Thermal Hall and heat capacity studies of topological magnetic insulators and superconductors

Abstract

This thesis describes the development, and implementation, of effective low temperature thermal measurements, and their application in investigating the thermal transport and heat capacity of topological systems. The first investigation concerns the quantum magnet SrCu2(BO3)2. This material is a physical realisation of the Shastry-Sutherland model - a two-dimensional square lattice, solved to have a ground state of singlets on diagonal bonds. A number of theoretical studies have suggested that the material should host a topological phase of magnetic excitations (or triplons), which may manifest a thermal Hall effect - arguably the best experimental probe of neutral, topologically non-trivial quasiparticles. However, within the measured experimental resolution, the material exhibits no thermal Hall signal. The theory relies on triplon-triplon interactions being negligible, such that the triplon number is conserved. The results suggest that - in the temperature range under investigation - this is not a fair assumption. Additionally, an identical procedure has been used to investigate Lu2V2O7 - a material that has been shown to exhibit a thermal Hall effect, of a similar magnitude to that which was predicted for SrCu2(BO3)2. This measurement was successful in replicating those previous results, and thus demonstrated the feasibility of detecting a similar signal in SrCu2(BO3)2.

The same experimental procedure has also been adapted in order to investigate the thermal transport properties of the heavy fermion superconductor UPt3 at mK temperatures. This experiment was intended to test a recent theoretical prediction that the low temperature B-phase should exhibit a zero-field anomalous thermal Hall effect. Observation of such an effect would be the first definitive proof of bulk chiral superconductivity. Although no zero-field signal was found, the material does exhibit a thermal Hall effect approaching the in-field B-C transition (between two superconducting phases). Further measurements have sought to clarify the origin of this effect, and in particular, whether it might be linked to the chirality of the superconducting state.

Finally, the specific heat of the heavy fermion compounds UAu2 and UTe2 has been measured at mK temperatures, with a bespoke heat capacity stage. In the former, the specific heat shows non-Fermi liquid behaviour, deep within an antiferromagnetically ordered phase. This contrasts with the majority of heavy fermion compounds, wherein non-Fermi liquid behaviour is attributed to a close proximity to a magnetic quantum critical point. Additionally, the specific heat indicates there is a magnetic phase transition at high fields, at which point the material reverts to a Fermi liquid behaviour. This implies the quantum critical behaviour in UAu2 is tied to the antiferromagnetic order. UTe2 is a heavy fermion superconductor, anticipated to host a topological form of superconductivity. For this material, specific heat measurements have been employed to demonstrate that the superconducting properties vary with composition. Most notably, the residual specific heat as T → 0 appears to be negatively correlated with the superconducting transition temperature. This contradicts a recent hypothesis that the residual contribution is due to only one spin direction being paired in the superconducting state.