Aperiodic Structures Research Articles

Effect of reducing the vibration of a frame-spacing arrangement

The methods of arranging frame spacing are explored for reducing the vibration of a cylindrical shell. According to the principle that an aperiodic structures will produce irregular reflections on structural waves, resulting in structural vibration attenuation, two methods of arranging frame spacing are adopted: periodic frame spacing and random frame spacing. Using the two methods, the frames of cylindrical shell models are arranged, and then, using the finite element method (FEM), the vibration response of the cylindrical shells is calculated and compared. It can be found that, for the finite-length cylindrical shells, the aperiodic arrangement of the frame spacing can decrease the vibration level of the cylindrical shells in some specific high-frequency band. For example, for the cylindrical shells with a diameter of 7.6 m and a length of 8.4 m, the frame-spacing arrangement is random, and the vibration level is reduced in about 350-600 Hz, with a maximum reduction of up to 9 dB.

Quasicrystal kirigami

Kirigami, the art of introducing cuts in thin sheets to enable articulation and deployment, has become an inspiration for a novel class of mechanical metamaterials with unusual properties. Here we complement the use of periodic tiling patterns for kirigami designs by showing that quasicrystals can also serve as the basis for designing deployable kirigami structures, and analyze the geometrical, topological and mechanical properties of these aperiodic kirigami structures.

Intermetallic phases and mechanical properties of a Mg–8Zn–6Al–1Sm (wt%) casting alloy

Intermetallic phases including precipitate constituents in both as-cast and peak-aged Mg–8Zn–6Al-1Sm (wt%) alloys were investigated using transmission electron microscopy. In the as-cast sample, grain boundary intermetallic phases are mainly Mg44Al15Zn41 icosahedral (i) phase, Mg5Zn2Al2 (φ) phase, Mg32(Al,Zn)49 (τ) phase, and Al2Sm phase. During aging treatment, a possible phase transformation from τ to i might occur. Additionally, precipitate constituents in α-Mg matrix of the peak-aged sample are mainly i phase, MgZnAl cubic phase, MgZn2 phase, Mg4Zn7 phase, and aperiodic structures. Two or more orientation relationships between i (or Mg4Zn7, or MgZnAl cubic phase) and Mg matrix were revealed, and ordering planar faults were observed for some MgZn2 precipitates. Finally, calculations indicate that the strengthening effect of Mg–Zn–Al precipitates is more efficient than that of Mg–Zn precipitates, and the high strength of the studied alloy sample is mainly attributed to the semi-continuous intermetallic skeletons and the Mg–Zn–Al based precipitates.

Hyperbolic metamaterial-assisted structured illumination microscopy using periodic sub-diffraction speckles

Structured illumination microscopy (SIM), as a wide-field, rapid, super-resolution imaging technology, is widely employed in the field of biology. In this work, we propose a hyperbolic metamaterial (HMM)-assisted super-resolution structured illumination microscopy technique. By utilizing the HMM, a sub-diffraction illumination pattern of bulk plasmon polariton (BPP) with pure and higher spatial frequency replaces the conventional laser interference fringes, thus the imaging resolution of BPPSIM could surpass that of conventional SIM and reach 65 nm for HMM with 8-layers periodic structure. Moreover, an HMM with 10-layers aperiodic structure designed with particle swarm optimization was obtained, and BPPSIM could bring the imaging resolution down to 60 nm (1/9 of the fluorescence wavelength), which is a 3.3-fold improvement compared with the diffraction-limited image. This BPPSIM would provide a super-resolution, wide field of view, and good bio-compatibility approach in biological imaging.

Optical computing of quantum revivals.

Interference is the mechanism through which waves can be structured into the most fascinating patterns. While for sensing, imaging, trapping, or in fundamental investigations, structured waves play nowadays an important role and are becoming the subject of many interesting studies. Using a coherent optical field as a probe, we show how to structure light into distributions presenting collapse and revival structures in its wavefront. These distributions are obtained from the Fourier spectrum of an arrangement of aperiodic diffracting structures. Interestingly, the resulting interference may present quasiperiodic structures of diffraction peaks on a number of distance scales, even though the diffracting structure is not periodic. We establish an analogy with revival phenomena in the evolution of quantum mechanical systems and illustrate this computation numerically and experimentally, obtaining excellent agreement with the proposed theory.

Agnostic Life Finder (ALF) for Large-Scale Screening of Martian Life During In Situ Refueling.

