Abstract
We consider an infinitely extended system of fermions on a d-dimensional lattice with (magnetic) translation-invariant short-range interactions. We further assume that the system has a gapped ground state. Physically, this is a model for the bulk of a generic topological insulator at zero temperature, and we are interested in the current response of such a system to a constant external electric field. Using the non-equilibrium almost-stationary states approach, we prove that the longitudinal current density induced by a constant electric field of strength ε is of order O(ε∞) , i.e. the system is an insulator in the usual sense. For the Hall current density we show instead that it is linear in ε up to terms of order O(ε∞) . The proportionality factor σH is by definition the Hall conductivity, and we show that it is given by a generalization of the well known double commutator formula to interacting systems. As a by-product of our results, we find that the Hall conductivity is constant within gapped phases, and that for d = 2 the relevant observable that "measures" the Hall conductivity in experiments, the Hall conductance, not only agrees with σH in expectation up to O(ε∞) , but also has vanishing variance. A notable difference to several existing results on the current response in interacting fermion systems is that we consider a macroscopic system exposed to a small constant electric field, rather than to a small voltage drop.