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%0 Thesis
%A Johnsen, Tjorven
%T Interplay of quantum Hall edge states in graphene with the tip-induced quantum dot and graphene sample fabrication techniques for advanced scanning tunneling microscopy
%I RWTH Aachen University
%V Dissertation
%C Aachen
%M RWTH-2022-11449
%P 1 Online-Ressource : Illustrationen, Diagramme
%D 2022
%Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University 2023
%Z Dissertation, RWTH Aachen University, 2022
%X The quantum Hall effect (QHE) is in large portion rationalized by the edge channel (EC) picture in which the charge transport takes place in stripe like channels along the edge of the sample. For the adequate parameters, ECs can run along a pn interface instead of the physical edge of the sample. Using such a gate-defined pn interface in graphene, compressible stripes that combined with incompressible stripes constitute the ECs are studied within this thesis by scanning tunneling microscopy (STM) and spectroscopy (STS). To this end, the Landau level (LL) features are investigated a tan interface of different filling factors for varying gate voltages. Charging lines (CLs) emerging in the measurements reveal that a tip-induced quantum dot (QD) develops below the STM tip. Poisson simulations of the experimental setting involving the tip, the graphene layer, and the electrostatic interface are conducted with the purpose to disentangle the contributions of tip and interface on the ECs. Together with tight binding (TB) calculations on the basis of the Poisson simulations, all experimental features are well understood establishing a useful method for the investigation of ECs. Flat plateaus of constant LL energy and about 40nm width close to the pn interface indicate electrostatic reconstruction of the ECs into compressible and incompressible stripes due to screening. Besides branches of LL features evolve at the pn interface interconnecting neighbouring LLs. Their explanation depends on the choice of sample and back gate voltage. If a QD is below the tip, its position with respect to the tip gets shifted at the pn interface whereby states at a different position within the QD are probed by the tip. These states are at different orbital energies within the QD. Their successive measurement leads to the observed LL branches. If in contrast no QD exits below the tip, the wave function of the compressible stripe is directly probed. Its position and width are influenced by the tip but without altering the structure of the wave function significantly. This enables to map the wave function of compressible stripes of several LLs for a specific choice of parameters. Likely, the structure of the compressible stripe wave function also reflects a lateral shift of the two sublattice components at the interface. Furthermore, various processes to fabricate graphene samples for combined electrical transport and STM investigations are described. Two of them are novelly conceived and integrate a graphite back gate into the sample structure. A third process employs a standard dry stacking technique followed by evaporating contacts through a shadow mask. A sample prepared by this method is characterized both by STM and transport measurements displaying interaction induced symmetry breaking of the LLs and the fractional 1/3 quantum Hall state yet only in transport. A different sample prepared by one of the novel methods has little improved quality and a similar outcome of symmetry broken states in transport measurements. Linking the STM experiments and sample fabrication processes, two procedures to locate a STM tip on a micron-sized graphene sample are presented and both successfully carried out. The first one relies on the optical alignment of tip and sample by means of a long distance microscope. In the second procedure the spatially varying and externally tunable electrostatic force between tip and sample measured by a tuning fork sensor guides the tip to the sample.
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%R 10.18154/RWTH-2022-11449
%U https://publications.rwth-aachen.de/record/860526