ABSTRACT
The moir Ìe superlattice is a type of artificial crystal structure that is created by stacking two layers of materials with a small relative twist angle. The term “moir Ìe” refers to the interference pattern that is created between the two layers when they are stacked in this way, which can give rise to new electronic properties that are not present in either of the individual layers. In twisted bilayer graphene, some of the interesting properties that have been observed in moir Ìe superlattices include the emergence of new electronic bands, the formation of unconventional superconductivity, and the nature of correlated insulating behaviours. Apart from twisted bilayer graphene, twisted bilayer transition metal dichalcogenides (TMDs) are a fascinating class of moir Ìematerials that have garnered a lot of attention in recent years due to their unique electronic properties. In these materials, two layers of TMDs are stacked on top of each other with a small twist angle. One of the most exciting properties of twisted bilayer TMDs is their ability to exhibit correlated electron behavior and signatures of superconductivity at low carrier density.
In Chapter 2, we show that twisted bilayer WSe2 (tWSe2) with uniaxial strain exhibits a large nonlinear Hall (NLH) response due to the non-trivial Berry curvatures of the flat bands. Moreover, the NLH effect is greatly enhanced near the topological phase transition point which can be tuned by a vertical displacement field. Importantly, the nonlinear Hall signal changes sign across the topological phase transition point and provides a way to identify the topological phase transition and probe the topological properties of the flat bands. The strong enhancement and high tunability of the NLH effect near the topological phase transition point renders tWSe2 and related moire materials available platforms for rectification and second harmonic generations.
In Chapter 3, by using scanning tunneling microscopy and spectroscopy, we show the emergence of multiple ultra-flat bands in twisted bilayer WSe2 when the twist angle is within 3â—¦ of 60â—¦. The ultra-flat bands are manifested as narrow tunneling conductance peaks with estimated bandwidth less than 10 meV, which is only a fraction of the estimated on-site Coulomb repulsion energy. The number of these ultra-flat bands and spatial distribution of the wavefunctions match well with the theoretical predictions, strongly evidencing that the observed ultra-flat bands are induced by lattice reconstruction. Our work provides a foundation for further study of the exotic correlated phases in TB-TMDs.
In Chapter 4, we systematically study the Berry curvature effects in moir Ìe TMD het-erobilayers. We point out that the moir Ìe potential of the remote conduction bands would induce a sizable periodic pseudomagnetic field (PMF) on the valence band. This periodic PMF creates net Berry curvature flux in each valley of the moir Ìe Brillouin zone. The combination of the effect of the Berry curvature and the spin-valley locking can induce the spin Hall effect being observed in the experiment. Interestingly, the valley-contrasting Berry curvature distribution generated by the PMF can be probed through shift currents, which are dc currents induced by linearly polarized light through nonlinear responses. Our work sheds light on the novel quantum phenomena induced by Berry curvature in moir Ìe TMD heterobilayers.
In Chapter 5, we show that the interaction-driven valley polarization, together with the trigonal warping of the Fermi surface, induce the Josephson diode effect (JDE) in twisted bilayer graphene being observed in experiments. The valley polarization, which lifts the degeneracy of the states in the two valleys, induces a relative phase difference between the first and the second harmonics of supercurrent and results in the JDE. We further show that the nontrivial current phase relation, which is responsible for the JDE, also generates the asymmetric Shapiro steps.