Transport Properties of the "Excitonic Insulator": Electrical Conductivity

Abstract

The dc conductivity of the "excitonic insulator" recently discussed in the literature is calculated in the semimetallic region. The calculations are based on recent work on the description of the excitonic phase in the presence of impurities. It is shown that the conductivity decreases below the transition temperature to the excitonic state. For low impurity concentrations the system acquires insulating properties. For higher impurity concentrations the conductivity is still nonzero at $T=0\mathrm{}$. Thus, metallic properties prevail in the excitonic phase. It is pointed out that this behavior depends essentially on the form of the excitation spectrum of the system, i.e., the presence or absence of a gap. At the transition temperature the conductivity-versus-temperature curve has a finite slope.