Variation Of Twist Angle Research Articles

Modulating interfacial thermal conductance of atomic-scale Si/Si and Al/Al interfaces via adjusting twist angles

While various methods have been proposed to modulate interfacial heat transport in semiconductors and metals, the intrinsic mechanisms by which twist angle affects interfacial thermal conductance (ITC) remain inadequately understood. This paper investigates the impact of twist angle variations on the ITC of atomic-scale Si/Si and Al/Al interfaces across different temperatures. The findings demonstrate that at large twist angles, the overlapping area of ​​​the phonon density of state at the interface diminishes, and the degree of phonon coupling weakens, thereby reducing the ITC. Besides, as the degree of interface mismatch improves, the interlayer distance gradually increases, further decreasing heat transfer efficiency. We also discover that high temperatures can facilitate heat transport across the interface. The reason is that high temperature excites more long-wave phonons to transfer heat, and the degree of interface phonon coupling is enhanced, thereby enhancing the ITC. Further investigation reveals that the reduction in phonon participation rate originates from the decrease in ITC at large twist angle or low temperature.

Transport effects of twist-angle disorder in mesoscopic twisted bilayer graphene

Magic-angle twisted bilayer graphene (TBG) is a tunable material with remarkably flat energy bands near the Fermi level, leading to fascinating transport properties and correlated states at low temperatures. However, grown pristine samples of this material tend to break up into landscapes of twist-angle domains, strongly influencing the physical properties of each individual sample. This poses a significant problem to the interpretation and comparison between measurements obtained from different samples. In this work, we study numerically the effects of twist-angle disorder on quantum electron transport in mesoscopic samples of magic-angle TBG. We find a significant property of twist-angle disorder that distinguishes it from onsite-energy disorder: it leads to an asymmetric broadening of the energy-resolved conductance. The magnitude of the twist-angle variation has a strong effect on conductance, while the number of twist-angle domains is of much lesser significance. We further establish a relationship between the asymmetric broadening and the asymmetric density of states of TBG at angles smaller than the first magic angle. Our results show that the qualitative differences between the types of disorder in the energy-resolved conductance of TBG samples can be used to characterize them at temperatures above the critical temperatures of the correlated phases, enabling systematic experimental studies of the effects of the different types of disorders also on the other properties such as the competition of the different types of correlated states appearing at lower temperatures.

Cryogenic nano-imaging of second-order moiré superlattices.

Second-order superlattices form when moiré superlattices with similar periodicities interfere with each other, leading to larger superlattice periodicities. These crystalline structures are engineered using two-dimensional materials such as graphene and hexagonal boron nitride, and the specific alignment plays a crucial role in facilitating correlation-driven topological phases. Signatures of second-order superlattices have been identified in magnetotransport experiments; however, real-space visualization is still lacking. Here we reveal the second-order superlattice in magic-angle twisted bilayer graphene closely aligned with hexagonal boron nitride through electronic transport measurements and cryogenic nanoscale photovoltage measurements and evidenced by long-range periodic photovoltage modulations. Our results show that even minuscule strain and twist-angle variations as small as 0.01° can lead to drastic changes in the second-order superlattice structure. Our real-space observations, therefore, serve as a 'magnifying glass' for strain and twist angle and can elucidate the mechanisms responsible for the breaking of spatial symmetries in twisted bilayer graphene.

