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High-pressure bulk crystal growth for scattering studies of quantum magnets


This thesis is concerned with the construction, commissioning and usage of a high-pressure, laser-based, floating-zone furnace for the purpose of synthesizing bulk single crystals of quantum magnets and studying them via neutron scattering. Bulk single crystals are some of the most valuable experimental platforms in condensed matter physics. This is particularly true for studies of quantum magnetism since it allows for the use of single-crystal neutron scattering, which is the premier technique for analyzing both the static and dynamic behaviors of magnetic materials in conjunction with resolution of underlying anisotropies. The floating-zone crystal growth method has been especially impactful in this regard, as it has the potential to create high-quality crystals with large volumes, an essential feature for compatibility with the flux limitations of neutron scattering. Accordingly, broadening the phase-space of materials accessible by this technique is a direct means of helping the field move forward. The furnace described in this thesis represents such an improvement. Its laser-based design allows for floating-zone crystal growth at record-setting gas pressures with simultaneous access to high temperatures via a well-defined heating zone. These features make it suitable for growth of highly volatile, metastable and low surface tension compounds, all of which are challenging for traditional floating zone-furnaces.

I begin with a detailed overview of the floating-zone growth technique, including its advantages, disadvantages and the manner in which it is conducted (Chapter 2). This sets the stage for a description of the furnace (Chapter 3), including its key design features and the conduction of a variety of successful commissioning growths to demonstrate its abilities. I also provide a brief reflection on challenges associated with high-pressure, laser-based, floating-zone crystal growth.

Following an attempt at defining quantum magnetism and an introduction of the basic principles of neutron scattering(Chapter 4) detailed studies of two of the commissioning materials (both quantum magnets) are presented. The first study (Chapter 5) elucidates the role of chemical disorder in modifying the fragmented magnetic state found in Nd2Zr2O7 through a combination of thermodynamic measurements, electron probe microanalysis and neutron scattering. It is demonstrated that growth under high-pressure allows for a notable reduction in chemical disorder, which modifies the temperature range over which the different spin correlations of the fragmented state are stabilized. The second study (Chapter 6) centers around a detailed survey of the magnetic excitations in a highly stoichiometric Li2CuO2 sample (enabled by the high-pressure growth environment) via inelastic neutron scattering. The resulting data allowed for refinement of the existing exchange model and also revealed the presence of multi-magnon bound states. This provides the first reported observation of multi-magnon bound sates in a ferromagnetic chain by neutron scattering.

The final study presented (Chapter 7) summarizes an attempt to grow single-crystal Eu2Ir2O7 via high-pressure floating-zone. Due to persistent decomposition at high temperatures, a detailed study on polycrystalline carrier-doped (Eu1−xCax )2Ir2O7 was carried out instead. Using a wide variety of experimental probes it is demonstrated that depression of the thermal metal-insulator transition by carrier doping leads to the formationof an electronically phase-separated state in these samples, without observable change in the lattice symmetry. Comparison to literature shows that formation of this phase-separated state is dependent on synthesis method, indicating that even small amounts of disorder can dramatically modify the properties.

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