Twisting Degrees Of Freedom Research Articles

Moiré circuits: Engineering magic-angle behavior

Moir\'e superlattices in the twisted bilayer graphene provide an unprecedented platform to investigate a wide range of exotic quantum phenomena. Recently, the twist degree of freedom has been introduced into various classical wave systems, giving rise to new ideas for the wave control. The question is whether twistronics and moir\'e physics can be extended to electronics with potential applications in the twist-enabled signal processing. Here, we demonstrate both in theory and experiment that lots of fascinating moir\'e physics can be engineered using electric circuits with extremely high degrees of freedom. By suitably designing the interlayer coupling and biasing of one sublattice for the twisted bilayer circuit, the low-energy flat bands with large bandgaps away from other states can be realized at various twist angles. Based on the moir\'e circuit with a fixed twist angle, we experimentally demonstrate the effect of band narrowing as well as the localization of electric energy when a magic value of the interlayer coupling is applied. Furthermore, the topological edge states, which originate from the moir\'e potential induced pseudomagnetic field, are also observed for the first time. Our findings suggest a flexible platform to study twistronics beyond natural materials and other classical wave systems, and may have potential applications in the field of intergraded circuit design.

Photonic analog of bilayer graphene

Drawing inspiration from bilayer graphene, this paper introduces its photonic analog comprising two stacked graphene-like photonic crystals, that are coupled in the near-field through spoof surface plasmons. Beyond the twist degree of freedom that can radically alter the band structure of the bilayer photonic graphene, the photonic dispersion can be also tailored via the interlayer coupling which exhibits an exponential dependence on the distance between the two photonic crystals. We theoretically, numerically, and experimentally characterize the band structures of AA- and AB-stacked bilayer photonic graphene, as well as for twisted bilayer photonic graphene with even and odd sublattice exchange symmetries. Furthermore, we numerically predict the existence of magic angles in bilayer photonic graphene, which are associated with ultra-flat bands resulted from interlayer hybridization. Finally, we demonstrate that the bilayer photonic graphene at a particular twist angle satisfying even sublattice exchange symmetry is a high-order photonic topological insulator. The proposed bilayer photonic graphene could constitute a useful platform for identifying new quantum materials and inspiring next-generation photonic devices with new degrees of freedom and emerging functionality.

A novel analytical method of windmill chiral cell structure (WCCS)

This paper proposes a novel analytical method based on a stiffness matrix. The static properties of a Windmill Chiral Cell Structure (WCCS) are evaluated using this method. The theoretical model for the WCCS, which with twist degree of freedom (DOF), is established by hierarchical construction, rigid constraints establishing and eliminating redundant nodes. The influence of different structural parameters on the DOF of WCSS is also investigated. Moreover, the correctness of the theoretical model is verified by the finite element analysis (FEA). The method can be applied to the analysis of linear properties of mechanical metamaterials and provide some reference for the 3D cellular structure design. The proposed WCCS structure can be used to make high precision rotary displacement stages.

Higher-Order Band Topology in Twisted Moiré Superlattice.

The two-dimensional (2D) twisted bilayer materials with van der Waals coupling have ignited great research interests, paving a new way to explore the emergent quantum phenomena by twist degree of freedom. Generally, with the decreasing of twist angle, the enhanced interlayer coupling will gradually flatten the low-energy bands and isolate them by two high-energy gaps at zero and full filling, respectively. Although the correlation and topological physics in the low-energy flat bands have been intensively studied, little information is available for these two emerging gaps. In this Letter, we predict a 2D second-order topological insulator (SOTI) for twisted bilayer graphene and twisted bilayer boron nitride in both zero and full filling gaps. Employing a tight-binding Hamiltonian based on first-principles calculations, three unique fingerprints of 2D SOTI are identified, that is, nonzero bulk topological index, gapped topological edge state, and in-gap topological corner state. Most remarkably, the 2D SOTI exists in a wide range of commensurate twist angles, which is robust to microscopic structure disorder and twist center, greatly facilitating the possible experimental measurement. Our results not only extend the higher-order band topology to massless and massive twisted moiré superlattice, but also demonstrate the importance of high-energy bands for fully understanding the nontrivial electronics.

Electrically Tunable Valley Dynamics in Twisted WSe_{2}/WSe_{2} Bilayers.

