抄録
We investigate the effects of the XY-like anisotropy on thermodynamic quantities of the two-dimensional XXZ model on the square lattice using a Schwinger boson mean-field approach. Our formulation leads to correct relations of the spin correlation functions; ‹ SixSjx› =‹ SiySjy› ≠ ‹ SizSjz› . The internal energy, specific heat, spin correlation functions, uniform susceptibilities and correlation length are obtained after we correct the transverse spin correlation function by an extra factor 1/2. At zero temperature our results agree with those of the conventional spin wave theory and those of numerical studies. At low temperatures thermal quantities, except for the transverse susceptibility for the ferromagnetic XXZ model and the correlation length, agree with those of numerical studies. In particular, we find that the transverse uniform susceptibility of the antiferromagnetic XXZ model behaves as a+bT3 at low temperatures in contrast to the T-linear behavior of the isotropic Heisenberg antiferromagnet.
