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Abstract: In contrast to semiconductors doped with transition metal magnetic elements,which become ferromagnetic at temperatures below ~ 100K, semiconductors dopedwith non-magnetic ions e.g. silicon doped with phosphorous have not shownevidence of ferromagnetism down to millikelvin temperatures. This is despitethe fact that for low densities the system is expected to be well modeled bythe Hubbard model, which is predicted to have a ferromagnetic ground state atT=0 on 2- or 3-dimensional bipartite lattices in the limit of strongcorrelation near half-filling. We examine the impurity band formed byhydrogenic centers in semiconductors at low densities, and show that it isdescribed by a generalized Hubbard model which has, in addition to strongelectron-electron interaction and disorder, an intrinsic electron-holeasymmetry. With the help of mean field methods as well as exact diagonalizationof clusters around half filling, we can establish the existence of aferromagnetic ground state, at least on the nanoscale, which is more robustthan that found in the standard Hubbard model. This ferromagnetism is mostclearly seen in a regime inaccessible to bulk systems, but attainable inquantum dots and 2D heterostructures. We present extensive numerical resultsfor small systems that demonstrate the occurrence of high-spin ground states inboth periodic and positionally disordered 2D systems. We consider howproperties of real doped semiconductors, such as positional disorder andelectron-hole asymmetry, affect the ground state spin of small 2D systems. Wealso discuss the relationship between this work and diluted magneticsemiconductors, such as Ga 1-xMn xAs, which though disordered, showferromagnetism at relatively high temperatures.



Autor: Erik Nielsen, R. N. Bhatt

Fuente: https://arxiv.org/







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