Test for Pairing Symmetry Based on Spin Fluctuations in the Organic Superconductorκ−(BEDT−T

Abstract

We propose that the superconducting pairing symmetry of organic superconductors κ-(BEDT-TTF)2X 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 dxy-like pairing states. In addition, we predict the nonexistence of the spin resonance mode in this compound, which is an important feature in high-Tc superconductors.PACS number: 74.70. Kn,74.20.Rp Layered organic superconductors based on the (BEDT-TTF) molecule are rich examples of strongly correlated electron systems in low dimensions [1]. Among them, κ-(BEDT-TTF) 2 X(κ-(ET)) has attracted considerable attention, because they share many similarities to cuprate superconductors [2], such as the proximity of its superconducting(SC) phase with the antiferromagnetic(AF) insulating phase which suggests that it may be an unconventional superconductor. Understanding its SC mechanism could therefore provide important insight into the origin of the unconventional superconductivity. As an essential step for this process, intensive experimental and theoretical investigations have been carried out to identify its SC pairing symmetry, but no consensus has been achieved yet [3]. The NMR [4] and millimeterwave transmission experiment [5] suggests d x 2 −y 2 symmetry (A different interpretation has been proposed for the latter result [6]). However, the recent thermal conductivity [7] and scan tunneling spectroscopy measurements [8] predict that it is more like d xy than d x 2 −y 2 . Moreover, a nodeless s-wave SC state is also suggested in recent specific-heat measurements [9]. Theoretically, the d x 2 −y 2 -like pairing symmetry is favored in the model in which the superconductivity is mediated by strong AF fluctuations [10,11] and also in a model of small-q phonon mediated superconductivity [12]. However, it is shown that the d xy -like superconductivity is also likely and can dominate over d x 2 −y 2 wave, when a more realistic model is considered [13].In this letter, we propose an orientational-dependent method aiming at a direct determination of the gap anisotropy in κ-(ET). It is to measure the q-space position of the incommensurate(IC) peaks of the dynamical spin susceptibility by inelastic neutron scatterings. Our proposal is based on: i) it is believed that AF spin fluctuations are strong in this material [1,2,4]; ii) the central issue at problem about the gap symmetry is what the direction of the gap nodes is or if there are gap nodes. The underlying physics is that the particle-hole excitations, which contribute to the spin susceptibility, are only available for the node-to-node transitions in the SC state, if the excitation energy is lower than 2∆ 0 (∆ 0 the magnitude of the SC gap). Therefore, t...