Nuclear magnetic resonance of novel type-II superconductors.

Abstract

Nuclear magnetic resonance was crucial in the vindication of the theory of superconductivity in 1957, it still continues to provide vital information today in the ongoing challenge that is the determination of the mechanism for type-II superconductors.

This forms the basis of this piece of work, which details the preparation, characterisation and cryogenic measurements of two new superconductors based on the cage like fullerides. In light of recent synthetic developments it has now become possible to encapsulate small molecules inside the fullerene cage, which may then be doped with alkali metals to form the superconducting materials. In this thesis the encapsulated molecules are water and hydrogen, and the topic is the study of normal and superconducting states from their response to the magnetic resonance technique.

The materials are made using a vapour doping method, and their characterisation suggests high purity, with the superconducting transition temperatures modestly affected by the larger water molecule. The spectral lineshape suggests a unique window into the vortices in the superconducting state from both endohedral molecules, and the magnetic shift decays concomitant with s-wave symmetry of superconducting gap. The nuclear spin lattice relaxation rate is vastly different between molecules, highlighting the unique information available from these new probes.

Magnesium diboride is also studied using a sample enriched with carbon-13, which allows a new window into this multiple-band superconductor. Measurements of the lineshape reveal the role of carbon as a flux pinning centre, and combined with Knight shift measurements suggest the doping procedure favours the chemical substitution scenario. Also ab-initio calculations yield results which match this scenario and agree well with experimental values.

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Identifiers

ORCID for Marina Carravetta: orcid.org/0000-0002-6296-2104

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