. Ferreira da Silva1 and R. Kishore2 1
Instituto de Física, Universidade Federal da Bahia, 40210 340 Salvador, Bahia, Brazil
2
Instituto Nacional de Pesquisas Espaciais –INPE/LAS 12210 970 São José dos Campos, São Paulo, Brazil.
ABSTRACT The study of ferromagnetic transition of Ga1-xMnxAs dilute magnetic semiconductor (DMS) is much of interest mainly due to the potential application in spintronic devices. Based on the mean field approach we present the average contribution of the hole spins by considering the holes in an impurity band (IB) and the critical concentration for the metal-insulator transition (MIT) in this semiconductor. In order to calculate the mean configuration of spins of impurities Mn+2 we use a formalism proposed for a spatial disordered system. The results for the metallic densities around the MIT transition are compared to experimental results and other theoretical findings.
INTRODUCTION The discovery of the ferromagnetic transition at temperature TC of Ga1-xMnxAs exceeding 100K, is much of interest to investigate the physical properties of dilute magnetic semiconductor (DMS) mainly in view of the potential application in spintronic devices[1]. In our study, we consider that the ions Mn+2 impurities replacing the Ga sites in GaAs semiconductor [2] have a null local moment angular, momentum for spin S = 5/2 and holes moderately bounded. Based on the mean field approach we calculate the average contribution of the hole spins by considering the holes in an impurity band (IB) and the critical concentration for the metal-insulator transition (MIT) in this semiconductor. In order to calculate the mean configuration of spins of impurities Mn+2 we used a formalism proposed for spatial disorder[3]. The best results, showed for intermediate and high concentrations (metallic phase systems), agree well with experimental results and other theoretical results[4,5], indicating a behavior character ferromagnetic phases. We may consider that our method is very useful in understanding the physical properties this magnetic material.
METAL-INSULATOR TRANSITION In Mott’s original model, the metal-insulator (MIT) transition occurs at a value of impurity critical concentration Nc, given by the relationship[6] N c1 / 3 a * ≈ 0.25 ,
where a * is the effective Bohr radius of the system.
(1)
In the Mott-Hubbard picture the MIT trasition is given by[7-9] ∆ω = 1.15 , U
(2)
where ∆ω is the unperturbed impurity bandwidth in units of Eb and U is the intra impurity coulomb interaction energy or Hubbard U , given by U = 0.96 Eb ( Eb being the binding energy of a single acceptor system) [10,11]. ∆ω is related to the hopping integral energy T , with adjacent sites i and j, as [9,10] ∆ω = 2 T ,
(3)
where T (R) , defined as the avarege hopping energy, is given by [9,10] T ( R) = ∫ T ( R ) P ( R )dR
(4)
In the equation above T ( R) and U are given by
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