Before the first humans depart for Mars in the next decade, hundreds of tons of martian water-ice must be harvested to produce propellant for the return vehicle, a process known as in situ resource utilization (ISRU). We describe here an instrument, the Agnostic Life Finder (ALF), that is an inexpensive life-detection add-on to ISRU. ALF exploits a well-supported view that informational genetic biopolymers in life in water must have two structural features: (1) Informational biopolymers must carry a repeating charge; they must be polyelectrolytes. (2) Their building blocks must fit into an aperiodic crystal structure; the building blocks must be size-shape regular. ALF exploits the first structural feature to extract polyelectrolytes from ∼10 cubic meters of mined martian water by applying a voltage gradient perpendicularly to the water's flow. This gradient diverts polyelectrolytes from the flow toward their respective electrodes (polyanions to the anode, polycations to the cathode), where they are captured in cartridges before they encounter the electrodes. There, they can later be released to analyze their building blocks, for example, by mass spectrometry or nanopore. Upstream, martian cells holding martian informational polyelectrolytes are disrupted by ultrasound. To manage the (unknown) conductivity of the water due to the presence of salts, the mined water is preconditioned by electrodialysis using porous membranes. ALF uses only resources and technology that must already be available for ISRU. Thus, life detection is easily and inexpensively integrated into SpaceX or NASA ISRU missions.

Selective generation of narrow-band harmonics by a relativistic laser pulse interaction with a detuned plasma grating.

The spectral characteristics of high-order harmonics generated by the interaction of a linearly polarized relativistic laser pulse with a plasma grating target are investigated. Through particle-in-cell simulations and an analytical model, it is shown that a plasma grating target with periodic structure can select special harmonics with integer multiples of the grating frequency, and that low-order harmonics with frequencies being integer times of the laser frequency are generated nearly parallel to the target surface from a Fresnel zone plate target with an aperiodic structure. Spectral control of the harmonics can be achieved by introducing a correction factor β to the radius formula of the Fresnel zone plate, which can create a slightly detuned plasma grating, and then only the narrow-band harmonics can be selected nearly parallel to the target surface. The center order of the narrow-band harmonics can be tuned by adjusting the correction factor β, while the bandwidth of the harmonics can be selected by adjusting the other parameter λ_{f} of the detuned plasma grating.

The Transformation from Surface Wave into Radiation Wave Using Composite Subwavelength Holes Array

Smith–Purcell radiation (SPR) is a phenomenon in that free electrons moving above a periodic structure deliver a part of their kinetic energies to electromagnetic waves. It plays an important role in the development of radiation sources and beam diagnosis. With the development of nanofabrication technology, various complex micro-nano-metallic periodic or aperiodic structures have been adopted to improve the efficiency and coherence of SPR. In general, when free electrons move above the metallic periodic structure, the localized surface wave and Smith–Purcell radiation wave are excited simultaneously. As the frequency of the surface wave is always lower than the threshold of Smith–Purcell radiation, the surface wave cannot be radiated. Here, the surface wave is transformed into SPR by using a novel composite subwavelength hole array, thus the SPR is enhanced and it becomes coherent and easy to tune. This work provides a competitive approach to achieve a coherent and high-efficiency THz radiation source.

Multidimensional color codes for chair tilings

Ordered aperiodic structures have been of interest to the crystallographic community for several decades, and study of them has in turn led to the study of lattice substitution systems, model sets and chair tilings. In this work a color code for chair tilings in arbitrary dimensions is presented. In two and three dimensions, it is expedient to translate the digital codes into colors. An explicit example of a three-dimensional color coding covering one octant is constructed. The tiling is then extended to the whole three-dimensional space and an indication is given of how to do this in arbitrary dimensions. Illustrations of some four-dimensional objects are also shown. The principle of color coding can be applied to other complex tilings such as brick tiling.

Quantifying the diverse wave effects in thermal transport of nanoporous graphene

Controlling thermal transport in nanoporous graphene through wave-like characteristics of phonons beyond the conventional particle regime has attracted widespread interest. Although the existence of wave-like coherent transport in nanoporous graphene has been proposed, the comprehensive impact on phonon transport remains unclear. In this work, we use a rigorous comparison between non-equilibrium molecular dynamics (NEMD) simulation which captures wave effects and mode-resolved phonon Boltzmann transport equation (BTE) which is a particle approach, to quantify the wave effect contribution in the periodic and aperiodic nanoporous graphene. We find that in periodic nanoporous graphene, the wave effect enhances the thermal conductivity compared to solely particle transport. While in aperiodic nanoporous graphene, we observe the coexistence of diverse wave effects that enhance or reduce thermal transport. The competition of these diverse wave effects can lead to an overall enhancement, decrease, or no impact on the thermal conductivity as compared to the particle transport. Our work reveals insights into wave transport in periodic and aperiodic nanoporous structures and provides important guidance for further tuning the thermal conductivity of nanostructures.

