Bond operator theory of Heisenberg ferrimagnetic spin chain

The bond operator theory is applied to study Heisenberg ferrimagnetic spin chain. We observe four branches of excited spectrum, the gap of excited spectrum and energy spectrum at T=0 KT=0 K. The energy gap is 1.64, which agrees quite well with the established numerical results. At finite temperature, the energy gaps, uniform susceptibility, internal energy and specific heat are calculated respectively. These results qualitatively agree with numerical results by other methods quite well.