2-fold Symmetry Research Articles

Quasicrystal phase evolution and mechanical properties of Mg-3.5Zn-0.6Gd alloy processed by high-pressure torsion

To explore the effect of high-pressure torsion (HPT) on the icosahedral quasicrystal phase (I-phase), the microstructure evolution and mechanical property of extruded Mg-3.5Zn-0.6Gd alloy after HPT were investigated in detail. The results showed that a large density of twins and dislocation were generated during HPT, which effectively promoted the microstructure refinement. The block-shaped I-phase was broken into chain structure and gradually evolved into granular I-phase with the increasing strain. The block and granular I-phase through HPT exhibited a more evenly distribution. After HPT for 7 turns, the average grain size was refined to round about 89 nm and microhardness reached to a maximum value about 119 HV. The 2-fold symmetry of icosahedral quasicrystal phase (I-phase) was unveiled by TEM. According to the HRTEM and IFFT, the [11 2‾ 0]Mg//[2-fold]I-phase and (0002)Mg//(2-fold)I-phase can be identified in the Mg-3.5Zn-0.6Gd alloy. The dislocation and defect with an angle nearly 135° were also observed after HPT for 9 turns. With the further increase of strain, the microhardness appeared to decrease due to the dissolution of granular I-phase. The modified strength mechanism was constructed to distinguish the contribution of grain boundary, dislocation and precipitates.

Investigation of microstructural development of liquid Nb in dependence of cooling rate: Molecular dynamics simulation study

In this study, the microstructural developments in the Niobium (Nb) bulk system cooled with different cooling rates from the liquid phase were investigated by the Molecular Dynamics (MD) method. The Embedded Atom Method (EAM), which includes many-body interactions, was used to calculate the interaction forces between atoms. The microstructural changes in the model system were tried to be determined by radial distribution function (RDF), voronoi polyhedra analysis (VP), determination of folded symmetries, spherical periodic order (SPO) and atomic images obtained from common neighbor analysis (CNA). Glassy structure transformations were observed for 1 × 1014 K/s, 5 × 1013 K/s, 1 × 1013 K/s cooling rates applied to the liquid Nb system, and crystal structure transformations were observed under 0 GPa pressure for 5 × 1012 K/s and 2 × 1012 K/s cooling rates. In addition, the glassy and crystalline transition temperatures at which these transformations take place were calculated. While an increase was observed in 5-fold symmetries expressing icosahedral short distance order in glassy structures obtained as a result of cooling processes, a decrease in 5-fold symmetries and an increase in 4 and 6-fold symmetries were determined in crystal structure transformations.

Mapping of foot-and-mouth disease virus antigenic sites recognized by single-domain antibodies reveals different 146S particle specific sites and particle flexibility.

Vaccination with intact (146S) foot-and-mouth disease virus (FMDV) particles is used to control FMD. However, 146S particles easily dissociate into stable pentameric 12S particles which are less immunogenic. We earlier isolated several single-domain antibody fragments (VHHs) that specifically bind either 146S or 12S particles. These particle-specific VHHs are excellent tools for vaccine quality control. In this study we mapped the antigenic sites recognized by these VHHs by competition ELISAs, virus neutralization, and trypsin sensitivity of epitopes. We included two previously described monoclonal antibodies (mAbs) that are either 12S specific (mAb 13A6) or 146S specific (mAb 9). Although both are 12S specific, the VHH M3F and mAb 13A6 were found to bind independent antigenic sites. M3F recognized a non-neutralizing and trypsin insensitive site whereas mAb 13A6 recognized the trypsin sensitive VP2 N-terminus. The Asia1 146S-specific site was trypsin sensitive, neutralizing and also recognized by the VHH M8F, suggesting it involves the VP1 GH-loop. The type A 146S-specific VHHs recognized two independent antigenic sites that are both also neutralizing but trypsin insensitive. The major site was further mapped by cross-linking mass spectrometry (XL-MS) of two broadly strain reactive 146S-specific VHHs complexed to FMDV. The epitopes were located close to the 2-fold and 3-fold symmetry axes of the icosahedral virus 3D structure, mainly on VP2 and VP3, overlapping the earlier identified mAb 9 site. Since the epitopes were located on a single 12S pentamer, the 146S specificity cannot be explained by the epitope being split due to 12S pentamer dissociation. In an earlier study the cryo-EM structure of the 146S-specific VHH M170 complexed to type O FMDV was resolved. The 146S specificity was reported to be caused by an altered conformation of this epitope in 12S and 146S particles. This mechanism probably also explains the 146S-specific binding by the two type A VHHs mapped by XL-MS since their epitopes overlapped with the epitope recognized by M170. Surprisingly, residues internal in the 146S quaternary structure were also cross-linked to VHH. This probably reflects particle flexibility in solution. Molecular studies of virus-antibody interactions help to further optimize vaccines and improve their quality control.

