Large Lattice Research Articles

A study on prenucleation and heterogeneous nucleation in liquid Pb on solid Al using molecular dynamics simulations.

In this paper, we investigate prenucleation and heterogeneous nucleation in the liquid Pb/solid Al system as an example of systems with large lattice misfit using molecular dynamics simulation. Solid Pb and Al have a large positive lattice misfit (f) of 18.2% along the densely packed [110] direction. This study reveals that prenucleation occurs at 600K (an undercooling of 15K), and a 2-dimensional (2D) ordered structure forms at the interface with a coincidence site lattice (CSL) between the first Pb and first Al layers. The CSL accommodates the major part of the f, and only a small residual lattice misfit (fr) of 1.9% remains. The formation of the CSL transforms the original substrate into a considerably potent nucleant, where the first Pb layer becomes the new surface layer of the substrate. At an undercooling of about 22K, nucleation proceeds by merging 2D ordered structure through structural templating: the second Pb layer is epitaxial to the CSL Pb layer, the third Pb layer largely accommodates the fr, and the fourth Pb layer is a nearly perfect crystalline plane. Further analysis indicates that the interface with the CSL has a lower interfacial energy than with a cube-to-cube orientation relationship. For the first time, we established that the CSL was an effective mechanism to accommodate the f for systems with a large positive misfits. Heterogeneous nucleation is governed not by a single mechanism (misfit dislocations in Turnbull's model), but instead by various mechanisms depending on f. This study sheds new light on the atomistic mechanism of heterogeneous nucleation.

Synchronization Transition of the Second-Order Kuramoto Model on Lattices

The second-order Kuramoto equation describes the synchronization of coupled oscillators with inertia, which occur, for example, in power grids. On the contrary to the first-order Kuramoto equation, its synchronization transition behavior is significantly less known. In the case of Gaussian self-frequencies, it is discontinuous, in contrast to the continuous transition for the first-order Kuramoto equation. Herein, we investigate this transition on large 2D and 3D lattices and provide numerical evidence of hybrid phase transitions, whereby the oscillator phases θi exhibit a crossover, while the frequency is spread over a real phase transition in 3D. Thus, a lower critical dimension dlO=2 is expected for the frequencies and dlR=4 for phases such as that in the massless case. We provide numerical estimates for the critical exponents, finding that the frequency spread decays as ∼t-d/2 in the case of an aligned initial state of the phases in agreement with the linear approximation. In 3D, however, in the case of the initially random distribution of θi, we find a faster decay, characterized by ∼t-1.8(1) as the consequence of enhanced nonlinearities which appear by the random phase fluctuations.

Effects of Alloying on the Interface Energy of the gamma ''-Phase in Nickel-Based Superalloys

Nickel-based superalloys of the 718 type have been extensively used in the production of engine parts which operate at high temperatures. The alloy is strengthened by the precipitation of small gamma ''-phase particles with a large lattice distortion in the gamma matrix. These gamma ''-phase particles, however, may transform to the more thermodynamically stable delta -phase. Such transformation might be mitigated by adding other alloying elements. In this work, we used density functional theory calculations at 0 K to study the influence of relevant alloying elements on the interface energy of the gamma ''/gamma coherent interfaces. By replacing an atom either in the gamma ''-phase or at the interface and calculating the corresponding interface segregation energy, we assessed the favorable replacement position of each alloying element. Implications of our results on the gamma ''-phase growth are also discussed.

Large mass field fluctuation and lattice anharmonicity effects promote thermoelectric and mechanical performances in NbFeSb half-Heusler alloys via Ti/Zr/Hf stepwise doping

Thermoelectric materials require not only high performance to maximize the energy-conversion efficiency but also good mechanical properties to guarantee machinability and reliable operation.

Two-Dimensional DNA Origami Lattices Assembled on Lipid Bilayer Membranes.

