Scanning tunnelling spectroscopy study of paramagnetic superconducting beta ''-ET4[(H3O)Fe(C2O4)(3)]center dot C6H5Br crystals

Scanning tunnelling spectroscopy (STS) and microscopy (STM) were performed on the paramagnetic molecular superconductor ?-ET4[(H3O)Fe(C2O4)3]·C6H5Br. Under ambient pressure, this compound is located near the boundary separating superconducting and insulating phases of the phase diagram. In spite of a strongly reduced critical temperature Tc (Tc = 4.0 K at the onset, zero resistance at Tc = 0.5 K), the low temperature STS spectra taken in the superconducting regions show strong similarities with the higher Tc ET ?-derivatives series. We exploited different models for the density of states (DOS), with conventional and unconventional order parameters to take into account the role played by possible magnetic and non-magnetic disorder in the superconducting order parameter. The values of the superconducting order parameter obtained by the fitting procedure are close to the ones obtained on more metallic and higher Tc organic crystals and far above the BCS values, suggesting an intrinsic role of disorder in the superconductivity of organic superconductors and a further confirmation of the non-conventional superconductivity in such compounds.