2 TYPES OF SELF-TRAPPED STATES FOR EXCITONS IN ONE-DIMENSION

When excitons (or electrons) have only one-dimensional freedom of motion on a discrete lattice, free states are always unstable, irrespective of the strength of the exciton-phonon (short-ranged) interaction. Excitons spontaneously relax into self-trapped (S) states, distorting the lattice. Two types of S states are realized: One is a large-radius type associated with a spatially extended lattice distortion, while another is a small-radius type associated with a spatially contracted distortion. As the interaction strength increases, the lowest S state changes its character abruptly, although continuously, from the former to the latter. This result is rigorous within the adiabatic approximation, calculated with a method presented previously by the present authors [H. Sumi and S. Higai, J. Phys. Soc. Jpn. 62 (1993) 1500].