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Twisted Bilayer Graphene
Motivated by recent experiments probing the wavefunctions of magic-angle twisted bilayer graphene (tBLG), we perform large-scale first-principles calculations of tBLG with full atomic relaxation across a wide range of twist angles down to 0.99°. Focusing on the magic angle, we compute wavefunctions of the low energy bands, resolving atomic-scale details and moiré-scale patterns that form triangular, honeycomb, and Kagome lattices. By tuning the interlayer interactions, we illustrate the formation of the flat bands from isolated monolayers and the emergence of the band inversion and fragile topology at a sufficiently large interaction strength. We identify strong indicators of a new topological phase transition with increasing interlayer interaction strength, achievable with external pressure or a decrease in the twist angle. When this transition occurs, the upper and lower flat bands exchange their wavefunction character, which may explain why superconductivity appears with electron doping below the magic angle. Our study demonstrates the feasibility of using first-principles wavefunctions to help interpret experimental signatures of topological and correlated phases in tBLG.
Physical Review Letters 133, 246703 (2024)
Editors' Suggestion
Two-Dimensional Multiferroics
Two-dimensional (2D) materials that exhibit spontaneous magnetization, polarization, or strain (referred to as ferroics) have the potential to revolutionize nanotechnology by enhancing the multifunctionality of nanoscale devices. However, multiferroic order is difficult to achieve, requiring complicated coupling between electron and spin degrees of freedom. We propose a universal method to engineer multiferroics from van der Waals magnets by taking advantage of the fact that changing the stacking between 2D layers can break inversion symmetry, resulting in ferroelectricity as well as magnetoelectric coupling. We illustrate this concept using first-principles calculations in bilayer NiI2, which can be made ferroelectric upon rotating two adjacent layers by 180° with respect to the bulk stacking. Furthermore, we discover a novel strong magnetoelectric coupling between the interlayer spin order and interfacial electronic polarization. Our approach is not only general but also systematic and can enable the discovery of a wide variety of 2D multiferroics with strong magnetoelectric coupling.
Sliding Ferroelectricity
Ferroelectric materials change polarization in response to an electric field and are useful for memory. However, these materials often fatigue as they are cycled many times, capping their lifetime. Bian et al. explored this behavior of a sliding ferroelectric, bilayer molybdenum disulfide, and found very little fatigue after switching the polarization a million times. Sliding ferroelectrics switch polarization differently than conventional ferroelectrics, giving rise to this behavior. These observations show why this type of material might be of interest for applications such as nonvolatile memory. Yasuda et al. investigated the fatigue behavior of another sliding ferroelectric, bilayer boron nitride, and found very little fatigue after switching the polarization 100 billion times. This number of switching cycles rivals other state-of-the-art conventional ferroelectric materials. The robust stability through this many cycles is due to the fact that the defects that occur in conventional ferroelectrics and degrade the electrical properties over time are not generated.
Nature Communications 14, 1629 (2023)
Editors' Highlights
Topological Polarization in Twisted Bilayers
Out-of-plane polar domain structures have recently been discovered in strained and twisted bilayers of inversion symmetry broken systems such as hexagonal boron nitride. Here we show that this symmetry breaking also gives rise to an in-plane component of polarization, and the form of the total polarization is determined purely from symmetry considerations. The in-plane component of the polarization makes the polar domains in strained and twisted bilayers topologically non-trivial, forming a network of merons and antimerons (half-skyrmions and half-antiskyrmions). For twisted systems, the merons are of Bloch type whereas for strained systems they are of Néel type. We propose that the polar domains in strained or twisted bilayers may serve as a platform for exploring topological physics in layered materials and discuss how control over topological phases and phase transitions may be achieved in such systems.
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