High Pressure Magnetotransport Studies of the Isostructural Organic Conductor Family: Alpha-Bis

This work is a comprehensive investigation of the magnetoresistance (MR) and Shubnikov-de Haas (SdH) effect in the synthetic organic conductors, alpha -(ET)_2MHg(-SCN) _4 (M = K, Rb, Tl and NH_4 ) under hydrostatic pressure. Although isostructural, their physical properties differ substantially. The NH _4 salt is superconducting at ~1K, whereas the other three salts are non -superconducting and have similar anomalous low temperature ground states (LTS). The nature of the anomalous LTS is a subject of great controversy. Several models have been proposed to describe the LTS, including: a spin density wave (SDW) state, a charge density wave (CDW) state, a metallic state with a Peierls type of transition, or a metallic state with a different band structure and Fermi surface (FS) from that of the NH_4 salt. These isostructural materials have been studied through systematic magnetotransport measurements to obtain information about the Fermi surfaces and their variation with pressure. First, it is found that the LTS is absent for pressures above ~6kbar and summarize these results in a unified phase diagram. Second, a drastic change of the SdH oscillation behavior with pressure is reported, which indicates a highly pressure-dependent FS topology. This allows for a test of the accuracy of the tight-binding band calculations which are commonly applied to organic conductors' electronic structure. Third, interband carrier tunneling (magnetic breakdown-MB) effects are observed in the Tl salt, which invalidate the accepted picture of the widely separated closed and open FS sections. In light of these results, an empirical FS for the Tl salt is constructed. In addition, results are presented on various phenomena including: giant MR with an anomalous maximum, splitting behavior in the SdH oscillations, and MR kink transition in the K, Rb, and Tl salts, and slow SdH oscillations in the NH_4 salt. Based on these observations, it is argued that a SDW state is unlikely to be the origin of the anomalous LTS. Rather, a metallic state with a modified band structure which allows MB as well as an inclination toward instabilities in the open FS section is a more plausible mechanism. This proposed ground state is discussed in light of the standard theory of magnetotransport and pressure effects on band structure and superconductivity.