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%0 Thesis
%A Sowmya Spandana, Somanchi
%T Transport through graphene quantum point contacts
%I RWTH Aachen University
%V Dissertation
%C Aachen
%M RWTH-2023-10276
%P 1 Online-Ressource : Illustrationen, Diagramme
%D 2023
%Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University
%Z Dissertation, RWTH Aachen University, 2023
%X This thesis investigates low temperature transport through graphene quantum point contacts (QPCs) encapsulated in hexagonal boron nitride (hBN) using the van der Waals technique. Single layer graphene (SLG)QPCs are fabricated using electron beam lithography followed by SF6 based reactive ion etching to define the shape and the width of the QPC. In such devices, we observe that the rough edges due to physical etching play an important role in the quantized conductance characteristics of the QPC particularly around the charge neutrality point (CNP). In order to be able to achieve better control over the charging of these localized edge states, we fabricate local top gates to see if it is possible to control the edges independently from the rest of the QPC channel. In one of the two devices measured, we use a pair of top gates in a split gate geometry that cover only the edges on either side of the QPC. Here, we not only observe quantized conductance kinks on the order of 2 - 3 e2/h but also a non-linear relative gate lever arm. This can be explaine dusing the fact that the edges are very likely to be terminated by Fluorine atoms after etching with SF6 whichresults in higher charge accumulation along the edges and consequently, a gate voltage dependent gate lever arm. In the other device, we employ a single top gate spanning the entire channel of the QPC except the edges. We measure the conductance as a function of both the top and the back gate voltages and observe conductance kinks that result in a linear relative gate lever arm. This dominant linear line denotes the charge neutrality of each individually measured conductance trace and its slope is referred to as the "major" slope. However, interestingly, we also observe several other features that evolve with a smaller "minor" slope. In both experiment and theoretical calculations using the tight binding model, we notice that sweeping the gates simultaneously along a direction with the minor slope results in a much cleaner conductance trace especially around the CNP where the edge disorder is the maximum. This suggests that the features corresponding to the minor slope are due to the effect of the electric field lines of the top gate on the edge states. Since these localized edge states are farther from the top gate as compared to the channel, they are tuned less strongly as compared to the Bloch states in the channel right under the top gate. This is further corroborated by the Landau fan measurements along both the directions with major and minor slope. Here,we observe that (i) the larger Landau level features at higher magnetic field appear to be unaffected by the direction of sweep. (ii) There are number of vertical straight lines that are unaffected by the magnetic field in the low magnetic field, low charge carrier density region around the CNP. These are the localized states due to the edges. (iii) The number of such vertical straight line features is lesser along the minor line than any other direction of sweep of the gates. (iv) In general, the evolution of the conductance kinks from the size quantization to their respective Landau levels is much cleaner along the direction of the minor line without a lot of interference from localized states. Thus, we have been able to use the top gate as a knob to disentangle the features related to edge disorder from size quantization. We then move to bilayer graphene (BLG) where we apply voltage on a pair of split gates to define the width of the QPC.We create a displacement field using the combination of the split gates and a graphite back gate. This depletes the charge carriers underneath the side gates, thereby creating a 250 nm wide channel in between the source and the drain. Using a layer of graphite as the back gate instead of the doped Si as in the case of the single layer QPCs ensures that the gate is much closer to BLG resulting in a far better tuning besides also screening any impurities from the surrounding SiO2 or hBN. We include three other finger gates along the length of the QPC channel to tune the charge carrier density locally. Conductance traces exhibit clear 4 e2/h steps that split into intermediate kinks at higher values of parallel magnetic field indicating spin degeneracy lifting. From the crossing points of spin-up and spin-down branches of successive sub-bands, we extract the values of sub-band spacing. More importantly, in the transconductance plots as a function of the finger gate voltage and the magnetic field, we observe discontinuities in the applied voltage at (i) 0 T between the spin-up and spin-down levels of the first sub-band. This is manifested in the form of a step at 2e2/h that remains unaffected by the magnetic field. (ii) Another gap in voltage is observed at a higher value of magnetic field at the crossing point of the spin up level of the first sub-band and the spin-down level of the second sub-band. This is evident in the form of step at around 1.5 × 4 e2/h that travels down 4 e2/h which was observed earlier in GaAs heterostructures and referred to as the 0.7 analog, similar to the 0.7 anomaly at 0 T as a result of exchange/electron - electron (e-e) interactions. In our device, we attribute the voltage gap at 0 T to a spin-orbit (SO) coupling of the Kane - Mele type that dominates the e-e interactions. While at higher magnetic field, this situation is reversed and the Zeeman effects quenches the SO interaction. Both these voltage gaps seem to evolve linearly with the displacement field.
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%R 10.18154/RWTH-2023-10276
%U https://publications.rwth-aachen.de/record/972605