Twist Angle Research Articles

Engineered moiré photonic and phononic superlattices.

Recent discoveries of Mott insulating and unconventional superconducting states in twisted bilayer graphene with moiré superlattices have not only reshaped the landscape of 'twistronics' but also sparked the rapidly growing fields of moiré photonic and phononic structures. These innovative moiré structures have opened new routes of exploration for classical wave physics, leading to intriguing phenomena and robust control of electromagnetic and mechanical waves. Drawing inspiration from the success of twisted bilayer graphene, this Perspective describes an overarching framework of the emerging moiré photonic and phononic structures that promise novel classical wave devices. We begin with the fundamentals of moiré superlattices, before highlighting recent studies that exploit twist angle and interlayer coupling as new ingredients with which to engineer and tailor the band structures and effective material properties of photonic and phononic structures. Finally, we discuss the future directions and prospects of this emerging area in materials science and wave physics.

In-silico heart model phantom to validate cardiac strain imaging

The quantification of cardiac strains as structural indices of cardiac function has a growing prevalence in clinical diagnosis. However, the highly heterogeneous four-dimensional (4D) cardiac motion challenges accurate “regional” strain quantification and leads to sizable differences in the estimated strains depending on the imaging modality and post-processing algorithm, limiting the translational potential of strains as incremental biomarkers of cardiac dysfunction. There remains a crucial need for a feasible benchmark that successfully replicates complex 4D cardiac kinematics to determine the reliability of strain calculation algorithms. In this study, we propose an in-silico heart phantom derived from finite element (FE) simulations to validate the quantification of 4D regional strains. First, as a proof-of-concept exercise, we created synthetic magnetic resonance (MR) images for a hollow thick-walled cylinder under pure torsion with an exact solution and demonstrated that “ground-truth” values can be recovered for the twist angle, which is also a key kinematic index in the heart. Next, we used mouse-specific FE simulations of cardiac kinematics to synthesize dynamic MR images by sampling various sectional planes of the left ventricle (LV). Strains were calculated using our recently developed non-rigid image registration (NRIR) framework in both problems. Moreover, we studied the effects of image quality on distorting regional strain calculations by conducting in-silico experiments for various LV configurations. Our studies offer a rigorous and feasible tool to standardize regional strain calculations to improve their clinical impact as incremental biomarkers.

Mechanical response of double-stranded DNA: Bend, twist, and overwind.

We employed all-atom molecular dynamics simulations to explore the mechanical response of bending, twisting, and overwinding for double-stranded DNA (dsDNA). We analyzed the bending and twisting deformations, as well as their stiffnesses, using the tilt, roll, and twist modes under stretching force. Findings indicate that the roll and twist angles vary linearly with the stretching force but show opposite trends. The tilt, roll, and twist elastic moduli are considered constants, while the coupling between roll and twist modes slightly decreases under stretching force. The effect of the stretching force on the roll and twist modes, including both their deformations and elasticities, exhibits sequence-dependence, with symmetry around the base pair step. Furthermore, we examined the overwinding path and mechanism of dsDNA from the perspective of the stiffness matrix, based on the tilt, roll, and twist modes. The correlations among tilt, roll, and twist angles imply an alternative overwinding pathway via twist-roll coupling when dsDNA is stretched, wherein entropic contribution prevails.

Twist-angle-tunable spin texture in WSe2/graphene van der Waals heterostructures.

Twist engineering has emerged as a powerful approach for modulating electronic properties in van der Waals heterostructures. While theoretical works have predicted the modulation of spin texture in graphene-based heterostructures by twist angle, experimental studies are lacking. Here, by performing spin precession experiments, we demonstrate tunability of the spin texture and associated spin-charge interconversion with twist angle in WSe2/graphene heterostructures. For specific twist angles, we detect a spin component radial with the electron's momentum, in addition to the standard orthogonal component. Our results show that the helicity of the spin texture can be reversed by twist angle, highlighting the critical role of the twist angle in the spin-orbit properties of WSe2/graphene heterostructures and paving the way for the development of spin-twistronic devices.

