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

Abstract

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.) Proto...