Quantum spin dynamics

Abstract:

We have measured angle-dependent magnetoresistance oscillations and the interplane millimetre-wave conductivity of the organic superconductor beta "-(ET)(2)SF5CH2CF2SO3 as a function of the orientation of the applied magnetic field. We observe harmonics of the cyclotron frequency in the real-space velocity of quasiparticles orbiting the Fermi surface (FS). The harmonic amplitudes depend on the field orientation, providing a new way to measure the quasi-two-dimensional FS topology.

Abstract:

The shape of the Fermi surface of organic metals can be measured by recording angle-dependent magnetoresistance oscillations. We review this technique and develop a model for parametrizing the shape of the quasi-two-dimensional Fermi surface sections which often appear in organic metals. Using this model, we show that it is possible to extract more detail about the quasi-two-dimensional pocket shape from angle-dependent magnetoresistance oscillations than in the traditional approximation which assumes an elliptical Fermi surface shape. We also consider the implications for cyclotron resonance experiments.

Abstract:

We describe a novel resonant cavity system which allows the bulk magnetoconductivity 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)2SF5CH2 CF2SO3, 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.

Abstract:

The magnetic behaviour and resistivity of single crystals of the layered organic superconductor, κ-(BEDT-TTF)2Cu(NCS)2, have been studied as a function of the orientation of the applied magnetic field. When the magnetic field lies in the quasi-two-dimensional planes of the material, there is strong evidence for a phase transition from the superconducting state into a Fulde-Ferrell-Larkin-Ovchinnikov state.