UC Riverside

The microscopic study of lattice vibrations is essential for regulating the thermal properties and understanding the phase transition of materials. As for the newly proposed and observed chiral phonons, they are significant in controlling the entanglement of quantum dots and generating the thermal Hall effect in materials. In layered transition metal chalcogenides and some other quantum materials, their lattice dynamics are mostly studied by first-principles calculations, the phonon measurement is relatively rare, especially with temperature and pressure dependence.

Phonon theory and experimental techniques, such as inelastic X-ray scattering, inelastic neutron scattering, and Raman scattering, for phonon measurement are briefly discussed. The phonon computational method is also reviewed. Phonon measurements and theoretical calculations were performed on some layered materials and other quantum materials.

Using millielectronvolt-resolution non-resonant inelastic X-ray scattering, we discovered that it could be utilized to directly probe phonon chirality throughout the whole Brillouin zone in tungsten carbide. The results show that phonon chirality and X-ray polarization play essential roles in the scattering process. The results also suggest that a revision to the textbook X-ray scattering function of phonons is needed.

To study the temperature and pressure dependence of lattice dynamics in materials, especially for layered transition metal chalcogenides, we performed the first temperature- and pressure-dependent inelastic X-ray scattering measurements on bulk tungsten diselenide and obtained the mode Grüneisen parameters. The results show monolayer-like lattice dynamics in the bulk tungsten diselenide. We also performed the pressure-dependent phonon measurement on palladium diselenide. A panoramic diamond anvil cell was used to generate the high hydrostatic pressure. We observed the pressure-dependent flexural phonons for the first time and quantified the elastic properties and interlayer van der Waals interactions in layered materials.

Using inelastic neutron scattering, temperature- and pressure-dependent phonon lattice dynamics measurements on p-terphenyl were studied. The results indicate strong anharmonic phonon dynamics and suggest a lack of phase transition in the region of 0~1.51 kbar and 10~30 K.

Using Raman scattering, the pressure- and temperature-dependent results on Fe3GeTe2 were performed, and a significant pressure-induced phonon energy shift was observed. The phonon energy shift may be related to the strong spin-phonon interactions, which may play important roles in its application for magnetic storage devices.