Vortex Phenomena in Unconventional Superconductors

Abstract

Unconventional superconductors are those which are not described by theBCS theory, and for which no known theoretical description currently exists. Thecareful study of the behaviour of superconducting vortices in such systems yieldscrucial insights into the underlying physics of these exciting materials. This thesis describes a series of magnetometry experiments conducted on three different unconventional superconductors: Sr2RuO4, MgB2 and Bi2Sr2CaCu2O8+; utilising two techniques: Scanning Hall probe microscopy and magneto-optical imaging.An exotic p-wave chiral order parameter is thought to exist in Sr2RuO4 and isexpected to produce several identifiable magnetic signatures. A search for thesesignals via scanning Hall probe measurements failed to detect any such signatures, but did capture a structural transition of the vortex lattice that is consistent with the proposed chiral order parameter. Studies of several samples also suggest that the vortex behaviour is strongly modified with even tiny amounts of disorder, a conclusion that has important consequences for interpretations of vortex patterns in Sr2RuO4.Several recent experiments have reported vortex configurations consistentwith a competing short-range repulsion and long-range attraction in the intervortex interaction in MgB2 single crystals. We observe the spontaneous formation of vortex chains and labyrinths in a 160nm MgB2 thin-film that are suggestive of a non-monotonic vortex interaction, but perhaps more indicative of an intermediate-range attraction in harness with short and long-range repulsions.The suitability of seven potential mechanisms of vortex attraction in MgB2 arereviewed in light of the unusually short electronic mean-free path of our sample.Finally, magneto-optical imaging has been used to study the penetration offlux into regular polygon-shaped Bi2Sr2CaCu2O8+ platelets with various geometries.The variation of HP with geometry qualitatively contradicts conventionalestimates of demagnetisation factors based on equivalent ellipsoids using inscribed circles. This work has important implications for the estimation of appropriate effective demagnetisation factors in arbitrarily shaped superconducting bodies.

Date of Award	19 Jul 2013
Original language	English
Awarding Institution	University of Bath
Supervisor	Simon Bending (Supervisor)