High-pressure Raman scattering studies of magnon-phonon interactions and ferroelastic phase transitions.

Abstract

Using high-pressure Raman spectroscopy, this dissertation investigates several areas of condensed matter physics. With metal thiophosphates (MnPS$\sb3$, NiPS$\sb3)$ as our reference systems, we investigate coupling between phonons and two-magnon continua. We find that MnPS$\sb3$'s two-magnon excitation can be tuned into resonance with the 155 cm$\sp{-1}$ phonon at a temperature near 60 K. In NiPS$\sb3$, we find that the two-magnon excitation has a linewidth broader than that predicted by standard two-magnon theory, reminiscent of the similar linewidth observed in the undoped cuprate superconductors. This observation calls into question the role quantum fluctuations associated with spin 1/2 play in the cuprates' two-magnon spectrum. Additionally, high-pressure Raman measurements of NiPS$\sb3$ yielded evidence of resonant enhancement of the two-magnon excitation--previously only observed in the cuprate superconductors. Additionally, we investigated the rutile-to-CaCl$\sb2$ ferroelastic phase transition occurring in RuO$\sb2.$ We observed the splitting of the (rutile) $E\sb{g}$ mode, and used this to find a transition pressure of 11.8 GPa. Based on the lower transition pressure found in previous work and on other results in the literature, we speculate that stoichiometry plays a critical role in determining the stability of the rutile or CaCl$\sb2$ phase of the metal dioxides. These experiments were performed with a variety of single-, double-, and triple-grating spectrometers (Renishaw, SPEX, and Dilor, respectively). The excitation sources used were primarily ion lasers (either argon or helium-neon). Pressures up to 35 GPa were achieved via a Mao-Bell style Diamond Anvil Cell.