Exciton condensation and its influence on the specific heat

In rare earth compounds with localized 4f states the observation of bound 4f-hole-5d-electron states, excitons, is questionable. On the other hand the same compounds exhibit p–d excitons, which are derived from itinerant bands. In rare earth compounds, which exhibit intermediate valence, 4f–5d hybridization produces a narrow, several 10 meV wide 4f band. Now 4f–5d excitons are possible and have been observed in TmSe0.45Te0.55 and similar compositions. The special band structure of these materials permits an enormous amount of excitons (≈1021 cm−3), which condense in a first order transition at low temperatures and high pressure. This static and immense concentration of excitons dominates the heat conductivity and the thermal diffusivity and even exhibits the phenomenon of superfluidity in a solid. The measured specific heat shows that phonons couple to these excitons forming exciton polarons.