(Georgia Institute of Technology, 2019-11-06)
Bai, Xiaojian
The basic theme of my Ph.D. research is understanding exotic magnetic phases of matter and investigate their collective low-energy excitations using neutron-scattering and quantitative modeling. In this thesis, I start with an attempt to answer a list of questions that I had in the beginning of my Ph.D. study, such as why we can use a simple effective model to describe this complex world, how to synthesize and characterize samples, how to analyze the data and find a good theoretical model and many more. There is no unique answer to these questions. I speak from experience and hope to provide a road map to whoever read my thesis and is interested in starting condensed matter research using neutron-scattering.
Next, I present two material projects that I assume a major role. In both projects, high resolution single-crystal inelastic neutron-scattering data enables me and my collaborators to make significantly advances in understanding complex dynamical responses of magnetic materials. In Chapter 2, I present our study on a canonical frustrated magnet MgCr2O4 in the deep cooperative paramagnetic regime. In experiment, we observe a highly structured elastic scattering pattern with continuous excitation spectrum. Using analytic and computational methods, we reveal the highly correlated spin state is proximate to a "spiral spin-liquid" phase and the collective excitations are predominantly fast harmonic precessions of spin on a slow-varying disordered background. In Chapter 3, I present our study on an enigmatic compound with prior investigations dated back to 1970s – FeI2. In experiment, we observe a bright and dispersive band with "quadrupolar" character, apparently at odds with the dipole selection rule. Using advance numerical techniques, we are able to fully account for this band via a novel hybridization mechanism involving off-diagonal symmetric exchange interactions.
In Chapter 4, I introduce detailed implementations of spin dynamics simulations and application to a realistic diamond-lattice system. This technique provides a simple framework to study finite temperature and non-linear effects of complex magnetic materials and has increasingly been used to study disordered and strongly-correlated spin systems. I close this thesis in Chapter 5 with an outlook of future directions.
(Georgia Institute of Technology, 2018-05)
Bender, Darian Marie
In my research, I wish to classify and identify a possible Quantum Spin-Liquid (QSL) phase on novel quantum materials. Materials of interest include the two triangular lattice materials, Li4CoTeO6 and Li4NiTeO6, in which Ni and Co ions with effective spin-1 and spin-1/2 each occupy a triangular lattice. We performed thermodynamic and magnetization measurements which indicate a possible exotic magnetic ground-state in both
materials. We then performed elastic neutron scattering, providing additional evidence for exotic magnetism in these materials. Inelastic neutron scattering measurements are still necessary to probe the nature of the magnetic correlations and to confirm a QSL phase.
Another material of interest is the kagomé lattice material, KFe3(OH)6(SO4)2 (known as Fe-Jarosite). This material is a popular QSL.1, 2 Small crystals of Fe-Jarosite have been created by hydrothermal synthesis in Mourigal Lab, and preliminary measurements of
magnetization are in good agreement with known values.1, 3 Neutron scattering is required to study this material’s spin-dynamics, however, scattering is weak. Therefore, further synthesis attempts must be performed in order to increase the size of single-crystals of Fe-Jarosite from 2.6 mm to 1.0 cm.