Elettra | Spectromicroscopy Giant band engineering in superlattices with rotation-symmetry-mismatched interfaces Two-dimensional (2D) moiré superlattices, formed by stacking individual layers such as 2H transition metal dichalcogenides (TMDs) with accurately twist angles, have garnered significant attention due to their potential to exhibit strongly correlated physical properties, including unconventional superconductivity, moiré excitons, Wigner crystalline states, and orbital ferromagnetism. The moiré potentials and flat bands play a key role in driving these correlated physical phenomena. Despite the rapid advancement of the field, several critical and emergent aspects of moiré physics in TMD-based heterostructures remain underexplored. Understanding these phenomena is essential for both advancing fundamental science and realizing applications in quantum electronics, spintronics, and valleytronics. Two pressing and largely unresolved issues in this field are highlighted below: 1) The realization of flat bands in moiré superlattices of TMDs has been primarily confined to specific “magic” twist angles. These twist angles are often narrow and difficult to reproducibly fabricate, creating a significant experimental constraint. Moreover, the observed flat bands typically reside below the valence band maximum (VBM), limiting the range of accessible physical phenomena, particularly those associated with the highest occupied states. Whether it is possible to generate flat bands at arbitrary angles and push these bands to the highest occupied states—where novel quantum phenomena are expected to emerge— remains an open challenge. 2) TMDs in the 2H phase exhibit an intrinsic three-fold rotational (C₃) symmetry that governs their electronic and optical properties. However, when a C₃ symmetric TMD is stacked onto a substrate with a two-fold rotational (C₂) symmetry—the composite heterostructure experiences a symmetry mismatch. This mismatch can lead to profound consequences, such as the breaking of valley degeneracy and spontaneous generation of photocurrent due to band hybridization between C3 and C2 layers. However, physics behind the C3 symmetry breaking and direct experimental evidence of the band hybridization remains understudied to date. These questions can be effectively addressed via a direct visualization of the electronic band structure. New developments in micro angle-resolved photoemission spectroscopy (microARPES) can now provide the necessary space resolution to study these reduced size heterostructures with the further advantage of providing both a high energy and momentum resolution. In our work, the first example of electronic band structure properties in a C3 layer/C2 layer stacked superlattice, MoS2-black phosphorus is studied. The atomic structure and the corresponding Brillouin zone of the superlattice with a twist angle of θ are shown in Fig. 1 (a). The twist angle θ between the layers is defined as the angle between the black phosphorus and MoS2 armchair directions (AC) or equivalently in the momentum space as the angle between the black phosphorus Γ-X direction and the MoS2 Γ-M direction. Regardless of the twist angles between MoS2 and black phosphorus, a breaking of the C3 symmetry for the highest occupied MoS2 states is directly observed in the constant-energy cut of 3D ARPES maps. This is demonstrated by comparing the pristine MoS2 (MoS2 on BN) with the MoS2-black phosphorus regions, as shown in Fig. 1 (b). Constant-energy cuts at E-EVBMΓ=0 meV and E-EVBMΓ=-100 meV present a significant deformation along the high symmetry directions of the bP crystal, indicating an effective coupling of the MoS2 monolayer with the black phosphorous substrate. Additionally, micro-ARPES reveals the formation of anisotropic flat bands and minigaps at the Γ valley, as seen in Fig. 1 (c), where these properties are highly dependent on the orientation of the underlying black phosphorus layer. The band structure of pristine monolayer MoS2 shows a parabolic dispersion at Γ from which a hole effective mass of m*=2.3±0.1 me. Instead, minigaps are clearly visible in the band structure of MoS2 obtained from the two stacks. As one moves from the AC to the zigzag (ZZ) black phosphorous directions, these minigaps shift to larger wave vectors and decrease in width. Intriguingly, the top valence band at Γ bands undergoes a significant transformation, becoming completely flat as shown for the AC±45° directions. Along the ZZ direction a small dispersion is preserved and the extracted holes effective masses are m*=8.1 ± 0.1 me in the θ=8.5° sample. Moreover, a giant energy renormalization of approximately 140 meV is visualized in Fig. 1 (d), leading to a shift of the VBM from the K point to the Γ point. As expected, in monolayer MoS2 the VBM at K is about 120 meV higher than at Γ. However in the MoS2 stacked on black phosphorus, the relative energy positions between Γ and K is inverted and the VBM occurs at Γ, where the flat band is located. All these effects are explained by Density Functional Theory (DFT) calculations, indicating that they arise from strong hybridization between the Mo dz2 orbitals of the monolayer MoS2 and the P px orbitals of the black phosphorous, as depicted in Fig. 1 (e). This understanding of the electronic structure modulation in monolayer MoS2, induced by the substrate black phosphorus through rotational symmetry breaking and interlayer hybridization, opens up possibilities for generating flat bands at arbitrary twist angles in 2D TMDs heterostructures. Moreover, such control over the symmetry and geometry of moiré superlattices could pave the way for next-generation quantum devices that exploit directionally tuned correlated phenomena, spontaneous symmetry breaking, and exotic ground states driven by strong coupling between symmetry-mismatched layers. Elettra Highlights 2024–2025 82
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