Anisotropic Dispersion Research Articles

Observation of surface wave patterns modified by sub-surface shear currents

We report experimental observations of two canonical surface wave patterns – ship waves and ring waves – skewed by sub-surface shear, thus confirming effects predicted by recent theory. Observed ring waves on a still surface with sub-surface shear current are strikingly asymmetric, an effect of strongly anisotropic wave dispersion. Ship waves for motion across a sub-surface current on a still surface exhibit striking asymmetry about the ship’s line of motion, and large differences in transverse wavelength for upstream versus downstream motion are demonstrated, all of which is in good agreement with theoretical predictions. Neither of these phenomena can occur on a depth-uniform current. A quantitative comparison of measured versus predicted average phase shift for a ring wave is grossly mispredicted by no-shear theory, but in good agreement with predictions for the measured shear current. A clear difference in wave frequency within the ring wave packet is observed in the upstream versus downstream direction for all shear flows, while wave dispersive behaviour is identical to that for quiescent water for propagation normal to the shear current, as expected. Peak values of the measured two-dimensional Fourier spectrum for ship waves are shown to agree well with the predicted criterion of stationary ship waves, with the exception of some cases where results are imperfect due to the limited wavenumber resolution, transient effects and/or experimental noise. Experiments were performed on controlled shear currents created in two different ways, with a curved mesh and beneath a blocked stagnant-surface flow. Velocity profiles were measured with particle image velocimetry, and surface waves with a synthetic schlieren method. Our observations lend strong empirical support to recent predictions that wave forces on vessels and structures can be greatly affected by shear in estuarine and tidal waters.

Interactions of benzene, naphthalene, and azulene with alkali-metal and alkaline-earth-metal atoms for ultracold studies

We consider collisional properties of polyatomic aromatic hydrocarbon molecules immersed into ultracold atomic gases and investigate intermolecular interactions of exemplary benzene, naphthalene, and azulene with alkali-metal (Li, Na, K, Rb, and Cs) and alkaline-earth-metal (Mg, Ca, Sr, and Ba) atoms. We apply the state-of-the-art ab initio techniques to compute the potential energy surfaces (PESs). We use the coupled cluster method restricted to single, double, and noniterative triple excitations to reproduce the correlation energy and the small-core energy-consistent pseudopotentials to model the scalar relativistic effects in heavier metal atoms. We also report the leading long-range isotropic and anisotropic dispersion and induction interaction coefficients. The PESs are characterized in detail, and the nature of intermolecular interactions is analyzed and benchmarked using symmetry-adapted perturbation theory. The full three-dimensional PESs are provided for the selected systems within the atom-bond pairwise additive representation and can be employed in scattering calculations. The present study of the electronic structure is the first step toward the evaluation of prospects for sympathetic cooling of polyatomic aromatic molecules with ultracold atoms. We suggest azulene, an isomer of naphthalene which possesses a significant permanent electric dipole moment and optical transitions in the visible range, as a promising candidate for electric field manipulation and buffer-gas or sympathetic cooling.

Dielectric-loaded black phosphorus surface plasmon polariton waveguides.

By using the effective-index method (EIM) and the finite element methods (FEM), a surface plasmon polariton (SPP) waveguide structured by a dielectric ridge placed on monolayer black phosphorus is proposed and analyzed in the infrared spectral region. It is found that the strong anisotropic dispersion of the black phosphorus (BP) gives rise to direction-dependent waveguide modes in a dielectric-loaded black phosphorus waveguide (DLBPW). The effective mode index, propagation loss and cutoff wavelength of higher order modes are investigated along the armchair (AC) and the zigzag (ZZ) directions of the black phosphorus. Moreover, the propagation characteristics of single-mode are investigated for different widths of the dielectric ridge and different polarization directions of the black phosphorus. Via tuning the carrier density, the electromagnetic field propagation features can be effectively modified. Also, the coupling effect by adding more dielectric bridges can tune the propagation properties. These results will lead to great applications in black phosphorus-based optical integrated devices.

