Lattice Effects Research Articles

Manipulation of photon states in photonic crystals based on silicon

Coupling between the active centers generated by localized states and the two-dimensional photonic crystals (2D-PCs) has been used to select model in the nano-laser and the optical interconnects on silicon chip. The photonic crystals have been investigated in the simulating calculation, and the physical model has been built in the manipulation of photon states. New quantum phenomena appear in the photonic crystals as well as in the atomic crystals. We have numerically studied on the band structure of 2D-PCs with different types of lattice and various shapes in the optical communication windows. The simulating calculation results show that the band gap of photon states can be effectively manipulated in 2D-PCs by using the competition between the quantum confinement effect and the lattice symmetry effect, in which the band gap width of the photon states will obviously increase by changing the lattice symmetry and by increasing the filling rate of the air hole to restrain photons in 2D-PCs on silicon. It is interesting to make a comparison between the transverse electric (TE) polarization and the transverse magnetic (TM) polarization, in which a wider band gap occurs in the TM mode. By adjusting the defect mode in 2D-PCs, the photon states can be localized into the second and third windows of optical communication for selecting modes and resonating on silicon chip.

Lattice rotation effect on the dislocation pattern of Cu deformed in tension

ABSTRACT The dislocation pattern of cold deformed FCC metals follows an orientation-dependent rule. However, only final orientation after deformation has been considered in related studies, i.e. without considering lattice rotation effect, although metals have undergone heavy lattice rotation during plastic deformation. In this work, the lattice rotation effect on the dislocation structures of Cu is investigated by combining electron backscatter diffraction, focused ion beam lift-out and scanning transmission electron microscopy techniques. The dislocation patterns of [110]-[110], [111]-[111] and [110]-[111] type grains deformed in tension under room temperature are compared. The notation [h1k1l1]-[h2k2l2] represents the lattice rotation history of a grain, meaning that the grain rotates from [h1k1l1] corner to [h2k2l2] corner during tension. It is found that for a given strain range, the dislocation cell structure is observed in [111]-[111] grain, while the dislocation cell block structure is dominant in [110]-[111] grain. This demonstrates that for Cu grains with a final orientation near [111] corner, dislocation pattern type is influenced by the lattice rotation feature. Same with [110]-[111] type grain, cell block structure is the main feature found in [110]-[110] grain, indicating that for grain with initial orientation near [110] corner, lattice rotation has a marginal effect on the dislocation structure. The relation between the dislocation structure, lattice rotation, slip system and strain level of Cu is discussed.

Monte Carlo simulations of nanotube filler in composite material: code optimization

The electrically conductive polymer composites (CPCs) have attracted intensive attention for several decades due to their flexibility and unique electrical properties. CPCs are potentially used in many applications such as flexible electrodes, batteries, and strain sensors. The percolated conductive pathways are formed by conductive filler in polymer matrix which is a major effect on the electrical behavior of CPCs. Computational simulations have been used to study the percolation phenomena of CPCs. The simulation algorithms need to be developed and optimized for reducing the simulation time-consuming. In this study, the in-house Monte Carlo simulation that used to estimate percolation threshold is optimized. To simulate in the large-scale system, cut-off distance will be defined to avoid unnecessary complex calculations. The calculation sequence within the code has been rearranged to omit the unnecessary calculation processes. Results show that the optimized software takes less processing time than the previous version around 5 times. Therefore, we can perform the large system to investigate the percolation phenomenon with less lattice confinement effect.

