Interplay between vortex dynamics and superconducting gap structure in quasi-two-dimensional organic superconductors

Over the past three decades, unconventional superconductivity with anisotropic gap structure has been found in strongly correlated electron systems such as heavy fermion compounds, high Tc cuprates, and organic conductors. Unconventional superconductivity is characterized by anisotropic superconducting gap, which may have zeros (nodes) along certain directions in the Brillouin zone. The nodal structure is closely related to the pairing interaction, and it is widely believed that the presence of nodes is a signature of magnetic or some other exotic, rather than conventional phonon mediated, pairing mechanism. Thus, experimental determination of the gap function is of fundamental importance. However, the detailed gap structure is an unresolved issue for many unconventional superconductors. In this context, we have demonstrated for layered organic superconductors that flux-flow resistance under a magnetic field rotated within conducting layers provide a clue for determining the nodal directions. Here we present the experimental results for the angular variation of the flux-flow resistance in d-wave organic superconductors β”-(ET)2SF5CH2CF2SO3 and λ-(BETS)2GaCl4. Nodal structures for these superconductors are briefly discussed.