Direct Observation of d-Wave Superconducting Gap in κ-(BEDT-TTF)_2Cu[N(CN)_2]Br with Scanning Tunneling Microscopy(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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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_<x^2-y^2>-wave. (If we take the same convention for the magnetic Brillouin zone as high-T_c cuprates with the d_<x^2-y^2>-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_<x^2-y^2>-wave symmetry.
社団法人日本物理学会の論文
2008-11-15