Compressive deformation behavior and energy absorption characteristic of additively manufactured sheet CoCrMo triply periodic minimal surface lattices

CoCrMo sheet triply periodic minimal surface (sheet-TPMS) lattices with two different topologies (Neovius, IWP) were fabricated by laser power bed fusion (LPBF). Compressive behavior and energy absorption characteristic for each topology and direction were investigated. The unique pore channel structures of the fabricated Neovius and IWP lattices were well formed. The microstructural observation revealed that fine cellular substructures existed inside the sheets, and most of their constituent phases were confirmed to be the FCC phase. The results of the compression tests demonstrated that Neovius lattice had higher yield strength and first peak strength than IWP lattice, regardless of the compressive direction. As for the stress–strain curves of the lattices, an abrupt stress drop was found in neither of the two topologies. A comparison of the energy absorption characteristics confirmed that Neovius lattice had higher energy absorption efficiency and ideality than IWP lattice. The deformed structures showed that Neovius lattice displayed a gradual collapse mode, whereas a layer-by-layer compaction mode was indicated in IWP lattice. After the deformation, Neovius showed an evenly distributed and high fraction of the HCP phase caused by strain-induced martensitic transformation. The topology of lattice and strain-induced martensitic transformation effects on the compressive mechanical responses of the LPBF-built TPMS lattices were also discussed.

Bloch oscillations and the lack of the decay of the false vacuum in a one-dimensional quantum spin chain

We consider the decay of the false vacuum, realised within a quantum quench into an anti-confining regime of the Ising spin chain with a magnetic field opposite to the initial magnetisation. Although the effective linear potential between the domain walls is repulsive, the time evolution of correlations still shows a suppression of the light cone and a reduction of vacuum decay {via suppression of the growth of nucleated bubbles. The suppression of the bubble growth is a lattice effect, and can be assigned to emergent Bloch oscillations.

Isotropic finite-difference approximations for phase-field simulations of polycrystalline alloy solidification

Phase-field models of microstructural pattern formation during alloy solidification are commonly solved numerically using the finite-difference method, which is ideally suited to carry out computationally efficient simulations on massively parallel computer architectures such as Graphic Processing Units. However, one known drawback of this method is that the discretization of differential terms involving spatial derivatives introduces a spurious lattice anisotropy that can influence the solid-liquid interface dynamics. We find that this influence is significant for the case of polycrystalline dendritic solidification, where the crystal axes of different grains do not generally coincide with the reference axes of the finite-difference lattice. In particular, we find that with the commonly used finite-difference implementation of the quantitative phase-field model of binary alloy solidification, both the operating state of the dendrite tip and the dendrite growth orientation are strongly affected by the lattice anisotropy. To circumvent this problem, we use known methods in both real and Fourier space to derive finite-difference approximations of leading differential terms in 2D and 3D that are isotropic at order h2 of the lattice spacing h. Importantly, those terms include the divergence of the anti-trapping current that is found to have a critical influence on pattern selection. The 2D and 3D discretizations use an approximated form of the anti-trapping current that facilitates the Fourier-space derivation of the associated isotropic differential operator at O(h2), but we also derive a 2D discretization of the standard form of this current. Finally, we present 2D and 3D phase-field simulations of alloy solidification, showing that the isotropic finite-difference implementations dramatically reduce spurious lattice anisotropy effects, yielding both the tip operating state and growth direction of the dendrite that are nearly independent of the angle between the crystal and finite-difference lattice axes.

Emerging Long‐Range Order from a Freeform Disordered Metasurface

Recently, disordered metasurfaces have attracted considerable interest due to their potential applications in imaging, holography, and wavefront shaping. However, how to emerge long-range ordered phase distribution in disordered metasurfaces remains an outstanding problem. Here, a general framework is proposed to generate a spatially homogeneous in-plane phase distribution from a disordered metasurface, by engineering disorder parameters together with topology optimization. As a proof-of-concept demonstration, an all-dielectric disordered supercell metasurface with relatively homogeneous in-plane phase fluctuation is designed by disorder parameter engineering, manifesting as polarization conversion-dependent random scattering or unidirectional transmission. Then, a topology optimization approach is utilized to overcome the lattice coupling effect and to further improve the homogeneity of complex electric field fluctuation. In comparison with the initial supercell metasurface, both the phase fluctuation range and the relative efficiency of the topology-optimized freeform metasurface are significantly improved, leading to a long-range ordered electric field distribution. Moreover, three experimental realizations are performed, all of which agree well with the theoretical results. This methodology may inspire more exotic optical phenomena and find more promising applications in disordered metasurfaces and disorderedoptics.

