Phonon anomalies in pyrochlores and 2D materials under extreme conditions: Insights from Raman spectroscopic studies

Abstract

In solids, electrons do not behave as separate particles but instead interact intensely to form quasiparticles, which couple with other quasiparticles such as phonon, plasmon, and exciton to generate emergent quantum phenomena. The emergent quantum phases in materials arise from the competition and cooperation of various interactions. Competition and cooperation describe a situation where two entities produce a result that cannot be achieved by one alone. Raman spectroscopy is a powerful tool to probe various interactions in solids across the quantum phase transitions. Using Raman studies, this thesis investigates emergent phenomena in novel quantum materials which include pyrochlore and two-dimensional materials. n this thesis, we have carried out Raman studies of the following samples: (Eu1−xBix)2Ir2O7, (Eu1−xBix)2Sn2O7, graphene, and tungsten disulphide (WS2) under extreme conditions of pressure and temperature. Using in-situ Raman measurements, we were able to identify phonon anomalies in the pyrochlore oxides under the perturbation of pressure and temperature. The X-ray studies showed the absence of structural change, conforming that these anomalies were caused by phonon renormalization due to various interactions. Low-temperature Raman measurements on (Eu1−xBix)2Ir2O7 were performed to investigate electron-phonon coupling and spin-phonon coupling across the metal-insulator transition. We have also shown that electron-phonon coupling plays an important role in the incoherency of the metallic phase in (Eu1−xBix)2Ir2O7. High pressure Raman measurements were done on single layers of graphene and WS2 to detect structural changes