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Register a new userTwister: Construction and structural relaxation of commensurate moire superlattices
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Introduction of a twist between layers of two-dimensional materials which leads to the formation of a moire pattern is an emerging pathway to tune the electronic, vibrational and optical properties. The fascinating properties of these systems is often linked to large-scale structural reconstruction of the moire pattern. Hence, an essential first step in the theoretical study of these systems is the construction and structural relaxation of the atoms in the moire superlattice. We present the Twister package, a collection of tools that constructs commensurate superlattices for any combination of 2D materials and also helps perform structural relaxations of the moire superlattice. Twister constructs commensurate moire superlattices using the coincidence lattice method and provides an interface to perform structural relaxations using classical forcefields.
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Twisted bilayers of van der Waals materials have recently attracted great attention due to their tunable strongly correlated phenomena. Here, we investigate the chirality-specific physics in 3D moire superlattices induced by Eshelby twist. Our direct DFT calculations reveal helical rotation leads to optical circular dichroism, and chirality-specific nonlinear Hall effect, even though there is no magnetization or magnetic field. Both these phenomena can be reversed by changing the structural chirality. This provides a way to constructing chirality-specific materials.
We develop parameters for the interlayer Kolmogorov-Crespi (KC) potential to study structural features of four transition metal dichalcogenides (TMDs): MoS$_2$, WS$_2$, MoSe$_2$ and WSe$_2$. We also propose a mixing rule to extend the parameters to their heterostructures. Moire superlattices of twisted bilayer TMDs have been recently shown to host shear solitons, topological point defects and ultraflatbands close to the valence band edge. Performing structural relaxations at the DFT level is a major bottleneck in the study of these systems. We show that the parametrized KC potential can be used to obtain atomic relaxations in good agreement with DFT relaxations. Furthermore, the moire superlattices relaxed using DFT and the proposed forcefield yield very similar electronic band structures.
Moire superlattices comprised of stacked two-dimensional materials present a versatile platform for engineering and investigating new emergent quantum states of matter. At present, the vast majority of investigated systems have long moire wavelengths, but investigating these effects at shorter, incommensurate wavelengths, and at higher energy scales, remains a challenge. Here, we employ angle-resolved photoemission spectroscopy (ARPES) with sub-micron spatial resolution to investigate a series of different moire superlattices which span a wide range of wavelengths, from a short moire wavelength of 0.5 nm for a graphene/WSe2 (g/WSe2) heterostructure, to a much longer wavelength of 8 nm for a WS2/WSe2 heterostructure. We observe the formation of minibands with distinct dispersions formed by the moire potential in both systems. Finally, we discover that the WS2/WSe2 heterostructure can imprint a surprisingly large moire potential on a third, separate layer of graphene (g/WS2/WSe2), suggesting a new avenue for engineering moire superlattices in two-dimensional materials.
Moire superlattices in graphene supported on various substrates have opened a new avenue to engineer graphenes electronic properties. Yet, the exact crystallographic structure on which their band structure depends remains highly debated. In this scanning tunneling microscopy and density functional theory study, we have analysed graphene samples grown on multilayer graphene prepared onto SiC and on the close-packed surfaces of Re and Ir with ultra-high precision. We resolve small-angle twists and shears in graphene, and identify large unit cells comprising more than 1,000 carbon atoms and exhibiting non-trivial nanopatterns for moire superlattices, which are commensurate to the graphene lattice. Finally, a general formalism applicable to any hexagonal moire is presented to classify all reported structures.
Moire superlattices in van der Waals heterostructures are gaining increasing attention because they offer new opportunities to tailor and explore unique electronic phenomena when stacking 2D materials with small twist angles. Here, we reveal local surface potentials associated with stacking domains in twisted double bilayer graphene (TDBG) moire superlattices. Using a combination of both lateral Piezoresponse Force Microscopy (LPFM) and Scanning Kelvin Probe Microscopy (SKPM), we distinguish between Bernal (ABAB) and rhombohedral (ABCA) stacked graphene and directly correlate these stacking configurations with local surface potential. We find that the surface potential of the ABCA domains is ~15 mV higher (smaller work function) than that of the ABAB domains. First-principles calculations based on density functional theory further show that the different work functions between ABCA and ABAB domains arise from the stacking dependent electronic structure. We show that, while the moire superlattice visualized by LPFM can change with time, imaging the surface potential distribution via SKPM appears more stable, enabling the mapping of ABAB and ABCA domains without tip-sample contact-induced effects. Our results provide a new means to visualize and probe local domain stacking in moire superlattices along with its impact on electronic properties.
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