UC Berkeley

The discovery of two-dimensional (2D) van der Waals heterostructure provided a highly tunable material platform with multiple knobs and hence extremely large phase space to study novel quantum phenomena. Particularly, the emergence of moiré heterostructures, formed by stacking two slightly mismatched atomic lattices, opened a new world to explore 2D interacting electrons owing to their new length and energy scale compared with conventional systems. On the one hand, the electron correlation effect in moiré heterostructure is greatly enhanced, leading to the emergence of a variety of correlated ground states such as correlated insulator, electron crystal, and unconventional superconductivity. On the other hand, the moiré potential also significantly impacts the excited states of interacting electrons such as generating novel excitonic states. Signatures of these novel quantum phenomena of moiré interacting electrons have been observed through mesoscopic measurements such as electrical transport and optical spectroscopy. However, their microscopic natures, such as local interaction strength and charge spatial distribution, remain mostly underexplored for lack of an effective probe tool. My PhD mainly has focused on exploring the interacting electrons in 2D moiré heterostructures through developing a series of novel imaging techniques based on scanning tunneling microscopy (STM). In this thesis I will discuss my efforts in the last five years in four parts. The first part briefly introduces the background knowledge used in the thesis. I will first discuss the material platform studied here: from 2D van der Waals materials to moiré heterostructure. Next, I will introduce a classic lattice model for correlated electrons: the Fermi-Hubbard model. Finally, I will talk about the main experimental instrument used here: STM and its various applications. The second part explores the fundamental atomic and electronic structures in moiré heterostructure that consists of two works. The first one studies the atomic and single-particle electronic structures of a moiré heterostructure microscopically. The moiré superlattice and electronic minibands in a WS2/WSe2 moiré heterostructure are imaged with STM and STS. A three-dimensional atomic reconstruction was found to be present in the moiré heterostructure and responsible for the moiré potential. The second work studies the electron Coulomb interactions in this moiré superlattice. Using STM tip as local gate, we can control the cascade discharge of nearby moiré electrons, through which the nearest-neighbor Coulomb interaction can be experimentally measured. The third part describes our efforts on imaging an exotic correlated ground state of interacting moiré electrons, generlazied Wigner crystal, through developing a novel non-invasive microscopic thermodynamic probe tool. We innovatively employed a non-invasive graphene-sensing-layer assisted STM probe method, and for the first time saw the images of electron crystals after its prediction about 90 years ago. We further developed this imaging technique and demonstrated it to be a local thermodynamic measurement of correlated electrons. Through controlling the STM tip bias, we can locally excite an electron or hole quasiparticle and measure the local thermodynamic gaps. This technique is the so far one of the thermodynamic probes with the highest spatial resolution (~nm). The last part studies the microscopic nature for photoexcited states of interacting electrons in TMD moiré superlattice. Through combing laser excitation with STM we realized the probe of transient photoexcited states. With this technique we directly imaged the internal electron and hole distributions within a new type of charge-transfer moiré excitons.