Pressure-driven semiconductor-metal transition in intermediate-valence ${\mathrm{TmSe}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Te}}_{\mathit{x}}$ and the concept of an excitonic insulator

This work studies the pressure-induced semiconductor-to-metal transition (SMT) in the TmSe-TmTe alloy system. This SMT is accompanied by a valence instability of the Tm ions. Single-crystalline semiconducting ${\mathrm{TmSe}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Te}}_{\mathit{x}}$ alloys are investigated under high pressure at low temperatures. Measurements of electrical resistivity, magnetic susceptibility, neutron diffraction, and optical properties are presented and discussed. A very unusual peak structure in the resistivity-pressure relation is observed at low temperatures. A discussion of the novel feature involves the concept of an excitonic insulator and f-d hybridization. The magnetic behavior of the compounds is significantly influenced by the SMT. This is thought to be mainly due to the additional coupling between the magnetic moments of Tm via free carriers which are present in the metallic state.