Landau’s Fermi Liquid theory states that the low energy excitations in many-electron systems subject to Coulomb interaction in solid materials may be approximately described as non- interacting quasiparticles. It is very successful and works well for most crystals. However, in systems with strong correlations, behavior of charge carriers may disobey Fermi Liquid theory. Strongly-correlated systems can host many intriguing phenomena, including cases where electrons are effectively ‘fractionalized’ into parts. Their study is however challenging, due to the rareness of both material systems and detection probes. 2D materials have recently become a new material platform to search for strongly-correlated phenomena with their flexibility, versality and high device quality. Among various 2D materials, WTe2 has a special anisotropic lattice structure and hosts strong correlation, topology and spin-orbit coupling simultaneously. In this thesis, I presented several experimental studies related to the strongly-correlated phenomena in monolayer WTe2 and its moir´e structures.

First, I present our discovery of a new quantum state, i.e. a 2D anisotropic Luttinger Liquid state in small angle twisted bilayer WTe2 (tWTe2). Its characteristics include an exceptionally high transport anisotropy, a power-law scaling behavior characteristic for Luttinger Liquid physics along the hard direction, and a non-linear differential resistance along the easy direction. Our results provide direct experimental evidence for 2D Luttinger liquid physics at ultralow temperatures, whose stability was a question under debate in theory. It opens up new possibilities to study non-Fermi liquid phenomena in a new regime. Second, novel excitations are observed in the insulating state of monolayer WTe2. Undoped mono- layer WTe2 is a quantum spin Hall insulator, but the insulating bulk state is beyond a simple band picture. I present evidence for the insulating WTe2 to be an excitonic insulator. More surprisingly, quantum oscillations mimicking the characteristic Shubnikov-de Haas (SdH) oscillations in metals are observed in the insulating state of monolayer WTe2. We discuss possible explanations and a comprehensive understanding of such states is important to the search for a class of new quantum matter such as neutral Fermi surface state in insulators.

Adviser: Sanfeng Wu