Unconventional Superconductors

Department of Physics (www2.warwick.ac.uk/fac/sci/physics)

University of Warwick (www2.warwick.ac.uk)

Superconductivity and Magnetism Groups (go.warwick.ac.uk/supermag)

The discovery of superconductivity by Kamerlingh Onnes over 100 years ago has offered physicists the opportunity to study this most intriguing macroscopically coherent state of electrons. In conventional superconductors, the superconductivity arises from electron-phonon coupling of electrons to form Cooper pairs, with a single s-wave, isotropic gap. In unconventional materials, the Cooper pairs may be coupled by other interactions, the pair wave function may be an odd-parity spin-triplet, or there may be multiple superconducting bands.

Unconventional superconductivity can be found in number of different classes of materials. These include superconductors with noncentrosymmetric crystal structures, i.e. systems without inversion symmetry where parity is no longer a meaningful label, in materials with strong spin-orbit coupling, e.g. those containing 4d and 5d metals, and in systems with unusual crystallographic geometries, e.g. kagome lattice superconductors. It will be these three groups of superconductors that will be the focus of this work. In unconventional superconductors, a variety of physical effects that may previously have been forbidden are now allowed. Examples include exotic superconducting gap structures (lines or nodes in the superconducting gap), upper critical fields exceeding the Pauli limit, time reversal symmetry breaking, and even topological effects. For some of our recent work in this area please see Refs. 1-8.

In this experimental project, you will study the superconducting properties of some of these superconducting materials. You will learn how to prepare polycrystalline and single crystal samples. The structure and composition of these samples will be studied using a suite of state-of-the-art x-ray diffractometers and electron microscopes. You will then examine the superconducting properties of these materials in the laboratory at low temperatures and in high magnetic fields. A range of neutron scattering and muon spectroscopy techniques available at national and international central facilities will also be used to investigate the physics of these superconductors.

This experimental project will offer an excellent training in several important aspects of modern condensed matter physics.

[1] R. P. Singh et al. Detection of time-reversal symmetry breaking in the noncentrosymmetric superconductor Re6Zr using muon-spin spectroscopy, Phys. Rev. Lett. 112, 107002 (2014).

[2] J. A. T. Barker et al. Unconventional superconductivity in La7Ir3 revealed by muon spin relaxation: introducing a new family of noncentrosymmetric superconductor that breaks time-reversal symmetry Phys. Rev. Lett. 115, 267001 (2015).

[3] J. A. T. Barker et al. Superconducting and normal-state properties of the noncentrosymmetric superconductor Re3Ta, Phys. Rev. B 98,

104506 (2018).

[4] D. A. Mayoh et al. Multigap superconductivity in chiral noncentrosymmetric TaRh2B2, Phys. Rev. B 98, 014502 (2018).

[5] P. K. Biswas et al. Chiral singlet superconductivity in the weakly correlated metal LaPt3P, Nature Communications 12, 2504 (2021).

[6] D. Das et al. Probing the superconducting gap structure in the noncentrosymmetric topological superconductor ZrRuAs, Phys. Rev. B

103, 144516 (2021).

[7] D. A. Mayoh et al. Evidence for the coexistence of time-reversal symmetry breaking and Bardeen-Cooper-Schrieffer-like

superconductivity in La7Pd3, Phys. Rev. B 103, 024507 (2021).

[8] D. G. C. Jonas et al. Quantum muon diffusion and the preservation of time-reversal symmetry in the superconducting state of type-I

rhenium, Phys. Rev. B 105, L020503 (2022).

This is a fully funded-PhD studentship at standard UK Research Council rates, for a 3.5 year period. The studentship covers university fees and a living stipend. For further information and details of how to apply, please see our postgraduate admissions website: Postgraduate- Department of Physics (warwick.ac.uk)