A high pressure low temperature study on rare earth compounds: Semiconductor to metal transition

Abstract

This work studies the pressure induced semiconductor to metal transition (SMT) in several rare earth compounds. This SMT is accompanied by a valence instability. Single crystalline semiconducting TmSe1−xTex, Tm1−xEuxSe and SmS1−xSex alloys are investigated under high pressure at low temperatures. Measurements of electrical resistivity, magnetic susceptibility, neutron diffraction, volume and optical properties are presented and discussed. A very unusual peak structure in the resistivity-pressure relation of TmSe1−xTex at low temperatures is observed. A discussion of the novel feature involves the concept of the excitonic insulator and f-d hybridization. The magnetic behavior of the Tm and Eu based compounds is significantly influenced by the SMT. This is thought to be mainly due to the additional coupling between the rare earth moments via free carriers which are present in the metallic state. In SmS1−xSex a considerable softening of the lattice is observed before the valence transition occurs. It is speculated that Poisson's ratio might become negative already in the semiconducting state.