S=1反铁磁海森堡链的热力学性质

应用二阶格式函数方法。研究了双二次相互作用项对反铁磁海森堡链激发谱和热力学性质的影响，目前用二阶格林函数方法研究此类问题尚未见报导。研究发现随着双二次相互作用的增强，反铁磁海森堡链的Ｈａｌｄａｎｅ能隙将降低，内能增加，比热曲线的峰质左移，并且存在一个双二次相互作用临界值ａｃ，当ａ〉ａｃ时，将得不到体系的激发谱。

THE THERMODYNAMIC PROPERTIES OF THE ANTIFERROMAGNETIC HEISENBERG CHAIN WITH SPIN S =1

The effect of the biquadratic exchange on the excitation spectra of the antifer-romagnetic Heisenberg chain (AFHC)is investigated by using the second-order Green's Function(GF)method.We have found that the Haldane gap will decrease with the in crease of the biquadratic exchange interaction,the internal energy will increase with the increase of the biquadratic exchange interaction,and with the increasing of the biquadratic exchange intheraction,the position of the maximum specific heat will shift fowards the left.It is interesting that there exists a critical value of the biquadratic exchange parameter a c =0.21 at T =0 K,when a>a c ,the magnons can not be excited,which is resulted by the competition between the bilinear and biquadratic exchange couplings.