Finite temperature density matrix and two-point correlations in the   antiferromagnetic XXZ chain
  We derive finite temperature versions of integral formulae for the two-point correlation functions in the antiferromagnetic XXZ chain. The derivation is based on the summation of density matrix elements characterizing a finite chain segment of length $m$. On this occasion we also supply a proof of the basic integral formula for the density matrix presented in an earlier publication. 
Frank G\"ohmann, Nils P. Hasenclever and Alexander Seel