Effects of spin fluctuations, charge fluctuations and lattice distortions on charge orders in θ- and α-type BEDT-TTF salts

Abstract

Effects of spin fluctuations, charge fluctuations and lattice distortions on charge orders in θ-(BEDT-TTF)2RbZn(SCN)4 and α-(BEDT-TTF)2I3 are investigated theoretically in a two-dimensional extended Peierls-Hubbard model. By using exact diagonalization, we have calculated hole-density distributions and transfer modulations with transfer integrals based on the corresponding high-temperature structures as a function of electron-phonon couplings. The results clearly show the origin of their lattice-effect differences, namely, the lattice effect on θ-(BEDT-TTF)2RbZn(SCN)4 is much larger than that on α-(BEDT-TTF)2I3. This finding is systematically explained by the strong-coupling perturbation theory. It is found that spin fluctuations induce lattice distortions in θ-(BEDT-TTF)2RbZn(SCN)4, but their effects are partially cancelled by charge fluctuations in α-(BEDT-TTF)2I3.