Molecular self-assembly has attracted much attention as a method to create novel supramolecular architectures. The scaffolded DNA origami method has enabled the construction of almost arbitrarily shaped DNA nanostructures, which can be further used as components of higher-order architectures. Here, we describe a method to construct and visualize two-dimensional (2D) lattices self-assembled from DNA origami tiles on lipid bilayer membranes. The weak adsorption of DNA origami tiles onto the mica-supported lipid bilayer allows their lateral diffusion along the surface, facilitating interactions among the tiles to assemble and form large 2D lattices. Depending on the design (i.e., shape, size, and interactions with each other) of DNA origami tiles, a variety of 2D lattices made of DNA are constructed.

A hollow urchin-like metal–organic framework with Ni–O-cluster SBUs as a promising electrode for an alkaline battery–supercapacitor device

A hollow NiPSC electrode with a large d-spacing lattice distance has been demonstrated as an active SC electrode in a supercapacitor–battery hybrid device to achieve superior electrochemical performance.

A new method to enhance the magnetocaloric effect in (Sc,Ti)Fe2 via magnetic phase separation

Magnetic refrigeration based on the magnetocaloric effect (MCE) is a novel refrigeration technology that will replace traditional vapor-compression refrigeration in the future. Improvement in the performance of MCE materials is crucial for the development of magnetic refrigeration technology. This study presents a new method that enhances the MCE performance of (Sc,Ti)Fe2 via magnetic phase separation. The maximum magnetic entropy change induced by the coexistence of an in-plane ferromagnetic phase (FMab) and a canting antiferromagnetic phase (CAFM) in Sc0.3Ti0.7Fe2 is twice that found in other (Sc,Ti)Fe2 compounds. Variable-temperature neutron diffraction experiments directly reveal that the large magnetic entropy change in Sc0.3Ti0.7Fe2 is dominated by the transformation from a highly ordered FMab state to a CAFM state with a lower magnetic order. The magnetic phase separation is a direct transition from a higher-ordered state with a larger lattice to a lower-ordered state with a smaller lattice that induces a large magnetic order change and lattice contraction. The combination of the metamagnetic transition and negative thermal expansion leads to enhanced MCE. This study suggests the possibility that magnetic phase separation can be an effective approach to achieving and controlling a large MCE in magnetic materials.

Two-dimensional Hubbard model at finite temperature: Weak, strong, and long correlation regimes

We investigate the momentum-resolved spin and charge susceptibilities, as well as the chemical potential and double occupancy in the two-dimensional Hubbard model as functions of doping, temperature and interaction strength. Through these quantities, we identify a weak-coupling regime, a strong-coupling regime with short-range correlations and an intermediate-coupling regime with long magnetic correlation lengths. In the spin channel, we observe an additional crossover from commensurate to incommensurate correlations. In contrast, we find charge correlations to be only short ranged for all studied temperatures, which suggests that the spin and charge responses are decoupled. These findings were obtained by a novel connected determinant diagrammatic Monte Carlo algorithm for the computation of double expansions, which we introduce in this paper. This permits us to obtain numerically exact results at unprecedentedly low temperatures $T\geq 0.067$ for interactions up to $U\leq 8$, while working on arbitrarily large lattices. Our method also allows us to gain physical insights from investigating the analytic structure of perturbative series. We connect to previous work by studying smaller lattice geometries and report substantial finite-size effects.

Density Functional Theory Study of the Point Defects on KDP (100) and (101) Surfaces.

Surface defects are usually associated with the formation of other forms of expansion defects in crystals, which have an impact on the crystals' growth quality and optical properties. Thereby, the structure, stability, and electronic structure of the hydrogen and oxygen vacancy defects (VH and VO) on the (100) and (101) growth surfaces of KDP crystals were studied by using density functional theory. The effects of acidic and alkaline environments on the structure and properties of surface defects were also discussed. It has been found that the considered vacancy defects have different properties on the (100) and (101) surfaces, especially those that have been reported in the bulk KDP crystals. The (100) surface has a strong tolerance for surface VH and VO defects, while the VO defect causes a large lattice relaxation on the (101) surface and introduces a deep defect level in the band gap, which damages the optical properties of KDP crystals. In addition, the results show that the acidic environment is conducive to the repair of the VH defects on the surface and can eliminate the defect states introduced by the surface VO defects, which is conducive to improving the quality of the crystal surface and reducing the defect density. Our study opens up a new way to understand the structure and properties of surface defects in KDP crystals, which are different from the bulk phase, and also provides a theoretical basis for experimentally regulating the surface defects in KDP crystals through an acidic environment.