Diffusive topological transport in spatiotemporal thermal lattices

Topological phases have been studied in photonic, acoustic and phononic metamaterials, promising a range of applications. Such topological modes usually stem from collective resonant effects in periodic lattices. One may, therefore, expect similar features to be forbidden for thermal diffusion that is purely dissipative and mostly incoherent, prohibiting collective resonances. Here we report the discovery of diffusion-based topological states supported by spatiotemporally modulated advections stacked over a fluidic surface. This arrangement imitates a periodic propagating potential in an effective thermal lattice. We observe edge states in topologically non-trivial and bulk states in topologically trivial lattices. Interface states form at boundaries between these two types of lattice, manifesting inhomogeneous thermal properties on the fluidic surface. Our findings establish a framework for topological diffusion and thermal edge or bulk states, and it may allow a distinct mechanism for the flexible manipulation of diffusive phenomena for robust heat and mass transfer.

Crystal field and lattice disorder effects on the low-temperature thermodynamic properties of TmF3

The temperature dependence of the heat capacity Cp(T), (2–300 K), and the lattice parameters a(T), b(T), and c(T) (5–300 K) TmF3 were experimentally studied. Anomalies in the studied characteristics were identified due to the influence on the thermal properties of trifluoride of the crystal field, and at the lowest temperatures (2–10 K), of a possible glass-like contribution due to disorder in the crystal lattice. As a result of our analysis of the excess contribution to the heat capacity (Schottky contribution, CSch(T)), we proposed a simplified scheme based on 1-1-7-5 splitting of the ground state of the Tm3+ ion, and determined the splitting parameters. The influence of the crystal field is also manifested through anomalies in the thermal expansion of TmF3. The temperature dependences of the unit cell volume of thulium trifluoride, and its heat capacity, were analysed using the Debye-Einstein model, and the model parameters are determined.

3D Self-Supported Binary PtCu Aerogel Boosted Methanol Oxidation

Precious metal aerogel has a broad application prospect in the field of fuel cell electrocatalysis because of its unique structure characteristics. In this work, Three-dimensional (3D) self-supporting porous PtCu aerogels have been successfully synthesized via a gelation strategy based on excess NaBH4. The mass activity of optimized PtCu aerogel in acid methanol solution is 6.56 times higher than that of JM-Pt/C. In addition, the optimized PtCu aerogel showed better stability than JM Pt/C. For the methanol oxidation reaction, the optimized PtCu aerogel still maintains the current density of 930 mA mg−1 Pt after 3600 s Chronoamperometric (CA) test. By X-ray photoelectron spectroscopy (XPS), we demonstrate the electron transfer between Cu and Pt. This would arouse the change of electronic state and atomic spacing on the surface of Pt and endow the PtCu aerogel with both prominent activity and excellent stability due to lattice stress and electron effect. This work provides a strategy for reasonable design high performance fuel cell electrocatalysts.

Formation and oxidation behavior of refractory high-entropy silicide (NbMoTaW)Si2 coating

A single high-entropy silicide (NbMoTaW)Si2 coating was prepared on NbMoTaW refractory alloy by a halide-activated pack cementation process. The formation mechanism and growth kinetics of the (NbMoTaW)Si2 coatings were analysed. The oxidation behavior of the (NbMoTaW)Si2 coatings was investigated at 1300 ℃ for 24 h in air. The results showed that the high entropy effect and sluggish effect of the high-entropy NbMoTaW alloy makes it form a single-phase silicide (NbMoTaW)Si2 during the Si diffusion growth process. The lattice distortion effect of substrate provides a channel for the diffusion of Si to accelerate its diffusion rate. The (NbMoTaW)Si2 coating effectively protects the NbMoTaW alloy from oxidation at 1300 ℃. The oxidation resistance of the coating is attributed to the preferential oxidation of Si to produce a dense continuous SiO2 scale and the precipitation of WSi2 to suppress the interdiffusion of the silicide layer and the alloy substrate.