Visualizing the Local Twist Angle Variation within and between Domains of Twisted Bilayer Graphene

Moiré superlattices in twisted two-dimensional materials have emerged as ideal platforms for engineering quantum phenomena, which are highly sensitive to twist angles, including both the global value and the spatial inhomogeneity. However, only a few methods provide spatial-resolved information for characterizing local twist angle distribution. Here we directly visualize the variations of local twist angles and angle-dependent evolutions of the quantum states in twisted bilayer graphene by scanning microwave impedance microscopy (sMIM). Spatially resolved sMIM measurements reveal a pronounced alteration in the local twist angle, approximately 0.3° over several micrometers in some cases. The variation occurs not only when crossing domain boundaries but also occasionally within individual domains. Additionally, the full-filling density of the flat band experiences a change of over 2 × 1011 cm−2 when crossing domain boundaries, aligning consistently with the twist angle inhomogeneity. Moreover, the correlated Chern insulators undergo variations in accordance with the twist angle, gradually weakening and eventually disappearing as the deviation from the magic angle increases. Our findings signify the crucial role of twist angles in shaping the distribution and existence of quantum states, establishing a foundational cornerstone for advancing the study of twisted two-dimensional materials.

Interlayer-coupling-engineerable flat bands in twisted MoSi2N4bilayers.

The two-dimensional layered semiconductor MoSi2N4, which has several advantages including high strength, excellent stability, high hole mobility, and high thermal conductivity, was recently successfully synthesized using chemical vapor deposition. Based on first-principles calculations, we investigate the effects of the twist angle and interlayer distance variation on the electronic properties of twisted bilayer MoSi2N4. The flat bands are absent for twisted bilayer MoSi2N4when the twist angleθis reduced to 3.89°. Taking twisted bilayer MoSi2N4withθof 5.09° as an example, we find that flat bands emerge as the interlayer distance decreases. As the interlayer distance can be effectively modulated by hydrostatic pressure, we propose hydrostatic pressure as a knob for tailoring the flat bands in twisted bilayer MoSi2N4. Our findings provide theoretical support for extending the applications of MoSi2N4in strong correlation physics and superconductivity.

Hydrodynamic design of a horizontal axis current turbine with passive flow control using vortex generator and inserted tube

ABSTRACT Energy from river currents is one of the available renewable energy sources to produce electricity. An improvement in the efficiency of the horizontal axis current turbine is required for the successful utilization of this energy. This paper compares the power production of a horizontal axis current turbine and a turbine with two passive flow control attachments, the vortex generator, and the inserted tube. Firstly, the NACA S1210 hydrofoil shape has been selected. Then the variation of chord length and twist angle along the blade span has been determined. Finally, a 5-meter-diameter, three-bladed, horizontal-axis current turbine has been designed using the blade element momentum theory.The rotor has a maximum power coefficient and a maximum power of 0.47 and 42.71 kW, respectively, at the current speed of 2.1 m/s and a tip speed ratio of 6 when the hydrofoil with tubes has been chosen to design the horizontal axis current turbine. It has been proven that including vortex generators and tubes on the turbine can help increase power production by 9.8% and 14.7%, respectively.

NACA2412 airfoil based method for design and aerodynamic analysis of small HAWT using modified BEM approach

Efficient utilization of wind energy depends on careful selection and suitable design of Wind Turbine (WT) blades. Small scale Wind Turbines (SWT) normally operate in the low range of Angle of Attack (AoA). This makes the task even more difficult for designing and optimization of WT blades. This article deals with an airfoil based computational approach to design the blade for a standalone small scale Horizontal Axis Wind Turbine (HAWT). A procedure has been proposed to find the important parameters and analyze the performance characteristics for a three bladed HAWT operating under the wake rotation. Computational code has been written to find optimum blade profile. Twist angle variation, chord length and other related parameters are determined with the help of program. Comparison between different types of airfoil has been made to figure out the most suitable one. Airfoil selection and design approach are intended to make Wind Turbine blade efficient specially under low range of AoA. Characteristics of the Wind Turbine obtained analytically from this procedure are compared with several other reported earlier in some of the literatures. The result obtained by the proposed procedure is simpler and more efficient than BEM theory – a method normally employed for blade design.

Raman imaging of twist angle variations in twisted bilayer graphene at intermediate angles

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 using a green excitation laser. The twist angles can directly be extracted from the moiré superlattice-activated Raman scattering process of the transverse acoustic (TA) phonon mode. Furthermore, we show that the width of the TA Raman peak contains valuable information on spatial twist angle variations on length scales below the laser spot size of ∼500 nm.

