Enhancements of Superconductivity and Insulating Electrical Resistivity under the Same Uniaxial Strain in a Molecular Conductor

We observed the temperature dependence of electrical resistivity under uniaxial strains in β′′-(ET)4[Ga(ox)3(H3O)]C6H5NO2, which has attracted attention as a superconductor showing charge fluctuation due to the intersite Coulomb repulsion (V). Not only superconducting transition temperature (TSC) but also the electrical resistivity and activation energy just above the TSC were enhanced by the small strain, indicating that superconductivity and insulating electrical resistivity have the same origin. This pressure dependence was more remarkable in the b-axis strain than in the a-axis strain. The b-axis is almost parallel to the direction of the largest transfer integral and almost normal to that of the largest V. The enhancements of insulating behavior and superconductivity by the b-axis strain is attributable to the change in the transfer integral. Therefore, not only the charge fluctuation due to V but also the lattice fluctuation due to the inhomogeneous transfer integral plays an important role in the superconductivity.