Bipolaronic Charge Density Waves, Polaronic Spin Density Waves and High Tc Superconductivity

Abstract

At large enough electron phonon coupling, the existence of bipolaronic, polaronic and mixed states is rigourously proven for the adiabatic Holstein model at any dimension and any band filling1. The ground-state is one of them which then prove the existence of insulating Bipolaronic Charge Density Waves (BCDW). The role of the quantum lattice fluctuations is analysed and found to be negligible in that regime but to become essential in case of phonon softening then favouring the occurrence of superconductivity2, 3. When a strong Hubbard term is also present, the bipolarons break into polarons and the ground state is expected to be a Polaronic Spin Density Wave (PSDW). If the repulsive Hubbard term is comparable to the electron-phonon coupling, the energy for breaking a bipolaron into two polarons can become small and we get instead of these two degenerate structures, a pair of polarons bounded by a spin resonance which we call “spin resonant bipolaron”. This resonant bipolaron is still strongly bound, but the role of the quantum lattice fluctuations becomes now very important and yields a sharp broadening of the band width of this resonant bipolaron. Thus, the strong quantum character of these resonant bipolarons could prevent their localization into real space structures which could be insulating BCDWs or PSDWs, then favouring the formation of a superconducting coherent state with a possible high Tc.(For details see ref. 4)