CHARGE-DENSITY WAVES IN AN ELECTRON-2 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 a pproximation (HFA) and Quantum Monte Carlo (QMC) simulations of 2 d la ttices and in the limit d = infinity. While the HFA provides a phase t ransition of first or second order (depending on the model parameters) to a charge density wave state, QMC simulations give evidence for an Ising-type phase transition at a much lower temperature. The results f or d = 2 and the d = infinity are in good agreement.