(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.