Finite-temperature phase diagram of mixed-stack charge-transfer complexes

Abstract

The mixed-stack organic charge-transfer complex, (BEDO–TTF)(Cl 2TCNQ), has shown a phase transition accompanied with a sharp drop in the magnetic susceptibility and with an increase in the intermolecular overlap, as the temperature is lowered. Here, we theoretically consider the possibility for a phase transition from the ionic phase to a neutral phase, which is driven by charge-transfer fluctuations. Using finite-temperature density-matrix renormalization-group calculations for the one-dimensional extended Hubbard model with alternating potentials, we show that, with increasing transfer integral t, a transition from the neutral phase to the ionic phase is induced by spin fluctuations for small t, while another transition from the ionic phase to the neutral phase is induced by charge-transfer fluctuations for large t. Near the phase boundary, the free energy is easily lowered by dimerization of transfer integrals or by staggered magnetic field. Thus, a further transition is also possible to a dimerized nonmagnetic phase or to an antiferromagnetic phase at a lower temperature. In two dimensions, we expect that the effect of charge-transfer fluctuations is larger and that the transition to the dimerized nonmagnetic phase is suppressed.