Charge density waves in an electron-two level system model

The Hubbard model with additional two level systems (TLS) which couple locally to the electronic density is investigated by a Hartree-Fock approximation (HFA) and Quantum Monte Carlo (QMC) simulations of 2 d lattices and in the limit d = {infinity}. While the HFA provides a phase transition of first or second order (depending on the model parameters) to a change density wave state, QMC simulations give evidence for an Ising-type phase transition at a much lower temperature. The results for d = 2 and d = {infinity} are in good agreement.