The authors report ' C NMR spin-lattice relaxation rates 1/Tl and Knight shifts Ks in the quasitwo-dimensional organic superconductor «-(ET)2Cu[N(CN)i]Br ( T, = 11.6 K), for an aligned single crystal. The normal-state behavior is reminiscent of the high-T, cuprates, in which antiferromagnetic Auctuations and spin-gap behavior dominate. In the superconducting state, the data rule out the BCS electron-phonon mechanism as the source of the superconductivity, but support an unconventional pairing state with possible nodes in the gap function.The discovery of superconductivity in organic chargetransfer salts based on the BEDT-TTF ("ET") molecule has stimulated interest in understanding the electronic structure of their normal and superconducting states. ' The ET compound with the highest ambient pressure critical temperature is «-(ET)zCu[N(CN)2]Br (T, =11.6 K). This salt has a layered structure and quasi-twodimensional (2D) electronic conduction, similar to the cuprate superconductors. The reduced dimensionality, and low carrier density (-10 ' cm ), indicate that electron correlation effects may be important. The nature of the superconducting state in the ET salts (i.e. , BCS or unconventional) is unsettled at present. The low-temperature magnetic penetration depth has been studied, with some experiments supporting the existence of an isotropic BCS gap, and others an anisotropic gap.We have utilized ' C NMR to study the normal and superconducting states of tc-(ET)zCu [N(CN) 2]Br [' K-(ET)&Br"]. In agreement with the work of Mayafre et al. and Kawamoto et al. , we find that the normalstate behavior is not that of a simple metal, and that antiferromagnetic Auctuations, and spin-gap behavior may be present. Our data for «. -(ET)2Br below T, favor spinsinglet pairing, and a highly anisotropic energy gap, such as is present for d-wave, or anisotropic s-wave, orbital pairing. Thus, in contrast to the A3C6o (A =K,Rb, Cs) "Buckyball" superconductors, the superconductivity in tc-(ET)2Br arises from a mechanism other than the conventional BCS electron-phonon coupling.In a metal, the hyperfine interactions of the nuclei with the spins of the conduction electrons dominate the NMR properties. We find that in «. -(ET)2Br, both the isotropic Fermi contact and anisotropic dipolar (2p, ) interactions are present. For a simple metal, the Korringa law relates the Knight shift E, to the spin-lattice relaxation time T& . .1/T, T o= K, . Below T"a gap opens at the Fermi level, producing dramatic changes in the NMR properties.For BCS spin-singlet s-wave superconductors, K, decays to zero as T~O in a characteristic way. Just below T" 1!T,rises ("coherence peak"), but falls off exponentially at low T. These features are found in the NMR studies of A3C6o, clearly identifying it as a conventional BCS superconductor with an isotropic energy gap.' We note that the above results are drastically modified in the case of an anisotropic gap function.Previously, we have reported 'H NMR in the superconducting state of tc-(ET)@Br. " We showed that the NM...

The authors report 13 C NMR and magnetization measurements on the magnetic state of oriented single crystals of the organic superconductor -(BEDT-TTF) 2 Cu͓N(CN) 2 ͔Cl. To understand these data a spin Hamiltonian based on the Pnma symmetry of the crystal is developed. When interpreted in the context of this Hamiltonian, the measurements provide a detailed picture of the spin ordering. It is found that the Dzialoshinskii-Moriya ͑DM͒ interaction is largely responsible for the details of the ordering above the spinflop field. Of particular note, the interplane correlations are determined by the intraplane DM interactions and the direction of the applied field.

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