d-wave Pair Symmetry in the Superconductivity of κ-(BEDT-TTF)2X

The superconducting phase of organic superconductors κ-(BEDT-TTF) 2 X was studied by the electron tunneling using scanning tunneling microscope (STM). Tunneling both at the conducting plane and lateral surfaces was performed on single crystals of κ-(BEDT-TTF) 2 Cu(NCS) 2 . Tunneling conductance curve obtained at the conducting plane is explained by the d -wave gap symmetry. The temperature dependence of tunneling spectra is also consistent with the d -wave symmetry. Tunneling spectra at lateral surfaces varies depending on the tunneling direction, suggesting the in-plane anisotropy of the gap. The directional dependence is also consistent with the d -wave symmetry with line nodes π/4 from the k b - and k c -axes. It indicates that the superconducting pair wave function has the d x 2 - y 2 -wave symmetry. (If we take the same conventions for the magnetic Brillouin zone as high- T c cuprates with the d x 2 - y 2 -wave symmetry, the gap symmetry of κ-(BEDT-TTF) 2 Cu(NCS) 2 is the d x y -wave symmetry.) Protonated and partially deuterated κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br were also studied. Tunneling spectra are consistent with the d -wave symmetry in all salts. Gap parameters obtained in each salt were discussed in the view point of the effective correlation. The large gap value suggests the strong coupling superconductivity. The pseudogap structure was found above T c . The zero bias conductance peak, which gives an evidence for nodes of the superconducting gap, was found.