Soliton Lattice State of Spin- Antiferromagnetic Chain in an External Magnetic Field**The project supported by National Natural Science Foundation of China

Abstract

One-dimensional spin- anisotropic Heisenberg antiferromagnet in a longitudinal external magnetic field is studied using bosonization method and Gaussian wave functional techniques which take account of the spatial structure. The magnetization and the energy of the ground state which depend on the external magnetic field are calculated. For the case of anisotropic parameter , increasing of the external magnetic field above the threshold value leads to the appearance of the soliton lattice state in the ground state, until to an another critical field where the ground state changes to the canted state phase. Therefore, with increasing external magnetic field, the ground state experiences four different phases successively, namely, antiferromagnetic Ising, soliton lattice state canted state, and magnetization saturated phases. When , the soliton lattice state phase does not appear, with increasing external field, the paramagnetic phase smoothly evolves into the canted state phase, finally reaches magnetization saturated phase.