High precision thermodynamic measurements and their application to quantum materials

Abstract

Discoveries in condensed matter physics are often driven by the development of new capabilities or pushing the sensitivity of existing ones. This thesis covers the development and application of two such capabilities, which have revealed new details in the phase diagram of CeRh₂As₂ as well as the behaviour of Sr₃Ru₂O₇ under both external and internal (chemical) stress.

We recently developed a nanocalorimeter using primary Coulomb blockade thermometry which improves accuracy for measurements of microcrystals in magnetic fields and at low temperatures. I extended the system to lower temperatures, performing a detailed study of a high-quality microcrystal of CeRh₂As₂. My measurements for the first time properly resolve the mixed SC2 + I phase and its associated heat capacity jump. A flattening of both T_c(H) and the jump in specific heat associated with the phase are observed, with implications as to the magnetic response of the mixed SC2 + I phase. Measurements below T_c(H) also reveal weak signatures of both SC1 + I → SC2 + I and I phase transitions, the first time these have been observed in specific heat as a function of magnetic field.

I also present the development of a high-resolution dilatometer. This has a resolution better than 1 pm / √H z and allows measurements to be performed with a truly stress-free sample mounting. I present measurements of quantum oscillations in strain for both Sr₂RuO₄ and Sr₃Ru₂O₇ which allow calculation of the stress sensitivities of the Fermi surfaces in these materials. These results confirm DFT calculations and provide insight into the evolution the ruthenates under uniaxial stress. For Sr₃Ru₂O₇ I also explore the band structure evolution caused by more complex doping induced distortions, and use these to tune the Van Hove singularity through the Fermi energy.

Finally I present initial work developing both new probes for use in the PPMS, increasing their accessibility and operational temperature range.

Date of Award	3 Jul 2026
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Andreas Rost (Supervisor) & Andrew Mackenzie (Supervisor)