Tunneling spectroscopy on κ-(BEDT-TTF)2Cu[N(CN)2]Br using STM

We report the electron tunneling spectroscopy on κ-(BEDT-TTF)2Cu[N(CN)2]Br using a low temperature STM. In the superconducting state, tunneling spectra show the energy gap structure clearly. The tunneling differential conductance shows linear dependence on energy inside the gap edge. The conductance curve is well fitted by the d-wave calculation with a little broadening. The d-wave gap parameter is obtained as Δ0=2.5–3.5 meV, correspondingly, 2Δ0/kTc=5.0–7.1. The pairing symmetry is consistent with that in κ-(BEDT-TTF)2Cu(NCS)2 as well as the gap parameter. The broad dip structure around the Fermi level is found in tunneling spectra above Tc. The pseudogap structure is observed below 50 K.