The interlayer twist effectively regulates interlayer excitons in InSe/Sb van der Waals heterostructure

The interlayer twist angle endows a new degree of freedom to manipulate the spatially separated interlayer excitons in van der Waals (vdWs) heterostructures. Herein, we find that the band-edge Γ-Γ interlayer excitation directly forms interlayer exciton in InSe/Sb heterostructure, different from that of transition metal dichalcogenides (TMDs) heterostructures in two-step processes by intralayer excitation and transfer. By tuning the interlayer coupling and breathing vibrational modes associated with the Γ-Γ photoexcitation, the interlayer twist can significantly adjust the excitation peak position and lifetime of recombination. The interlayer excitation peak in InSe/Sb heterostructure can shift ~400 meV, and the interlayer exciton lifetime varies in hundreds of nanoseconds as a periodic function of the twist angle (0°–60°). This work enriches the understanding of interlayer exciton formation and facilitates the artificial excitonic engineering of vdWs heterostructures.

Experimental Investigation on Pure Torsion Behavior of Concrete Beams Reinforced with Glass Fiber-Reinforced Polymer Bars

The failure mechanism of torsional concrete beams with fiber-reinforced polymer (FRP) bars is essential for developing the design method. However, limited experimental research has been conducted on the torsion behavior of concrete beams with FRP bars. Therefore, the pure torsion test of four large-scale FRP-RC beams (2800 mm × 400 mm × 200 mm) was conducted to investigate the influence of the stirrup ratio (0, 0.49%, and 0.98%) and longitudinal reinforcement ratio (3.01%, 4.25%) on torsion behavior. The test results indicated that three typical failure patterns, including concrete cracking failure, stirrup rupturing failure, and concrete crushing failure, were observed in specimens without stirrups (stirrup ratio 0), partially over-reinforced specimens (stirrup ratio 0.49%), and over-reinforced specimens (stirrup ratio 0.98%), respectively. The tangent angle of spiral cracks at the midpoint of the long side of the cross-section was approximately 45° initially for all specimens. The torque–twist angle curves exhibited a linear and bilinear behavior for specimens without stirrups and specimens with stirrups, respectively. As the stirrup ratio increased from 0 to 0.98%, torsion capacity increased from 24.9 kN∙m to 27.8 kN∙m, increased by 12%, ultimate twist angle increased from 0.0018 rad/m to 0.0403 rad/m. As the longitudinal reinforcement ratio increased from 3.01% to 4.25%, the torsion capacity increased from 27.8 kN∙m to 28.3 kN∙m, and the ultimate twist angle decreased from 0.0403 rad/m to 0.0244 rad/m. Based on test results, the stirrup strain limit of 5200 με and spiral crack angle of 45° was suggested for torsion capacity calculation. In addition, based on the database of torsion tests, the performance of torsion capacity provisions was assessed.

Parametric Optimization Study of Novel Winglets for Transonic Aircraft Wings

This paper deals with the topic of reducing drag force acting on aircraft wings by incorporating novel winglet designs, such as multi-tip, bird-type, and twisted. The high-speed NASA common research model (CRM) was selected as the baseline model, and winglet designs were retrofitted while keeping the projected wingspan constant. Computational analysis was performed using RANS coupled with the Spalart–Allmaras turbulence model to determine aerodynamic coefficients, such as CL and CD. It was observed that the multi-tip and bird-type designs performed exceptionally well at a low angle of attack (0°). A parametric study was conducted on multi-tip winglets by tweaking the parameters such as sweep angle (Λ), tip twist (Є), taper ratio (λ), and cant angle (Φ). The best combination of parameters for optimal aerodynamic performance while maintaining the wing root bending moment was determined using both the Taguchi method and Taguchi-based grey relational analysis (T-GRA) coupled with principal component analysis (PCA). Also, the percentage contribution of each parameter was determined by using the analysis of variance (ANOVA) method. At the design point, the optimized winglet design outperformed the baseline design by 18.29% in the Taguchi method and by 20.77% in the T-GRA coupled with the PCA method based on aerodynamic efficiency and wing root bending moment.

