Universität zu Köln

Borgwardt, Nick (2019). Optics on materials with strong spin-orbit coupling: topological insulators Bi2-xSbxTe3-ySey and the j=1/2 compounds Na2IrO3 and alpha-RuCl3. PhD thesis, Universität zu Köln.

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Abstract

We performed temperature-dependent infrared transmittance and reflectance measurements on topological insulators and spin-orbit-entangled Mott insulators. Both material classes exhibit strong spin-orbit coupling which gives rise to topologically protected phases. Potential applications are hindered, due to various deviations of the synthesized crystals from the ideal topological insulator or spin-orbit-entangled Mott insulators, respectively. In this thesis several promising candidates for the realization of either models are investigated. For topological insulators the bulk conductivity poses a problem. We report on thicknessdependent data on thin films of Bi2Te3 which show that the bulk optical conductivity σ1(ω) superimposes the surface conductance. Therefore, we continue the search for signatures of the surface state on compensated (Bi0.57Sb0.43)2Te3, for which we report on a very low σ1(ω), but we find no direct evidence for the topologically protected surface state. The compensation of topological insulators can be performed more sophisticated by partly substituting Te with Se. Thereby, the family of topological insulators Bi2-xSbxTe3-ySey with insulating behavior is created. Nevertheless, compensated topological insulators have a reduced activation energy compared to the intrinsic gap, which is usually attributed to impurity band. Lately, the formation of charge puddles in compensated topological insulators, due to the random distribution of defects, thus reducing the activation energy, was predicted. We report on the experimental verification of charge puddles and reveal the highly non-monotonic temperature dependence of them. Additionally, we describe, in collaboration with the group of Prof. Rosch of the Institute for Theoretical Physics at the University of Cologne, the observations semiquantitatively by Monte Carlo simulations. Thereby, we gain insight into, e.g., the defect density and the level of compensation depending on the composition which allows to choose the composition best suited the each purpose. One of the thrilling phenomena in Kitaev physics is the occurrence of a quantum spin liquid in the ground state. However, finite Heisenberg exchange due to a trigonal crystal field or finite mixing of the t2g and eg states, causes deviations from pure j=1/2 states, which results in a zigzag-ordered ground state instead of the sought-after quantum spin liquid. Due to these deviations even the formation of quasi-molecular orbitals instead of local j = 1=2 states was suggested for RuCl3 and Na2IrO3. We investigate σ1(ω) and identify different excitations, thereby gaining insight into the underlying physics. In α-RuCl3 we reveal the single, double, and triple spin-orbit exciton. Hence, local j=1/2 states are formed in this compound. For Na2IrO3 precise predictions on the optical spectrum depending on the underlying physics allow us to conclude from σ1(ω) that the local j=1/2 scenario is appropriate. Furthermore, we report on signatures of a controversially discussed peak obtained by RIXS, in the optical spectrum including magnon sidebands.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Borgwardt, Nick borgwardt@ph2.uni-koeln.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-91874
Date:	2019
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics II
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language Topological Insulator English puddles English Spin-orbit Mott Insulator English
Date of oral exam:	19 June 2018
Referee:	Name Academic Title Grüninger, Markus Prof. Dr. Loosdrecht, Paul H.M. van Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/9187