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

Abstract

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 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 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.