Low Energy Electrodynamics Of Quantum Magnetism

Abstract

The manner in which localized spins interact in magnetic insulators is a strongly correlated many-body problem. To date, very few exactly solvable models of such spin interactions exist and those that do are often highly idealized. Therefore, intense experimental effort is devoted to characterizing the ground states of such “quantum magnets.” As nature offers a plethora of materials with various symmetries, geometries, and interactions, the resultant ground states vary from conventional, where the spins essentially behave as classical vectors, to truly quantum, where the quantum mechanical nature of spin cannot be ignored. This thesis focuses on the characterization of magnetic ground states by examining their low energy excitations through their interaction with light. A method for extracting the complex magnetic susceptibility in a transmission measurement is developed and utilized to study the magnetic field and temperature dependence of such excitations. A combination of techniques which probe from the microwave to the terahertz range are used to provide a holistic view of the low energy electrodynamics of these materials. By comparing our results to existing models, we uncover unique magnetic ordering due to low symmetry environments, classical grounds states with unconventional interactions, and quantum ground states with long range entanglement and exotic fractionalized excitations

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