Published in last 50 years
A lower bound to the free energy of an anisotropic linear Heisenberg chain is obtained at nonzero temperatures by using a variational principle involving only the two particle density matrix.
We show that the transfer matrix for Yang’s two-dimensional model of a hydrogen-bonded crystal in the presence of an electric field commutes with a generalized form of the Hamiltonian of the anisotropic Heisenberg chain. Thus the connection between the two problems initially found for the ground-state eigenvector is extended to all the eigenvectors.