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 beta ''-(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 (T-SC) but also the electrical resistivity and activation energy just above the T-SC 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.