Unsupervised machine learning based on non-negative tensor factorization for analyzing reactive-mixing

Analysis of reactive-diffusion simulations representing complex mixing processes requires a large number of independent model runs. For each high-fidelity model simulation, the model inputs are varied and the predicted mixing behavior is represented by temporal and spatial changes in species concentration. It is then required to discern how the model inputs (such as diffusivity, dispersion, anisotropy, and velocity field properties) impact the mixing process. This task is challenging and typically involves interpretation of large model outputs representing temporal and spatial changes of species concentration within the model domain. However, the task can be automated and substantially simplified by applying Machine Learning (ML) methods. In this paper, we present an application of an unsupervised ML method (called NTFk) using Non-negative Tensor Factorization (NTF) coupled with a custom clustering procedure based on k-means to reveal the temporal and spatial features in product concentrations. An attractive and unique aspect of the proposed ML method is that it ensures the extracted features are non-negative, which is important to obtain a meaningful deconstruction of the mixing processes. The ML methodology is applied to a large set of high-resolution finite-element model simulations representing anisotropic reaction-diffusion processes in perturbed vortex-based velocity fields. The applied finite-element method ensures that spatial and temporal species concentration are always non-negative, even in the case of high anisotropic contrasts. The simulated reaction is a fast irreversible bimolecular reaction A+B→C, where species A and B react to form species C. The reactive-diffusion model input parameters that control mixing include properties of the velocity field (such as vortex structures), anisotropic dispersion, and molecular diffusion. We demonstrate the applicability of the ML feature extraction method to produce a meaningful deconstruction of model outputs to discriminate between different physical processes impacting the reactants, their mixing, and the spatial distribution of the product C. The presented ML analysis allowed us to identify additive temporal and spatial features that characterize mixing behavior. The application of the proposed NTFk approach is not limited to reactive-mixing. NTFk can be readily applied to any observed or simulated datasets that can be represented as tensors (multi-dimensional arrays) and have separable latent signatures or features.

Triclinic Pedial Sarcosinium Hydrogen l(+)‐Tartrate—Crystal Growth, Pyroelectricity, Thermal Expansion, and Linear Optical Properties

AbstractSingle crystals of optical quality and dimensions up to 25 × 25 × 40 mm3 of sarcosinium hydrogen l(+)‐tartrate and up to 15 × 20 × 30 mm3 of sarcosinium hydrogen d(−)‐tartrate are grown from aqueous solution during growth periods of 10–14 weeks. For sarcosinium hydrogen l(+)‐tartrate refractive indices and their dispersion in the wavelength range 365–1083 nm are determined. The biaxial positive crystals show marked optical anisotropy with maximum birefringence at 365 nm of Δn = 0.089 and only small orientation dispersion of the principal axes of the optical indicatrix. Based on the linear optical data, phase matching conditions for collinear second harmonic generation (SHG) are analyzed. The optical properties allow SHG phase matching for both, type I and type II processes. Temperature‐dependent coefficients of the tensor of thermal expansion [αij] between 145 and 300 K show pronounced anisotropy and moderate orientation dispersion of thermal expansion, with principal values at 293 K of = −3.3(1) × 10−6 K−1, = 35.7(1) × 10−6 K−1, and = 66.6(1) × 10−6 K−1. The absolute value of the pyroelectric vector |pσ| of sarcosinium hydrogen l(+)‐tartrate amounts to about 20 µC K−1 m−2 and varies only slightly with temperature in the temperature range 100–320 K.

Structural and electronic anisotropy, negative Poisson’s ratio, strain-sensitive Dirac-like cone in monolayer α-CSe: Tailoring electronic properties

