Perfect Lattice Research Articles

Puzzling Low-Temperature Behavior of the Van Der Waals Friction Force between Metallic Plates in Relative Motion

This paper presents the results of calculating the van der Waals friction force (dissipative fluctuation-electromagnetic force) between metallic (Au) plates in relative motion at temperatures close to 1 K. The stopping tangential force arises between moving plates along with the usual Casimir force of attraction, which has been routinely measured with high precision over the past two decades. At room temperatures, the former force is 10 orders of magnitude less than the latter, but at temperatures T<50 K, friction increases sharply. The calculations have been carried out in the framework of the Levin-Polevoi-Rytov fluctuation electromagnetic theory. For metallic plates with perfect crystal lattices and without defects, van der Waals friction force is shown to increase with decreasing temperature as T−4. In the presence of residual resistance ρ0 of the metal, a plateau is formed on the temperature dependence of the friction force at T→0 with a height proportional to ρ0−0.8. Another important finding is the weak force-distance dependence ~a−q (with q<1). The absolute values of the friction forces are achievable for measurements in AFM-based experiments.

Support vector machines for predicting the compressive response of defected 3D printed polymeric sandwich structures

Purpose This study aims to investigate the prediction of the nonlinear response of three-dimensional-printed polymeric lattice structures with and without structural defects. Unlike metallic structures, the deformation behavior of polymeric components is difficult to quantify through the classical numerical analysis approach as a result of their nonlinear behavior under mechanical loads. Design/methodology/approach Geometric models of periodic lattice structures were designed via PTC Creo. Imperfections in the form of missing unit cells are introduced in the replica of the lattice structure. The perfect and imperfect lattice structures have the same dimensions – 10 mm × 14 mm × 30 mm (w × h × L). The fused deposition modelling technique is used to fabricate the parts. The fabricated parts were subjected to physical compression tests to provide a measure of their transverse compressibility resistance. The ensuing nonlinear response from the experimental tests is deployed to develop a support vector machine surrogate model. Findings Results from the surrogate model’s performance, in terms of correlation coefficient, rose to as high as 99.91% for the nonlinear compressive stress with a minimum achieved being 98.51% across the four datasets used. In the case of deflection response, the model accuracy rose to as high as 99.74% while the minimum achieved is 98.56% across the four datasets used. Originality/value The developed model facilitates the prediction of the quasi-static response of the structures in the absence and presence of defects without the need for repeated physical experiments. The structure investigated is designed for target applications in hierarchical polymer packaging, and the methodology presents a cost-saving method for data-driven constitutive modelling of polymeric parts.

MoP3SiO11 : A 4d3 honeycomb antiferromagnet with disconnected octahedra

We report the crystal structure and magnetic behavior of the $4d^3$ spin-$\frac32$ silicophosphate MoP$_3$SiO$_{11}$ studied by high-resolution synchrotron x-ray diffraction, neutron diffraction, thermodynamic measurements, and ab initio band-structure calculations. Our data revise the crystallographic symmetry of this compound and establish its rhombohedral space group ($R\bar 3c$) along with the geometrically perfect honeycomb lattice of the Mo$^{3+}$ ions residing in disconnected MoO$_6$ octahedra. Long-range antiferromagnetic order with the propagation vector $\mathbf k=0$ observed below $T_N=6.8$ K is a combined effect of the nearest-neighbor in-plane exchange coupling $J\simeq 2.6$ K, easy-plane single-ion anisotropy $D\simeq 2.2 $ K, and a weak interlayer coupling $J_c\simeq 0.8$ K. The 12% reduction in the ordered magnetic moment of the Mo$^{3+}$ ions and the magnon gap of $\Delta\simeq 7$ K induced by the single-ion anisotropy further illustrate the impact of spin-orbit coupling on the magnetism. Our analysis puts forward single-ion anisotropy as an important ingredient of $4d^3$ honeycomb antiferromagnets despite their nominally quenched orbital moment.

