In this publication, details of the calculation of heat conductivity and thermal diffusivity, compressibility, sound velocity and exciton‐polaron dispersion will be shown. The properties of excitons, coupling to phonons, producing thus polarons, but also blocking the phonons as running waves lead to an exciton condensation or exciton liquid. Surprisingly, this exciton liquid is contained in a macroscopic crystal, a solid, nevertheless, which becomes extremely hard due to the exciton liquid and finally exhibits a strange type of superfluid in a two fluid model, where the superfluid phase increases more and more below about 20 K until the whole exciton liquid becomes a superfluid at zero temperature. Never else a superfluid phase has been observed at such high temperatures.
Part of the book: Excitons