Using density functional theory (DFT), a designed isoelectronic counterpart of phosphorene, black-phosphorus-analogous monolayer CSe (α-CSe) was confirmed to be stable. α-CSe displays apparently anisotropic behaviors of in-plane mechanics through calculating ideal tensile strength and critical strain, which thus causes anisotropic response to in-plane strains in its electronic and transport properties. It was found that the monolayer α-CSe can endure an ultimate stress of 4.008 N/m along armchair direction at εx=0.28, and 10.96 N/m along zigzag direction at εy=0.22. Intriguingly, the strain praxis of α-CSe under zigzag deformation highlights a negative out-of-plane Poisson’s ratio of −0.76 rooted in its puckered structure. Direct-to-indirect bandgap transitions and reverse transitions occurred alternatively under uniaxial and biaxial strains, which in essence issued in drastic variation of effective mass in k-place. Thus, the prior transport direction of carriers can be feasibly tuned by imposing biaxial strain. However, the preferred armchair transport direction is irrespective of strain size and present good mechanical stability, under the uniaxial strain. The anisotropic Dirac-like dispersions in α-CSe were identified at appropriate strain value, which unveiled the ballistic conduction along Γ-Y. Our numerical results offered meritorious information of puckered sheets by insight into α-CSe, and revealed the prospects of α-CSe in nano-devices applications.

Effect of Ga composition on the static and dynamic magnetic properties of Fe100-Ga films (18.5 ≤ x ≤ 33.4)

Abstract This paper describes the effect of Ga composition (x) on the static and dynamic magnetic properties of 50-nm-thick Fe100-xGax films (18.5 ≤ x ≤ 33.4). The in-plane damping constant (α//), the perpendicular (out-of-plane) damping constant (α⊥), the saturation magnetostriction (λs) and the maximum piezomagnetic coefficient ((dλ/dH)max) of these films are dependent on x, whereas the tendencies of α// and α⊥ for x are quite opposite to those of λs and (dλ/dH)max for x. For every x, the value of α// is much larger than that of α⊥, suggesting that both the intrinsic damping and the extrinsic damping coexists in the in-plane dynamics of each Fe100-xGax film because of magnetic inhomogeneities such as two magnon scattering, anisotropy dispersion and so on. These results also demonstrate that only the intrinsic damping contributes to the out-of-plane dynamics of each Fe100-xGax film.

Large-scale velocity dispersion and the cosmic web

Gravitational collapse in cosmological context produces an intricate cosmic web of voids, walls, filaments and nodes. The anisotropic nature of collisionless collapse leads to the emergence of an anisotropic velocity dispersion, or stress, that absorbs most of the kinetic energy after shell-crossing. In this paper, we measure this large-scale velocity dispersion tensor $\sigma^2_{ij}$ in $N$-body simulations using the phase-space interpolation technique. We study the environmental dependence of the amplitude and anisotropy of the velocity dispersion tensor field, and measure its spatial correlation and alignment. The anisotropy of $\sigma^2_{ij}$ naturally encodes the collapse history and thus leads to a parameter-free identification of the four dynamically distinct cosmic web components. We find this purely dynamical classification to be in good agreement with some of the existing classification methods. In particular, we demonstrate that $\sigma^2_{ij}$ is well aligned with the large-scale tidal field. We further investigate the influence of small scale density fluctuations on the large scale velocity dispersion, and find that the measured amplitude and alignments are dominated by the largest perturbations and thus remain largely unaffected. We anticipate that these results will give important new insight into the anisotropic nature of gravitational collapse on large scales, and the emergence of anisotropic stress in the cosmic web.

Kinematical evolution of Globular Clusters

AbstractWe present several results of the study of the evolution of globular clusters’ internal kinematics, as driven by two-body relaxation and the interplay between internal angular momentum and the external Galactic tidal field. Via a large suite of N-body simulations, we explored the three-dimensional velocity space of tidally perturbed clusters, by characterizing their degree of velocity dispersion anisotropy and their rotational properties. These studies have shown that a cluster’s kinematical properties contain distinct imprints of the cluster’s initial structural properties, dynamical history, and tidal environment. Building on this fundamental understanding, we then studied the dynamics of multiple stellar populations in globular clusters, with attention to the largely unexplored role of angular momentum.

