In-Plane Anisotropy of Upper Critical Field and Flux-Flow Resistivity in Layered Organic Superconductor β′′-(ET)2SF5CH2CF2SO3

We report the in-plane anisotropy of the upper critical field (Hc2) and the flux-flow resistivity (FFR) for a layered organic superconductor β′′-(ET)2SF5CH2CF2SO3 with the incoherent nature of the interlayer transport. The in-plane angular dependence of Hc2 showed a fourfold oscillation with maxima in the directions around H || b and a axes. This result is compatible with a \(d_{x^{2} - y^{2}}\) order parameter. The determined nodal structure is consistent with theoretical predictions of superconductivity mediated by charge fluctuations. Moreover, the dependence of the FFR on in-plane field-orientation showed a fourfold symmetry with cusp-like minima in the directions around H || b and a axes, which is very similar to the FFR for κ-(ET)2Cu(NCS)2 with d-wave pairing symmetry. From these results, we claim that the vortex dynamics is strongly affected by the superconducting gap structure for highly two-dimensional superconductors with d-wave pairing symmetry.