Pairing in planar organic superconductors

The nature of superconductivity in planar organics is still a controversial problem. We investigate two types of d-wave and extended s-wave pairing proposed theoretically in a model of κ-(BEDT-TTF)2X organic superconductors. As probes of the pairing symmetry, we calculate the density of states, the tunnelling conductance, the specific heat, and the spin susceptibility. The sensibility of superconductivity to the sample preparation suggests that beside the symmetric dx2-y2 gap, which appears to be consistent with some experiments (for the specific heat, see Nakazawa and Kanoda, Phys. Rev. B 55 (1995) R8670, for STM, Arai et al., Phys. Rev. B 63 (2001) 194518, and for NMR, Mayaffre et al., Phys. Rev. Lett. 75 (1995) 4122) the asymmetric dxy-like or extended s-wave gaps could occur. We show that in comparison with the dx2-y2 case this should be manifested by a more rapid increase of the specific heat at low temperature, by the presence of inner peaks on the density of states and conductance curves at low temperature, and by a different curvature of the spin susceptibility temperature dependence.