Moiré disorder effect in twisted bilayer graphene

We theoretically study the electronic structure of magic-angle twisted bilayer graphene with disordered moir\'e patterns. By using an extended continuum model incorporating non-uniform lattice distortion, we find that the local density of states of the flat band is hardly broadened, but splits into upper and lower subbands in most places. The spatial dependence of the splitting energy is almost exclusively determined by the local value of the effective vector potential induced by heterostrain, whereas the variation of local twist angle and local moir\'e period give relatively minor effects on the electronic structure. We explain the exclusive dependence on the local vector potential by a pseudo Landau level picture for the magic-angle flat band, and we obtain an analytic expression of the splitting energy as a function of the strain amplitude.

Flat band carrier confinement in magic-angle twisted bilayer graphene

Magic-angle twisted bilayer graphene has emerged as a powerful platform for studying strongly correlated electron physics, owing to its almost dispersionless low-energy bands and the ability to tune the band filling by electrostatic gating. Techniques to control the twist angle between graphene layers have led to rapid experimental progress but improving sample quality is essential for separating the delicate correlated electron physics from disorder effects. Owing to the 2D nature of the system and the relatively low carrier density, the samples are highly susceptible to small doping inhomogeneity which can drastically modify the local potential landscape. This potential disorder is distinct from the twist angle variation which has been studied elsewhere. Here, by using low temperature scanning tunneling spectroscopy and planar tunneling junction measurements, we demonstrate that flat bands in twisted bilayer graphene can amplify small doping inhomogeneity that surprisingly leads to carrier confinement, which in graphene could previously only be realized in the presence of a strong magnetic field.

Modulation of trithiophene-based chalcone positional isomers by twist angle variation: Ultrafast nonlinear optical properties and excited-state dynamics

The nonlinearities involves the interaction between multiple sources, to minimize variable factors, four trithiophene chalcone isomeric molecules were designed and synthesized to investigate the modulation of torsion angle changes (6°, 25°, 37°, 60°) on optical nonlinearities. The electronic excitation properties and relevant parameters were calculated through quantum chemical methods. Femtosecond Z-scan measurements showed that the four molecules exhibited configuration-dependent reverse saturation absorption (RSA) at 600 and 700 nm and had ≥1 figure of merit (F) at 700 nm. Combined with the orbital delocalization index, the results indicated that the spatial delocalization of electrons is gradually suppressed with system twisting. Transient absorption spectra revealed a transition from the locally excited (LE) state to the charge transfer (CT) state, and the planarized structure promotes the relaxation of excited particles. This study suggest that these novel trithiophene derivatives possess potential in nonlinear optical applications and provide an approach for the optimization of nonlinear functional isomeric materials.

Graphene bilayers with a twist.

Near a magic twist angle, bilayer graphene transforms from a weakly correlated Fermi liquid to a strongly correlated two-dimensional electron system with properties that are extraordinarily sensitive to carrier density and to controllable environmental factors such as the proximity of nearby gates and twist-angle variation. Among other phenomena, magic-angle twisted bilayer graphene hosts superconductivity, interaction-induced insulating states, magnetism, electronic nematicity, linear-in-temperature low-temperature resistivity and quantized anomalous Hall states. We highlight some key research results in this field, point to important questions that remain open and comment on the place of magic-angle twisted bilayer graphene in the strongly correlated quantum matter world.