Exploring the Esterase Catalytic Activity of Minimalist Heptapeptide Amyloid Fibers.

This paper investigates the esterase activity of minimalist amyloid fibers composed of short seven-residue peptides, IHIHIHI (IH7) and IHIHIQI (IH7Q), with a particular focus on the role of the sixth residue position within the peptide sequence. Through computational simulations and analyses, we explore the molecular mechanisms underlying catalysis in these amyloid-based enzymes. Contrary to initial hypotheses, our study reveals that the twist angle of the fiber, and thus the catalytic site's environment, is not notably affected by the sixth residue. Instead, the sixth residue interacts with the p-nitrophenylacetate (pNPA) substrate, particularly through its -NO2 group, potentially enhancing catalysis. Quantum mechanics/molecular mechanics (QM/MM) simulations of the reaction mechanism suggest that the polarizing effect of glutamine enhances catalytic activity by forming a stabilizing network of hydrogen bonds with pNPA, leading to lower energy barriers and a more exergonic reaction. Our findings provide valuable insights into the intricate interplay between peptide sequence, structural arrangement, and catalytic function in amyloid-based enzymes, offering potentially valuable information for the design and optimization of biomimetic catalysts.

Twist Angle-Dependent Phonon Hybridization in WSe2/WSe2 Homobilayer.

The emerging moiré superstructure of twisted transition metal dichalcogenides (TMDs) leads to various correlated electronic and optical properties compared to those of twisted bilayer graphene. In such a versatile architecture, phonons can also be renormalized and evolve due to atomic reconstruction, which, in turn, depends on the twist angle. However, observing this reconstruction and its relationship to phonon behavior with conventional, cost-effective imaging methods remains challenging. Here, we used noninvasive Raman spectroscopy on twisted WSe2/WSe2 (t-WSe2) homobilayers to examine the evolution of phonon modes due to interlayer coupling and atomic reconstruction. Unlike in the natural bilayer (NB), ∼0° as well as ∼60° t-WSe2 samples, the nearly degenerate A1g/E2g mode in the twisted samples (1-7°) split into a doublet in addition to the nondegenerate B2g mode, and the maximum splitting is observed around 2-3°. Our detailed theoretical calculations qualitatively capture the splitting and its dependence as a function of the twist angle and highlight the role of the moiré potential in phonon hybridization. Additionally, we found that around the 2° twist angle, the anharmonic phonon-phonon interaction is higher than the natural bilayer and decreases for larger twist angles. Interestingly, we observed anomalous Raman frequency softening and line-width increase with the decreasing temperature below 50 K, pointing to the combined effect of enhanced electron-phonon coupling and cubic anharmonic interactions in moiré superlattice.

Spatial arrangements and types of dislocations in interfacial networks obtained by Si(001) wafer bonding at low twist angle: A TEM characterization

Two interfacial dislocation networks were obtained in a Si–Si material system by wafer bonding process for low twist angles around 0.2°. For one sample, one of the two initial wafers was rotated with an additional angle of 90° before the bonding. TEM analyses of samples prepared for planar and transverse views show different characteristics in terms of spatial arrangement and dislocation types. Long climbing segments slightly deviated from the [100] axis and short segments a/2<101> are arranged in the dislocation network obtained with an additional rotation and at the lowest twist angle (Ψ = 0.18°) from a <110> axis. Long screw segments a/2<110> and short partial Shockley segments are arranged in the dislocation network obtained without any additional rotation at the highest twist angle (Ψ = 0.27°) from a <110> axis. In both cases, the long and short segments form elongated hexagonal patterns, which are accurately ordered in one set paving the <100> directions or randomly ordered in two perpendicular sets paving the <110> directions, respectively. The results are discussed taking into account the arangement of the surface miscut steps in the initial wafers before the bonding.

