  We consider the Haldane model, a 2D topological insulator whose phase is
defined by the Chern number. We study its phases as temperature varies by means
of the Uhlmann number, a finite temperature generalization of the Chern number.
Because of the relation between the Uhlmann number and the dynamical transverse
conductivity of the system, we evaluate also the conductivity of the model.
This analysis does not show any sign of a phase transition induced by the
temperature, nonetheless it gives a better understanding of the fate of the
topological phase with the increase of the temperature, and it provides another
example of the usefulness of the Uhlmann number as a novel tool to study
topological properties at finite temperature.