Transport across twist angle domains in moiré graphene

Many of the experiments in twisted bilayer graphene (TBG) differ from each other in terms of the details of their phase diagrams. Few controllable aspects aside, this discrepancy is largely believed to be arising from the presence of a varying degree of twist angle inhomogeneity across different samples. Real space maps indeed reveal TBG devices splitting into several large domains of different twist angles. Motivated by these observations, we study the quantum mechanical tunneling across a domain wall (DW) that separates two such regions. We show that the tunneling of the moir\'e particles can be understood by the formation of an effective step potential at the DW. The height of this step potential is simply a measure of the difference in twist angles. These computations lead us to identify the global transport signatures for detecting and quantifying the local twist angle variations. In particular, Using Landauer-B\"uttiker formalism we compute single-channel conductance ($dI/dV$) and Fano factor for shot noise (ratio of noise power and mean current). A zero-bias, sub-meV transport gap is observed in the conductance which scales with the height of the step potential. One of the key findings of our work is that transport in presence of twist angle inhomogeneity is "noisy", though sub-Poissonian. In particular, the differential Fano factor peaks near the van Hove energies corresponding to the domains in the sample. The location and the strength of the peak is simply a measure of the degree of twist angle inhomogeneity.

Aerodynamic investigation of twist angle variation based on wing smarting for a flying wing

In this paper, the effects of twist angle variation on aerodynamic coefficients and flow field on the wing with wing smarting approach are studied using numerical simulation. The simulation was performed using incompressible Reynolds-Averaged Navier-Stokes (RANS) equations based on the two-equation k-ω Shear Stress Transport (SST) turbulent model for flow speed 30 m/s and a Reynolds number of 69000. Investigations have been carried out for several twist angles and at a specific range of angles of attack. The twist applied is the type of geometric twist (wash-out), which is linearly distributed along the span. The test case is a lambda-shaped tailless aircraft with a wing fracture on the trailing edge, and a sweep angle 56°. The results show that with increasing twist angle, the aerodynamic efficiency improves over a wide range of angles of attack, but at 0° angle of attack it will decrease significantly. By increasing the angle of attack, the effect of twist on the flow field and aerodynamic coefficients will gradually decrease; hence, at a certain amount of angle of attack, the effect of twist will stop, that angle is called the neutral brink angle. Longitudinal stability analysis shows that by growing the twist angle, the conditions required for longitudinal stability are satisfied, and the pitch-up phenomenon will be delayed.

Exact solution of the Heisenberg XX model with twisted boundary conditions

We develop a method to solve the Heisenberg XX model in a transverse field with twisted boundary conditions. We investigate the twist effects on the ground state for the different external magnetic fields. The results indicate that the boundary energy is symmetric under the reflection of θ=π and obeys a scaling behavior. The properties of the ground state strongly depend on the coupling between the particle number operator and the twist angle. The twist effects can be controlled by the external magnetic field only when the system is in the magnetic order phase. We also calculate the specific heat and the nearest neighbor entanglement of the boundary sites, and find the behaviors of the boundary sites are sensitive to the twist angle. The boundary sites exhibit different behaviors compared with the normal sites with the variation of twist angle, which can be tuned by the external magnetic field.

Crystal structure of calcium vanadate-phosphate fluoride

Apatite-type materials AI4AII6(BO4)6X2 have two unique cations sites AI and AII, which can host large mono-, di- tri- and tetra-valent cations. The average cation radii will affect the twist angle and lattice constants. However, there are few reports on the influence of B site substitutions on the twist angle and lattice parameters. It is believed that the lattice constant variation as a function of B site substitutions may not follow the same twist-angle model as proposed for A site. This work reports our results on the crystal chemistry of synthetic apatite Ca10(VxP1−xO4)6F2 obtained through the crystal structure characterization using Rietveld refinement and high-resolution transmission electron microscopy. The quantification of vanadium/phosphorus partitioning in BO4 tetrahedra showed that equilibrium with more than 70% substitution of phosphorous by vanadium was difficult to achieve unless longer annealing (about 1 week at 900 °C) was employed. In comparison with the apatites with different ionic radii at AI and AII sites, Ca10(VxP1−xO4)6F2 apatites with different ionic radii at B site show little twist angle variation for the whole series, which indicates that the dilation of unit cell constants is mainly because of the expansions of BO4 tetrahedra when A site cation is fixed.

