Pressure-driven metal-insulator transition in La-doped SmS: Excitonic condensation

The long-predicted excitonic condensed phase, the excitonic insulator, has been verified once more. After the discovery of two intermediate valent ${\mathrm{TmSe}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Te}}_{\mathit{x}}$ alloys a few years ago, for which the excitonic phase has been verified under pressure at low temperatures, another rare-earth chalcogenide alloy system, namely, ${\mathrm{Sm}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{La}}_{\mathit{x}}$S, has now turned out to be also an excitonic insulator under pressure at low temperatures. The effect is caused by Coulomb interaction of electron and holes in an intermediate valent state when pressure is decreasing the interaction volume and low temperature is preventing the ionization of the condensed phase.