Direct Observation of d-Wave Superconducting Gap in κ-(BEDT-TTF)₂Cu[N(CN)₂]Br with Scanning Tunneling Microscopy

Abstract

Conducting and lateral surfaces of two-dimensional organic superconductor κ-(BEDT-TTF)₂Cu[N(CN)₂]Br were studied by electron tunneling using scanning tunneling microscopy. The V-shape tunneling conductance curve was observed at the conducting plane in the superconducting phase, attributed to the d-wave pairing symmetry. Tunneling spectra were successfully obtained at various lateral surfaces of crystals. The tunneling conductance curve varies systematically depending on tunneling direction. The conductance curve and its directional dependence are explained by the d-wave with nodes along the direction near π⁄4 from the k_a- and k_c-axes. The pairing symmetry is of the d_x²−y²-wave. (If we take the same convention for the magnetic Brillouin zone as high-T_c cuprates with the d_x²−y²-symmetry, the pairing symmetry of κ-(BEDT-TTF)₂Cu[N(CN)₂]Br is the d_xy-symmetry.) This is the same symmetry as that in the superconducting gap of κ-(BEDT-TTF)₂Cu(NCS)₂. The zero bias conductance peak (ZBCP) was observed at the tunneling along near π⁄4 from the a- and c-axes. This ZBCP result reinforces the conclusion of the d_x²−y²-wave symmetry.