Electronic Transport of a 2D Quantum Material WTe2

Abstract

Quantum materials, such as superconductors, topological insulators, magnets, and Weyl semimetals, are materials whose properties cannot be explained without quantum mechanics. Strong correlation and band topology play very important roles in quantum materials. The study of two-dimensional (2D) quantum materials was triggered by the discovery of graphene, the first atomically thin crystal wherein electrons are confined in a single 2D sheet. In this regime, critical material parameters such as carrier density, electron correlation strength and topology can be modified by electrostatic gate, screening effect and magnetic field, respectively. Moreover, the stacking of different 2D quantum materials enables creation of novel structures which can host new electronic phenomena and realize new device functionalities. All of these make 2D quantum materials a perfect platform to study the physics of strong correlation, magnetism, and topology. This thesis focuses on electronic properties of a 2D quantum material WTe2. In the first part, I will discuss the discovery of the first ferroelectric metal in few-layer WTe2. As WTe2 is thinned down to trilayer and bilayer, it exhibits coexistence of ferroelectric and metallic phases. Then, I will discuss the quantum spin Hall (QSH) edge state of a monolayer WTe2 in three aspects. The spin axis is first established by study of the anisotropy of conductance in an external magnetic field. Then I will show the magnetic proximity effect between the QSH edge and a 2D magnet CrI3. Finally, I will summarize the experiment of spin current injection into the QSH edge.