Spin and Charge Fluctuations and Lattice Effects on Charge Ordering in α-(BEDT-TTF)2I3

Abstract

The effects of spin and charge fluctuations and electron–phonon couplings on charge ordering in α-(BEDT-TTF) 2 I 3 [BEDT-TTF = bis(ethylenedithio)-tetrathiafulvalence] are investigated theoretically for an anisotropic triangular lattice at 3/4 filling. By the exact-diagonalization method, we have calculated the hole density distributions and the modulations of transfer integrals from high-temperature values as a function of electron–phonon coupling strength. The results clearly show that the lattice effect on α-(BEDT-TTF) 2 I 3 is weak compared with that on θ-(BEDT-TTF) 2 RbZn(SCN) 4 , as previously found by experiments. This finding, which is also consistent with recent mean-field results, is systematically explained by strong-coupling perturbation theory; the effects of spin fluctuations are partially canceled by charge fluctuations in α-(BEDT-TTF) 2 I 3 .