Exotic Thermal Transport in a Kitaev Magnet

Abstract

In 2006, Alexei Kitaev presented an exactly solvable spin model that is capable of exhibiting a novel spin liquid phase with non-abelian Majorana fermion excitations whose physics is strongly relevant to topological quantum computing. At the heart of this model is a unique type of magnetic exchange interaction that can potentially produce the phenomenology of Kitaev’s solution as well as other exotic magnetic phases. RuCl3 is currently the most well-known and intensely studied material candidate for realizing this model. In this thesis, we present results from a variety of experiments aimed at studying this material using thermal transport. Transport measurements have long been a key tool for studying excitations in condensed matter systems. Thermal transport measurements are used instead of more traditional electrical ones because RuCl3 is electrically insulating and its excitations are charge neutral. The first major finding presented in this dissertation is the observation of magneto-oscillations in the material’s longitudinal thermal conductivity κ_xx. Such an effect may imply Landau quantization of charge-neutral quasiparticles and is therefore possibly the strongest piece of evidence yet for the long-sought spinon Fermi surface state. The second major finding is a planar thermal Hall conductivity κ_xy with a temperature-dependence consistent with the presence of topological bosonic edge modes at high field. This result is the first clear observation of a topological bosonic quasiparticle in a transport measurement and therefore has substantial implications for both spintronic technologies and topological physics more broadly. We also discuss at length the various technical and materials challenges associated with these notoriously difficult experiments.