Tunneling Spectroscopy in Organic Superconductor κ-(BEDT-TTF-d[3,3])2Cu[N(CN)2]Br

Abstract

We performed tunneling spectroscopy (STS) measurement in an organic superconductor, partially deuterated κ-(BEDT-TTF-d[3,3])2Cu[N(CN)2]Br. We carried out angle-resolved STS on cut lateral surfaces by the focused ion beam (FIB) method as well as as-grown single-crystal surfaces. It was found that the d[3,3]-Br salt is a strong-coupling d-wave superconductor and that the nodal direction is at an angle of π/4 from the a*-axis, corresponding to the \(d_{x^{2} - y^{2}}\) symmetry in the bulk superconducting phase. It is understood that the electron correlation in the d[3,3]-Br salt is not strong enough on the basis of the spin fluctuation mechanism. On the other hand, we also observed two types of superconducting gap. This suggests the coexistence of the \(d_{x^{2} - y^{2}}\) and dxy symmetries. This indicates a change in the symmetry from \(d_{x^{2} - y^{2}}\) in the bulk superconducting phase to dxy around the insulating region with increasing electron correlation.