Twisted bilayers of two-dimensional (2D) materials are proving a fertile ground for investigating strongly correlated electron phases. This is because the moiré pattern introduced by the relative twist between layers introduces long-wavelength effective potentials which lead to electron localization. Here, we develop a generalized continuum model for the electronic structure of moiré patterns, based on first-principles calculations and tailored to capture the physics of twisted bilayer 2D semiconductors. We apply this model to a database of eighteen 2D crystals covering a range of atomic relaxation and electronic structure features. Many of these materials host topologically insulating (TI) moiré bands in a certain range of twist angles, which originate from the competition between triangular and hexagonal moiré patterns, tuned by the twist angle. The topological phases occur in the same range as the maximally flat moiré bands.
Twisted two-dimensional (2D) layered materials exhibit many novel and unique phenomena, such as insulation and superconductivity transition, and superlubricity. However, the effect of twisting on these phenomena remains unclear. A key challenge is the lack of linkage between the twisting and the material properties. Here, twisting induced moiré pattern is directly correlated with the configuration entropy of the bilayer graphene, based on which a new twisting phase transition in layered materials is uncovered. Most critically, the twisting phase transition is found size dependent. The diversit…
The highly anisotropic thermal conductivity in layered materials is crucial for a broad range of applications such as thermal management of electronic devices, thermal insulation, and thermoelectrics. Understanding of anisotropic thermal transport in layered materials largely depends on atomistic simulations based on density functional theory (DFT) or empirical potentials, which however suffer either low computational efficiency or accuracy. Recently, machine learning interatomic potentials (MLIPs) are emerging as a powerful tool to bridge the gap. Despite the recent progress in developing MLI…
Two-dimensional (2D) layered materials offer intriguing possibilities for novel physics and applications. Before any attempt at exploring the materials space in a systematic fashion, or combining insights from theory, computation and experiment, a formal description of information about an assembly of arbitrary composition is required. Here, we introduce a domain-generic notation that is used to describe the space of 2D layered materials from monolayers to twisted assemblies of arbitrary composition, existent or not-yet-fabricated. The notation corresponds to a theoretical materials concept of…
When two-dimensional van der Waals materials are stacked to build heterostructures, moiré patterns emerge from twisted interfaces or from mismatch in lattice constant of individual layers. Relaxation of the atomic positions is a direct, generic consequence of the moiré pattern, with many implications for the physical properties. Moiré driven atomic relaxation may be naively thought to be restricted to the interfacial layers and thus irrelevant for multi-layered heterostructures. However, we provide experimental evidence for the importance of the three dimensional nature of the relaxation in tw…
Focusing on the twist angle for the minimal commensurate structure, we perform nonperturbative calculations of electron dynamics in the twisted bilayer graphene (TBG) under intense laser fields. We show that the TBG exhibits enriched high-harmonic generation that cannot occur in monolayer or conventional bilayers. We elucidate the mechanism of these nonlinear responses by analyzing dynamical symmetries, momentum-resolved dynamics, and roles of interlayer coupling. Our results imply nonlinear "optotwistronics," or controlling optical properties of layered materials by artificial twists.
Layered two-dimensional materials exhibit rich transport and optical phenomena in twisted or lattice-incommensurate heterostructures with spatial variations of interlayer hybridization arising from moiré interference effects. Here, we report experimental and theoretical studies of excitons in twisted heterobilayers and heterotrilayers of transition metal dichalcogenides. Using MoSe$_2$-WSe$_2$ stacks as representative realizations of twisted van der Waals bilayer and trilayer heterostructures, we observe contrasting optical signatures and interpret them in the theoretical framework of interlay…
Research progress in high-pressure tuning of layered magnetic materials
Yanshan University（燕山大学）；Center for High Pressure Science, State Key Laboratory of Metastable Materials Science and Technology, Yanshan University（燕山大学高压科学中心亚稳态材料科学与技术国家重点实验室）；Hebei Key Laboratory of Microstructure Materials Physics, School of Science, Yanshan University（燕山大学理学院河北省微观结构材料物理重点实验室）
The incommensurate 30$^{\circ}$ twisted bilayer graphene (BG) possesses both relativistic Dirac fermions and quasiperiodicity with 12-fold rotational symmetry arising from the interlayer interaction [\href{https://science.sciencemag.org/content/361/6404/782}{Ahn et al., Science \textbf{361}, 782 (2018)} and \href{https://www.pnas.org/content/115/27/6928}{Yao et al., Proc. Natl. Acad. Sci. \textbf{115}, 6928 (2018)}]. Understanding how the interlayer states interact with each other is of vital importance for identifying and subsequently engineering the quasicrystalline order in the layered stru…
It is shown that twisted $n$-layers have an intrinsic degree of freedom living on $2n$-tori, which is the phason supplied by the relative slidings of the layers and that the twist generates pseudo magnetic fields. As a result, twisted $n$-layers host intrinsic higher dimensional topological phases and those characterized by second Chern numbers can be found in a twisted bi-layer. Indeed, our investigation of phononic lattices with interactions modulated by a second twisted lattice reveals Hofstadter-like spectral butterflies in terms of the twist angle, whose gaps carry the predicted topologic…
