Theory of Thermal Conductivity on Excitonic Insulator.

We study the thermal conductivity in the excitonic insulator using a simple quasi one-dimensional two-band model consisting of electron and hole bands with the Coulomb interactions between these bands. Based on the linear response theory within a mean-field scheme, we develop a method to identify the contributions to thermal conductivity driven by excitonic insulator. It is found that there is an additional heat current operator owing to the excitonic phase transition, and that it gives contributions to thermal conductivity which are not expressed in the form of Sommerfeld-Bethe relations written in the form of an imaginary-time derivative of the electric current operator. Finally, we discuss the relationship between the newly-found additional contribution and the heat current carried by excitons.