GHz magnetoconductivity and spin resonance studies of strongly correlated systems

This thesis describes a series of experiments probing the millimetre-wave response of the low-dimensional organic superconductors β"-(BEDT-TTF)4[(H3O)-M(C2O4)3]S, κ-(BEDT-TTF)2Cu(NCS)2 and (TMTSF)2 PF6 in applied magnetic fields. Chapter 1 introduces the physics of charge carriers in the presence of a mag- netic field and field-induced resonant phenomena in the conductivity of metals. The basic concept of Fermi liquid theory, that of the existence of quasiparticles with effective mass, is presented, as well as the principles of electron spin resonance. Chapter 2 outlines the experimental technique of millimetre-wave spectroscopy, the principle of guided radiation and the use of metallic resonators and for perturbative measurements. Some results on novel dielectric whispering-gallery resonators and hybrid "tunnel-gap" resonators for potential use in pulsed and destructive field experiments are also presented. Chapter 3 is a detailed angle-, temperature- and frequency- dependent investigation of electron spin resonance in the family of quasi-two-dimensional charge transfer salts β"-(BEDT-TTF)4[(H3O)M(C2O4)3]S, where M = Cr3+ and Fe3+. The effective spin Hamiltonians for the Cr3+ and Fe3+ ions are identified, and the presence of weak antiferromagnetic correlations established; insight is gained into the relaxation mechanism and the effect of the superconducting state on the localised spins. Chapter 4 reports the angle-dependent magneto-optical response of isotopically substituted β"-(BEDT-TTF)2Cu(NCS)2. Both normal and superconducting states are investigated, and Josephson plasma resonances and Fermi-surface traversal resonances are identified. Chapter 5 presents some preliminary results from the investigation of the quasi- one-dimensional Bechgaard salt (TMTSF)2PF6 under hydrostatic pressure and in applied magnetic field. The millimetre-wave response is investigated as a function of frequency as the field-induced spin-density-wave cascade is crossed. Possible evidence for resonant absorptions due to collective spin excitations is reported.