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講演者：Prof. D.M. Edwards (Imperial College, London)

日時： １０月２７日（金）　午後４時半から
場所： 青山学院大学　理工学部（世田谷キャンパス）　一号館　５階　１５３８号室

講演題目： Ferromagnetism and Transport in the Double Exchange Model, with and without Phonons
要旨：
Recently the observed colossal magnetoresistance in manganites has caused attention to be focussed on the so-called double exchange (DE) model of these materials. In this s-d model electrons in a conduction band interact with local manganese spins and, in the more general Holstein-DE model, with local phonons. In the classical limit for the local spins and phonons these models are one-electron problems which can be solved almost analytically within the approximation of dynamical mean field theory (DMFT). The standard coherent potential approximation (CPA) is equivalent to DMFT for these one-electron problems. For the many-body case of quantum local spins and phonons, where DMFT is intractable analytically, we have developed a many-body CPA which is exact in the atomic limit and reduces to DMFT in the classical limit. We use our CPA to discuss quantum effects on the electronic spectrum, Curie temperature and resistivity, comparing our results with experimental results on manganites. Good semi-quantitative agreement is obtained for the temperature and field dependence of the resistivity.