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arXiv:cond-mat/9804188 · Advances in solid state physics

Disorder and impurities in hubbard-antiferromagnets

M. Ulmke, P. J. H. Denteneer, V. Janiš, R. T. Scalettar, A. Singh, D. Vollhardt, G. T. Zimanyi

Published Jan 1, 1999• 8 citations
arXivHTMLPDF
DOI
10.1007/bfb0107630

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We study the influence of disorder and randomly distributed impurities on the properties of band-antiferromagnets. To this end the Hubbard model with (i) random potentials, (ii) random hopping elements, and (iii) randomly distributed values of interaction is treated using quantum Monte Carlo and dynamical mean-field theory. In cases (i) and (iii) weak disorder can lead to an enhancement of antiferromagnetic (AF) order: in case (i) by a disorder-induced delocalization, in case (iii) by binding of free carriers at the impurities. For strong disorder or large impurity concentration antiferromagnetism is eventually destroyed. Random hopping leaves the local moment stable but AF order is suppressed by local singlet formation. Random potentials induce impurity states within the charge gap until it eventually closes. Impurities with weak interaction values shift the Hubbard gap to a density off half-filling. In both cases an antiferromagnetic phase without charge gap is observed. 1
Recent numerical studies have shown that antiferromagnetism (AF) prevails widely near half filling in the two-dimensional Hubbard ( t - t '- U ) model, contrary to the experiments on cuprate superconductors. In cuprates, disorders owing to the dopants, which are usually disregarded in theory, are possibly intrinsic and considerable. Thus, as a first step, we consider the effects of point-type impurity potentials, which are randomly distributed on the lattice sites, on the stability of AF. It is found that a moderate impurity potential does not modify the stability of AF, but a sufficiently strong attractive potential V ( V ≲ − U ) appreciably weakens an antiferromagnetic state owing to an insufficient screening of V .
The Hubbard model is studied in which disorder is introduced by putting the on-site interaction to zero on a fraction f of (impurity) sites of a square lattice. Using Quantum Monte Carlo methods and Dynamical Mean–Field Theory we find that antiferromagnetic long–range order is initially enhanced at half–filling and stabilized off half-filling by the disorder. The Mott-Hubbard charge gap of the pure system is broken up in two pieces by the disorder: one incompressible state remains at average density n = 1 and another can be seen slightly below n = 1 + f. Qualitative explanations are provided.