Anisotropic Fully Gapped Superconductivity Possibly Mediated By Charge Fluctuations In A Nondimeric Organic Complex

We investigate low-temperature electronic properties of the nondimeric organic superconductor beta ''-(ET)(4) [(H3O)Ga(C2O4)(3)]PhNO2 . By examining ultrasonic properties, charge disproportionation (CD) without magnetic field dependence is detected below T-CD similar to 8 K just above the superconducting critical temperature T-c similar to 6 K. From quantum oscillations in high fields, we fmd variation in the Fermi surface and mass enhancement induced by the CD. Heat capacity studies elucidate that the superconducting gap function is fully gapped in the Fermi surface, but anisotropic with fourfold symmetry. We point out that the pairing mechanism of the superconductivity is possibly dominated by charge fluctuations.