Superconducting and normal-state gaps in κ-(BEDT-TTF)2Cu(NCS)2 studied by STM spectroscopy

Scanning tunneling microscope (STM) spectroscopy on the b–c plane of single crystals of an organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 has been performed in superconducting and normal states with varying temperature. The temperature dependence of the zero-bias conductance dI/dV|V=0 is well explained by the d-wave gap model considering the T2 dependence of the broadening parameter of the one-electron level Γ(T), which is obtained from the fitting of the dI/dV–V curves to the d-wave gap model. The dI/dV–V curve shows a broad dip around the Fermi energy above Tc=10.4 K. This pseudogap structure remains up to about 45 K. The magnitude of the pseudogap is much larger than that of the superconducting gap Δp∼3 meV.