Cathodoluminescence from interlayer excitons in a 2D semiconductor heterobilayer.

Photoluminescence has widely been used to study excitons in semiconducting transition metal dichalcogenide (MX$_2$) monolayers, demonstrating strong light matter interactions and locked spin and valley degrees of freedom. In heterobilayers composed of overlapping monolayers of two different MX$_2$, an interlayer exciton can form, with the hole localised in one layer and the electron in the other. These interlayer excitons are long-lived, field-tunable, and can be trapped by moir'e patterns formed at small twist angles between the layers. Here we demonstrate that emission from radiative recombination of interlayer excitons can be observed by cathodoluminescence from a WSe$_2$/MoSe$_2$ heterobilayer encapsulated in hexagonal boron nitride. The higher spatial resolution of cathodoluminescence, compared to photoluminescence, allows detailed analysis of sample heterogeneity at the 100s of nm lengthscales over which twist angles tend to vary in dry-transfer fabricated heterostructures.

Investigation of the wind loads and flow patterns of a high-rise building under twisted wind flows based on LES

The twisted wind effect has attracted increasing attention among scholars due to its significant influence on the wind loads and wind induced responses of high-rise buildings. Nowadays, it is still challenging to simulate twisted wind flows (TWFs) with larger wind twist angles (WTAs) in the wind tunnel test. Compared to wind tunnel test limitations, the large eddy simulation (LES) offers an effective tool to simulate a broader WTA range. Several state-of-the-art numerical studies have generated TWFs that rely on multiple velocity inflow boundary (MVIB), but it results in vast computational domains and substantial abnormal pressure fluctuations. This paper proposes a single velocity inflow boundary (SVIB) method for modeling TWFs, which can significantly reduce the computational domain size without losing accuracy. Based on the SVIB method, we simulate TWFs with four WTAs (i.e., 0°, 25°, 35°, 45°) and conduct a comprehensive analysis of the influence of WTA and wind direction angle on both the wind load characteristics of and the flow patterns around a high-rise building. The results show that the TWFs will twist the vortex topology and trajectory, and the maximum extreme local resultant force coefficient under 35TWF is larger than that under SWF by around 10 %. The positive and negative wind pressures can simultaneously occur on a special surface at large WTAs (e.g., 35°, 45°), which may impose significant shear forces on the intermediate layers. The wind veering effect enhances the contribution of vortex shedding to the longitudinal fluctuating wind loads and the influence of incoming turbulence on lateral fluctuating wind loads. This study contributes to providing a more effective LES method to simulate TWFs with large WTAs and to reveal the effect of large WTA on the wind load characteristics of high-rise buildings under TWFs and the underlying flow mechanism, which is beneficial to the future wind-resistant design of skyscrapers.

Twist-assisted all-antiferromagnetic tunnel junction in the atomic limit

Antiferromagnetic spintronics1,2 shows great potential for high-density and ultrafast information devices. Magnetic tunnel junctions (MTJs), a key spintronic memory component that are typically formed from ferromagnetic materials, have seen rapid developments very recently using antiferromagnetic materials3,4. Here we demonstrate a twisting strategy for constructing all-antiferromagnetic tunnel junctions down to the atomic limit. By twisting two bilayers of CrSBr, a 2D antiferromagnet (AFM), a more than 700% nonvolatile tunnelling magnetoresistance (TMR) ratio is shown at zero field (ZF) with the entire twisted stack acting as the tunnel barrier. This is determined by twisting two CrSBr monolayers for which the TMR is shown to be derived from accumulative coherent tunnelling across the individual CrSBr monolayers. The dependence of the TMR on the twist angle is calculated from the electron-parallel momentum-dependent decay across the twisted monolayers. This is in excellent agreement with our experiments that consider twist angles that vary from 0° to 90°. Moreover, we also find that the temperature dependence of the TMR is, surprisingly, much weaker for the twisted as compared with the untwisted junctions, making the twisted junctions even more attractive for applications. Our work shows that it is possible to push nonvolatile magnetic information storage to the atomically thin limit.