Imperfect comb construction reveals the architectural abilities of honeybees

Honeybees are renowned for their perfectly hexagonal honeycomb, hailed as the pinnacle of biological architecture for its ability to maximize storage area while minimizing building material. However, in natural nests, workers must regularly transition between different cell sizes, merge inconsistent combs, and optimize construction in constrained geometries. These spatial obstacles pose challenges to workers building perfect hexagons, but it is unknown to what extent workers act as architects versus simple automatons during these irregular building scenarios. Using automated image analysis to extract the irregularities in natural comb building, we show that some building configurations are more difficult for the bees than others, and that workers overcome these challenges using a combination of building techniques, such as: intermediate-sized cells, regular motifs of irregular shapes, and gradual modifications of cell tilt. Remarkably, by anticipating these building challenges, workers achieve high-quality merges using limited local sensing, on par with analytical models that require global optimization. Unlike automatons building perfectly replicated hexagons, these building irregularities showcase the active role that workers take in shaping their nest and the true architectural abilities of honeybees.

The influence of defects on the elastic response of lattice structures resulting from additive manufacturing

Stiffness prediction for additively manufactured (AM) lattices is necessary for lightweight components design. For a given lattice structure, the Gibson and Ashby (GA) model can predict relative stiffness as a function of its relative density. However, volumetric porosity and surface roughness defects that are commonplace in AM lattices depreciate the quality of prediction made by the GA model. This is because such defects complicate the elastic behavior of lattice structures. In this work, a modified GA model is proposed that accounts for these defects. A Bayesian inferencing framework is constructed to delineate the influence of spatial distributions of these defects on resulting mechanical response. Principal component analysis (PCA) is used to identify differences between elastic strain fields (∊11, ∊22, and ∊12) resulting from perfect and defective lattices. The insights obtained can provide a viable approach to predict the mechanical response of as-received AM lattices that are often defective, and thereby enable systematic approaches for their design.

On the geometry of nearly orthogonal lattices

Nearly orthogonal lattices were formally defined in [4], where their applications to image compression were also discussed. The idea of “near orthogonality” in 2-dimensions goes back to the work of Gauss. In this paper, we focus on well-rounded nearly orthogonal lattices in Rn and investigate their geometric and optimization properties. Specifically, we prove that the sphere packing density function on the space of well-rounded lattices in dimension n≥3 does not have any local maxima on the nearly orthogonal set and has only one local minimum there: at the integer lattice Zn. Further, we show that the nearly orthogonal set cannot contain any perfect lattices for n≥3, although it contains multiple eutactic (and even strongly eutactic) lattices in every dimension. This implies that eutactic lattices, while always critical points of the packing density function, are not necessarily local maxima or minima even among the well-rounded lattices. We also prove that a (weakly) nearly orthogonal lattice in Rn contains no more than 4n−2 minimal vectors (with any smaller even number possible) and establish some bounds on coherence of these lattices.

Crystal growth and magnetic properties of quantum spin liquid candidate KErTe2 * *Project supported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0302904 and 2016YFA0300504), the National Natural Science Foundation of China (Grant Nos. U1932215 and 11774419), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB33010100), and Postdoctoral Science Foundation of

Recently rare-earth chalcogenides have been revealed as a family of quantum spin liquid (QSL) candidates hosting a large number of members. In this paper we report the crystal growth and magnetic measurements of KErTe2, which is the first member of telluride in the family. Compared to its cousins of oxides, sulfides and selenides, KErTe2 retains the high symmetry of and Er3+ ions still sit on a perfect triangular lattice. The separation between adjacent magnetic layers is expectedly increased, which further enhances the two dimensionality of the spin system. Specific heat and magnetic susceptibility measurements on KErTe2 single crystals reveal no structural and magnetic transition down to 1.8 K. Most interestingly, the absorption spectrum shows that the charge gap of KErTe2 is roughly 0.93±0.35 eV, which is the smallest among all the reported members in the family. This immediately invokes the interest towards metallization even superconductivity using the compound.

Localization and coalescence of imperfect planar FCC truss lattice metamaterials under multiaxial loadings

This study investigates the effect of stress triaxiality on the failure mechanisms of an-isotropic perfect and imperfect planar FCC (Face Centred Cubic) truss lattice metamaterials. Three types of imperfection have been considered in the numerical modelling, namely, distorted struts, missing struts, and strut diameter variation. In order to maintain constant stress triaxiality during the simulations, a novel numerical framework was developed to overcome computational difficulties within the existing numerical approaches beyond elastic region. Three modes of microscopic localization were observed in perfect and imperfect lattices before failure: crushing band, shear band and void coalescence. A clear separation exists between the three modes of localization depending upon the type and level of defects, as well as the stress triaxiality. Under compressive loading, all lattices fail owing to crushing band; the distorted lattices are prone to shear band localization with increase in distortion, whereas missing lattices majorly fail due to void coalescence at high missing struts defect. Strut diameter variation, within the range of the strut diameters selected, shows no significant influence on the macroscopic mechanical response and strain localization. This work may open the door for predicting failure mechanisms of imperfect lattices under variety of loading conditions.

