Elementary excitations in one-dimensional spin-orbital models: neutral and charged solitons and their bound states

Sektion Physik, Ludwig-Maximilians Universit¨at Mu¨nchen, Theresienstr. 37, D-80333 Mu¨nchen, Germany(August 22, 2000)We study, both numerically and variationally, the interplay between different types of elementaryexcitations in the model of a spin chain with anisotropic spin-orbit coupling, in the vicinity of the“dimer line” with an exactly known dimerized ground state. Our variational treatment is foundto be in a qualitative agreement with the exact diagonalization results. Soliton pairs are shown tobe the lowest excitations only in a very narrow region of the phase diagram near the dimer line,and the phase transitions are always governed by magnon-type excitations which can be viewed assoliton-antisoliton bound states. It is shown that when the anisotropy exceeds certain critical value,a new phase boundary appears. In the doped model on the dimer line, the exact elementary chargeexcitation is shown to be a hole bound to a soliton. Bound states of those “charged solitons” arestudied; exact solutions for N-hole bound states are presented.75.10.Jm,75.30.Kz,72.10.Fk,71.70.EjI. INTRODUCTION