Corrections to scaling in the 3D Ising model: A comparison between MC and MCRG results

Corrections to scaling in the 3D Ising model are studied based on Monte Carlo (MC) simulation results for very large lattices with linear lattice sizes up to [Formula: see text]. Our estimated values of the correction-to-scaling exponent [Formula: see text] tend to decrease below the usually accepted value about 0.83 when the smallest lattice sizes, i.e. [Formula: see text] with [Formula: see text], are discarded from the fits. This behavior apparently confirms some of the known estimates of the Monte Carlo renormalization group (MCRG) method, i.e. [Formula: see text] and [Formula: see text]. We discuss the possibilities that [Formula: see text] is either really smaller than usually expected or these values of [Formula: see text] describe some transient behavior which, eventually, turns into the correct asymptotic behavior at [Formula: see text]. We propose refining MCRG simulations and analysis to resolve this issue. Our actual MC estimations of the critical exponents [Formula: see text] and [Formula: see text] provide stable values [Formula: see text] and [Formula: see text], which well agree with those of the conformal bootstrap method, i.e. [Formula: see text] and [Formula: see text].

The numerical value for a universal quantity of a two-dimensional dimerized quantum antiferromagnet

The numerical value of a universal quantity associated with the quantum critical regime, namely χuc2/T, for a two-dimensional (2D) dimerized spin-1/2 antiferromagnet is calculated using the quantum Monte Carlo simulations (QMC). Here χu, c, and T are the uniform susceptibility, the spin-wave velocity, and the temperature, respectively. By simulating large lattices at moderately low temperatures, we find χuc2/T∼0.32. Our estimation of χuc2/T deviates from the related analytic prediction but agrees with recent numerical calculations of other 2D dimerized spin-1/2 antiferromagnets.

On the continuum limit for the discrete nonlinear Schrödinger equation on a large finite cubic lattice

In this study, we consider the nonlinear Schödinger equation (NLS) with the zero-boundary condition on a two- or three-dimensional large finite cubic lattice. We prove that its solution converges to that of the NLS on the entire Euclidean space with simultaneous reduction in the lattice distance and expansion of the domain. Moreover, we obtain a precise global-in-time bound for the rate of convergence. Our proof heavily relies on Strichartz estimates on a finite lattice. A key observation is that, compared to the case of a lattice with a fixed size (Hong et al., 2021), the loss of regularity in Strichartz estimates can be reduced as the domain expands, depending on the speed of expansion. This allows us to address the physically important three-dimensional case.

The lattice distortion, mechanical and thermodynamic properties of A(Zr0.2Sn0.2Ti0.2Hf0.2Nb0.2)O3 (A = Sr, Ba) high-entropy perovskite with B-site disorder: First principles prediction

High-entropy perovskite oxides are novel high-entropy ceramics developing recently. In this work, the local lattice distortion, mechanical and thermodynamic properties of perovskites A(Zr0.2Sn0.2Ti0.2Hf0.2Nb0.2)O3 (A = Sr, Ba) are explored using the first principle investigation. The equilibrium lattice parameter and bulk moduli of high-entropy perovskites obtained by fitting approximately satisfy the rule of mixture of the components, while the entropy effect of component mixing is very small. The bond length distribution reveals the B-site disorder results in large local lattice distortion. The atomic displacement indicates larger average displacement of O atoms contributes mainly to the wider bond length distribution for most B-O bond. The distortion degree at B-site is naturally associated with A-site. The obtained elastic constants of high-entropy perovskites are also closed to the rule of mixing of the components. Compared with ternary SrTiO3 and BaTiO3 perovskites, B-site mixing for high-entropy perovskites enhances their toughness at expense of strength and stiffness, showing the elastic moduli can be modified and adjusted through multi-component design strategy. Relevant thermodynamic property reveals the B-site disorder is benefit to improve the resistance to softening and suppress volume expansion at high temperature. Electronic structures show the insulator–metal transition takes place, also uncover that the interaction of B-O bond is stronger.