In-plane optical anisotropy of quasi-one-dimensional layered semiconductor Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystal

Transition-metal phosphorous chalcogenide <i>M</i>PS (<i>M</i> = transition metal), an emerging type of two-dimensional (2D) van der Waals material with the unique optical and opto-electronic properties, has received much attention. The quasi-one-dimensional chain structure of Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> will possess the strong anisotropic optical and photoelectric properties. Therefore, the single crystal and low-dimensional materials of Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> have potential applications in new polarization controllers, polarization-sensitive photoelectronic detectors, etc. However, there is still a lack of research on the anisotropic optical properties of the high-quality Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystals. Herein, the millimeter-sized Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystals are successfully prepared by the chemical vapor transport method. The chemical composition, the crystal structure and the anisotropic optical properties of the Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystals are carefully analyzed. The energy dispersive X-ray spectroscopy results show that the element distribution is uniform and the element ratio is close to the stoichiometric ratio. The X-ray diffraction and the transmission electron microscopy results show a good crystallinity. The absorption spectra shows that the optical band gap of the Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystal is 1.8 eV. Interestingly, the Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystal can be mechanically exfoliated to obtain few-layer material. The thickness-dependent Raman spectra show that the Raman vibration peaks of bulk and few-layer Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> each have only a weak shift, indicating a weak interlayer interaction in the Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystal. In order to make an in-depth study of the optical properties of Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystals, the polarized-dependent Raman spectra and the femtosecond transient absorption (TA) spectra by using pump pulses and probe pulses with a wavelength of 400 nm and a wavelength range of 500–700 nm are recorded. Importantly, the polarized-dependent Raman scattering spectra with the angle-dependent measurements reveal that the intensity of Raman peak at 202 cm<sup>–1</sup> and at 489 cm<sup>–1</sup> show a 2-fold symmetry and a 4-fold symmetry in the parallel and vertical polarization configurations, respectively. Moreover, the results of ultrafast carrier dynamics with the in-plane rotation angles of Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystals in the parallel polarization configurations, clearly indicate that both the hot carrier number and the relaxation rate after photoexcitation have the in-plane anisotropic properties. These results are useful in understanding the in-plane anisotropic optical properties of Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystal, which can further promote their applications in the low-dimensional angle-dependent optoelectronics.

Flexing infinite frameworks with applications to braced Penrose tilings

A planar framework -- a graph together with a map of its vertices to the plane -- is flexible if it allows a continuous deformation preserving the distances between adjacent vertices. Extending a recent previous result, we prove that a connected graph with a countable vertex set can be realized as a flexible framework if and only if it has a so-called NAC-coloring. The tools developed to prove this result are then applied to frameworks where every 4-cycle is a parallelogram, and countably infinite graphs with $n$-fold rotational symmetry. With this, we determine a simple combinatorial characterization that determines whether the 1-skeleton of a Penrose rhombus tiling with a given set of braced rhombi will have a flexible motion, and also whether the motion will preserve 5-fold rotational symmetry.

Spontaneous flows and dynamics of full-integer topological defects in polar active matter.

