Barker, Joel (2017) Muon studies of unconventional superconductors. PhD thesis, University of Warwick.

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Abstract

This thesis presents muon spin rotation and relaxation (μSR) studies of various superconducting materials. μSR studies have been complemented with laboratory measurements of heat capacity, resistivity, and magnetization. This work contributes to the body of work surrounding noncentrosymmetric superconductors (NCS) and unconventional superconductivity.

The intermetallic compound La₇Ir₃ is found to break time-reversal symmetry upon transitioning into the superconducting state, elucidated by muon spin relaxation. The pairing symmetry appears fully gapped, and is well described by an isotropic s-wave model. Further theoretical analysis of the point group symmetry could investigate the allowed superconducting states potentially existing here.

The physical properties of the NCS Re₃Ta are well described by the conventional BCS theory of superconductivity. μSR suggests unusual narrowing of the vortex lattice signal in a region close to Tc, interpreted as motional narrowing due to thermal motion of vortices. This is supported by irreversible magnetization in a sizable region in the phase diagram. Ginzburg number and quantum resistance calculations place Re₃Ta intermediate between the high-Tc and conventional low-Tc superconductors.

LuRuB₂ and YRuB₂, superconducting members of the ternary boride family of materials, are studied using μSR. Zero-field measurements detect spin fluctuations exhibiting critical slowing down as the temperature is lowered, suggesting that these materials lie near a quantum phase transition. These weak fluctuations coexist with the superconductivity, which is well described by an isotropic, s-wave model for the pairing symmetry.

The semi-metal Lu₃Os₄Ge₁₃ is investigated using transverse-field and zerofield μSR. The superfluid density is well described by a model containing two superconducting gaps, supporting previously reported heat capacity measurements. Zero-field measurements below Tc reveal a signal hinting at broken time-reversal symmetry, however, it does not coincide with the bulk superconducting transition. Further theoretical work could determine whether this is the first observation of a novel three-gap multiband superconducting ground state.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	Superconductors -- Thermal properties, Superconductors -- Magnetic properties, Muon spin rotation, Superconductivity
Official Date:	April 2017
Dates:	Date Event April 2017 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Paul, Don McK. ; Hillier, A. D. (Adrian D.)
Format of File:	pdf
Extent:	xvi, 141 leaves : illustrations, charts
Language:	eng

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