UC San Diego

This dissertation reports on ``infrared" measurements of exotic phases in condensed matter systems: topological insulators (TIs), rare earth nickelates (RENs), and the Mn-pnictides. In the first part, I will discuss the experimental interrogation of TI systems, where electrons are confined to two dimensional surface states (SS) and the bulk is anticipated to be semiconducting. To elucidate the properties of TIs, we investigated the variation of bulk properties in TIs as a function of growth parameters. Later, mathematical constraints were placed on the maximum electronic response that could be expected from the SS electrons in general TI systems, with experimental work culminating in the measurement of an infrared absorption, consistent with these expectations. Subsequent magneto-optical investigation clarified infrared response, and identified a significant contribution from the exotic significant SS. In the second part, I will discuss the Mn-pnictide LaMnPO, which is a material highly similar to the recently discovered family of Fe-pnictide, high temperature superconductors. Guided by theoretical predictions, we used high pressures to induce a correlated metallic state, similar to the state preceding high temperature superconductivity in the Fe-pnictides. The last part of this thesis discusses recent, high resolution imaging experiments of the insulator-metal transition (IMT) in RENs. In these measurements, we revealed the discrete nature of the IMT. Additionally, we uncovered a conducting boundary between insulating, and magnetically ordered domains, which undergoes a distinct phase transition from the rest of the sample.