Universität zu Köln

Stein, Philipp Lars ORCID: 0000-0001-5202-3181 (2025). Raman Spectroscopic View on Quantum Materials: Local Symmetry Breaking in K2ReCl6; Raman Optical Activity in Ni3TeO6; and Electronic Excitations in Ruthenates. PhD thesis, Universität zu Köln.

PDF 01PublicationVersion.pdf - Accepted Version Download (8MB)

Abstract

This thesis aims to demonstrate different application possibilities for Raman spectroscopy in solid state physics and develop and showcase some rarely used extensions of the method. Measurements on Ni3TeO6 show that the scattering intensity of some phonons is different for left- and right circularly polarised light. This is quantified by calculating the Raman optical activity (ROA), which for most phonons yields values between 0.01 and 0.2. These values are giant compared to commonly reported values of 0.001 - 0.0001 for chiral molecules. More generally, these results are among the very few cases where the method was successfully applied to crystals. Calculations show that the strong chiral scattering contributions in this material originates from dynamic modification of the Berry curvature of the electronic band structure through phonon excitation. In K2ReCl6, polarisation selective Raman spectroscopy is performed in the temperature range between 5 K and room temperature. Besides showing the effect of the crystallographic phase transitions on the Raman active phonons, results also establish that the global cubic Fm-3m symmetry is locally broken by disordered tilting of the ReCl6 octahedra. High energy Raman spectroscopy on Ruthenium dimer systems is successfully demonstrated, following construction of a new Raman setup. In Ba3EuRu2O9 and Ba3CeRu2O9, a number of crystal field excitations are observed in the energy range between 100 and 600 meV. The competition between crystal fields, spin-orbit coupling and intra-cluster hopping leads to a large number of states that can be scattered from, which impedes exact identification of the observed peaks. Nevertheless, comparison with a calculated energy level diagram shows some agreement for Ba3CeRu2O9. Polarisation dependent measurements suggest that neither trigonal crystal field nor spin-orbit coupling are dominating, and that intra-cluster hopping plays a large role.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Stein, Philipp Lars UNSPECIFIED https://orcid.org/0000-0001-5202-3181 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-797365
Date:	2025
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics II
Subjects:	Natural sciences and mathematics Physics
Uncontrolled Keywords:	Keywords Language Condensed Matter Physics English Raman Spectroscopy English Chirality English Raman Optical Activity English Cluster-Mott insulator English
Date of oral exam:	25 November 2025
Referee:	Name Academic Title van Loosdrecht, Paul H.M. Prof. Dr. ir. Dressel, Martin Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/79736