Microscopic Theory of the CDW State of 1T-TiSe₂

Physical properties of the CDW state of 1 T -TiSe 2 such as electronic band structures are investigated on the basis of the band-type Jahn-Teller model by extending our microscopic theory of lattice instability in the normal phase. The results are summarized as follows: (i) The triple- q CDW state is more stable than the single- q CDW state as observed. (ii) The magnitude of the lattice distortion for the triple- q structure at T =0 K is close to the observed value. (iii) The largest CDW gap is obtained to be about 0.2 eV, which agrees in order of magnitude with the optically determined gap 0.4 eV. (iv) On formation of the CDW the density-of-states at E F decreases considerably and a peak develps at 0.1 eV below E F , which is consistent with the observation by photoemission.