Universität zu Köln

Röper, Torsten ORCID: 0000-0003-0013-9662 (2026). Dynamics and breakdown in quantum Hall systems and quantum dots. PhD thesis, Universität zu Köln.

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Abstract

High-frequency transport experiments provide access to relaxation, coherence, and interaction effects beyond the reach of dc measurements. The central objective of this thesis is to develop and apply radio-frequency techniques to probe the dynamical properties of charge transport in two complementary material platforms: quantum anomalous Hall (QAH) insulators and gate-defined quan- tum dots in bilayer graphene (BLG). In the first part, edge plasmon propagation in QAH edge states formed in thin films of V-doped (Bi,Sb)2Te3 is investigated using broadband, phase-resolved microwave measurements. The velocity and dissipation of edge plasmons are characterized as a function of frequency, excitation voltage, temperature, and magnetic field. Despite dissipationless transport in dc measurements, finite losses are observed at microwave frequencies, which are attributed to coupling between the chiral edge channel and localized bulk states. The breakdown of quantized transport under strong electric fields is studied in both QAH and quantum Hall devices. Breakdown measurements reveal a crossover from a non-Ohmic to an Ohmic transport regime with increasing electric field. In QAH samples, Joule heating of localized bulk states is identified as the dominant dissipation mechanism. In the second part, microwave transport in BLG quantum dots is explored as a route toward controlled single-electron emission at zero magnetic field. Low-frequency transport measurements establish the formation and tunability of gate-defined quantum dots, while radio-frequency excitation enables dynamical control of the dot potential. Although quantized single-electron emission is not yet achieved, we identify the key limitations of the current device architecture and outline a path toward a reliable single-electron source in BLG. Together, these results establish microwave-based transport techniques as a powerful tool for studying both collective and single-particle charge dynamics in low-dimensional quantum conductors. Potential future applications of the high-frequency methods developed in this thesis include reduced-dissipation QAH plasmonics, interaction-induced charge fractionalization, and time-resolved investigations of hybrid BLG–superconductor systems.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Röper, Torsten torstenroeper@yahoo.de https://orcid.org/0000-0003-0013-9662 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-804581
Date:	2026
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 Experimental solid state physics English Radio-frequency experiments English Cryogenic experiments English Quantum anomalous Hall English Quantum Dots English Topological insulator English Graphene English
Date of oral exam:	5 May 2026
Referee:	Name Academic Title Bocquillon, Erwann Prof. Dr. Feve, Gwendal Prof. Dr. Egger, Reinhold Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/80458