Fractional Chern insulators with a non-Landau level continuum limit

Recent developments in fractional quantum Hall (FQH) physics highlight the importance of studying FQH phases of particles partially occupying energy bands that are not Landau levels. FQH phases in the regime of strong lattice effects, called fractional Chern insulators, provide one setting for such studies. As the strength of lattice effects vanishes, the bands of generic lattice models asymptotically approach Landau levels. In this paper, we construct lattice models for single-particle bands that are distinct from Landau levels even in this continuum limit. We describe how the distinction between such bands and Landau levels is quantified by band geometry over the magnetic Brillouin zone and reflected in the electromagnetic response. We analyze the localization-delocalization transition in one such model and compute a localization length exponent of 2.57(3). Moreover, we study interactions projected to these bands and find signatures of bosonic and fermionic Laughlin states. Most pertinently, our models allow us to isolate conditions for optimal band geometry and gain further insight into the stability of FQH phases on lattices.

Metal-insulator transition and local-moment collapse in negative charge transfer CaFeO3 under pressure

We compute the electronic structure, spin and charge state of Fe ions, and the structural phase stability of paramagnetic ${\mathrm{CaFeO}}_{3}$ under pressure using a fully self-consistent in charge density $\mathrm{DFT}+$ dynamical mean-field theory method. We show that at ambient pressure ${\mathrm{CaFeO}}_{3}$ is a negative charge transfer insulator characterized by strong localization of the Fe $3d$ electrons. It crystallizes in the monoclinic $P{2}_{1}/n$ crystal structure with a cooperative breathing mode distortion of the lattice. While the Fe $3d$ Wannier occupations and local moments are consistent with robust charge disproportionation of Fe ions in the insulating $P{2}_{1}/n$ phase, the physical charge density difference around the structurally distinct Fe $A$ and Fe $B$ ions with the ``contracted'' and ``expanded'' oxygen octahedra, respectively, is rather weak, of $\ensuremath{\sim}0.04$. This implies the importance of the Fe $3d$ and O $2p$ negative charge transfer and supports the formation of a bond-disproportionated state characterized by the Fe $A 3{d}^{5\ensuremath{-}\ensuremath{\delta}}{\underline{L}}^{2\ensuremath{-}\ensuremath{\delta}}$ and Fe $B 3{d}^{5}$ valence configurations with $\ensuremath{\delta}\ensuremath{\ll}1$, in agreement with strong hybridization between the Fe $3d$ and O $2p$ states. This complex interplay between electronic correlations, strong covalency, and lattice effects, resulting in bond disproportionation, is in many ways reminiscent of the behavior of rare-earth nickelates, $R{\mathrm{NiO}}_{3}$ $(R=\text{rare earth})$. Upon compression, ${\mathrm{CaFeO}}_{3}$ undergoes the metal-to-insulator phase transition (MIT) which is accompanied by a structural transformation into the orthorhombic $Pbnm$ phase. The phase transition is accompanied by suppression of the cooperative breathing mode distortion of the lattice and, hence, results in the melting of bond disproportionation of the Fe ions. Our analysis suggests that the MIT transition is associated with orbital-dependent delocalization of the Fe $3d$ electrons and leads to a remarkable collapse of the local magnetic moments. Our results imply the crucial importance of the interplay of electronic correlations and structural effects to explain the properties of ${\mathrm{CaFeO}}_{3}$.

Second-Sphere Lattice Effects in Copper and Iron Zeolite Catalysis.

Transition-metal-exchanged zeolites perform remarkable chemical reactions from low-temperature methane to methanol oxidation to selective reduction of NOx pollutants. As with metalloenzymes, metallozeolites have impressive reactivities that are controlled in part by interactions outside the immediate coordination sphere. These second-sphere effects include activating a metal site through enforcing an "entatic" state, controlling binding and access to the metal site with pockets and channels, and directing radical rebound vs cage escape. This review explores these effects with emphasis placed on but not limited to the selective oxidation of methane to methanol with a focus on copper and iron active sites, although other transition-metal-ion zeolite reactions are also explored. While the actual active-site geometric and electronic structures are different in the copper and iron metallozeolites compared to the metalloenzymes, their second-sphere interactions with the lattice or the protein environments are found to have strong parallels that contribute to their high activity and selectivity.