First-principles study of vacancy defects in TiVTa and TiVTaNb concentrated solid-solution alloys

The formation and migration energies of vacancy defects are studied systematically by first-principal calculations method in three concentrated solid-solution alloys (CSAs), namely equimolar TiVTa alloy, TiVTa50 alloy with Ta content increased to 50%, and equimolar TiVTaNb alloy. It is found that the vacancy formation and migration in TiVTaNb alloy are more difficult than that in TiVTa and TiVTa50 alloys. The TiVTa alloy has the larger local lattice distortion than TiVTa50 and TiVTaNb alloys, which would lead to an irregular energy landscape and make it easier to form vacancies. Vacancies are easily formed in Ti-rich environments and the trend reverses for in V-rich environments in these three alloys, originating from the large local lattice distortion in Ti-rich environments and strong binding interactions between Ti atoms and vacancies. Ti atoms with the largest size tend to migrate to neighboring vacancies in these three alloys. The addition of Nb enhances the electron interactions between atoms in TiVTaNb alloy. Compared to TiVTa and TiVTa50 alloys, the migration barrier of atoms diffusing into vacancies in TiVTaNb alloy is improved due to the combination of strong electron interactions between atoms and lattice distortion. It is implied that the influence of alloy element combination on vacancy formation and migration energies may be greater than that of alloy element content in Ti-V-Ta and Ti-V-Ta-Nb alloys. These results provide fundamental insights into the defects evolution of CSAs with body-centered cubic (bcc) structure and supply the scientific basis for the composition design of bcc CSAs.

Microstructure and properties of the solid solution ceramic coating by high speed laser cladding

It is facing challenge to prepare protective ceramic coatings on the surfaces of metal parts. In this study, two types of ceramic coatings, ZrB2 and ZrB2 + TiB2, were prepared on 304 stainless steel using high speed laser cladding (HSLC) technology, and a comparative study was conducted on them. The results show that additional TiB2 were melted sufficiently during the HSLC process and reacted with ZrB2 to generate acicular (Zrx, Ti1-x)B2 structure. The existence of (Zrx, Ti1-x)B2 resulted in the increase of hardness from 719 HV0.5 to 1130 HV0.5, and the decrease of wear rate from 8.89 × 10-6 mm3 / N·m to 5.0 × 10-6 mm3 / N·m of the coating. It can be attributed to the large lattice distortion inside (Zrx, Ti1-x)B2, which improved the hardness and wear resistance compared with ZrB2. In addition, the friction coefficient of the coating decreased from 0.73 to 0.68 due to lubricating effect of the oxide debris generated by (Zrx, TI1-x) B2 in the friction process.

Supersolid-like square- and triangular-lattice crystallization of dipolar droplets in a box trap

Using a beyond-mean-field model including a Lee-Huang-Yang-type interaction, we demonstrate a supersolid-like spatially-periodic square- and triangular-lattice crystallization of droplets in a polarized dipolar condensate confined by an appropriate three-dimensional (3D) box trap. In this paper we consider a rectangular box (cuboid) trap, a square box (cuboid with two equal sides) trap, a cylindrical box trap and a hexagonal box (hexagonal prism) trap. The droplet lattice is always formed in the $x$-$y$ plane perpendicular to the polarization $z$ direction of dipolar atoms. In contrast to a harmonic trap, the box traps allow the formation of a large clean supersolid-like spatially-periodic crystallization in free space without any distortion. Moreover, a droplet lattice can be formed in a 3D box trap with a significantly reduced number of atoms than in a harmonic trap, which could facilitate the experimental observation of droplet lattice in a box trap. With present know-how such a supersolid-like crystallization of dipolar droplets in a 3D box trap can be realized in a laboratory thus allowing the study of a large periodic lattice of dipolar droplets in free space bounded by rigid walls.

