Quasicrystalline 30° Twisted Bilayer Graphene as an Incommensurate Superlattice with Strong Interlayer Coupling (1804.05372v2)

Abstract: The interlayer coupling can be used to engineer the electronic structure of van der Waals heterostructures (superlattices) to obtain properties that are not possible in a single material. So far research in heterostructures has been focused on commensurate superlattices with a long-ranged Moir\'e period. Incommensurate heterostructures with rotational symmetry but not translational symmetry (in analogy to quasicrystals) are not only rare in nature, but also the interlayer interaction has often been assumed to be negligible due to the lack of phase coherence. Here we report the successful growth of quasicrystalline 30{\deg} twisted bilayer graphene (30{\deg}-tBLG) which is stabilized by the Pt(111) substrate, and reveal its electronic structure. The 30{\deg}-tBLG is confirmed by low energy electron diffraction and the intervalley double-resonance Raman mode at 1383 cm$^{-1}$. Moreover, the emergence of mirrored Dirac cones inside the Brillouin zone of each graphene layer and a gap opening at the zone boundary suggest that these two graphene layers are coupled via a generalized Umklapp scattering mechanism, i.e. scattering of Dirac cone in one graphene layer by the reciprocal lattice vector of the other graphene layer. Our work highlights the important role of interlayer coupling in incommensurate quasicrystalline superlattices, thereby extending band structure engineering to incommensurate superstructures.

Quasicrystalline 30° Twisted Bilayer Graphene as an Incommensurate Superlattice with Strong Interlayer Coupling

This paper presents an investigation into the unique electronic properties of a 30° twisted bilayer graphene (30°-tBLG), characterized as an incommensurate van der Waals heterostructure with a quasicrystalline order. The paper challenges previous assumptions that incommensurate heterostructures have negligible interlayer interactions, revealing significant modifications in electronic structures due to interlayer coupling.

Key Findings and Methodology

The research demonstrates that the 30°-tBLG, when epitaxially grown on a Pt(111) substrate, exhibits a distinctive electronic structure, including the emergence of mirrored Dirac cones. This outcome is primarily attributed to the coherent interlayer coupling that occurs even in the absence of long-range translational symmetry. The paper characterizes the 30°-tBLG using angle-resolved photoemission spectroscopy (ARPES) and NanoARPES to illuminate its electronic properties, supported by first-principles calculations.

1. Growth and Stability: The 30°-tBLG was grown using carbon segregation from a Pt(111) substrate. The stability of this structure, despite being noncommensurate, is enhanced by substrate interactions, notably a coupling between the carbon p orbital and the platinum d orbital.
2. Characterization Techniques: Low-energy electron diffraction (LEED) verified the structure, and Raman spectroscopy identified a double-resonance Raman mode, indicating novel phonon scattering processes intrinsic to the twisted bilayer. ARPES and NanoARPES analyses further supported the observation of mirrored Dirac cones and the presence of a band gap at the zone boundary caused by Umklapp scattering.
3. Electronic Structure and Band Gap: The paper revealed a mirrored Dirac cone formation, attributed to Umklapp scattering processes between the graphene layers. The presence of a band gap at the crossing point of original and mirrored Dirac cones, approximately 280 meV at its maximum, suggests a significant interlayer electronic hybridization.

Implications and Future Developments

The findings underscore the potential for band structure engineering in incommensurate superlattices, highlighting the importance of considering interlayer coupling in designs previously assumed negligible. Specifically, the strong interlayer interaction in such quasicrystalline formations could pave the way for novel functionalities in electronic devices leveraging van der Waals heterostructures. This work opens a pathway toward advanced applications, extending the conceptual design of material properties in two-dimensional materials through tailored growth techniques.

Future research could expand on this work by exploring various substrate interactions and further quantifying the role of interlayer coupling in other incommensurate systems. The continued exploration into quasicrystalline order in two-dimensional heterostructures will likely yield insights pertinent to both theoretical advancements and practical electronic applications, such as in quantum computing and nanotechnology.

In summary, this paper provides critical experimental and theoretical insights into the electronic behavior of incommensurate twisted bilayer graphene, reinforcing the notion that interlayer interactions can significantly modulate electronic properties even in the absence of long-range periodicity.