Pressure-driven metal-insulator transition in La- and Tm-doped SmS by exciton condensation

Abstract

In analogy with well-known intermediate-valent TmTeSe alloys, Sm 1− x La x S, with x = (0.10, 0.25, 0.35), and Sm 1− y Tm y S, with y = (0.10, 0.15), have also turned out as excitonic insulators under high pressure and low temperatures. The condensation of excitons results from strong Coulomb attraction between 4f holes and 5d conduction electrons, which is enhanced by external hydrostatic pressure. By hybridization in the intermediate-valent ground state, a narrow 4f band forms and lends the holes a high effective mass. Thus, the mobility of the excitons is reduced, and low temperature prevents their thermal dissociation. A sufficient exciton density triggers a Bose condensation. In the cryogenic range, the electrical resistivity of the pressurized samples indicates a gap of meV magnitude, corresponding to the activation energy of the excitons. Further pressure being applied closes the gap continuously, since more and more valence electrons pass into the conduction band and screen the Coulomb interaction. Finally, the integer-valent metallic phase is reached.