Upper critical magnetic fields in quasi-one-dimensional layered superconductors

This thesis presents a theoretical analysis of upper critical magnetic fields in quasi-one-dimensional (Q1D), layered superconductors with highly anisotropic electron spectra. It is shown quantitatively how the temperature dependence and spacial orientation of the upper critical magnetic fields, Hc2(T), can reveal important microscopic properties of such superconductors, including the nature of their pairing symmetry. The results obtained show that highly anisotropic, layered compounds can possess exotic superconducting properties such as: non-analytical angular dependence in the upper critical fields at low temperature, the rare spin-triplet Cooper pairing, and a novel quantum limit reentrant superconducting phase occurring in Q1D compounds under ultra-high magnetic fields. For this purpose, two unconventional superconductors are examined: the highly anisotropic Q1D organic superconductor (DMET)2I3, and the layered transition metal oxide superconductor Li0.9Mo6O 17. In the first case, an angular dependence of H c2 that varies as theta3/2 is predicted in (DMET) 2I3 for small angles and low temperatures, in contrast to the well-established (Ginzburg-Landau) quadratic angular dependence near the transition temperature. For Li0.9Mo6O17, spin-triplet pairing is shown to be the most likely scenario, supported by theoretical analysis of the recent experimental data on H c2(T when the field is aligned parallel to the most conducting axis. Furthermore, in Li0.9Mo6O17, a novel quantum limit (QL) superconducting phase is theoretically predicted as a consequence of dimensional crossover in ultra-high magnetic field. If confirmed experimentally, the QL phase would be the first example of existence of superconductivity in magnetic fields greater than 100 Tesla, and in addition would unequivocally confirm spin-triplet Cooper pairing in Li0.9Mo 6O17.