Exploring the magneto-transport of planar and cylindrical graphite in quantizing fields

Narayan Kunchur

AutorIn

Narayan Kunchur

Titel

Exploring the magneto-transport of planar and cylindrical graphite in quantizing fields

Zitierfähige Url:

https://nbn-resolving.org/urn:nbn:de:bsz:14-qucosa2-968084

Erstveröffentlichung

2025

Datum der Einreichung

15.02.2024

Datum der Verteidigung

20.06.2024

Abstract (EN)

The quantum Hall effect (QHE) observed in a two-dimensional electron gas (2DEG) stands as a pivotal discovery in twentieth-century physics. This phenomenon propelled a significant advancement in understanding electrons confined to a 2D space subjected to a transverse magnetic field (B) and their interactions with disorder and sample boundaries. The precisely quantized value of the Hall resistivity led to a new definition of the resistance standard, incorporating the concept of topology into condensed matter physics. This breakthrough not only initiated a new realm of exploration into topological materials but also inspired in-depth theoretical and experimental investigations across various facets of the observation. Traditionally, the QHE was believed to be exclusively confined to a 2DEG. However, recent measurements have challenged this long-standing belief by quantitatively explaining the plateau-like features in the Hall conductivity (σxy) of low carrier density semi-metals and lightly doped semiconductors with a 3D Fermi surface. This is termed the quasi-quantized Hall effect (qQHE). The hypothesis underlying the qQHE suggests that, in the quantum limit, σxy approximately scales with the Fermi wave-vector along B. Predictions also include the presence of localized states, and surface states analogous to the edge states in the QHE. The qQHE is declared as a generic phenomenon applicable to all low carrier density systems possessing a 3D Fermi surface. The first part of this thesis delves into investigating whether the qQHE quantitatively explains the σxy in graphite. The motivation arises from the following reasons. Firstly, prior magneto-transport studies have revealed plateau-like features in the σxy of graphite; however, the quantitative explanation of its height remains unexplained. Secondly, graphite, being semi-metal with low carrier density, aligns with the prerequisites set forth by previous qQHE investigations. Thirdly, the electronic properties of graphite are accurately captured by the Slonczewski-Weiss-McClure (SWM) model. This model facilitates the theoretical estimation of σxy as hypothesized within the qQHE framework, enabling a direct comparison with experimental data to scrutinize the validity of the hypothesis. Moreover, this study goes beyond the conventional boundaries by extending its scope to material systems featuring multiple Fermi pockets-a domain yet unexploredpocketshe context of the qQHE. Intriguingly, the investigation reveals a spread in the σxy values across samples with similar Fermi surfaces, approaching the predicted qQHE values. This challenges existing explanations derived from the qQHE. The thesis delves into a thorough discussion of various possibilities aimed at addressing the σxy in graphite. Notably, a novel power-law-like scaling of σxy is observed, hinting at potential analogies with the power-law scaling known from the anomalous Hall effect—a facet that adds an interesting layer to the exploration. In the second part of this thesis, the focus shifts toward examining whether hypothesized surface states exist in the qQHE. Graphite’s ability to be processed into various 3D shapes is noted, and a cylindrical geometry is chosen. For an out-of-plane orientation relative to the cylinder length axis, the number of occupied Landau bands is angle-dependent due to the angle-dependent Landau quantization. In 2D, regions that indicate a change in the number of occupied Landau levels must necessarily host edge states because of a topological phase transition. The examination aims to discern whether analogous effects arise due to hypothesized surface states and the parallels drawn between the qQHE and QHE, particularly concerning Berry curvature effects. During the course of my measurements, a significant gap in understanding the experimental signatures of charge transport under the combined influence of B and curvature is identified. Specifcally, I noted that current redistribution plays a crucial role in this process. To address this issue, I developed a network model that qualitatively explains the longitudinal and Hall measurements in cylindrical graphite

Freie Schlagwörter (EN)

Hall effect, Quantum transport, Graphite, Topology

Klassifikation (DDC)

530

Klassifikation (RVK)

UK 1200

GutachterIn