Electrical and Optical Manifestations of Flat Band Physics in Van der Waals Materials

The scientific progress is significantly transforming contemporary society with the introduction and widespread application of technologies like artificial intelligence and quantum computing. Despite their profound impact, these technologies necessitate enhanced energy consumption and sophisticated heat management strategies for efficient functioning. Given the current technological limitations in meeting the rapidly increasing energy demands, it is imperative to delve into the exploration of novel materials. This is crucial for identifying previously unknown physical properties that are promising for practical applications, thereby addressing the critical gap in sustainable energy solutions. In this context, this doctoral thesis explores the electrical and optical properties of an emergent class of materials, the layered metal monochalcogenide family.

The thesis begins by discussing the fabrication of air-sensitive devices based on the metal monochalcogenide indium selenide. Once a stable device is achieved, a comprehensive investigation into the unique properties and potential applications of these materials in various domains such as optical, electrical, chiral, thermoelectric, and magnetic is presented.

Key findings include the optical investigation of flat band-induced many-body interactions probed by exciton complexes, followed by the first observation of ambipolar transport in few-layer indium selenide. The last finding enabled the discovery of an electrical method based on the observation of an onset in the out-of-plane tunneling current which can be used to detect the flat band in a fast, reliable, and cost-effective way. This technique is applicable to any flat band material in a field effect structure, marking an important milestone in the study of flat band physics in 2D materials. Such a method allowed the discovery of chirality-sensitive tunneling differential conductance in an originally achiral system, and the experimental realization of spin-polarized hole transport at the valence band edge, which was theoretically predicted almost a decade ago. The thermoelectric and thermomagnetic properties of this class of materials are further explored, with the realization of the first electrically tunable giant Nernst effect operating at ultra-low temperatures.

The work highlights metal monochalcogenides as a promising class of materials for future optical and electrical applications, unlocking properties and functionalities currently unexplored by the research community. This study contributes to the field of material science and technology, addressing urgent energy challenges and paving the way for the development of next-generation energy-efficient devices and systems.