Upper Critical Magnetic Fields in Quasi-One-Dimensional Layered Superconductors

摘要

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 Li₀.₉Mo₆O₁₇. In the first case, an angular dependence of Hc₂ that varies as ϴ³/² 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 Hc₂(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 Li₀.₉Mo₆O₁₇.