Suppressing torsional buckling in auxetic meta-shells

Take a thin cylindrical shell and twist it; it will buckle immediately. Such unavoidable torsional buckling can lead to systemic failure, for example by disrupting the blood flow through arteries. In this study, we prevent this torsional buckling instability using a combination of auxeticity and orthotropy in cylindrical metamaterial shells with a holey pattern. When the principal axes of the orthotropic meta-shell are relatively aligned with that of the compressive component of the applied stress during twisting, the meta-shell uniformly shrinks in the radial direction as a result of a local buckling instability. This shrinkage coincides with a softening-stiffening transition that leads to ordered stacking of unit cells along the compressive component of the applied stress. These transitions due to local instabilities circumvent the usual torsional instability even under a large twist angle. This study highlights the potential of tailoring anisotropy and programming instabilities in metamaterials, with potential applications in designing mechanical elements for soft robotics, biomechanics or fluidics. As an example of such applications, we demonstrate soft torsional compressor for generating pulsatile flows through a torsion release mechanism.

Development of a novel characterization model for innovative bionic helical carbon fiber tows

The design of bionic helical structures significantly enhances load-bearing capacity and impact resistance, demonstrating great potential for mechanical property improvement. However, there is still a lack of precise characterization models for the helical structures of carbon fibers, which constrains their potential in engineering applications. This paper successfully fabricated helical carbon fiber tows using a carbon fiber twisting technique and introduced a novel intersecting circular cross-section model to thoroughly investigate the influence of the helical structure on the mechanical properties of carbon fiber tows. Tensile test results show that at a twist angle of 5°, the helical structure increases the tensile strength and fracture toughness of carbon fiber tows by 13.6% and 34.3%, respectively. Additionally, the introduction of complex structures such as helices significantly influences the material’s modulus, and precise modulus prediction is crucial for material optimization and practical application. Based on the newly proposed model, a modulus prediction model is developed, demonstrating higher predictive accuracy than traditional models with a prediction error of 2.8%. Finite element simulations further validate the practicality and superiority of the intersecting circular cross-section model. Compared to traditional models, the predictions of the novel model are closer to experimental data, with a prediction error below 2%, emphasizing the significant impact of microstructures on macroscopic properties and offering a new perspective for modeling complex material systems. These findings provide crucial theoretical and methodological support for the design and performance optimization of carbon fiber materials.

Magnetostriction, piezomagnetism and domain nucleation in a Kagome antiferromagnet

Whenever the elastic energy of a solid depends on magnetic field, there is a magnetostrictive response. Field-linear magnetostriction implies piezomagnetism and vice versa. Here, we show that Mn3Sn, a non-collinear antiferromanget with Weyl nodes, hosts a large and almost perfectly linear magnetostriction even at room temperature. The longitudinal and transverse magnetostriction, with opposite signs and similar amplitude are restricted to the kagome planes and the out-of-plane response is negligibly small. By studying four different samples with different Mn:Sn ratios, we find a clear correlation between the linear magnetostriction, the spontaneous magnetization and the concentration of Sn vacancies. The recently reported piezomagnetic data fits in our picture. We show that linear magnetostriction and piezomagnetism are both driven by the field-induced in-plane twist of spins. A quantitative account of the experimental data requires the distortion of the spin texture by Sn vacancies. We find that the field-induced domain nucleation within the hysteresis loop corresponds to a phase transition. Within the hysteresis loop, a concomitant mesoscopic modulation of local strain and spin twist angles, leading to twisto-magnetic stripes, arises as a result of the competition between elastic and magnetic energies.

