The Lipkin model at finite temperature

Abstract

We consider a system of N fermions described by the Lipkin model and investigate their behaviour at finite temperature. We study the in uence of the interactions on thermodynamic properties. The calculations have been done by a numerical diagonalization of the Hamiltonian. Once the full spectrum is known we consider the thermal occupation of the states and calculate the different thermodynamic properties. The extension of the Hellmann-Feynman theorem at finite temperature requires the derivative of the free energy instead of the internal energy used for zero temperature. The importance of the entropy in the fullfilment of the Hellmann-Feynman theorem is analyzed