Polar active matter of self-propelled particles sustain spontaneous flows through the full-integer topological defects. We study theoretically the incompressible flow profiles around ±1 defects induced by polar and dipolar active forces. We show that dipolar forces induce vortical flows around the +1 defect, while the flow around the -1 defect has an 8-fold rotational symmetry. The vortical flow changes its chirality near the +1 defect core in the absence of the friction with a substrate. We show analytically that the flow induced by polar active forces is vortical near the +1 defect and is 4-fold symmetric near the -1 defect, while it becomes uniform in the far-field. For a pair of oppositely charged defects, this polar flow contributes to a mutual interaction force that depends only on the orientation of the defect pair relative to the background polarization, and that enhances defect pair annihilation. This is in contradiction with the effect of dipolar active forces which decay inversely proportional with the defect separation distance. As such, our analyses reveals a long-ranged mechanism for the pairwise interaction between topological defects in polar active matter.

Partial hypoglossal-facial neurorrhaphy: a surgical technique.

This is a video demonstration of hemihypoglossal-facial neurorrhaphy. Any irreversible facial palsy more than House-Brackmann grade II is aesthetically problematic. Also, the functional and psychological impact associated with a higher grade of facial palsy is even worse. In this video, the procedure is demonstrated in a female patient with irreversible grade VI facial palsy and corneal opacity due to exposure keratitis 1 year following surgical excision of cerebellopontine angle (CPA) schwannoma. Postoperative outcomes are shown in another patient who underwent a similar procedure for establishing outcomes. The nasolabial fold and eye closure symmetry can be achieved with minimal or no hypoglossal morbidity. The video can be found here: https://stream.cadmore.media/r10.3171/2022.10.FOCVID2290.

Structural Origin of Sinjar Anticline, NW Iraq

The Sinjar anticline is a double plunging, trending almost E-W in the northwestern part of Iraq. It extends in Syria for about 42 km, whereas in Iraq, its length is about 91 km, and the width is about 31 km. The northern limb (45° - 80°) is steeper than the southern limb (15° - 25°), with average plunges dip of 35° and axial plane dipping of 47.5° southwards. The exposed rocks in the anticline range in age from Upper Cretaceous, represented by the Shiranish Formation, to Upper Miocene, represented by the Injana Formation. Google Earth image was used to calculate structural data, which were used to indicate the structural origin of the Sinjar anticline. This was achieved by calculating the Aspect Ratio (AR), Fold Symmetry Index (IFS or IFS), and length of the mountain front (FS). Accordingly, it was found that the structural origin of the Sinjar anticline is a fault-bend fold.

Electrical discharge triggers quasicrystal formation in an eolian dune

We report the discovery of a dodecagonal quasicrystal Mn72.3Si15.6Cr9.7Al1.8Ni0.6-composed of a periodic stacking of atomic planes with quasiperiodic translational order and 12-fold symmetry along the two directions perpendicular to the planes-accidentally formed by an electrical discharge event in an eolian dune in the Sand Hills near Hyannis, Nebraska, United States. The quasicrystal, coexisting with a cubic crystalline phase with composition Mn68.9Si19.9Ni7.6Cr2.2Al1.4, was found in a fulgurite consisting predominantly of fused and melted sand along with traces of melted conductor metal from a nearby downed power line. The fulgurite may have been created by a lightning strike that combined sand with material from downed power line or from electrical discharges from the downed power line alone. Extreme temperatures of at least 1,710 °C were reached, as indicated by the presence of SiO2 glass in the sample. The dodecagonal quasicrystal is an example of a quasicrystal of any kind formed by electrical discharge, suggesting other places to search for quasicrystals on Earth or in space and for synthesizing them in the laboratory.

A Twisted Chiral Cavitand with 5-Fold Symmetry and Its Length-Selective Binding Properties.

Controlling bottom-up syntheses from chiral seeds to construct architectures with specific chiralities is currently challenging. Herein, a twisted chiral cavitand with 5-fold symmetry was constructed by bottom-up synthesis using corannulene as the chiral seed and pillar[5]arene as the chiral wall. After docking between the seed and the wall, their dynamic chiralities (M and P) are fixed. Moreover, the formed hedges also exhibit M and P chirality. Through dynamic covalent bonding, the thermodynamically stable product is obtained selectively as a pair of enantiomers (MMM and PPP), where all three subcomponents, i.e., the corannulene, hedges, and pillar[5]arene, are tilted in the same direction. Furthermore, the twisted cavitand exhibits length-selective binding to alkylene dibromides, with three maximum binding constants being unexpectedly observed.

