Effect of magnetic fields on strongly correlated d-wave superconductors and disordered s-wave superconductors

Datta, Anushree (2021) Effect of magnetic fields on strongly correlated d-wave superconductors and disordered s-wave superconductors. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Anushree Datta (14RS024)) 14RS024.pdf - Submitted Version Restricted to Repository staff only Download (6MB)

Abstract

In this thesis, we study two forms of interplay in superconductors: between a magnetic field and strong electronic correlations in d-wave superconductors; and between a magnetic field and disorder in type-II s-wave superconductors. As a first direction, we investigate the role of strong correlations on the magnetic field induced states in d-wave superconductors. We capture the effects of strong electronic correlations within a Gutzwiller formalism augmented with Hartree-Fock-Bogoliubov (HFB) meanfield theory. First, we investigate the Zeeman effects of magnetic fields in a Fulde-Ferrell- Larkin-Ovchinnikov (FFLO) state in strongly correlated d-wave superconductors. We find that the crucial effects of strong correlations lie in shifting the Bardeen-Cooper-Schrieffer (BCS)-FFLO phase boundary towards a lower Zeeman field and thereby enlarging the window of the FFLO phase. In the FFLO state, we find that strong correlations profoundly impact the features of the density of states and order parameters. Second, we analyze a vortex lattice induced by the orbital effect of magnetic fields in a strongly correlated d-wave superconductor. We find that strong correlations promote Mott insulating vortex cores, in contrast to the findings from weak-coupling descriptions, where the vortex core regions behave as a normal metal. The change in the nature of the normal state inside the core has prominent effects on the zero-bias conductance peak of the local density of states, elucidating a longstanding puzzle of tunneling spectroscopic measurements in vortex cores of cuprate superconductors. Our calculations indicate that strong correlations cause a non-monotonic variation of the vortex size with doping in sharp contrast with weak-coupling descriptions. In a second direction of the thesis, we study type-II s-wave superconductors in the simultaneous presence of orbital magnetic field and disorder. Within a HFB mean-field theory, we find the effect of disorder is intriguing in an s-wave vortex lattice. At weak disorder strengths, the superfluid density and the superconducting energy gap collapse simultaneously. However, as the disorder strength increases, the two critical fields corresponding to the vanishing of the superfluid density and collapsing of the energy gap, start diverging from each other, resulting in a paired insulator state. Our results also have important consequences for the strong magneto-resistance peak observed in disordered superconducting thin films.

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