Lattice relaxation effect in RbxMA(1−x)PbBr3 single crystal to enhance optoelectronic performance of perovskite photodetectors

Lattice relaxation effect in RbxMA(1−x)PbBr3 single crystal to enhance optoelectronic performance of perovskite photodetectors

Asymmetric cooperative motion in one dimension

We prove distributional convergence for a family of random processes on $\mathbb{Z}$, which we call asymmetric cooperative motions. The model generalizes the "totally asymmetric hipster random walk" introduced in [Addario-Berry, Cairns, Devroye, Kerriou and Mitchell, 2020]. We present a novel approach based on connecting a temporal recurrence relation satisfied by the cumulative distribution functions of the process to the theory of finite difference schemes for Hamilton-Jacobi equations [Crandall and Lyons, 1984]. We also point out some surprising lattice effects that can persist in the distributional limit, and propose several generalizations and directions for future research.

Magnetic properties and neutron spectroscopy of lanthanoid-{tetrabromocatecholate/18-crown-6} single-molecule magnets

Lanthanoid single-molecule magnets (Ln-SMMs) exhibit slow magnetic relaxation at low temperatures. This arises from an energy barrier to magnetisation reversal associated with the crystal field (CF) splitting of the Ln(III) ion. The magnetic relaxation is impacted by the interaction of the molecule with the crystal lattice, so factors including particle size and crystal packing can play an important role. In this work, a family of compounds of general formula [Ln(18-c-6)(NO3)(Br4Cat)]·X (Ln = La, Tb, Dy; 18-c-6 = 18-crown-6; Br4Cat2− = tetrabromocatecholate) has been studied by inelastic neutron scattering (INS) and magnetometry to elucidate the effects of crystal packing on the slow magnetic relaxation of the Tb(III) and Dy(III) compounds. The deuterated analogues [Ln(18-c-6-d24)(NO3)(Br4Cat)]·CH3CN-d3 (1-LnD; Ln = La, Tb, Dy) have been synthesised, with 1-TbD and the diamagnetic analogue 1-LaD measured by INS. The dynamic magnetic properties of 1-TbD and 1-DyD have also been measured and compared for two samples with different particle sizes. To probe packing effects on the slow magnetic relaxation, two new solvatomorphs of the hydrogenous compounds [Ln(18-c-6)(NO3)(Br4Cat)]·X (2-Ln: X = CH2Cl2; 3-Ln: X = 0.5 toluene) have been obtained for Ln = Tb and Dy. The CF splitting between the ground and first excited CF pseudo-doublets has been experimentally determined for 1-TbD by INS, and strongly rare earth dependent and anharmonic lattice vibrational modes have also been observed in the INS spectra, with implications for slow magnetic relaxation. Dynamic magnetic measurements reveal significant particle-size dependence for the slow magnetic relaxation for 1-TbD, while a previously reported anomalous phonon bottleneck effect in the 1-DyD analogue does not change with particle size. Further dynamic magnetic measurements of 2-Ln and 3-Ln show that the slow magnetic relaxation in these Ln-SMMs is strongly dependent on lattice effects and crystal packing, which has implications for the future use of Ln-SMMs in devices.

Adiabatic models for the quantum dynamics of surface scattering with lattice effects.

