We describe a novel resonant cavity system which allows the bulk magneto-conductivity of single crystals of anisotropic metals to be measured at GHz frequencies. The cavity can be made to rotate in a static magnetic field, permitting detailed studies of the dependence on the magnetic field orientation of the high-frequency magnetoconductivity. In this paper, the apparatus is used to measure the Fermi-surface topology of two organic superconductors; the details revealed are inaccessible to conventional fermiological techniques such as the de Haas-van Alphen effect. In κ-(BEDT-TTF)2Cu(NCS)2, Fermi-surface traversal resonances (FTRs) are observed. The angle dependence of the FTRs shows that the quasi-one-dimensional (Q1D) Fermi sheets of this material possess two distinct corrugations, with corrugation axes making angles of 17.3° and -19.4° with the ka-axis. Such data form important input parameters for current models of superconductivity in the organics, which invoke spin-density-wave-like fluctuations caused by partial nesting of the Q1D Fermi sheets. In β''-(BEDT-TTF)2SF5CH2CF2SO3, cyclotron resonance is observed, along with its second and third harmonics. The detailed angle dependence of the intensities of the various cyclotron harmonics allows the elongation and orientation of the closed section of the Fermi surface to be deduced, and strongly suggests that the interplane transport is coherent in this material. The effective mass deduced from the cyclotron resonance measurements is greater than that determined from magnetic quantum oscillations, in agreement with recent theoretical predictions.

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We have used a millimeter-wave magneto-optical technique to study the angle dependence of the high-frequency conductivity of the molecular superconductor k-͑BEDT-TTF͒ 2 Cu͑NCS͒ 2 . The data strongly suggest that the superconducting gap has nodes directed along the b and c directions of the crystal, in agreement with recent theoretical predictions. This supports the idea that the superconductivity in k-͑BEDT-TTF͒ 2 Cu͑NCS͒ 2 is d wave in nature, and is mediated by spin fluctuations. PACS numbers: 74.25.Gz, 74.20.Mn, 74.25.Nf, 74.70.Kn The family of superconducting charge-transfer salts k-͑BEDT-TTF͒ 2 X, where X can, for example, be Cu͑NCS͒ 2 , Cu͓N͑CN͒ 2 ͔Br, or I 3 , has attracted considerable recent attention. A variety of experiments have suggested that the superconducting gap function may contain nodes at certain points on the Fermi surface; e.g., the 13 C NMR spin-lattice relaxation rate [1] varies as T 3 and the thermal conductivity [2] is proportional to T below the superconducting critical temperature T c . In addition, microwave penetration-depth studies [3] show a non-BCS-like behavior of the penetration depth as a function of T and the electronic component of the specific heat [4] has an unconventional field dependence below T c . However, Shubnikov-de Haas [5,6], magnetic breakdown [7], and angle-dependent magnetoresistance oscillation [8] experiments demonstrate that these salts have well-defined quasi-two-dimensional (Q2D) Fermi surfaces, indicating that the quasiparticles can be described by Fermi-liquid theory at low temperatures (the Fermi surface is shown schematically as an inset of Fig. 1). Furthermore, it has been possible to fit experimental Fermi-surface-topology data to a simplified model of the tight-binding band structure (the so-called effective dimer model) to a good degree of accuracy [5,[9][10][11].The combination of unconventional superconductivity and a tractable analytical representation of the band structure makes the k-͑BEDT-TTF͒ 2 X superconductors attractive for theoretical studies, and a number of authors [9,10,12,13] have explored the possibility of d-wave superconductivity mediated by spin fluctuations. Such approaches predict that the superconducting order parameter will have four, roughly perpendicular, gap nodes, the exact orientation of the nodes depending on the underlying details of the Fermi surface [10,12]. Experiments performed thus far have merely detected the probable presence of the gap nodes, without giving information about their relative orientation. In this paper, we use a millimeter-wave magneto-optical technique to determine the orientation of the gap nodes in k-͑BEDT-TTF͒ 2 Cu͑NCS͒ 2 . The data appear to be in good qualitative agreement with the predictions of Schmalian [10].The experiments were carried out on a number of crystals of k-͑BEDT-TTF͒ 2 Cu͑NCS͒ 2 , grown by FIG.

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