Uniaxial stress tuning and high-sensitivity probes for unconventional superconductors

Abstract

Uniaxial stress is a powerful tuning parameter for correlated quantum materials because it can continuously modify the electronic structure while preserving chemical composition. In unconventional superconductors, it also acts as a symmetry-selective perturbation and can therefore be used both to tune transition temperatures and to test the symmetry of the superconducting state. However, conventional thermodynamic probes such as specific heat are difficult to implement in the constrained geometry of piezoelectric uniaxial-stress apparatus.

In this thesis, I develop a quantitative framework for alternating-current elastocaloric effect (ECE) measurements under in situ uniaxial stress. The central advance is a method to reconstruct the absolute entropy over the full temperature–strain phase diagram. This is achieved by combining the absolute accuracy available in a newly identified strong-coupling regime at low frequencies with the high signal-to-noise ratio of quasi-adiabatic measurements at high frequencies.

The method is first applied to Sr₂RuO₄ under ⟨100⟩ compression. These measurements provide the first quantitative entropy map of the strain-tuned phase diagram and yield specific-heat-like information throughout the temperature–stress plane, including across phase transitions. The results place the peak in 𝑇_c and the surrounding phase behaviour on a direct thermodynamic footing.

I then study Sr₂RuO₄ under ⟨110⟩ stress, which probes a different symmetry channel from ⟨100⟩ compression. Elastocaloric and susceptibility measurements in this geometry show no transition splitting and no cusp in 𝑇_c(𝜎) within experimental resolution. These results place an important symmetry-selective constraint on proposed multi-component superconducting order parameters.

Finally, I investigate La_2-𝑥Sr_𝑥CuO₄ using susceptibility under uniaxial stress applied along ⟨100⟩, ⟨110⟩, and ⟨001⟩. For 𝑥 = 0.14, the measurements establish the anisotropic stress dependence of 𝑇_c along all three directions and provide a basis for comparison with earlier hydrostatic-pressure studies.

Taken together, the results show that quantitative thermodynamic and susceptibility measurements under uniaxial stress provide a powerful route to studying the phase behaviour and symmetry response of unconventional superconductors.

Date of Award	1 Dec 2026
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Andrew Mackenzie (Supervisor) & Andreas Rost (Supervisor)