Surface Engineering and Controlled Ripening for Seed‐Mediated Growth of Au Islands on Au Nanocrystals

AbstractEngineering the nucleation and growth of plasmonic metals (Ag and Au) on their pre‐existing seeds is expected to produce nanostructures with unconventional morphologies and plasmonic properties that may find unique applications in sensing, catalysis, and broadband energy harvesting. Typical seed‐mediated growth processes take advantage of the perfect lattice match between the deposited metal and seeds to induce conformal coating, leading to either simple size increases (e.g., Au on Au) or the formation of core–shell structures (e.g., Ag on Au) with limited morphology change. In this work, we show that the introduction of a thin layer of metal with considerable lattice mismatch can effectively induce the nucleation of well‐defined Au islands on Au nanocrystal seeds. By controlling the interfacial energy between the seed and the deposited material, the oxidative ripening, and the surface diffusion of metal precursors, we can regulate the number of islands on the seeds and produce complex Au nanostructures with morphologies tunable from core‐satellites to tetramers, trimers, and dimers.

Surface Engineering and Controlled Ripening for Seed-Mediated Growth of Au Islands on Au Nanocrystals.

Engineering the nucleation and growth of plasmonic metals (Ag and Au) on their pre-existing seeds is expected to produce nanostructures with unconventional morphologies and plasmonic properties that may find unique applications in sensing, catalysis, and broadband energy harvesting. Typical seed-mediated growth processes take advantage of the perfect lattice match between the deposited metal and seeds to induce conformal coating, leading to either simple size increases (e.g., Au on Au) or the formation of core-shell structures (e.g., Ag on Au) with limited morphology change. In this work, we show that the introduction of a thin layer of metal with considerable lattice mismatch can effectively induce the nucleation of well-defined Au islands on Au nanocrystal seeds. By controlling the interfacial energy between the seed and the deposited material, the oxidative ripening, and the surface diffusion of metal precursors, we can regulate the number of islands on the seeds and produce complex Au nanostructures with morphologies tunable from core-satellites to tetramers, trimers, and dimers.

A new interatomic potential describing Fe-H and H-H interactions in bcc iron

We present a new many-body interatomic potential for H in body-centered cubic (bcc) Fe. The potential is developed based on extensive energetics and atomic configurations of an H atom and H-H interactions in Fe from density functional theory calculations. In detail, the potential is parameterized by fitting not only to a single H atom in the perfect bcc Fe lattice and to the properties of H trap binding to a vacancy and surfaces as being done by previous studies, but also to multiple H trapping to a vacancy and H-H interaction in Fe lattice. With such a fitting strategy, the developed potential outperforms existing potentials in its ability not only describing the behaviors of a single H atom in Fe, but also capturing the features of H-H interaction reliably, which is of key importance in revealing H behaviors in local H accumulation around dislocation cores, grain boundaries and crack tips.

Atomic Simulations of Packing Structures, Local Stress and Mechanical Properties for One Silicon Lattice with Single Vacancy on Heating.

The effect of vacancy defects on the structure and mechanical properties of semiconductor silicon materials is of great significance to the development of novel microelectronic materials and the processes of semiconductor sensors. In this paper, molecular dynamics is used to simulate the atomic packing structure, local stress evolution and mechanical properties of a perfect lattice and silicon crystal with a single vacancy defect on heating. In addition, their influences on the change in Young’s modulus are also analyzed. The atomic simulations show that in the lower temperature range, the existence of vacancy defects reduces the Young’s modulus of the silicon lattice. With the increase in temperature, the local stress distribution of the atoms in the lattice changes due to the migration of the vacancy. At high temperatures, the Young’s modulus of the silicon lattice changes in anisotropic patterns. For the lattice with the vacancy, when the temperature is higher than 1500 K, the number and degree of distortion in the lattice increase significantly, the obvious single vacancy and its adjacent atoms contracting inward structure disappears and the defects in the lattice present complex patterns. By applying uniaxial tensile force, it can be found that the temperature has a significant effect on the elasticity–plasticity behaviors of the Si lattice with the vacancy.

