DIRECT EVIDENCE FOR D-BAND INVOLVEMENT IN THE TISE2 PHASE-TRANSITION

We have investigated the Ti d-electronic states of stoichiometric TiSe2 for k-vectors in the neighborhood of the L-point at temperatures above and below the critical temperature using angle-resolved photoemission. Strong changes observed in the spectra upon lowering the temperature definitely demostrate that the lattice-instability phase transition involves Ti d-electrons as required by an excitonic-insulator mechanism. The hypothesis that TiSe2 at low temperatures is the first and only known example of an exitonic insulator, therefore, remains plausible after this crucial test.