Evidence that the liquid structure affects the nucleation of the primary metastable L12-Al3Zr in additive manufacturing

The strategy aiming at promoting the precipitation of the primary metastable L12-Al3X phase is the most widely used to refine grain size in Al alloys designed for additive manufacturing. It is generally admitted that Al grains nucleate onto the primary L12-Al3X precipitates by epitaxy resulting in a cube-cube orientation relationship. Here we report TEM observations showing clusters of precipitates of this primary L12-Al3X phase. Using Automated Crystallographic Orientation Mapping in the TEM, we show that such clusters consist of various orientations. Analysis of those clusters reveals that some orientations share common 5-fold rotational symmetry axes suggesting that they have nucleated from an icosahedral template. We conclude that Icosahedral Short Range Order (ISRO) in the liquid can play a role in the nucleation of the primary metastable L12-Al3Zr phase through the ISRO-mediated nucleation mechanism.

Discrete chiral organic nanotubes by stacking pillar[5]arenes using covalent linkages

Owing to their unique one-dimensional hollow structures, organic nanotubes have been widely explored in recent years. Covalent organic nanotubes (CONs) can be prepared by stacking building blocks, such as macrocycles, through covalent linkages. However, because of the mismatched covalent connections, controllable synthesis of the discrete CONs with clear structures, such as sidewall and chirality, is a challenging target. In this work, by coupling two pillar[5]arenes through dynamic covalent bonds, thermodynamically stable discrete CONs with 5-fold symmetry are successfully prepared. Three different chiral CONs are separated, including homo-CONs, consisting of two enantiomers (pR, pR and pS, pS), and hetero-CON, consisting of the meso form (pR, pS). These CONs show negative allosteric binding affinities toward guest molecules, which are not observed in individual pillar[5]arenes.

Friends in need: How chaperonins recognize and remodel proteins that require folding assistance.

Chaperonins are biological nanomachines that help newly translated proteins to fold by rescuing them from kinetically trapped misfolded states. Protein folding assistance by the chaperonin machinery is obligatory in vivo for a subset of proteins in the bacterial proteome. Chaperonins are large oligomeric complexes, with unusual seven fold symmetry (group I) or eight/nine fold symmetry (group II), that form double-ring constructs, enclosing a central cavity that serves as the folding chamber. Dramatic large-scale conformational changes, that take place during ATP-driven cycles, allow chaperonins to bind misfolded proteins, encapsulate them into the expanded cavity and release them back into the cellular environment, regardless of whether they are folded or not. The theory associated with the iterative annealing mechanism, which incorporated the conformational free energy landscape description of protein folding, quantitatively explains most, if not all, the available data. Misfolded conformations are associated with low energy minima in a rugged energy landscape. Random disruptions of these low energy conformations result in higher free energy, less folded, conformations that can stochastically partition into the native state. Two distinct mechanisms of annealing action have been described. Group I chaperonins (GroEL homologues in eubacteria and endosymbiotic organelles), recognize a large number of misfolded proteins non-specifically and operate through highly coordinated cooperative motions. By contrast, the less well understood group II chaperonins (CCT in Eukarya and thermosome/TF55 in Archaea), assist a selected set of substrate proteins. Sequential conformational changes within a CCT ring are observed, perhaps promoting domain-by-domain substrate folding. Chaperonins are implicated in bacterial infection, autoimmune disease, as well as protein aggregation and degradation diseases. Understanding the chaperonin mechanism and the specific proteins they rescue during the cell cycle is important not only for the fundamental aspect of protein folding in the cellular environment, but also for effective therapeutic strategies.

