NMR Investigation of Pauli Limited Superconductivity

A magnetic field interacts with a spin singlet superconductor via coupling to either the orbital motion of the superconducting electrons, or the spin of the superconducting electrons. When the coupling to the electron spin is the dominant interaction, the superconductor is referred to as Pauli limited, due to the limitations of the magnetic field strength which superconductivity can be maintained. When the external magnetic field strength is increased above the Pauli limiting field (Hp), the normal metallic state, associated with Pauli paramagnetism, becomes the energetically favorable state. The organic superconductor kappa-(BEDT-TTF)2Cu(NCS) 2 is a Pauli limited superconductor owing to its highly two dimensional structure. When the magnetic field is applied parallel to the superconducting layers, the orbital motion of the superconducting electrons is suppressed, allowing the interaction with the electron spin to dominate.;In the limit where a magnetic field couples only to the electron spin, the field dependence of the Fermi surface properties can be used to probe the superconducting gap structure, and in particular, determine whether the k-space gap contains nodal points. This is because, for a superconductor containing gap nodes, the Pauli limiting condition would be reached continuously in k-space for regions near the gap nodes, and thus the percentage of k-space where the Fermi surface has been restored would increase as the applied field is increased. In contrast, if the superconducting gap spanned the entire Fermi surface, the Fermi surface properties would be independent of applied field, until the bulk Pauli limit is reached. The Nuclear Magnetic Resonance (NMR) spin lattice relaxation rate experimentally probes the density of states at the Fermi level, and thus its field dependence can be used to determine whether nodes exist within the superconducting gap by measuring whether an applied field restores the Fermi surface. The field dependence of the relaxation rate in kappa-(BEDT-TTF)2Cu(NCS)2, is presented in this thesis, and demonstrates that this material's gap structure contains nodal points.;In addition, a Pauli limited superconductor has been theoretically shown to exhibit unconventional superconductivity at high magnetic fields. The work of Fulde and Ferrell [1], as well as Larkin and Ovchinnikov [2], demonstrated theoretically that the superconducting state can be maintained as the ground state for fields in excess of the Pauli limit by converting from the conventional zero-momentum pairing state described by Bardeen, Cooper, and Schrieffer (BCS), to a new pairing state with a net finite momentum. Evidence for this exotic high field phase, termed the FFLO state, has recently been reported in transport and thermodynamic measurements in kappa-(BEDT-TTF)2Cu(NCS) 2. The NMR relaxation rate and Knight shift presented in this thesis at high magnetic fields exhibit evidence for a phase transition within the superconducting state occurring at H=215 kOe, and demonstrate qualitative agreement within the theoretical framework of the FFLO state.