Magnetic Excitations in Transport Measurements of Novel Quantum Materials
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Quantum materials offer rich possibilities for quasiparticles that can revolutionalize our technology. The discovery of novel delocalized magnetic excitations in transport measurements implies that such quasiparticles are mobile, which may lead to technology applications that take advantage of the magnetic or charge properties of the collective excitations. Importantly, testing theoretical predictions of such quasiparticles allows us to understand how to realize such predictions in real materials. The purpose of this thesis is to investigate novel magnetic excitations in quantum materials, primarily by using thermal transport measurements. We investigate the origin of experimentally reported novel charge-neutral quasiparticles in the topological Kondo insulator candidate SmB₆ to address seemingly contradictory analyses of experimental data and motivate theoretical models. Then we test the theoretical prediction of heat-conducting photon excitations in the quantum spin ice candidate Pr₂Hf₂O₇. We primarily use thermal transport to investigate these magnetic excitations. Our standard steady state setup has a directional dependence that is optimized to investigate any anisotropic effects in single crystals by orienting the magnetic field and heat current directions along high-symmetry crystallographic directions, such as when investigating a spin ice anisotropy in the quantum spin ice candidate. After obtaining our data, a careful analysis of thermal conductivity allows us to distinguish the heat conduction of novel quasiparticles from that of more conventional heat carriers, such as phonons and conduction electrons. We also introduce magnetostriction measurements, which is a bulk thermodynamic measurement that is used to detect quantum oscillations in SmB₆. While the search for novel quasiparticles in quantum materials underlies the methods and analysis in this thesis, the materials themselves are unrelated and hence are covered in separate sections. We first summarize theoretical predictions and experimental evidence on the topological Kondo insulator candidate SmB₆ to motivate our investigation of charge-neutral quasiparticles in the bulk of the material. Then we report evidence that charge-neutral excitations are most easily distinguished in the thermal transport of samples with minimal bulk disorder, which can be quantified between different samples using thermal conductivity analysis. Having addressed conflicting interpretations of thermal conductivity in differing samples, we further investigate the field anisotropy and quantum oscillations in the magnetostriction to gain more information about the underlying properties of bulk charge-neutral excitations. After discussing our results for SmB₆, we introduce spin ice materials and the predicted excitations of the U(1) quantum spin liquid theory as applied to pyrochlore spin ices. Then we motivate the search for such excitations in the quantum spin ice candidate Pr₂Hf₂O₇ with a particular emphasis on the gapless, linearly dispersive photon excitation that has yet to be unambiguously realized in real materials. Instead, we report a lack of a heatconducting photon excitation in thermal conductivity, which places limits on the realization of the U(1) quantum spin liquid state in this material. Nonetheless, we also explore the temperature and field dependence of a suppressed thermal conductivity that suggests strong phonon scattering with an Ising anisotropy that could be related to strong spin fluctuations interacting with phonons. Our results investigating magnetic quasiparticles experimentally through thermal transport test theoretical predictions and motivate the continued study of these materials. Importantly, we corroborate evidence that charge-neutral, heat-conducting excitations and bulk quantum oscillations are present in high quality samples of SmB₆ in spite of its insulating bulk state, which require further testing of theoretical motivations to uncover the mechanism behind this phenomena. Then, in Pr₂Hf₂O₇, despite expectations of a U(1) quantum spin liquid ground state with heat-conducting photon excitations, we instead observe a thermal conductivity that is most consistent with conventional phonon excitations as the sole heat carriers. Such experimental tests of theoretical predictions are important to understand the realization of novel phases of matter in real materials in order to use such materials in technological applications.
Cite this version of the work
Jennifer Reid (2023). Magnetic Excitations in Transport Measurements of Novel Quantum Materials. UWSpace. http://hdl.handle.net/10012/19349