Twist, strain, and magnetic control on the electronic and excitonic states in van der Waals Moiré Systems /c by Huiyuan Zheng

Abstract

In the field of condensed matter physics, moiré systems involving stacked two-dimensional materials have become a particularly intriguing platform in recent years. These structures introduce a new degree-of-freedom in the form of the orbital layer component of particles or quasiparticles, which can be flexibly manipulated through various techniques. In electronic systems, the layerhybridized electrons in doubly stacked configurations exhibit layer pseudospin, leading to distinct transport and polarization behaviors. Similarly, in optical systems, interlayer excitons display unique optical characteristics. This dissertation provides a multifaceted investigation of the twist, strain, and magnetic control of the electronic and excitonic states in van der Waals moiré materials. Four major works will be divided into four individual chapters.

In Chapter 2, the study of the electronic transport in heterobilayer systems unveils the intrinsic planar Hall effect induced from the layer pseudospin. The concept of the in-plane orbital magnetic moment will be introduced. We provide the formalism of the response coefficients in EB- and EEB-order. Moreover, we will demonstrate the effects of two orders in twisted bilayer graphene. The formalism about semiclassical theory and the derivation of the response coefficients are placed in Appendix B.

In Chapter 3, the electronic response related to the anomalous layer polarization will be introduced from the origin of the band geometry. Unlike conventional layer polarization, it is induced by the in-plane electric field. The formalism of dipole and quadrupole polarization in the linear and nonlinear order of the electric field will be shown, along with the corresponding symmetry restriction. We will also present results of the linear effect in twisted MoTe2 homobilayer with uniaxial strain. In addition, in twisted MoTe2 homotrilayer, a comprehensive study of dipole and quadrupole responses will be demonstrated. A topological phase transition is also discovered.

In Chapter 4, we will systematically study the mini-band dispersion and optical properties of interlayer exciton in heterostrain-induced moiré patterns, contrasting them with those in moiré patterns caused by various twist angles. We also show that the bright excitonic states within the light cones evolve from localized wave packets to extended Bloch states as the moiré periodicity decreases. We investigate various strain configurations and its interplay with twisting, and present comprehensive diagrams illustrating the optical properties of moiré interlayer excitons in the strain-parameter space.

In Chapter 5, the magnetic control of the bright interlayer excitons will be demonstrated, where the moiré potential is quenched via a Boron Nitride spacer. We first introduce the magneto-Stark effect and its impact on the light cone displacement. Further, the g-tensor can be dictated through the magneto-Stark shift from in-plane magnetic field as well as the Zeeman shift from the out-of-plane magnetic field. To detect the anomalous magneto-optical response, we practically consider the exciton hybridization. Finally, the chiral light-matter interface can be realized through the resonant excitation.