Anomalous Magnetoresistance in a Spin-Fermion Model with Quantum Fluctuation

Abstract

Electron transport is investigated theoretically for an electron which is coupled to a localized spin system. The dc electrical conductivity of spin-fermion models in one dimension is studied by the direct numerical estimation of the Kubo formula. The dependence of the conductivity on temperature and external magnetic field is investigated. When the localized spins are antiferromagnetically coupled on a chain, the conductivity shows a monotonic increase with magnetic field, which is ordinary magnetoresistance. However, in a system where the localized spins form an antiferromagnetically coupled two-leg ladder, the conductivity shows an anomalous non-monotonic field dependence. Analysis of the spin configuration of the localized spin system reveals that quantum fluctuation determines the conductivity through a dimerized-rung spin configuration, and the formation of local triplets on the ladder rungs causes the decrease in conductivity.