Test for Pairing Symmetry Based on Spin Fluctuations in Organic Superconductors $\kappa$-(BEDT-TTF)$_{2}$X

Abstract

We propose that the superconducting pairing symmetry of organic superconductors $\kappa$-(BEDT-TTF)$_{2}$X can be determined by measuring the position in momentum space of the incommensurate peaks of the spin susceptibility. We show that the positions of these peaks are uniquely determined by the symmetry of the superconducting state and the geometry of Fermi surface. Based on the effective Hubbard model on an anisotropic triangular lattice, we demonstrate the different incommensurate patters of spin responses for $d_{x^{2}-y^{2}}$- and $d_{xy}$-like pairing states. In addition, we predict the nonexistence of the spin resonance mode in this compound, which is an important feature in high-$T_{c}$ superconductors.