Effects of the intramolecular structural change on the superconducting phase of the TMTTF systems

Abstract

The tetramethyltetrathiafulvalene (TMTTF) system is one of the key materials to reveal mechanisms for unconventional superconductivity which is driven by electronic correlation. Recent experimental studies show that only the SbF6 salt, which exhibit the strongest charge order among the PF6 and AsF6 and SbF6 salts have anomalously high superconducting phase transition pressure for the insulating-superconducting phase and superconducting-metallic phase transitions. To investigate the relationship between the superconductivity and the strength of the charge order of these salts, we constructed an extended Hubbard model with the periodic boundary condition that takes into account the site-deformation potential driven by the intramolecular structural change which causes the nonuniform charge order. The superconductivity was examined by the Drude weight and the charge gap obtained by exact diagonalization. It was found that site-deformation potentials driven by the intramolecular conformational change, even small changes, move the insulator-superconductor transition point to higher pressure, i.e. too large transfer integrals. Without the site-deformation potentials, the transfer integral at the transition point is too small as compared with those values deduced from the first-principles calculations at the structures under atmospheric pressure and it is difficult to reproduce the transition.