Putative rhythms in attentional switching can be explained by aperiodic temporal structure

The neural and perceptual effects of attention were traditionally assumed to be sustained over time, but recent work suggests that covert attention rhythmically switches between objects at 3–8 Hz. Here I use simulations to demonstrate that the analysis approaches commonly used to test for rhythmic oscillations generate false positives in the presence of aperiodic temporal structure. I then propose two alternative analyses that are better able to discriminate between periodic and aperiodic structure in time series. Finally, I apply these alternative analyses to published datasets and find no evidence for behavioural rhythms in attentional switching after accounting for aperiodic temporal structure. The techniques presented here will help clarify the periodic and aperiodic dynamics of perception and of cognition more broadly.

Competition between single ion and single chain magnetism in stoichiometric spin chain oxides [Sr4Mn2CoO9]n [Sr5Mn3CoO12

Stoichiometric spin chain “Mn4+-Co2+” oxides [Sr4Mn2CoO9]n[Sr5Mn3CoO12] have been synthesized for n ​= ​1, 2, 3, 4, 5, ∞. The X-ray powder diffraction study of these oxides also formulated Sr1+x(Mn1-xCox)O3, shows that their hexagonal aperiodic structure (a≈9.6 ​Å, c1 ​~ ​2.6 ​Å, c2 ​~ ​3.9 ​Å; modulation vector q ​= ​γc1∗, with γ ​= ​c1/c2) consists of blocks of multiple (n) Mn2CoO9 trimeric units that alternate with single Mn3CoO12 tetrameric units. Competition between single ion (SIM) and single chain magnetism (SCM) is established from ac-susceptibility magnetic measurements: introduction of tetrameric units in trimeric chains hinders SIM signal which decreases with n and disappears for n ​= ​1. A model is proposed for the origin of SIM based on the presence of [Mn4+2Co2+Mn4+2O18]18−groups in the structure. This scheme is supported by the SIM signal observed for the n ​= ​∞ member Sr2Ca2Mn2Co0.5Zn0.5O9 that contains a majority of isolated [Mn4+2Co2+Mn4+2O18] groups.

The giant enhancement of nonreciprocal radiation in Thue-morse aperiodic structures

Nonreciprocal radiation has potential applications in the fields of aerospace, energy conversion, and communications. In this paper, the strong nonreciprocal radiation effect in Thue-morse aperiodic structure is systematically investigated. It is achieved by placing a Thue-morse aperiodic magnetophotonic crystal atop a metallic mirror backed with a dielectric substrate. The giant enhancement of nonreciprocal radiation is achieved at the wavelength of 16 μm when the applied magnetic field is 3 T and the incident angle is 60°. This giant enhancement of nonreciprocity is attributed to the Tamm plasmon polaritons (TPPs) excited at the interface between the Thue-morse aperiodic magnetophotonic crystal and the metal mirror, and is confirmed by investigating the distribution of the normalized magnetic field amplitude. Besides, the performance of the enhanced nonreciprocal radiation could be expanded to the waveband smaller than 16 μm. The novelty of this work is that a new scheme is proposed to achieve giant enhancement of nonreciprocal radiation with lamellar structure. It is believed such structure can be employed to verify the Kirchhoff’s law with nonreciprocal mediums and will prompt the development of light harvesting devices and thermal emitters.

EEG spectral exponent as a synthetic index for the longitudinal assessment of stroke recovery

ObjectiveQuantitative Electroencephalography (qEEG) can capture changes in brain activity following stroke. qEEG metrics traditionally focus on oscillatory activity, however recent findings highlight the importance of aperiodic (power-law) structure in characterizing pathological brain states. We assessed neurophysiological alterations and recovery after mono-hemispheric stroke by means of the Spectral Exponent (SE), a metric that reflects EEG slowing and quantifies the power-law decay of the EEG Power Spectral Density (PSD). MethodsEighteen patients (n = 18) with mild to moderate mono-hemispheric Middle Cerebral Artery (MCA) ischaemic stroke were retrospectively enrolled for this study. Patients underwent EEG recording in the sub-acute phase (T0) and after 2 months of physical rehabilitation (T1). Sixteen healthy controls (HC; n = 16) matched by age and sex were enrolled as a normative group. SE values and narrow-band PSD were estimated for each recording. We compared SE and band-power between patients and HC, and between the affected (AH) and unaffected hemisphere (UH) at T0 and T1 in patients. ResultsAt T0, stroke patients showed significantly more negative SE values than HC (p = 0.003), reflecting broad-band EEG slowing. Most important, in patients SE over the AH was consistently more negative compared to the UH and showed a renormalization at T1. This SE renormalization significantly correlated with National Institute of Health Stroke Scale (NIHSS) improvement (R = 0.63, p = 0.005). ConclusionsSE is a reliable readout of the neurophysiological and clinical alterations occurring after an ischaemic cortical lesion. SignificanceSE promise to be a robust method to monitor and predict patients’ functional outcome.

