Superfluidity below 20 K in condensed excitons

In the condensed excitonic phase of intermediate valent TmSe0.45Te0.55 the heat conductivity and the thermal diffusivity have been obtained between 300 and 1.5 K and between ambient pressure and 17 kbar (1.7 GPa), as a first experiment of its kind. Pressure and temperature are used to navigate in three different phases of the material, the intermediate valent semiconducting phase, the condensed excitonic phase and the intermediate valent metallic phase. In the condensed excitonic phase the heat conductivity λ increases strongly below about 20 K, suggesting a superfluid phase for the lowest temperatures. In a solid under equilibrium conditions this has never been seen before. Also the thermal diffusivity a strongly increases below 20 K, giving evidence for second sound. The quotient λ/ρa represents the specific heat, which thus can be calculated, for the first time as a function of pressure and for a large temperature range. When entering the condensed excitonic phase under pressure and from high temperatures (100–250 K), the Dulong–Petit value of the specific heat drops precipitously to about half its value. Also this phenomenon is unprecedented. In addition the longitudinal sound velocity has been measured under pressure and as a function of temperature. Entering the excitonic phase the sound velocity drastically increases by about a factor 2.