Numerical study of the fluid-structure interaction of composite hydrofoil with different plying angles in steady flow

The object of this paper is to numerically investigate the fluid-structure interaction of composite hydrofoil. The aim is to validate 3D numerical simulation model with fluid-structure interaction of composite materials comparing to experimental data, and study the fluid-structure interaction of composite hydrofoil. Numerical results are presented for a composite hydrofoil. The hydrofoil has unswept trapezoidal planform of aspect ratio 3.33. The numerical model has been validated and showed reasonable agreement with the experiment measurements. Then the forces and deformations of composite hydrofoil with different plying angles have been investigated at Re=1x106 and initial angle of attack α=6°. The results show that lift and drag coefficients and tip twist angle decrease and get minimum at about plying angle θ=35°. According to Classical Laminate Theory, the intrinsic bending-twisting coupling of composite structures leads to the variation of twist angle for different plying angles. The relation expression on elements of matrices D has been derived to estimate twist angle at tip qualitatively.

Coordinated predictive control for wind farm with BESS considering power dispatching and equipment ageing

This study presents a supervisory model predictive control (MPC) scheme for coordinated active power control of a wind farm with a centralised battery energy storage system (BESS). The control target is wind turbine generator (WTG) and BESS coordination to respond to transmission system operator power dispatching, along with equipment ageing deceleration. To alleviate the WTG mechanical fatigue and extend the BESS expected lifetime, the twist angle variation and an index introduced by a weighted ampere-hour (Ah) throughput model are considered in the cost function. The damage equivalent load and weighted Ah throughput are used for the equipment ageing evaluation. Simulations and comparisons are conducted to demonstrate the effectiveness and applicability of the proposed MPC-based supervisory controller design.

Conformational Control of Ultrafast Molecular Rotor Property: Tuning Viscosity Sensing Efficiency by Twist Angle Variation.

Fluorescent molecular rotors find widespread application in sensing and imaging of microscopic viscosity in complex chemical and biological media. Development of viscosity-sensitive ultrafast molecular rotor (UMR) relies upon the understanding of the excited-state dynamics and their implications for viscosity-dependent fluorescence signaling. Unraveling the structure-property relationship of UMR behavior is of significance toward development of an ultrasensitive fluorescence microviscosity sensor. Herein we show that the ground-state equilibrium conformation has an important role in the ultrafast twisting dynamics of UMRs and consequent viscosity sensing efficiency. Synthesis, photophysics, and ultrafast spectroscopic experiments in conjunction with quantum chemical calculation of a series of UMRs based on dimethylaniline donor and benzimidazolium acceptor with predefined ground-state torsion angle led us to unravel that the ultrafast torsional dynamics around the bond connecting donor and acceptor groups profoundly influences the molecular rotor efficiency. This is the first experimental demonstration of conformational control of small-molecule-based UMR efficiencies which can have wider implication toward development of fluorescence sensors based on the UMR principle. Conformation-controlled UMR efficiency has been shown to exhibit commensurate fluorescence enhancement upon DNA binding.

Modelling and Analysis of Aeroelastic Tailoring Blade Wind Turbine Systems

Modelling and performance analysis of wind turbine control systems with aeroelastic tailoring blades (ATBs) are investigated. An industrial scale horizontal axis wind turbine (HAWT) model with rigid blades is firstly developed as the baseline model using the blade element momentum (BEM) theory. Designed twist angle variation distributions along blades are then introduced to the baseline model to characterise the ATB nature. The developed ATB wind turbine models are analysed by employing a baseline control system. The performances of the ATB wind turbine systems are compared with that of the baseline turbine using both nonlinear models and linearised models at selected wind speeds. The impacts of ATB design can be clearly observed from the simulation studies. Preliminary results suggest that with ATB design in wind turbines, the blade fatigue loads and the pitching activities can be reduced for large turbines without compromising the energy capture performance.