The statistical mechanics of the related S=1/2anisotropic and S=1 pure biquadratic quantum spin chains

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Abstract
The authors consider a quantum spin chain with S=1/2 and anisotropic exchange (XXZ model) with Delta =-3/2. This integrable system has a partial mapping to an S=1 chain with pure biquadratic exchange. They calculate the magnetisation curves at zero temperature which are identical for the two systems. They also use the integral equations obtained by Gaudin to study the specific heat as a function of temperature and magnetic field. The results are compared with numerical calculations for short chains of the S=1 system. The maximum occurs at approximately the same position for the two systems, but the shapes of the curves differ considerably.