Radiation Pattern Synthesis of the Coupled almost Periodic Antenna Arrays Using an Artificial Neural Network Model

This paper proposes radiation pattern synthesis of almost periodic antenna arrays including mutual coupling effects (extracted by Floquet analysis according to our previous work), which in principal has high directivity and a large bandwidth. For modeling the given structures, the moment method combined with the generalized equivalent circuit (MoM-GEC) is proposed. The artificial neural network (ANN), as a powerful computational model, has been successfully applied to antenna array pattern synthesis. Our results showed that multilayer feedforward neural networks are rugged and can successfully and efficiently resolve various distinctive, complex almost periodic antenna patterns (with different source amplitudes) (in particular, both periodic and randomly aperiodic structures are taken into account). An ANN is capable of quickly producing the synthesis results using generalization with the early stopping (ES) method. Significant advantages in speed and memory consumption are achieved by using this method to improve the generalization (called early stopping). To justify this work, several examples are shown and discussed.

Material research from the viewpoint of functional motifs.

As early as 2001, the need for the ‘functional motif theory’ was pointed out, to assist the rational design of functional materials. The properties of materials are determined by their functional motifs and how they are arranged in the materials. Uncovering functional motifs and their arrangements is crucial in understanding the properties of materials and rationally designing new materials of desired properties. The functional motifs of materials are the critical microstructural units (e.g. constituent components and building blocks) that play a decisive role in generating certain material functions, and can not be replaced with other structural units without the loss, or significant suppression, of relevant functions. The role of functional motifs and their arrangement in materials, with representative examples, is presented. The microscopic structures of these examples can be classified into six types on a length scale smaller than ∼10 nm with maximum subatomic resolution, i.e. crystal, magnetic, aperiodic, defect, local and electronic structures. Functional motif analysis can be employed in the function-oriented design of materials, as elucidated by taking infrared non-linear optical materials as an example. Machine learning is more efficient in predicting material properties and screening materials with high efficiency than high-throughput experimentation and high-throughput calculations. In order to extract functional motifs and find their quantitative relationships, the development of sufficiently reliable databases for material structures and properties is imperative.

Interlaced wire medium with quasicrystal lattice

We propose a design of an interlaced wire medium with a quasicrystalline lattice based on fivefold rotation symmetry Penrose tiling. The transport properties of this structure are studied. We distinguish two transport regimes, namely, a propagation regime related to the low-frequency interval and localization regime in the high-frequency interval. While the former is observed in structures both with and without the translation symmetry property, the latter is exclusive for aperiodic structures only. We show that the localization regime is promising for many applications, including the engineering of effective multichannel devices for telecommunication and imaging systems.

Machine learning-assisted low-frequency and broadband sound absorber with coherently coupled weak resonances

An artificial broadband sound absorber composed of multiple components is of significant interest in the physics and engineering communities. The existence of coherently coupled weak resonances (CCWRs) makes it difficult to achieve optimal broadband sound absorption, especially in the presence of complex and aperiodic components. Here, we present and experimentally implement a machine learning-assisted subwavelength sound absorber with CCWRs using an improved Gauss–Bayesian model, which exhibits flexible, high-efficient, and broadband properties at low frequencies (<500 Hz). The proposed aperiodic structure comprises three parallel split-ring units, which enable a quasi-symmetric resonant mode to be generated and effectively dissipate energy because of the huge phase difference between each component at the coupled resonant frequency. With high algorithmic efficiency (no more than 80 iterations), the improved Gauss–Bayesian model inversely determines the optimal CCWRs, realizing a reconfigurable high sound absorption spectrum (α > 0.9) from 229 to 457 Hz. The optimal configuration of sound spectrum characteristics and the unit cell structure can be confirmed flexibly. Good agreement between numerical and experimental results verifies the effectiveness of the proposed method. To further exhibit broadband and multiparameter optimization, a nine-unit sound absorber (27 parameters) is numerically simulated and shown to achieve high acoustic absorption and a relatively broad bandwidth (44.8%). Our work lifts the restrictions on analytic models of complex and aperiodic components with coherent coupling effects, paving the way for combining machine learning with the optimal design of metamaterials.

Формирование естественных композитов из расплавов: расчет и управление параметрами апериодических примесных профилей

The method for purposefully creating aperiodic radial impurity structures and controlling the shape of their spatial profiles in dilute alloys is presented. That is derived based on an analytical approach supported by the results of numerical calculations.

Formation of natural composites from melts, calculations and control of parameters of aperiodic impurity profiles

The method for purposefully creating aperiodic radial impurity structures and controlling the shape of their spatial profiles in dilute alloys is presented. That is derived based on an analytical approach supported by the results of numerical calculations. Keywords: unsteady directional controlled solidification, mobile impurities, aperiodic impurity profiles.