The twist degree of freedom provides a powerful new tool for engineering the electrical and optical properties of van der Waals heterostructures. Here, we show that the twist angle can be used to control the spin-valley properties of transition metal dichalcogenide bilayers by changing the momentum alignment of the valleys in the two layers. Specifically, we observe that the interlayer excitons in twisted WSe_{2}/WSe_{2} bilayers exhibit a high (>60%) degree of circular polarization (DOCP) and long valley lifetimes (>40 ns) at zero electric and magnetic fields. The valley lifetime can be tuned by more than 3 orders of magnitude via electrostatic doping, enabling switching of the DOCP from ∼80% in the n-doped regime to <5% in the p-doped regime. These results open up new avenues for tunable chiral light-matter interactions, enabling novel device schemes that exploit the valley degree of freedom.

Monopole-antimonopole: interaction, scattering and creation.

The interaction of a magnetic monopole-antimonopole pair depends on their separation as well as on a second 'twist' degree of freedom. This novel interaction leads to a non-trivial bound state solution known as a sphaleron and to scattering in which the monopole-antimonopoles bounce off each other and do not annihilate. The twist degree of freedom also plays a role in numerical experiments in which gauge waves collide and create monopole-antimonopole pairs. Similar gauge wavepacket scatterings in the Abelian-Higgs model lead to the production of string loops that may be relevant to superconductors. Ongoing numerical experiments to study the production of electroweak sphalerons that result in changes in the Chern-Simons number, and hence baryon number, are also described but have not yet met with success. This article is part of a discussion meeting issue 'Topological avatars of new physics'.

Optimization of flapping wing mechanism of bionic eagle

The present study is focused on optimization of a double-crank-rocker flapping wing mechanism to imitate the motion of eagle wing divided into inner and outer wing with flapping, folding and twist degree of freedom, and structural flexibility. Considering the multibar dimensions of the flapping mechanism as design variables, combinatorial optimization method was applied to maximize the aerodynamic efficiency. Based on the optimization results, a bionic eagle model of 1.5 m wing span was manufactured to perform flight tests. The study results show that the optimized kinematic of motion can be achieved by using the multibar flapping mechanism; the inner wing plays the key role of producing lift while the outer wing producing thrust; as a result of the optimization, the lift was increased by 2.8% from 3.85 N up to 3.96 N and the thrust increased by 12.5% from 0.16 N up to 0.18 N, which was verified by the flight tests. The flight test also demonstrated a sustainable level flight of the bionic eagle model at a flapping frequency of 2 Hz.

α-helix↔random coil phase transition: analysis of ab initio theory predictions

In the present paper we present results of calculations obtained with the use of the theoretical method described in our preceding paper [Eur. Phys. J. D, DOI: 10.1140/epjd/e2007-00328-9] and perform detail analysis of α-helix↔random coil transition in alanine polypeptides of different length. We have calculated the potential energy surfaces of polypeptides with respect to their twisting degrees of freedom and construct a parameter–free partition function of the polypeptide using the suggested method [Eur. Phys. J. D, DOI: 10.1140/epjd/e2007-00328-9]. From the build up partition function we derive various thermodynamical characteristics for alanine polypeptides of different length as a function of temperature. Thus, we analyze the temperature dependence of the heat capacity, latent heat and helicity for alanine polypeptides consisting of 21, 30, 40, 50 and 100 amino acids. Alternatively, we have obtained same thermodynamical characteristics from the use of molecular dynamics simulations and compared them with the results of the new statistical mechanics approach. The comparison proves the validity of the statistical mechanic approach and establishes its accuracy.

Conformational statistics of ribbonlike semiflexible polymer chains

The conformational statistics of ribbonlike polymers possessing bending and twist rigidity are considered on the basis of a lattice model of directed self-correlated walks. It was assumed that local properties of the ribbonlike chain are strongly anisotropic: bending is possible only in the plane of the ribbon (the orientation of this plane can vary due to twist). The generating function for the distribution of a chain segment that is non-Gaussian is constructed. It is shown that in the isotropic environment the twist degree of freedom has no effect on the state of such macromolecules as a whole, and the consideration of rigidity alone in bending results in correct statistical features of a polymer chain. This model is suitable in accounting for the twist degree of freedom in the system with broken rotary symmetry.