Quantitative insights into the dislocation source behavior of twin boundaries suggest a new dislocation source mechanism

Pop-in statistics from nanoindentation with spherical indenters are used to determine the stress required to activate dislocation sources in twin boundaries (TBs) in copper and its alloys. The TB source activation stress is smaller than that needed for bulk single crystals, irrespective of the indenter size, dislocation density and stacking fault energy. Because an array of pre-existing Frank partial dislocations is present at a TB, we propose that dislocation emission from the TB occurs by the Frank partials splitting into Shockley partials moving along the TB plane and perfect lattice dislocations, both of which are mobile. The proposed mechanism is supported by recent high resolution transmission electron microscopy images in deformed nanotwinned (NT) metals and may help to explain some of the superior properties of nanotwinned metals (e.g. high strength and good ductility), as well as the process of detwinning by the collective formation and motion of Shockley partial dislocations along TBs.Graphic abstract

Spinon Fermi Surface Spin Liquid in a Triangular Lattice Antiferromagnet NaYbSe2

Triangular lattice of rare-earth ions with interacting effective spin-$1/2$ local moments is an ideal platform to explore the physics of quantum spin liquids (QSLs) in the presence of strong spin-orbit coupling, crystal electric fields, and geometrical frustration. The Yb delafossites, NaYbCh$_2$ (Ch=O, S, Se) with Yb ions forming a perfect triangular lattice, have been suggested to be candidates for QSLs. Previous thermodynamics, nuclear magnetic resonance, and muon spin rotation measurements on NaYbCh$_2$ have supported the suggestion of the QSL ground states. The key signature of a QSL, the spin excitation continuum, arising from the spin quantum number fractionalization, has not been observed. Here we perform both elastic and inelastic neutron scattering measurements as well as detailed thermodynamic measurements on high-quality single-crystalline NaYbSe$_2$ samples to confirm the absence of long-range magnetic order down to 40 mK, and further reveal a clear signature of magnetic excitation continuum extending from 0.1 to 2.5 meV. The comparison between the structure of the magnetic excitation spectra and the theoretical expectation from the spinon continuum suggests that the ground state of NaYbSe$_2$ is a QSL with a spinon Fermi surface.

Deep insight into the lithium transportation mechanism and lithium deintercalation study on ε-LiVOPO4 cathode material by atomistic simulation and first-principles method

In this work, atomistic simulation together with density functional theory (DFT) method is applied to study the concerted motion of lithium ions and the intercalation/deintercalation mechanism in cathode material ε-LiVOPO4. Two kinds of one dimensional Li + diffusion paths along the direction of →a−→b, that is Li1-2iO5 chains and Li2-2iO5 chains with an energy barrier of 0.066 eV and 0.35 eV, are revealed by molecular dynamics (MD) and bond-valence-energy-landscape mapping (BVEL). At higher temperature, extra diffusion paths between different Li1-2iO5 chains and Li2-2iO5 chains are found. MD studies show that Li + migration along a-axis and b-axis own the same energy barrier of 0.128 eV and the estimated diffusion coefficient at room temperature (RT) is 1.4 × 10−6 cm2 s−1 at the fully lithiated state. The lithium mobility is extremely high for the perfect lattice of ε-LiVOPO. In the process of lithium ion deintercalation, the lithium mobility is gradually depressed. The state of charge (SOC) dependency of Li+ diffusion coefficients shows a downward tendency.

Wave Packet Dynamical Simulation of Quasiparticle Interferences in 2D Materials

Materials consisting of single- or a few atomic layers have extraordinary physical properties, which are influenced by the structural defects. We present two calculation methods based on wave packet (WP) dynamics, where we compute the scattering of quasiparticle WPs on localized defects. The methods are tested on a graphene sheet: (1) We describe the perfect crystal lattice and the electronic structure by a local atomic pseudopotential, then calculate the Bloch eigenstates and build a localized WP from these states. The defect is represented by a local potential, then we compute the scattering by the time development of the WP. (2) We describe the perfect crystal entirely by the kinetic energy operator, then we calculate the scattering on the local defect described by the potential energy operator. The kinetic energy operator is derived from the dispersion relation, which can be obtained from any electronic structure calculation. We also verify the method by calculating Fourier transform images and comparing them with experimental FFT-LDOS images from STM measurements. These calculation methods make it possible to study the quasiparticle interferences, inter- and intra-valley scattering, anisotropic scattering, etc., caused by defect sites for any 2D material.

