Intermediate valence and the possibility of a magnetic excitonic insulator

The long predicted existence of an excitonic condensed phase-i.e. the excitonic insulator-has first been verified experimentally under pressure and low temperature on the intermediate valent system TmSe_0.45Te_0.55 and TmSe_0.32Te_0.68 by Neuenschwander and Wachter (1990) and Bucher, Steiner and Wachter (1991). Now additional compounds such as Sm_0.75La_0.25S and YbO and YbS have been shown to fall under pressure into the category of excitonic insulators. The problem whether magnetism plays a role in the condensation will be discussed