Effects of Strain Induced Topological Changes on the Superconducting Critical Temperature

Abstract

This body of work outlines the results of a theoretical investigation into the effects of
external strain on superconductivity. This effect is explored for various gap pairings, for
superconductors that can be modelled on a square lattice. Using some general predictions of
group theory and a microscopic negative-U Hubbard model, the change in the critical temperature
(Tc), with applied strain, via hopping anisotropy, is systematically explored for some s, p and
d-wave models of superconductivity. The pairing states explored are done in conjunction with
parameters that reproduce the properties of Sr2RuO4, a material surrounded in uncertainty and
with a detailed set of external strain experiments. In direct comparison with these experiments
it is found that a d + ig pairing state best describes Sr2RuO4 with all available experimental
evidence considered. The generality of the model is of key importance as it is used to make
predictions for the change in Tc with applied strain for La2−xSrxCuO4 (LSCO). The applied
strain pushes the Fermi surface through a topological change known as the Lifshitz transition.
The leading cause for the change in Tc, in all cases, is found to be the orientation of symmetry
imposed superconducting gap nodes. If they are coincident with the opening of the Fermi surface
there’s an expected decrease in Tc or if they are non-coincident a predicted increase in Tc is seen
as the system passes through the topological Lifshitz point causing a van Hove singularity (vHs)
in the Density of States (DOS) to move through the Fermi level or not. The framework outlined
within this thesis provides a general model applicable to many pairing states in two dimensions
and applications to other superconductors can be readily applied.

Date of Award	18 Jun 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	James F Annett (Supervisor) & Martin Gradhand (Supervisor)