Strongly-interacting Excitonic Phases in van der Waals Heterostructures of Two Dimensional Semiconductors Pubblico

Abstract

Phenomena of spontaneous collective arrangement of strongly-interacting quantum particles have long been a fascinating area of physics, and have allowed us to understand many spectacular natural phenomena, ranging from fractionalization of charge and topological order to high-temperature superconductivity. Two-dimensional materials have emerged as a powerful new platform for the study of strongly-correlated physics, as the reduced dimensionality enhances particle interactions relative to their kinetic energy. Within these, Transition-Metal Dichalcogenide (TMD) monolayers are premier semiconductors, especially in the context of optics, since they couple strongly to light. In this dissertation, we use this property to optically generate large populations of excitons, quasiparticles that are the low-energy excitation of semiconductors, formed when an electron is excited and bound to the hole it leaves behind. We then study the possible many-body phases of interacting excitons, as they interact strongly via their dipolar moment and through exchange interactions.

The samples we study are heterostructures of TMD monolayers, made of different TMD species. We observe three distinct excitonic phases in TMD heterobilayers: an excitonic Mott insulator, a mixed electron-exciton Mott insulator and an in-plane excitonic ferroelectric phase, and in heterotrilayers we observe a novel quasiparticle, the out-of-plane quadrupolar exciton, with tantalizing signatures that they exhibit out-of-plane antiferroelectric correlations at high densities. The phases are identified through optical spectroscopy, emphasizing the power of relatively simple techniques in identifying microscopic details of the systems under study, due to the TMDs' excellent optical properties. We observe unusual collective emission from moiré excitons which is dependent on the stacking configuration between the monolayers. We also showcase the remarkable tunability of these systems, with integrated electric field and doping control. This work underscores the potential of the 2D platform in further advancing our understanding of strongly-correlated physics, particularly in a driven-dissipative system, and the significant role of TMDs and optics in the discovery and characterization of moiré quantum materials.

Table of Contents

Introduction 1

Thesis Outline 4

Background 6

Transition Metal Dichalcogenide Monolayers 6

Crystallographic Properties of TMD Monolayers 7

Electronic Properties of TMD Monolayers 10

Excitons in TMD Monolayers 19

Optical Selection Rules in TMD Monolayers 20

TMD Heterostructures 25

The Moiré Potential 26

Optical properties of Type-II TMD Heterostructures 32

The Moiré Potential and Optical Selection Rules 36

Field Modulation of Electronic and Excitonic Properties of TMD Devices 39

TMD Device Design 40

Monolayer Effects: Electrostatic Doping and the Valley Zeeman Effect 42

Excitonic Stark Effect 45

Heterostructure Valley Zeeman Effect 47

Electronic Mott Insulator and Generalized Wigner Crystals 49

TMD Heterostructure Fabrication 52

Flake Exfoliation 53

Exfoliation of thin flakes on SiO2 substrates 53

Flake identification and selection 55

Heterostructure Stacking 56

Patterned Substrate Fabrication 57

Polycarbonate-based dry fabrication method 60

Optical Measurements of TMD Heterostructures 70

Measurement Setup 70

Setup 1 - ‘Bluefors’ 71

Setup 2 - ‘Attodry’ 75

Measurement Techniques 75

Photoluminescence (PL) Spectroscopy 75

Photoluminescence Excitation (PLE) Spectroscopy 76

Reflectance Spectroscopy 76

Differential Reflectance Spectroscopy 76

Lifetime Measurements 77

Time-Resolved PL Spectroscopy 77

Excitonic Mott Insulator and Electron-Exciton Composite Crystals in TMD Bilayers 79

Excitonic Mott Insulator 79

Mixed Exciton-Electron Mott Insulator 86

Excitonic Lifetime in Mixed Exciton-Electron Moir´e Systems 90

Conclusion 94

Excitonic Ferroelectric Phase 97

The In-plane Quadrupolar Exciton 97

Density-dependent Redshift of IX 100

Modelling the IX Decay Dynamics 107

Conclusion 118

Out-of-plane Quadrupolar Excitons and the Excitonic Antiferroeletric Correlations in TMD Trilayers 119

Out-of-plane Quadrupolar Excitons 120

Tunable Lifetime of Quadrupolar Excitons 129

Excitonic Out-of-plane Antiferroeletric Correlations 131

Quadrupolar to Dipolar Density-Driven Transition 131

Modelling the Antiferroelectric State 136

Conclusion 142

Summary and Outlook 143

Appendix A Transfer Matrix Determination of Optimal hBN Thickness 147

Appendix B Monte-Carlo Simulation of Dipolar and Quadrupolar Exciton Decay Dynamics 149

Appendix C DFT Calculations of Trilayer Wavefunctions 156

Bibliography 157

About this Dissertation

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School	Laney Graduate School
Department	Physics
Degree	Ph.D.
Submission	Dissertation
Language	English
Research Field	Physics, Condensed Matter Physics, Quantum Physics, Optics
Parola chiave	Two Dimensional Semiconductor Moire Exciton 2D TMD
Committee Chair / Thesis Advisor	Ajit Srivastava, Emory University
Committee Members	Connie Roth, Emory University Carlos Sa de Melo, Georgia Institute of Technology Hayk Harutyunyan, Emory University Luiz Santos, Emory University

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