From band insulator to Mott insulator in one dimension

We derive the phase diagram for the one-dimensional model of a ferroelectri c perovskite recently analyzed by Egami, Ishihara, and Tachiki [Science 261 , 1307 (1993)]. We show that the interplay between covalency, ionicity, and strong correlations results in a spontaneously dimerized phase which separ ates the weak-coupling band insulator from the strong-coupling Mott insulat or. The transition from the band insulator to the dimerized phase is identi fied as an Ising critical point. The charge gap vanishes at this single poi nt with the optical conductivity diverging as sigma(omega) similar to omega (-3/4). The spin excitations are gapless above the second transition to the Mott insulator phase.