Electronic structures of twisted two-dimensional materials studied by angle-resolved photoemission spectroscopy
Keywords: ARPES
2D materials

This thesis is a summary of my DPhil research projects, in which we describe the angle-resolved photoemission spectroscopy (ARPES) study on the electronic structure of twisted two-dimensional and other crystalline materials:
1. Twisted van der Waals heterostructures have recently been proposed as a condensed-matter platform for realizing controllable quantum models due to the low-energy moiré bands with specific charge distributions in moiré superlattices. Here, using angle-resolved photoemission spectroscopy with sub-micron spatial resolution (μ-ARPES) and scanning tunneling microscopy (STM), we performed a systematic investigation on the electronic structure of 5.1° twisted bilayer WSe2, which was previously reported to host correlated insulating and zero-resistance states. We only observe the Γ-valley moiré bands but not the proposed K-valley moiré band. In addition, we have unambiguously identified the real-space honeycomb- and kagome-shaped charge distributions at the moiré length scale, which are associated with Γ-valley moiré bands. Our results not only reveal the valley dependent moiré-modified electronic structure, but also highlight the Γ-valley moiré bands as a promising platform for exploring strongly correlated physics in emergent honeycomb and kagome lattices.
2. The graphene-based heterostructures host competing periodicities that are favorable for Umklapp scattering. Using μ-ARPES, we have revealed that Umklapp scattering is a universal process in the large angle twisted graphene regardless of the twist angle and the number of layers, by showing the replica Dirac cones in the 30° twisted trilayer and 33° twisted bilayer graphene. We have also confirmed the existence of Umklapp scattering in the graphene/WSe2 heterostructures with 11° and 14° twist angles. Interestingly, we find that 11° could be a magic angle for the graphene/WSe2 heterostructure, which can lead to the formation of flat band via the second order Umklapp scattering.
3. BiCuChO (Ch=S and Se) family are promising thermoelectric materials with a high figure of merit, and also have been predicted as possible parent compounds for unconventional superconductivity. Using high-resolution ARPES, we have confirmed the co-existence of heavy and light hole-bands near the valence band maximum, which is favorable for maintaining the large Seebeck coefficient and the good electrical conductivity simultaneously. We also observed the predicted flat bands closing to the Fermi level, which can dramatically enhance the charge density upon p-type doping. Further polarization dependent study reveals the quasi-one-dimensional orbital configuration of the valence band top. Our results contribute to a better understanding of the thermoelectric properties and serve as an important reference for searching for superconductivity in BiCuChO family.
4. Chiral semimetals the PtGa family (PtGa, PtAl,PdGa) are expected to host topologically non-trivial electronic structure, such as the multi-fold band crossings with large Chern number at high symmetry points connected by long Fermi-arc surface states. Using ARPES in different energy ranges (from vacuum ultraviolet to soft X-ray), we report a comprehensive study on their electronic structures. By comparing with our ab initio calculation, we identified the six-fold and fourfold fermions at the R and Γ points, respectively. Besides, we observed iconic topological surface states with chiral edge modes around the Γ point, forming long Fermi-arc winding across the Brillouin zone. Surprisingly, we found the anomalous polarization dependence of the observed arc states in PtGa, the origin of which could be rooted in the chiral crystal structure.