Millimetre-wave measurements of the bulk magnetoconductivity of anisotropic metals: application to the organic superconductors kappa-(BEDT-TTF)(2)Cu(NCS)(2) and beta ''-(BEDT-TTF)(2)SF5CH2CF2SO3 (BEDT-TTF bis(ethylene-dithio)tetrathiafulvalene)

We describe a novel resonant cavity system which allows the bulk magnetocon ductivity of single crystals of anisotropic metals to be measured at GHz fr equencies. The cavity can be made to rotate in a static magnetic field, per mitting detailed studies of the dependence on the magnetic field orientatio n of the high-frequency magnetoconductivity. In this paper, the apparatus i s used to measure the Fermi-surface topology of two organic superconductors ; the details revealed are inaccessible to conventional fermiological techn iques such as the de Haas-van Alphen effect. In kappa-(BEDT-TTF)(2)Cu(NCS)( 2), Fermi-surface traversal resonances (FTRs) are observed, The angle depen dence of the FTRs shows that the quasi-one-dimensional (Q1D) Fermi sheets o f this material possess two distinct corrugations, with corrugation axes ma king angles of 17.3 degrees and -19.4 degrees with the k(a)-axis. Such data form important input parameters for current models of superconductivity in the organics, which invoke spin-density-wave-like fluctuations caused by p artial nesting of the Q1D Fermi sheets. In beta "-(BEDT-TTF)(2)SF5CH2CF2SO3 , cyclotron resonance is observed, along with its second and third harmonic s. The detailed angle dependence of the intensities of the various cyclotro n 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 magn etic quantum oscillations, in agreement with recent theoretical predictions .