Heat capacity measurements of Sr₂RuO₄ under uniaxial stress

Abstract

The most-discussed pairing symmetry in Sr₂RuO₄ is chiral p-wave, 𝑝_𝓍 ± 𝑖𝑝_𝑦, whose degeneracy is protected by the lattice symmetry. When the lattice symmetry is lowered by the application of a symmetry-breaking field, the degeneracy can be lifted, potentially leading to a splitting of the superconducting transition. To lift the degeneracy, the symmetry breaking field used in this study is uniaxial stress. Uniaxial stress generated by a piezo-electric actuator can continuously tune the electronic structure and in situ lower the tetragonal symmetry in Sr₂RuO₄.

Previous studies of magnetic susceptibility and resistivity under uniaxial stress have revealed that there is a strong peak in 𝑇_c when the stress is applied along the a-axis of Sr₂RuO₄. In addition, it
has been proposed that the peak in 𝑇_c coincides with a van Hove singularity in the band structure, and measurements of 𝐻_𝑐2 at the maximum 𝑇_c indicate the possibility of an even parity condensate for Sr₂RuO₄ at the peak in 𝑇_c.

In this thesis, the heat capacity approach is used to study the thermodynamic behavior of Sr₂RuO₄ under uniaxial stress applied along the crystallographic a-axis of Sr₂RuO₄. The first thermodynamic evidence for the peak in 𝑇_c is obtained, proving that is a bulk property. However, the experimental data show no clear evidence for splitting of the superconducting transition; only one phase transition can be identified within the experimental resolution. The results impose strong constraints on the existence of a second phase transition, i.e. the size of the second heat capacity jump would be small or the second 𝑇_c would have to be very close to the first transition. In addition to these results, I will present heat capacity data from the normal state of Sr₂RuO₄. The experimental results indicate that there is an enhancement of specific heat at the peak in 𝑇_c, consistent with the existence of the van Hove singularity. The possibility of even parity superconductivity at the maximum 𝑇_c has also been investigated. However, the heat capacity measurements are shown to be relatively insensitive to such a change, so it has not been possible to obtain strong and unambiguous evidence for whether it takes place or not.

Date of Award	6 Dec 2018
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Andrew Mackenzie (Supervisor), Michael Nicklas (Supervisor) & Clifford Hicks (Supervisor)