Hexagonal moiré superlattices, twist-angle disorder and rhombohedral-stacking in a commercially available grafoil-film

We report an advanced investigation on the structural properties of graphitic interfaces within a commercially available film of grafoil, by employing a combination of high-resolution transmission electron microscopy (HRTEM), selective area electron diffraction (SAED) and Raman mapping and point spectroscopy. We identify large-area hexagonal moiré superlattices with dimensions in the order of 2 μm ∗ 2 μm, with a periodicity ranging from D ∼ 12 nm to D ∼ 22 nm, which correspond to a variable twist angle parameter θtwist from 0.65° to 1.15°. These sample regions identified with HRTEM evidence the presence of local structural disorder (twist-angle moiré disorder). By employing SAED we identify the possible presence of a disordered A|ABAB|BCBC|CACA|A stacking sequence, which corresponds to a disordered rhombohedral phase, previously identified in nitrates-intercalated graphitic systems. The structural components and parameters associated to this phase were analysed by employing a combination of techniques, including HRTEM, SAED, Raman point and mapping spectroscopy, X-ray diffraction (XRD) and Rietveld refinements. In particular, XRD, Rietveld refinements and calculated diffractograms evidence the presence of coexisting disordered and crystalline rhombohedral phases in the analysed film.

Investigation on the performance of a novel heat transfer structure based on a new twisted airfoil fin array

Printed circuit heat exchanger (PCHE) is a type of micro-channel heat exchanger with high performance, which has a broad application in microelectronics, solar energy, and aerospace. Airfoil fins are widely recognized due to their exceptional thermo-hydraulic performance, but there are few studies on their shape adjustment in three dimensions. In this study, a novel twisted airfoil fin was proposed and applied to the heat exchanger. The heat transfer and flow resistance of the twisted airfoil fin PCHE with the traditional PCHE are compared using the numerical simulation method, and the influence of the twist angle, fin spacing, and arrangement is investigated. The results show that the new fins have excellent flow and heat transfer performance. Under the multiple Re number conditions, the Nu number is improved up to 52%, and the performance evaluation criteria is increased up to 16%. Also, increasing the twist angle of the fins, decreasing the fin spacing, and staggering the fins according to diverse twist directions can improve the performance of the heat exchanger.

Moiré Superlattices of Two-Dimensional Materials toward Catalysis

In recent years, there has been a surge in twistronics research, uncovering diverse emergent properties in twisted two-dimensional (2D) layered materials. Vertically stacking these materials with slight azimuthal deviation or lattice mismatch creates moiré superlattices, optimizing the structure and energy band and leading to numerous quantum phenomena with applications in electronics, optoelectronics, photonics, and twistronics. Recently, the superior (opto)electronic properties of these moiré superlattices have shown potential in catalysis, providing a platform to manipulate catalytic activity by adjusting twist angles. Despite their potential to revolutionize 2D catalysts, their application in catalysis is limited to simple reactions, and the mechanisms behind their catalytic performance remain unclear. Therefore, a comprehensive perspective on recent studies is needed to understand their catalytic effects for future research.

Two-dimensional electrons at mirror and twistronic twin boundaries in van der Waals ferroelectrics

Semiconducting transition metal dichalcogenides (MX2) occur in 2H and rhombohedral (3R) polytypes, respectively distinguished by anti-parallel and parallel orientation of consecutive monolayer lattices. In its bulk form, 3R-MX2 is ferroelectric, hosting an out-of-plane electric polarisation, the direction of which is dictated by stacking. Here, we predict that twin boundaries, separating adjacent polarisation domains with reversed built-in electric fields, are able to host two-dimensional electrons and holes with an areal density reaching ~ 1013cm−2. Our modelling suggests that n-doped twin boundaries have a more promising binding energy than p-doped ones, whereas hole accumulation is stable at external surfaces of a twinned film. We also propose that assembling pairs of mono-twin films with a ‘magic’ twist angle θ* that provides commensurability between the moiré pattern at the interface and the accumulated carrier density, should promote a regime of strongly correlated states of electrons, such as Wigner crystals, and we specify the values of θ* for homo- and heterostructures of various TMDs.