Dynamical models of spheroidal multi-component stellar systems

Abstract. We present a significantly improved version of our numerical code JASMINE, that can now solve the Jeans equations for axisymmetric models of stellar systems, composed of an arbitrary number of stellar populations, a Dark Matter halo, and a central Black Hole. The stellar components can have different structural (density profile, flattening, mass, scale length), dynamical (rotational support, velocity dispersion anisotropy), and population (age, metallicity, Initial Mass Function, mass-to-light ratio) properties. These models, when combined with observations, will allow to investigate important issues, such as quantifying the systematic effects of IMF variations, of mass-to-light ratio gradients, and of different stellar kinematic components (e.g. counter rotating disks, kinematically decoupled cores) on luminosity-weighted properties. The developed analytical and numerical framework aims at modeling Early-Type Galaxies, but it can also be applied to dwarf Spheroidal galaxies and Globular Clusters.

Novel electric field effects on Landau levels in multi-Weyl semimetals

Multi-Weyl semimetals (WSMs) have an anisotropic non-linear dispersion along a 2-D plane and a linear dispersion in an orthogonal direction. They have topological charge ‘n' and are created when two or multiple Weyl points or nodes with nonzero net monopole charge are brought together onto a high-symmetry point. We study the perturbation corrections up to second order of such multi-WSMs in crossed electric (E) and magnetic (B) fields in the low electric field approximation. As a result, the first order correction lifts only n-fold degeneracy of the lowest Landau levels (LLs) while the higher Landau levels are modified due to the second order perturbation. We study the signatures of these corrections due to electric fields on the density of states (DOS) subjected to a magnetic field.

Disorder-induced exceptional points and nodal lines in Dirac superconductors

We consider the effect of disorder on the spectrum of quasiparticles in the point-node and nodal-line superconductors. Due to the anisotropic dispersion of quasiparticles disorder scattering may render the Hamiltonian describing these excitations non-Hermitian. Depending on the dimensionality of the system, we show that the nodes in the spectrum are replaced by Fermi arcs or Fermi areas bounded by exceptional points or exceptional lines, respectively. These features are illustrated by first considering a model of a proximity-induced superconductor in an anisotropic two-dimensional (2D) Dirac semimetal, where a Fermi arc in the gap bounded by exceptional points can be realized. We next show that the interplay between disorder and supercurrents can give rise to a 2D Fermi surface bounded by exceptional lines in three-dimensional (3D) nodal superconductors.

Heterogeneity, suspension, and yielding in sparse microfibrous cellulose gels 2: strain rate-dependent two-fluid behavior

Complex fluids are widely used in formulated products to impart rheological properties like stability, performance, and aesthetics. Yield stress fluids are a particularly important example, enabling applications like particle suspension, surface coating, and therapeutic delivery. Recent work has shown that particle suspension in yield stress fluids can be a strong function of yielding and flow heterogeneities, especially in anisotropic fiber dispersions. This work uses a microbubble technique to study the deformation before and during yielding of microfibrous cellulose suspensions. We note significant variations in suspension performance as a result of fiber rearrangement and heterogeneities. Strong strain rate dependencies are found to vary local network yield strength, and confocal microscopy quantifies structural reinforcement and deformation rate effects. The observed behavior indicates a two-fluid interpretation may help interpret sparse network flow and suspension properties.

Effect of Heat Treatment on the Dispersion of the Magnetic Anisotropy of MnSb Nanoinclusions Embedded in Thin GaMnSb Films

We observed a temperature-controlled increase in the magnetic anisotropy and its dispersion in thin GaMnSb films with MnSb nanoinclusions obtained by pulsed laser deposition. The data of transmission electron microscopy indicate that in the samples, a transition of the crystalline structure of magnetic MnSb nanoinclusions from hexagonal (spatial group (sp. gr.) P63/mmc) to cubic (sp. gr. F-43m) takes place. Analysis of the temperature dependences of the magnetic moment m(T), measured using a SQUID magnetometer, obtained for both unannealed and annealed samples cooled in a zero magnetic field and a magnetic field of 10 kOe, indicates that this mechanism is not unique. In unannealed samples, the distribution of the magnetic anisotropy of MnSb nanoinclusions, determined from the dependences of m(T), is unimodal. In the annealed samples, the same dependence becomes multimodal. This means that several thermally activated processes occur in the samples during annealing, resulting in several “populations” of nanoinclusions present in the annealed thin films. The contribution to the increase in the magnetic anisotropy during annealing can result in the structural phase transition, the mismatch of the crystal lattices between MnSb and GaSb, an increase in the average volume of MnSb nanoinclusions, and a change in their stoichiometry.

