Raman Studies of Topological Crystalline Insulator, Natural Heterostructures, Excitonic Insulator and Layered Oxide Under Pressure

Abstract

The theme of this thesis is Raman spectroscopic study of a variety of exotic states of matter under extreme conditions, such as, hydrostatic pressure as high as 25 GPa and a wide temperature range from 77 K to 390 K. Raman spectroscopy and in some cases, X-ray diffraction studies were performed on systems such as; Topological Crystalline Insulator (TCI) SnTe, normal semiconductor SnSe, natural van der Waals heterostructures from the (SnTe)m(Bi2Te3)n (with m = 1 and n = 1, 2) homologous family, which are also predicted to be Topological Insulators (TI), then Excitonic Insulator (EI) Ta2NiSe5, and its S-counterpart, a normal semiconductor Ta2NiS5. In addition, a Ru2O6-layer honeycomb lattice compound, the silver ruthenium oxide AgRuO3 was also studied. The work presented in this thesis is divided into three parts. (1) Pressure dependent Raman studies were performed to look for signatures of topological phase transitions in SnTe, and a comparative study with a normal semiconductor SnSe. High-pressure Raman studies were also performed on SnBi2Te4 and SnBi4Te7 to look for electronic topological transitions as well as structural phase transitions. For structural characterization as a function of pressure, X-ray diffraction measurements using synchrotron source have been pursued. (2) Pressure and temperature dependent Raman studies were performed to look for signatures of stability of the excitonic insulating phase in Ta2NiSe5 and a comparative temperature dependent study on the S-counterpart, Ta2NiS5, to look for signatures of any phase transition. (3) Lastly, our temperature dependent Raman studies on AgRuO3 reveal a signature of subtle phase transition in addition to an antiferromagnetic transition. For pressure-dependent structural characterization, Raman and X-ray diffraction measurements have been pursued.