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

Conducting and lateral surfaces of two-dimensional organic superconductor kappa-(BEDT-TTF)(2)Cu[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 file d-wave pairing symmetry. Tunneling spectra were Successfully obtained at various lateral surfaces of crystals. The tunneling conductance curve varies systematically depending oil tunneling direction. The conductance curve and its directional dependence are explained by the d-wave with nodes along the direction near pi/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 syrnmetry of kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br is the d(xy)-symmetry.) This is the same symmetry as that in the superconducting gap of kappa-(BEDT-TTF)(2)Cu(NCS)(2). The zero bias conductance peak (ZBCP) was observed at the tunneling along near pi/4 front the a- and c-axes. This ZBCP result reinforces the conclusion of the d(x2-y2)-wave symmetry.