On the atomic structure of the [formula omitted] precipitate by density functional theory

The substitution of elements into the β′′ precipitate in the 6xxx-series of aluminium alloys is an interface sensitive problem. The strain caused by the misfit between the precipitate and the matrix interacts with the size misfit of solute substitutions. The full cross-section of the β′′ precipitate and interface was simultaneously studied without the influence of periodic images by applying rigid boundary conditions in a cluster-based model that contained two regions, one fixed and one relaxed. An optimised geometry of the fixed region allows partially occupied atomic columns which enable a more precise description of the various precipitate configurations. A subtle shift in the atomic columns occurred during relaxation from the initial atomic positions in the β′′ precipitate, which breaks the notion of a 4-fold symmetry of the β′′ eyes. The underlying C2/m symmetry was intact through this shift. The methodology was applied to calculate the relative formation enthalpy of substituting lithium and copper at different atomic sites and benchmarked against previously published density functional theory and transmission electron microscopy studies. The results correspond well with expectations based on the experimental studies available.

Texture Evolution of a Single Crystal Cu-8% at. Al Subjected to the Drawing Process

Single crystals of Cu-8.% at. Al were subjected to cold drawing up to 1.1 true strain. Development of the crystallographic texture was analysed at various stages of the drawing process. Investigations revealed significant crystal rotation and reorientation due to a twinning process, while general 4-fold symmetry around the drawing direction was preserved. Local texture analysis performed on cross-sections perpendicular to the drawing direction showed that due to crystal rotation, the original crystal was initially fragmented into four quadrants. However, within the volume near the central axis the initial cubic orientation was preserved. Subsequent deformation within dominating twinning systems led to further fragmentation which divided the crystal into sectors one eighth the size of the full circular cross-section. The obtained results also indicate that approximation of the drawing process stress state by the tensor having only normal components cannot be used during analysis of the activation of deformation systems.

Additive manufacturing of a Co-Cr-W alloy by selective laser melting: In-situ oxidation, precipitation and the corresponding strengthening effects

• The in-situ oxidization and precipitation behaviors in laser powder bed fusion (LPBF) processed Co-Cr-W alloy were investigated. • The corresponding strengthening behaviors of observed nano-oxide inclusions and nano-precipitates were studied. • A nano-oxide with high Si content, together with a bcc type Co5Cr3Si2 nano-precipitates were reported for the first time in LPBF Co-Cr-W alloys. • The strengthening mechanism of widely dispersed nano-oxides was disclosed, whose strengthening contribution was calculated to be ∼158 MPa. Additive manufacturing exhibits great potentials for the fabrication of novel materials due to its unique non-equilibrium solidification and heating process. In this work, a novel nano-oxides dispersion strengthened Co28Cr9W1.5Si (wt.%) alloy, fabricated by laser powder bed fusion (LPBF), was comprehensively investigated. During the layer-by-layer featured process, in-situ formation of Si rich, amorphous, nano-oxide inclusions was observed, whose formation is ascribed to the high affinity of Si to oxygen. Meanwhile, distinctive body-centered cubic (BCC) Co5Cr3Si2 nano-precipitates with an 8-fold symmetry were also confirmed to appear. The precipitates, rarely reported in previous studied Co-Cr alloys, were found to tightly bond with the in-situ oxidization. Furthermore, the morphologies, the size distributions as well as the microstructure of the interface between the matrix and the inclusions were investigated in detail and their influence on the tensile deformation was also clarified. The existence of transition boundaries between these inclusions and the matrix strongly blocked the movement of dislocations, thereby increasing the strength of the alloy. It was understood that when the plastic deformation proceeds, the fracture occurs in the vicinity of the oxide inclusions where dislocations accumulate. A quantitative analysis of the strengthening mechanism was also established, in which an additional important contribution to strength (∼ 169 MPa) caused by the effects of in-situ formed oxide inclusions was calculated.

