Numerical study on electrical conductivity of spin-fermion models

The DC electrical conductivity is studied numerically in finite-size spin-fermion models with antiferromagnetic couplings between localized spins by a direct numerical estimation of the Kubo formula. In contrast with an ordinary magnetoresistance, in a system where the localized spins form an antiferromagnetically coupled two-leg ladder, conductivity shows an anomalous non-monotonic field dependence. It is attributed to a spin configuration where two spins on the ladder rung dimerize into a spin singlet. In addition, we study conductivity of a square lattice and observe the direction dependence of conductivity. We obtained remarkable and qualitative differences in behavior of conductivity depending on the direction of current