Electronic and Magnetic Materials Under External Stimuli

The interaction between spin, charge, and lattice degrees of freedom leads to exotic and useful properties in multifunctional materials. This delicate balance of energy scales allows external stimuli such as temperature, magnetic field, or pressure to drive to novel phases. As a local probe technique, spectroscopy can provide insight into the microscopic mechanism of the phase transitions. In this dissertation I present spectroscopic studies of functional materials under extreme conditions.

Nanomaterials have attracted attention because nanoscale confinement affects various material properties and often reduces energy scales or suppress phase transitions. Combining Raman and infrared spectroscopies reveals that the breakdown mechanism of tungsten disulfide nanowires, a dichalcogenide widely used as a solid state lubricant, under compression is mainly driven by a breathing mode as revealed by its high sensitivity to compression. The optical properties of nanoscale hematite, a model antiferromagnet, reveal a size-dependent vibronic coupling behind the activation of the iron on-site excitation. Moreover, spin-charge coupling is enhanced below a critical size until the superparamagnetic limit is reached.

Molecule-based magnets offer opportunities to probe coupling processes due to their soft lattices and overall low energy scales. As an example, I reveal how the antiferromagnetic to ferromagnetic crossover of a copper halide coordination polymer originates from the formation of hydrogen bonds with applied pressure that increase the dimensionality of the copper-copper magnetic superexchange network. Finally, combining temperature and pressure spectroscopy techniques with theoretical calculations of manganese dicyanimide revealed a temperature-pressure-magnetic field phase diagram that contains many competing magnetoelastic phases, and indicates possible quantum behavior despite the typical classical treatment of Mn(II). Together, these studies provide insight on structure-property relations, spin-charge-lattice coupling, and phase transitions in nanomaterials and molecule-based magnets, and by extension higher energy scale materials like bulk oxides.