Commensurate-Incommensurate Phase Transition in Quasi One-Dimensional Electron-Phonon System: Nearly Half-Filled Band

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Abstract
Within a mean-field approximation for phonon fields one soliton formation energy Fsol is calculated at finite temperatures in the limit of weak coupling for the quasi one-dimensional electron-phonon system with a nearly half-filled band. The phase diagram on the (µ, T) plane is given on the assumption that the commensurate-incommensurate transition is of second order. Here µ is the Fermi energy as measured from the middle of a conduction band. For the system without an electron reservoir there are two successive phase transitions at proper values of µ, from the normal (N) to the commensurate (C) phase and the commensurate to the incommensurate (IC) phase as T is decreased to zero. For the case of finite reservoir there exists the region of the C-phase a T = 0, since the electron reservoir suppresses the increase of the chemical potential. For the case of infinite reservoir the chemical potential does not change with T and the phase transition occurs from the IC-to the C-phase as T is lowered to zero. Near the triple point analytical calculations are made to obtain the slope of the TCI (C-IC transition temperature) line. The C-phase becomes more stable and occupies a larger region on the (µ, T) plane as the size of electron reservoir increases.

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