In this contribution, we review models for the lattice effects in quantum dynamics calculations on surface scattering, which is important to modeling heterogeneous catalysis for achieving an interpretation of experimental measurements. Unlike dynamics models for reactions in the gas phase, those for heterogeneous reactions have to include the effects of the surface. For manageable computational costs in calculations, the effects of static surface (SS) are firstly modeled as this is simply and easily implemented. Then, the SS model has to be improved to include the effects of the flexible surface, that is the lattice effects. To do this, various surface models have been designed where the coordinates of the surface atoms are introduced in the Hamiltonian operator, especially those of the top surface atom. Based on this model Hamiltonian operator, extensive multi-dimension quantum dynamics calculations can be performed to recover the lattice effects. Here, we first review an overview of the techniques in constructing the Hamiltonian operator, which is a sum of the kinetic energy operator (KEO) and potential energy surface (PES). Since the PES containing the coordinates of the surface atoms in a cell is still expensive, the SS model is often accepted. We consider a mathematical model, called the coupled harmonic oscillator (CHO) model, to introduce the concepts of adiabatic and diabatic representations for separating the molecule and surface. Under the adiabatic model, we further introduce the expansion model where the potential function is Taylor expanded around the optimized geometry of the surface. By an expansion model truncated at the first and second order, various coupling surface models between the molecule and surface are derived. Moreover, by further and deeply understanding the adiabatic representation, an effective Hamiltonian operator is obtained by optimizing the total wave function in factorized form. By this factorized form of wave function and effective Hamiltonian operator, the geometry phase of the surface wave function is theoretically found. This theoretical prediction may be measured by carefully designing experiments. Finally, discussions on the adiabatic representation, the PES construction, and possibility of the classical-dynamics solutions are given. Based on these discussions, a simple outlook on the dynamics of photocatalytics is finally given.

Lattice relaxation and substrate effects of graphene moiré superlattice

When two two-dimensional (2D) materials with different lattice constants or with different rotation angles are superimposed, a moiré superlattice can be constructed. The electronic properties of the superlattice are strongly dependent on the stacking configuration, twist angle and substrate. For instance, theoretically, when the rotation angle of twisted bilayer graphene is reduced to a set of specific values, the so-called magic angles, flat bands appear near the charge neutrality, and the electron-electron interaction is significantly enhanced. The Mott insulator and unconventional superconductivity are detected in the twisted bilayer graphene with a twist angle around 1.1°. For a moiré pattern with a large enough periodicity, lattice relaxation caused by an interplay between van der Waals force and the in-plane elasticity force comes into being. The atomic relaxation forces atoms to deviate from their equilibrium positions, and thus making the system reconstructed. This review mainly focuses on the effects of the lattice relaxation and substrates on the electronic properties of the graphene superlattices. From both theoretical and experimental point of view, the lattice relaxation effects on the atomic structure and electronic properties of graphene-based superlattices, for example, the twisted bilayer graphene, twisted trilayer graphene, graphene-hexagonal boron nitride superlattice and twisted bilayer graphene-boron nitride superlattice are discussed. Finally, a summary and perspective of the investigation of the 2D material superlattice are presented.

Lattice relaxation effect in CsxMA(1-x)PbBr3 single crystal to enhance optoelectronic performance of perovskite photodetectors

Single crystal (SC) MAPbBr3 perovskite is regarded as a promising material to fabricate high-performance photodetectors (PDs) due to its impressive optoelectronic properties and good chemical stability. Cs+ doping has been proved to be an effective method to significantly improve the optoelectronic properties of MAPbBr3 SC. However, the intrinsic effect of Cs+ on the crystal structure is still unclear. Herein, we grew a series CsxMA(1-x)PbBr3 SCs. X-ray rocking curves (XRC) of CsxMA(1-x)PbBr3 SCs prove that appropriate Cs-doping concentration could reduce defect density effectively. X-ray photoelectron spectroscopy (XPS) measurements indicate that Cs-doping enhances the atoms interaction in CsxMA(1-x)PbBr3 lattice due to isoelectronic impurity effect. Single-crystal PDs were fabricated to compare the optoelectronic properties of MAPbBr3 SCs with different Cs-doping concentration, and the 2% Cs-doped CsxMA(1-x)PbBr3 PD shows the optimum performance. Density functional theory (DFT) calculations demonstrate that Cs-doping leads to lattice relaxation effect which is benefit to reduce trap density of CsxMA(1-x)PbBr3 SCs. The Pb-6p level of CsxMA(1-x)PbBr3 slightly shifts to lower energy region compared with MAPbBr3, resulting in the decrease of band gap of CsxMA(1-x)PbBr3.