C-Band Septum Polarizers With Polynomial Profile and Accurate Axial Ratio Characterization in Back-to-Back Configuration

Septum polarizers have been used in waveguide systems for over 50 years and are very popular in communication satellites operating in circular polarization. With the progress of computational electromagnetics and computer hardware, combined with advances in manufacturing techniques, there has been a growing interest in septum polarizers with smooth profiles. In particular, a design using Legendre polynomials was recently introduced by the authors. This paper reports on the first experimental validation of this component and introduces also alternative profiles based on cubic splines. The new designs account for considerations on the overhang angle, which is an important parameter in additive layer manufacturing. The axial ratio of these designs is evaluated using a simple and accurate technique based on a back-to-back configuration, which can be implemented outside an anechoic chamber. The proposed characterization technique relies on the custom flanges implemented on the circular common port of septum polarizers to interface with horn antennas. These flanges typically have an 8-fold rotational symmetry, which enables to perform multiple measurements of a back-to-back configuration without requiring further equipment. The measured data is then processed to obtain the axial ratio of the two septum polarizers. This characterization technique is particularly advantageous when a large number of feeds needs to be tested, which is relevant in current and future broadband communication satellites. The technique is validated by comparing with measured data acquired in an anechoic chamber using a standard technique. Various prototypes, designed for operation in C-band communication satellite downlink, are characterized and promising results are obtained, showing very good agreement between simulated and measured data.

Analisis Learning Obstacles Pada Materi Simetri Lipat dan Simetri Putar Untuk Kelas III Sekolah Dasar

This study aims to analyze the types and causes of learning obstacles experienced by elementary school class students on the material of folding symmetry and rotational symmetry. The approach used is a qualitative approach with descriptive research methods. Data collection techniques used in this study were tests, interviews, observation, and documentation. The results showed that there were three learning obstacle factors experienced by the third grade on the material of folding symmetry and rotational symmetry, namely 77% of students experienced ontogenical obstacles, 81% of students experienced didactical obstacles, and 100% of students experienced epistemological obstacles. The cause of students experiencing ontogenical obstacles is the lack of student interest in participating in learning, lack of readiness to accept learning, both psychological and material readiness. The learning presented by the teacher is too difficult and the students do not understand the prerequisite material. The causes of students experiencing didactical obstacie are due to the lack of teacher readiness in the learning process, limited material delivery, and the teacher does not provide practice questions. The cause of students experiencing epistemological obstacles is because the teacher does not give non-routine questions and there is a jump in the prerequisite material.

Structural basis for the activity regulation of a potassium channel AKT1 from Arabidopsis

The voltage-gated potassium channel AKT1 is responsible for primary K+ uptake in Arabidopsis roots. AKT1 is functionally activated through phosphorylation and negatively regulated by a potassium channel α-subunit AtKC1. However, the molecular basis for the modulation mechanism remains unclear. Here we report the structures of AKT1, phosphorylated-AKT1, a constitutively-active variant, and AKT1-AtKC1 complex. AKT1 is assembled in 2-fold symmetry at the cytoplasmic domain. Such organization appears to sterically hinder the reorientation of C-linkers during ion permeation. Phosphorylated-AKT1 adopts an alternate 4-fold symmetric conformation at cytoplasmic domain, which indicates conformational changes associated with symmetry switch during channel activation. To corroborate this finding, we perform structure-guided mutagenesis to disrupt the dimeric interface and identify a constitutively-active variant Asp379Ala mediates K+ permeation independently of phosphorylation. This variant predominantly adopts a 4-fold symmetric conformation. Furthermore, the AKT1-AtKC1 complex assembles in 2-fold symmetry. Together, our work reveals structural insight into the regulatory mechanism for AKT1.

Bistable H2Pc Molecular Conductance Switch on Ag(100)

Scanning tunneling microscopy (STM) and density functional theory (DFT) were used to study the tautomerization reaction of an H2Pc molecule adsorbed on a Ag(100) surface. The presence of two hydrogen atoms in the cavity of the H2Pc molecule enforces the existence of two molecular tautomers. It causes a reduction from 4- to 2-fold symmetry in STM images that can be recorded as two current states over the H2Pc molecule with a high-to-low current state ratio of ∼1.2. These findings are confirmed by the spatial distributions of the all-atom electron charge density calculated by using DFT and transmission maps together with tunneling current ratios (∼1.2) determined from the nonequilibrium Green’s function transport calculations. Therefore, we demonstrate that an H2Pc molecule adsorbed on a Ag(100) surface is a good candidate for a bistable molecular conductance switch since neither the presence of the Ag(100) surface nor that of the STM tip alters the tautomerization.