INSULATOR-METAL TRANSITION VERSUS TEMPERATURE FOR THE CDW OF THE 1D ADIABATIC HOLSTEIN MODEL

We perform a numerical study of the classical atomic configuration whi ch minimizes the electronic free energy at finite temperature of the I D adiabatic Holstein model. For a fixed incommensurability, the ground -states which are insulating bipolaronic incommensurate charge density wave (CDW) may undergo a second-offer transition into a conducting Pe ierls-Frohlich CDW when the temperature increases (inverse transition by breaking of analyticity). The global phase diagram is calculated as a function of temperature and electronic chemical potential mu. When increasing the temperature, the high commensurability and incommensura te CDWs progressively disappear. The same phase diagram represented as a function of temperature and electronic band filling exhibits phase separations between different commensurate structures also involving c harge separations (which are physically unacceptable). Thus it is demo nstrated that at moderate electron-phonon couplings which are in the r ange of those of real systems, the standard mean field theory of CDW I s inconsistent with the real behaviour of the model. Copyright (C) 199 8 Elsevier Science B.V.