ET charge transfer salts (where ET is <en>bis(ethylenedithio)- tetrathiafulvalene) have relatively simple quasi two-dimensional Fermi surface topologies, making them ideal for the study of the relationship between bandstructure and properties such as superconductivity. Experimental studies of the Fermi surface areas and associated effective masses have been carried out using the Shubnikov-de Haas (SdH) and de Haas-van Alphen (dHvA) effects. By comparing the experimental results to theoretical bandstructure calculations the strength of many body interactions has been estimated. High pressure magnetotransport experiments have been carried out on the superconductor κ-ET<sub>2</sub>Cu(NCS)<sub>2</sub>. The observation of SdH and magnetic breakdown oscillations has allowed the pressure dependences of the Fermi surface topology and effective masses to be deduced and compared with simultaneous measurements of the superconducting critical temperature. The data strongly suggest that the enhancement of the effective mass and the superconducting behaviour are directly connected. The results are compared with several current theories of superconductivity. The dHvA effect has been used to probe the superconducting mixed state of κ-ET<sub>2</sub>Cu(NCS)<sub>2</sub>. A recent model of the superconducting mixed state is applied to the experimental data in an attempt to determine the value and symmetry of the superconducting energy gap. SdH measurements up to 30 T have been used to study spin densitywave formation in α-ET<sub>2</sub>KHg(SCN)<sub>4</sub>, and the reasons why a very slight increase of the unit cell volume (i.e. replacing the K in α-ET<sub>2</sub>KHg(SCN)<sub>4</sub> by NH<sub>4</sub>) stabilises a superconducting state. Galvanomagnetic techniques have been used to measure the quasi onedimensional Fermi surface orientation below the spin-density-wave transition, and to accurately determine the shape of the quasi twodimensional Fermi surface above it. The application of pressure has been used to gradually reduce the onset temperature of a metal-insulator transition and to eventually stabilise a superconducting state in ET<sub>3</sub>Cl<sub>2</sub>2H<sub>2</sub>O. The bandstructure of ET<sub>3</sub>C1<sub>2</sub>2H<sub>2</sub>O has been investigated using the SdH effect whilst hydrostatic pressure has been used to pass through the superconducting part of the phase diagram.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:240621 |
| Date | January 1994 |
| Creators | Caulfield, Jason M. |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:5fbf2599-96d8-4eac-b882-ac74213ac3a5 |
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