Low-temperature properties of a spin-1 antiferromagnetic chain.

Abstract

We study the low-temperature properties of a spin-1 Heisenberg antiferromagnetic chain in an external magnetic field by an extended self-consistent mean-field approach developed earlier based on Anderson's spin-wave theory. At fixed magnetic field, the excitation energies increase, while the correlation length decreases with the increasing temperatures. The magnetic field leads to the Zeeman splitting of the excitations with the upper branch going up and the lower branch going down with the increasing fields. At zero temperature the correlation length is a constant and the gap closes at a critical magnetic field, while at finite temperature the correlation length decreases and a small gap seems to survive at the critical magnetic field. These results are consistent with those of other theoretical studies and agree with the experiments quite well. Also, we show that the system shares very similar properties with the valence-bond-solid state even at finite temperatures. The magnetization is studied and the coexistence of the hidden long-range order and the field-induced ferromagnetic order is discussed.