High-frequency magneto-conductivity studies of low-dimensional organic conductors

In this thesis I present experimental studies of the millimetre-wave magneto-conductivity of the organic charge-transfer salts κ-(BEDT-TTF)2Cu(NCS)2, α-(BEDT-TTF2KHg(SCN)4 and (TMTSF)2ClO4. A rotating resonant cavity insert was developed to investigate the angle dependence of resonances in the magneto-conductivity. The technique revealed a new kind of magnetic resonance, the Fermi-surface traversal resonance (FTR), which is described by a semiclassical model. The FTRs give information about the topology of the quasi-one-dimensional (Q1D) Fermi-surface sections. The Q1D Fermi-surface sections play an important role in forming the ground states in organic conductors giving rise, for example, to nesting associated with a density-wave. Chapter 1 gives a brief historical overview of the development of organic conductors and provides basic information about crystal structures and Fermi surfaces of BEDT-TTF based systems. Chapter 2 introduces the reader to basic solid state physics used later in the thesis. In Chapter 3 I discuss the operation of the experimental techniques and apparatus, including millimetre-wave techniques. In addition, Chapter 3 contains a description of the design of the FTR rotating cavity insert which is unique of its kind. In Chapter 4 a model of oscillating real-space velocity vectors is presented. The model is used to explain the origin of the FTRs observed in the high-frequency magneto- conductivity data. In Chapter 5 I report two studies of the angle dependence of FTRs in the high-frequency magneto-conductivity. The FTRs in κ-(BEDT-TTF)2Cu(NCS)2 and α-(BEDT- TTF)2KHg(SCN)4 show two previously unknown corrugations in the Q1D Fermi-surface sections of the two materials. The FTRs in α-(BEDT-TTF)2KHg(SCN)4 are investigated both in the density-wave state and near its collapse into a high-temperature, high-field state. In Chapter 6 a study of the millimetre-wave properties of (TMTSF)2ClO4 at low temperatures is described. Finally, in Chapter 7 I present a study of the angle dependence of the superconductor order parameter in κ-(BEDT-TTF)2Cu(NCS)2 with a new millimetre-wave technique. (author)