2D selenium allotropes from first principles and swarm intelligence

Combining the particle-swarm optimization method with first-principles calculations, we explore a new category of two-dimensional (2D) monolayers composed of solely the element selenium. Three stable structures are screened from outputs of crystal search computations, namely T-Se (1T-MoS2-like), C-Se (tiled 1D helical chain), and S-Se (square structure). Phonon calculations, as well as formation energy calculations have been performed to confirm the stability of the three phases. The electronic structure calculations show that both T-Se and C-Se are indirect-band-gap semiconductors, with gap values of 1.11 eV and 2.64 eV respectively when using the hybrid HSE06 functional. In particular, C-Se has a centrosymmetry-breaking structure which provides a spontaneous in-plane ferroelectric polarization of about 2.68 × 10−10 C m−1 per layer. Interestingly, S-Se has a Dirac cone that can open up a band gap of 0.11 eV if spin–orbit coupling is included. The tilted Dirac cone of S-Se shows anisotropic band dispersion as characterized with different Fermi velocities of 1.26 × 106 and 0.24 × 106 m s−1 around the Dirac point. Our works enrich the family of 2D materials of selenium allotropes and show that their versatile properties could give rise to potential application in various fields.

Anomalous Photothermoelectric Transport Due to Anisotropic Energy Dispersion in WTe2.

Band structures are vital in determining the electronic properties of materials. Recently, the two-dimensional (2D) semimetallic transition metal tellurides (WTe2 and MoTe2) have sparked broad research interest because of their elliptical or open Fermi surface, making distinct from the conventional 2D materials. In this study, we demonstrate a centrosymmetric photothermoelectric voltage distribution in WTe2 nanoflakes, which has not been observed in common 2D materials such as graphene and MoS2. Our theoretical model shows the anomalous photothermoelectric effect arises from an anisotropic energy dispersion and micrometer-scale hot carrier diffusion length of WTe2. Further, our results are more consistent with the anisotropic tilt direction of energy dispersion being aligned to the b-axis rather than the a-axis of the WTe2 crystal, which is consistent with the previous first-principle calculations as well as magneto-transport experiments. Our work shows the photothermoelectric current is strongly confined to the anisotropic direction of the energy dispersion in WTe2, which opens an avenue for interesting electro-optic applications such as electron beam collimation and electron lenses.

Quantum impurity in topological multi-Weyl semimetals

Multi-Weyl semimetals are new types of topological semimetals which have anisotropic nonlinear energy dispersion and whose topological charge can be 2 or more depending on the value of the winding number $J$. Here we investigate the Kondo effect of a spin-$1/2$ magnetic impurity in a multi-Weyl semimetal of different $J$ by using the variational wave function method. For different $J$, the binding energy is always positive in the presence of broken inversion symmetry, and a bound state is favored to form between the impurity and the host electrons. It is found that the formed bound state is more stable for larger $J$. Due to the spin-orbit coupling, the components of spin-spin correlation functions ${J}_{uv}(\mathbf{r})\phantom{\rule{4pt}{0ex}}(u,v=x,y,z)$ show strong anisotropy in the coordinate space. The spin-spin correlation indicates distinct decay behaviors along different directions due to the anisotropic dispersion in the energy band. In the case of $J=2$ and $J=3$, the rotational symmetries of the spatial spin-spin correlations are displayed. Especially, the components ${J}_{xx}$ and ${J}_{yy}$ and ${J}_{xy}$ and $\ensuremath{-}{J}_{yx}$ can be related through a $\ensuremath{\pi}/(2J)$ rotation.

