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A note on the high temperature expansion of the density matrix for the isotropic Heisenberg chain

Göhmann, Klümper and Seel derived the multiple integral formula of the density matrix of the $XXZ$ Heisenberg chain at finite temperatures. We have applied the high temperature expansion (HTE) method to isotropic case of their formula in a finite magnetic field and obtained coefficients for several short-range correlation functions. For example, we have succeeded to obtain the coefficients of the HTE of the 3rd neighbor correlation function $<σ_{j}^{z}σ_{j+3}^{z}>$ for zero magnetic field up to the order of 25. These results expand our previous results on the emptiness formation probability [Z.Tsuboi, M.Shiroishi, J. Phys.A: Math. Gen. 38(2005) L363; cond-mat/0502569] to more general correlation functions.