Ultrahigh piezoelectric performance benefiting from quasi-isotropic local polarization distribution in complex lead-based perovskite

Materials with high piezoelectricity have always been the pursuit of both industrial production and scientific research. Designing new high-piezoelectricity materials and establishing the structure-performance relationship are crucial for those two aspects. Here, a complex perovskite system with excellent comprehensive properties (d33 = 920 pC/N, d33* = 902 pm/V, Tm = 120 °C, kp = 0.62) is developed, and taken as a case study to explain the origins of high piezoelectricity utilizing atomic-resolution scanning transmission electron microscopy and in situ high-energy synchrotron X-ray diffraction. Correlated local multiple polar symmetries coexistence with a quasi-isotropic local polarization distribution result in a long-range average pseudo-cubic phase. The reduced local polarization anisotropy and the flexible polarization configuration promote a continuous polarization rotation between different polarities, leading to a large field-induced lattice strain and ultimately generating high piezoelectric performance. This work provides a perspective that decreased polarization anisotropy at the local scale is beneficial to the enhancement of high piezoelectricity, which will facilitate the design and development of new piezoelectric materials.

Weak coupling limit of U(1) lattice model in Fourier basis

The transfer-matrix of the U(1) lattice model is considered in the Fourier basis and in the weak coupling limit. The issues of Gauss law constraint and gauge invariant states are addressed in the Fourier basis. In particular, it is shown that in the strong coupling limit the gauge invariant Fourier states are effectively the finite size closed loop currents. In the weak coupling limit, however, the link-currents along periodic or infinite spatial directions find comparable roles as gauge invariant states. The subtleties related to the extreme weak coupling of the transfer-matrix in the Fourier basis are discussed. A careful analysis of the zero eigenvalues of the matrix in the quadratic action leads to a safe extraction of the diverging group volume in the limit $g\to 0$. By means of the very basic notions and tools of the lattice model, the spectrum at the weak coupling limit for any dimension and size of lattice is obtained analytically. The spectrum at the weak coupling limit corresponds to the expected one by the continuum model in the large lattice limit.

Enormous Increases in Swir Detection for Gesn Strips Detector with Graphene Hybrid Structure

GeSn materials with tunable bandgaps covering the full shortwave infrared (SWIR) band from 1 to 2.6 μm present a new paradigm for silicon-based SWIR photodetection. However, crystalline quality remains a major limitation for the realization of high-performance photodetectors owing to the large lattice, thermal expansion coefficient mismatching, and Sn segregation. Here, the Sn self-catalyzed growth of lateral GeSn strips on Si(111) substrates by MBE is investigated. These GeSn strips are defect-free by relaxing the large lattice mismatch and introducing (111)-parallel planar dislocations at the GeSn/Si interface. A graphene-GeSn strip hybrid structure photodetector is fabricated, which shows more than 3000 times enhancement in photocurrent and an apparent improvement in temporal response compared with the detector without graphene. A high responsivity of 1.23 A/W and 1.08 A/W at 1310 nm and 1550 nm are achieved, respectively. This work presents a new approach for micro-scale but high crystal quality material in the integration on a silicon-based platform and shows future applications in the SWIR field.

Graphene‐Assisted Epitaxy of High‐Quality GaN Films on GaN Templates

AbstractGraphene (Gr)‐assisted epitaxy of semiconductors has been demonstrated as a revolutionary strategy for the fabrication of nitride materials and flexible devices. However, till now, most of the GaN epitaxy assisted by Gr is performed on thermally stable foreign substrates because Gr suffers obvious deterioration due to the decomposition of GaN substrate. Thus, large lattice and thermal mismatches still exist and inevitably lead to a high density of dislocations in nitrides. To address this issue, here, the Gr‐assisted epitaxy of high‐quality GaN films on GaN templates is demonstrated by constructing specially designed self‐organized defective (SOD) Gr as a buffer layer. The SOD Gr allows for the spontaneous relaxation of strain in GaN epilayers by weakening the interfacial interactions; and thus, blocks the upward spread of threading dislocations in GaN templates. Therefore, screw and edge dislocation densities in GaN epilayers show reductions of 51% and 62%, respectively, compared to that in homoepitaxy. Based on the GaN film grown on SOD Gr, the as‐fabricated blue light‐emitting diode exhibits a 17% enhancement of light output power and a better emission wavelength stability than that without Gr. This work opens a practical pathway for the growth of high‐quality nitrides and manufacturing of high‐performance nitride‐based devices.