Intermediate valent Tm compounds: A superfluid in a condensed excitonic state
Prof. Dr. Peter Wachterl
Laboratorium für Festkörperphysik, Eidgenössische Technische Hochschule (ETH), Zürich, Schweiz
vollständiger Text
Exciton condensation in the form of an excitonic insulator has been postulated by Mott already in 1961. The experimental realization of such a state in intermediate valent TmSe1-xTex compounds, however, had to wait until 1990 [J. Neuenschwander, P. Wachter, Phys. Rev. B 41, 12693 (1990)]. Now it appears essential that an intermediate valent state is a prerequisite for the exciton condensation because a narrow hybridized 4f band is necessary. The new and exciting facts are thermodynamic measurements on TmSe0.45Te0.55 single crystals at low temperatures and high pressure. At ambient conditions this material is a narrow gap semiconductor which can be transformed under high pressure (14 kbar) into an intermediate valent metal. Below 250 K and between 5 and 13 kbar the excitonic state is formed. In this case, the thermal conductivity shows a behavior which has only been observed so far in the superfluid phase of He II.