Cryogenic Photo-Magneto Imaging of ZrTe5 Crystals in Quantizing Fields

Erica F. Warth Pérez Arias

AutorIn

Erica F. Warth Pérez Arias

Titel

Cryogenic Photo-Magneto Imaging of ZrTe5 Crystals in Quantizing Fields

Zitierfähige Url:

https://nbn-resolving.org/urn:nbn:de:bsz:14-qucosa2-1011160

Erstveröffentlichung

2025

Datum der Einreichung

17.07.2025

Datum der Verteidigung

01.10.2025

Abstract (EN)

In the past two decades, the rise of topological and quantum materials has placed increasing emphasis on understanding the phenomena they exhibit-both theoretically and experimentally. However, most studies rely on conventional magnetotransport measurements, which inherently lack spatial resolution and overlook key spatial dependencies. A well-known example is the formation of (in-)compressible strips in the quantum Hall regime of a two-dimensional electron gas. To compensate for this blind spot, global transport measurements are often complemented by local or transport-decoupled techniques such as ARPES or reflectivity. While the former lacks spatial resolution, the latter often misses dynamic information. To date, no method has successfully bridged the gap between local and global characterization within the parameter space relevant to topological and quantum materials. In this thesis, I address this experimental gap by assembling a Photo-Magneto Imaging platform that combines local optical excitation with global voltage readout under cryogenic and high magnetic field conditions. The system integrates a 6 T optical cryostat, pixelsynchronous acousto-optic modulation, enabling rapid (2-20 min) two-dimensional photovoltage imaging of mm2-scale crystals with ∼2-3 μm resolution at 2.2-300 K. The setup generates three-dimensional data cubes (∼ 108 voxels) with milli-Tesla field resolution and optional control over temperature and laser power, pushing the experimental State-of-the-Art to spatially resolve quantum oscillations. As a test system, ZrTe5 offers a fertile research platform. Since its prediction as a two-dimensional topological insulator in 2014, it has been classified as a Dirac semimetal, a weak or strong topological insulator, or even a mixture of these phases simultaneously. ZrTe5 also exhibits a range of exotic phenomena, including the 3D quantum Hall effect, chiral anomaly, giant Nernst response, anomalous Hall effect, among others. These findings raise the question: is ZrTe5 finely tuned to a quantum critical point, or do current models and measurements fail to fully capture its complexity? Applying the Photo-Magneto Imaging technique to ZrTe5 revealed two phase-separable photo-induced signals not previously reported within a single crystal and measurement. The first is a uniformly distributed signal across the sample, producing Seebeck- and Nernstlike voltages that track quantum oscillations. Together with its spatial morphology, this signal is attributed to the global Photo-Thermoelectric Effect. The second is a long-ranged response extending over mm-scales, featuring inhomogeneous puddle-like structures and edge signals that oscillate with the magnetic field. To interpret this signal, I applied the wellestablished Shockley-Ramo theorem. For this purpose, I extended the SRT framework to include anisotropic electrical conductivity and external magnetic fields. While the model qualitatively reproduced the behavior at the sample edges, it does not explain the signal separation, nor the internal puddle structures. Consequently, a critical analysis revealed that charge-heat coupling and anisotropy in the thermopower, may play a crucial role in the photo-response of anisotropic crystals - factors omitted in the model. These mechanisms can give rise to thermoelectric eddy currents, influenced by carrier density gradients and local temperature-induced variations in the chemical potential. Such local temperatures may be driven by the Bridgman effect, which scales linearly with current density. Cross-reference measurements on isotropic InAs further hinted towards the significance of anisotropy in the photo-induced response. These findings suggest that anisotropy and thermoelectric dynamics can strongly influence the observed photo-induced response, effects relevant to transport in general. In addition to uncovering previously unreported photo-induced behavior, this work establishes a novel experimental platform and conceptual approach for investigating quantum materials through the lens of coupled charge and heat transport.

Freie Schlagwörter (DE)

Zrte5, Photokryogenik, Photomagneto-Quantisierung, Magnetotransport

Freie Schlagwörter (EN)

Zrte5, Photo-Cryogenic, Photo-Magneto Quantization, Magnetotransport

Klassifikation (DDC)

530

Klassifikation (RVK)

UX 3300

GutachterIn