Collective Modes in Pressure-Tuned Charge Density Wave Systems

Abstract

This thesis presents an investigation of the dynamics of collective modes in charge density wave (CDW) materials, specifically SmNiC_2 and CeTe_3, utilising ultrafast pump-probe spectroscopy and hydrostatic high-pressure tuning as experimental tools. Ultrafast spectroscopy is an ideal method for observing and understanding dynamics on the femtosecond timescale in materials exhibiting strongly correlated phenomena. The initial experiments focused on understanding the ultrafast pump-probe technique and the use of diamond anvil cell pressure tuning. Ultrafast pump-probe spectroscopy was performed on the room-temperature CDW material CeTe_3, with varying pressure until the CDW melting was observed. Pressure tuning induces lattice compression, which in turn affects the CDW order parameters. Upon careful investigation of the optical response from collective modes, one strongly coupled amplitude mode showed softening, while two additional amplitude modes exhibited hardening. Coherent modes were observed to disappear at high pressures, which is attributed to the lowering of the transition temperature below room temperature and the subsequent melting of the CDW order. Subsequently, we extended this approach to study the optical responses from the ternary compound SmNiC_2, beginning with temperature profiling of the phase diagram. Coherent oscillations were found to appear abruptly at T_M = 18 K, indicating a first-order transition from ferromagnetic order to CDW. In the CDW phase, the collective response in the reflectivity signal was analysed, and a single amplitude mode was extracted, showing strong softening with increasing temperature. The rare earth nickel carbide exhibits competing orders and magnetic phases at low temperatures, making it an excellent candidate for exploring fundamental correlation mechanisms. Through these studies, the thesis enhances the understanding of correlated materials and examines the potential for manipulating CDW/FM states via ultrafast optical excitation and external strain/pressure. The insights gained contribute to the broader field of quantum materials, offering prospects for novel electronic applications based on metastable states and ultrafast control of symmetry-broken phases.