Scanning Tunneling Spectroscopy on κ-(BEDT-TTF)2Cu[N(CN)2]Br

We report the electron tunneling spectroscopy on partially deuterated κ-(BEDT-TTF-d[3,3])2Cu[N(CN)2]Br using a low temperature STM. In the superconducting state, tunneling spectra show the energy gap structure clearly; the conductance around the zero bias voltage is well reduced. V-shaped tunneling conductance curve inside the gap edge suggests the gap anisotropy. The linear dependence on the energy near zero bias is explained by the d-wave pairing with line nodes. By fitting to the d-wave gap model, we obtain the gap parameter as Δ=1.4–3.3 meV. Correspondingly, 2Δ/kTc is estimated as 2.7–6.4. The value is almost the same as that in κ-(BEDT-TTF)2Cu(NCS)2, d[0,0] and d[2,2] salts. The zero bias anomaly, which is an evidence for nodes of the superconducting gap, was found. The d-wave symmetry is strongly supported.