A new perturbative approach to the study of quantum systems at finite temperature
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By starting from the formal expansion of the matrix elements ofz=exp [x+λy] in powers of thec-number λ, a general algorithm is derived, for the expansion of the partition function of a lattice system with nearest-neighbour interaction. This is applied to the calculation of the free energy of a spin-1/2 Hamiltonian up to the fourth perturbative order.
KeywordsPACS 02.10.Gd Order lattices and ordered algebraic structures PACS 05.30.Ch Quantum ensemble theory
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