Helicity Dynamics, Inverse, and Bidirectional Cascades in Fluid and Magnetohydrodynamic Turbulence: A Brief Review

We briefly review helicity dynamics, inverse and bidirectional cascades in fluid and magnetohydrodynamic (MHD) turbulence, with an emphasis on the latter. The energy of a turbulent system, an invariant in the nondissipative case, is transferred to small scales through nonlinear mode coupling. Fifty years ago, it was realized that, for a two‐dimensional fluid, energy cascades instead to larger scales and so does magnetic excitation in MHD. However, evidence obtained recently indicates that, in fact, for a range of governing parameters, there are systems for which their ideal invariants can be transferred, with constant fluxes, to both the large scales and the small scales, as for MHD or rotating stratified flows, in the latter case including quasi‐geostrophic forcing. Such bidirectional, split, cascades directly affect the rate at which mixing and dissipation occur in these flows in which nonlinear eddies interact with fast waves with anisotropic dispersion laws, due, for example, to imposed rotation, stratification, or uniform magnetic fields. The directions of cascades can be obtained in some cases through the use of phenomenological arguments, one of which we derive here following classical lines in the case of the inverse magnetic helicity cascade in electron MHD. With more highly resolved data sets stemming from large laboratory experiments, high‐performance computing, and in situ satellite observations, machine learning tools are bringing novel perspectives to turbulence research. Such algorithms help devise new explicit subgrid‐scale parameterizations, which in turn may lead to enhanced physical insight, including in the future in the case of these new bidirectional cascades.

The relationship between the morphology and kinematics of galaxies and its dependence on dark matter halo structure in EAGLE

We investigate the connection between the morphology and internal kinematics of the stellar component of central galaxies with mass $M_\star > {10}^{9.5} {\rm M}_\odot$ in the EAGLE simulations. We compare several kinematic diagnostics commonly used to describe simulated galaxies, and find good consistency between them. We model the structure of galaxies as ellipsoids and quantify their morphology via the ratios of their principal axes. We show that the differentiation of blue star-forming and red quiescent galaxies using morphological diagnostics can be achieved with similar efficacy to the use of kinematical diagnostics, but only if one is able to measure both the flattening and the triaxiality of the galaxy. Flattened oblate galaxies exhibit greater rotational support than their spheroidal counterparts, but there is significant scatter in the relationship between morphological and kinematical diagnostics, such that kinematically-similar galaxies can exhibit a broad range of morphologies. The scatter in the relationship between the flattening and the ratio of the rotation and dispersion velocities ($v/\sigma$) correlates strongly with the anisotropy of the stellar velocity dispersion: at fixed $v/\sigma$, flatter galaxies exhibit greater dispersion in the plane defined by the intermediate and major axes than along the minor axis, indicating that the morphology of simulated galaxies is influenced significantly by the structure of their velocity dispersion. The simulations reveal that this anisotropy correlates with the intrinsic morphology of the galaxy's inner dark matter halo, i.e. the halo's morphology that emerges in the absence of dissipative baryonic physics. This implies the existence of a causal relationship between the morphologies of galaxies and that of their host dark matter haloes.

Modelling the curing process in particle-filled electro-active polymers with a dispersion anisotropy

Even for a moderate actuation, a large electric voltage requirement hinders the application of electro-active polymers (EAPs) in many areas. Hence, among other mechanisms, the actuation enhancement in EAPs is performed via inclusions of high-dielectric-permittivity fillers in the matrix material in the uncured stage. Moreover, to obtain an optimum advantage from the high-dielectric-permittivity fillers, an electric field can be applied during the curing process which helps the particles to align in a preferred direction. To be specific, recent experimental evidences show that these particles form a dispersed anisotropy rather than a perfect transverse anisotropic structure. The polymer curing process is a complex (visco-) elastic phenomenon where a liquid polymer gradually transforms into a solid macromolecular structure due to cross-linking of the initial solution of short polymer chains. This phase transition comes along with an increase in the material stiffness and a volume shrinkage. In this paper we present a phenomenologically inspired large strain framework for simulating the curing process of particle-filled electro-active polymers with a dispersion-type anisotropy that can work under the influence of an electro-mechanically coupled load. The application of the proposed approach is demonstrated with some numerical examples. These examples illustrate that the model can predict common features in particle-filled dispersed electro-active polymers undergoing curing processes in the presence of an electro-mechanically coupled load.