Unexpected Dependence of Photonic Band Gap Size on Randomness in Self-Assembled Colloidal Crystals.

Using computer simulations, we explore how thermal noise-induced randomness in a self-assembled photonic crystal affects its photonic band gaps (PBGs). We consider a two-dimensional photonic crystal composed of a self-assembled array of parallel dielectric hard rods of infinite length with circular or square cross section. We find that PBGs can exist over a large range of intermediate packing densities and the largest band gap does not always appear at the highest packing density studied. Remarkably, for rods with square cross section at intermediate packing densities, the transverse magnetic (TM) band gap of the self-assembled (i.e., thermal) system can be larger than that of identical rods arranged in a perfect square lattice. By considering hollow rods, we find the band gap of transverse electric modes can be substantially increased while that of TM modes show no obvious improvement over solid rods. Our study suggests that particle shape and internal structure can be used to engineer the PBG of a self-assembled system despite the positional and orientational randomness arising from thermal noise.

Casimir entropy and nonlocal response functions to the off-shell quantum fluctuations

The nonlocal response functions to quantum fluctuations are used to find asymptotic expressions for the Casimir free energy and entropy at an arbitrarily low temperature in the configuration of two parallel metallic plates. It is shown that by introducing an alternative nonlocal response to the off-the-mass-shell fluctuations the Lifshitz theory is brought into agreement with the requirements of thermodynamics. According to our results, the Casimir entropy calculated using the nonlocal response functions, which take into account dissipation of conduction electrons, remains positive and monotonously goes to zero with vanishing temperature, i.e., satisfies the Nernst heat theorem. This is true for both plates with perfect crystal lattices and for lattices with defects of structure. The obtained results are discussed in the context of the Casimir puzzle.

Time–temperature superposition for cavitation resistance of metals with nonequilibrium vacancy concentrations

Here, we provide a systematic molecular dynamics (MD) study of the pulse duration and temperature dependence of spall strength in Al and Mg with a nonequilibrium concentration of vacancies. A superconcentration of single vacancies are randomly introduced into an otherwise perfect lattice, and the systems are allowed to evolve for up to 500 nanoseconds as a proxy for the pulse duration of a shock compression wave. Since these systems are nonequilibrium in nature, a time-dependent and temperature-dependent evolution of the microstructure ensues, which leads to a time- and temperature-dependent softening of the spall strength. Here, we report a large parametric study consisting of 196 MD calculations to quantify this softening in Al and Mg. We invoke the notion of time–temperature superposition from the field of viscoelasticity, and show that the results of our 196 MD calculations remarkably collapse onto a single master curve when normalized by an appropriate relaxation timescale. Lastly, a favorable agreement between the MD calculations and experimental measurements of thermal softening of spall strength is reported.

Epitaxially grown MOF membranes with photocatalytic bactericidal activity for biofouling mitigation in desalination

In this study, we demonstrated the antifouling MOF membrane via the in-situ growth of metal azolate framework-4 (MAF-4) on polyethersulfone (PES) substrates and then epitaxial growth of MAF-7. The basal MAF-4 layer, which was confirmed to bind to the substrate by chelation, was flawless to ensure the desalination performance. The upper MAF-7 shell, which was confirmed to possess crystal characteristics and leave an unbound nitrogen atom, was hydrophilic and smooth to subserve the membrane permeability and antiadhesion. The as-prepared MAF-7@4/PES membrane exhibited exceptional salt rejection (98.7% for NaCl) and preferable permeability (1.24 L m−2 h−1 MPa−1), along with a flux decline ratio of 22.4% by employing bovine serum albumin (BSA) as the model foulant. The synergy of MAF-4 and MAF-7 enhanced the photocatalytic activity of membranes probably imputed to the almost perfect lattice match at the junction interface of epitaxial growth, leading to a bactericidal rate of nearly 100%. The main bactericidal mechanism was proved to be massive detected reactive oxygen species (•OH, H2O2 and •O2−) generated under visible light, rather than the role of leached metal ions and ligand precursor as recognized. Finally, the significant reduction in flux loss and the inhibition of biofilm formation in dynamic filtration indicated MAF-7@4/PES membrane had the potential for the application of desalination with a prominent antifouling property.