Stacking two-dimensional layered materials such as graphene and transitional metal dichalcogenides with nonzero interlayer twist angles has recently become attractive because of the emergence of novel physical properties. Stacking of one-dimensional nanomaterials offers the lateral stacking offset as an additional parameter for modulating the resulting material properties. Here, we report that the edge states of twisted bilayer zigzag graphene nanoribbons (TBZGNRs) can be tuned with both the twist angle and the stacking offset. Strong edge state variations in the stacking region are first reve…
Moire superlattices in twisted bilayer graphene (TBG) and its derived structures can host exotic correlated quantum phenomena because the narrow moire flat minibands in those systems effectively enhance the electron-electron interaction. Correlated phenomena are also observed in 2H-transitional metal dichalcogenides moire superlattices. However, the number of moire systems that have been explored in experiments are still very limited. Here we theoretically investigate a series of two-dimensional (2D) twisted bilayer hexagonal materials (TBHMs) beyond TBG at fixed angles of 7.34 and 67.34 degre…
Layered van der Waals materials have risen as powerful platforms to artificially engineer correlated states of matter. Here we show the emergence of a multiferroic order in a twisted dichalcogenide bilayer superlattice at quarter-filling. We show that the competition between Coulomb interactions leads to the simultaneous emergence of ferrimagnetic and ferroelectric orders. We derive the magnetoelectric coupling for this system, which leads to a direct strong coupling between the charge and spin orders. We show that, due to intrinsic spin-orbit coupling effects, the electronic structure shows a…
Light-matter interactions in layered materials and heterostructures: from moiré physics and magneto-optical effects to ultrafast dynamics and hybrid meta-photonics
Commensurable twisted bilayers can drastically change the magnetic properties of chromium trihalide layered compounds, which opens novel opportunities for tuning magnetic states through layer rotations. Here, we introduce a mathematical approach to obtain moiré patterns in twisted hexagonal bilayers by performing a certain commensurable rotation $θ$ over one layer. To test our approach, we apply it to find moiré structures with $θ=21.79^{\circ}$ and $32.20^{\circ}$ in the phases R$\bar{3}$ and C2/m of CrI$_{3}$. For comparison purposes, we also consider a non-shifted CrI$_{3}$ structure. Elect…
Van der Waals layered materials with well-defined twist angles between the crystal lattices of individual layers have attracted increasing attention due to the emergence of unexpected material properties. As many properties critically depend on the exact twist angle and its spatial homogeneity, there is a need for a fast and non-invasive characterization technique of the local twist angle, to be applied preferably right after stacking. We demonstrate that confocal Raman spectroscopy can be utilized to spatially map the twist angle in stacked bilayer graphene for angles between 6.5° and 8° when…
Two-dimensional layered materials offer the possibility to create artificial vertically stacked structures possessing an additional degree of freedom - $the$ $interlayer$ $twist$. We present a comprehensive optical study of artificially stacked bilayers (BLs) MoS$_2$ encapsulated in hexagonal BN with interlayer twist angle ranging from 0 to 60 degrees using Raman scattering and photoluminescence spectroscopies. It is found that the strength of the interlayer coupling in the studied BLs can be estimated using the energy dependence of indirect emission versus the A$_\textrm{1g}$-E$_\textrm{2g}^1…
Understanding the mechanisms governing the optical activity of layered-stacked materials is crucial to the design of devices aimed at manipulating light at the nanoscale. Here, we show that both twisted and slid bilayer graphene are chiral systems that can deflect the polarization of linear polarized light. However, only twisted bilayer graphene supports circular dichroism. Our calculation scheme, which is based on the time-dependent Schrödinger equation, is particularly efficient for calculating the optical-conductivity tensor. Specifically, it allows us to show the chirality of hybridized st…
We introduce a theoretical framework for the new concept of three-dimensional (3D) twistronics by developing a generalized Bloch band theory for 3D layered systems with a constant twist angle $θ$ between successive layers. Our theory employs a nonsymmorphic symmetry that enables a precise definition of an effective out-of-plane crystal momentum, and also captures the in-plane moiré pattern formed between neighboring twisted layers. To demonstrate the novel topological physics that can be achieved through 3D twistronics, we present two examples. In the first example of chiral twisted graphite, …
Many-body benchmarking of DFT local-registry energetics in bilayer InSe
The Anthony J. Leggett Institute for Condensed Matter Theory, Department of Physics, University of Illinois at Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校物理系安瑟尼·J·雷格特凝聚态理论研究所）；Center for Nanophase Materials Sciences, Oak Ridge National Laboratory（橡树岭国家实验室纳米材料科学中心）；Applied Physics Program, Northwestern University（西北大学应用物理项目）；Department of Physics and Astronomy, Northwestern University（西北大学物理与天文学系）；Computational Science Division, Argonne National Laboratory（阿贡国家实验室计算科学部）
Broadband phonon-velocity suppression and a finite anisotropic crossover in twisted bilayer SnSe
Beihang University（北京航空航天大学）；Tianmushan Laboratory, Beihang University（天目山实验室）；Center for Bioinspired Science and Technology, Hangzhou International Innovation Institute, Beihang University（杭州国际创新学院仿生科学与技术中心）
Twisted layers of atomically thin two-dimensional materials realize a broad range of novel quantum materials with engineered optical and transport phenomena arising from spin and valley degrees of freedom and strong electron correlations in hybridized interlayer bands. Here, we report experimental and theoretical studies of WSe$_2$ homobilayers obtained in two stable configurations of 2H ($60^\circ$ twist) and 3R ($0^\circ$ twist) stackings by controlled chemical vapor synthesis of high-quality large-area crystals. Using optical absorption and photoluminescence spectroscopy at cryogenic temper…
