Moiré physics in the semiconductor MoSe2/WS2 heterostructure

Polovnikov, Borislav (2024): Moiré physics in the semiconductor MoSe2/WS2 heterostructure. Dissertation, LMU München: Fakultät für Physik

Abstract

Atomically thin Transition Metal Dichalcogenides are direct band gap semiconductors and present a very fertile ground for both optoelectronic applications and fundamental quantum research. In their monolayer limit, they are characterized by inherently strong light-matter coupling mediated by tightly bound electron-hole pairs known as excitons. Multilayer heterostructures, on the other hand, have attracted considerable interest as model systems for band engineering and implementations of previously elusive correlated electronic states. The geometric moiré interference effect in stacked van der Waals heterostructures plays an integral role in this development, since critical parameters of the long-range moiré potential can be engineered via the constituent layer materials and their rotational alignment. The MoSe2/WS2 heterostructure is characterized by near-resonant alignment of its conduction band edges which implies significant hybridization effects and electrostatic tunability of its energetic ground state. For small twist angles, the presence of a moiré potential leads to the emergence of distinct moiré excitons that allow to study electronic states optically, with signatures of Fermi-Hubbard model physics, correlation-induced magnetism, or correlated Mott-insulator states. The complex interplay of intralayer exciton mixing with interlayer electron tunneling, however, has long obscured the fundamental band structure and the nature of the bright moiré excitons. In this thesis, we clarify these questions by presenting a comprehensive theoretical and experimental study of angle-aligned MoSe2/WS2 heterostructures. We employ a dual gate field-effect device architecture that allows us to simultaneously control both out-of-plane electric fields as well as charge carrier doping. In combination with additional tunable parameters such as temperature or perpendicular magnetic fields, we study the behavior of the moiré excitons in an extended parameter range and develop theoretical models in order to explain the observed experimental data. First, we determine the nature of the bright moiré excitons by studying their dispersion with perpendicular electric fields in white light reflectance and narrow-band modulation spectroscopy. We present a continuum model that allows us to accurately describe the low-energy exciton physics and which we subsequently build upon to study the exciton behavior in the presence of electron doping. By means of electrostatic modeling, we establish evidence of a previously unseen charging behavior that proceeds layer-by-layer, with the first and second electron per moiré cell consecutively occupying moiré potential pockets first in the MoSe2 and then in the WS2 layer. Finally, we study the doping-dependent spin-susceptibility of the emerging bilayer electron lattice that suggests the presence of electrostatically tunable Ruderman-Kittel-Kasuya-Yosida magnetism. Our results establish MoSe2/WS2 as a unique system to explore Coulomb-correlated states in bilayer spin-charge lattices and provide compelling motivation for future experimental and theoretical work on many-body phenomena in MoSe2/WS2.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: 2D materials, TMDs, moiré heterostructures, excitons, MoSe2/WS2
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 13. Juni 2024
• 1. Bericht­erstatter:in: Högele, Alexander