Anisotropic Dispersion Research Articles

Anisotropic polaritons in van der Waals materials

AbstractPolaritons in two‐dimensional (2D) materials continues to garner significant attention due to their favorable ability of field‐confinement and intriguing potential for low‐loss and ultrafast optical and photonic devices. The recent experimental observation of in‐plane anisotropic dispersion in natural van der Waals materials has revealed much richer physics as compared to isotropic plasmonic materials, which provides new insight to manipulate the polaritons and manufacture flat optical devices with unprecedented controls. Herein, we give an overview of the recent progress in in‐plane anisotropic polaritons launched and visualized in the near‐field range in 2D layered van der Waals materials. Furthermore, future prospects in this promising but emerging field are featured on the basis of its peculiar applications. This review article will stimulate the scientific community to explore other hyperbolic materials and structures in order to develop optical technologies with novel functionalities and further improve the understanding of the exotic photonic phenomena.image

Improved fibre dispersion estimation using b-tensor encoding

Measuring fibre dispersion in white matter with diffusion magnetic resonance imaging (MRI) is limited by an inherent degeneracy between fibre dispersion and microscopic diffusion anisotropy (i.e., the diffusion anisotropy expected for a single fibre orientation). This means that estimates of fibre dispersion rely on strong assumptions, such as constant microscopic anisotropy throughout the white matter or specific biophysical models. Here we present a simple approach for resolving this degeneracy using measurements that combine linear (conventional) and spherical tensor diffusion encoding.To test the accuracy of the fibre dispersion when our microstructural model is only an approximation of the true tissue structure, we simulate multi-compartment data and fit this with a single-compartment model. For such overly simplistic tissue assumptions, we show that the bias in fibre dispersion is greatly reduced (~5x) for single-shell linear and spherical tensor encoding data compared with single-shell or multi-shell conventional data. In in-vivo data we find a consistent estimate of fibre dispersion as we reduce the b-value from 3 to 1.5 ms/μm2, increase the repetition time, increase the echo time, or increase the diffusion time. We conclude that the addition of spherical tensor encoded data to conventional linear tensor encoding data greatly reduces the sensitivity of the estimated fibre dispersion to the model assumptions of the tissue microstructure.

Sensitive chemical potential sensor based on graphene hyperbolic metamaterials

We have studied the anisotropic dispersion properties of hyperbolic metamaterials having a graphene/dielectric periodic structure and proposed a sensitive chemical potential sensor. It is found that the equifrequency contour of the structure will transit from an ellipse to a hyperbola as the chemical potential of graphene increases over a certain critical value. Interestingly, as the chemical potential increases close to the critical value, the transmittance varies very sensitively and nearly linearly in a wide range but with a small change of chemical potential. Based on these unique properties, we design a sensitive chemical potential sensor with advantages of fast response and high sensitivity simultaneously. Additionally, the measurement range of chemical potential can be expanded by adjusting the working frequency conveniently. These results have potential in many application fields such as beam control and sensing.

Nematic ordering of hard rods under strong confinement in a dense array of nanoposts.

The effect of confinement on the behavior of liquid crystals is interesting from a fundamental and practical standpoint. In this work, we report Monte Carlo simulations of hard rods in an array of hard nanoposts, where the surface-to-surface separations between nanoposts are comparable to or less than the length of hard rods. This particular system shows promise as a means of generating large-scale organization of the nematic liquid by introducing an entropic external field set by the alignment of nanoposts. The simulations show that nematic ordering of hard rods is enhanced in the nanopost arrays compared with that in bulk, in the sense that the nematic order is significant even at low concentrations at which hard rods remain isotropic in bulk, and the enhancement becomes more significant as the passage width between two nearest nanoposts decreases. An analysis of local distribution of hard-rod orientations at low concentrations with weak nematic ordering reveals that hard rods are preferentially aligned along nanoposts in the narrowing regions between two curved surfaces of nearest nanoposts; hard rods are less ordered in the passages and in the centers of interpost spaces. It is concluded that at low concentrations the confinement in a dense array of nanoposts induces the localized nematic order first in the narrowing regions and, as the concentration further increases, the nematic order spreads over the whole region. The formation of a well-ordered phase at low concentrations of hard rods in a dense array of nanoposts can provide a new route to the low-concentration preparation of nematic liquid crystals that can be used as anisotropic dispersion media.

Topological superconductivity from transverse optical phonons in oxide heterostructures

A topological superconductor features at its boundaries and vortices Majorana fermions, which are potentially applicable for topological quantum computations. The scarcity of the known experimentally verified physical systems with topological superconductivity, time-reversal invariant ones in particular, is giving rise to a strong demand for identifying new candidate materials. In this research, we study a heterostructure consisting of a transition metal oxide two-dimensional electron gas (2DEG) sandwiched by insulators near the paraelectric (PE) / ferroelectric (FE) phase transition. Its relevant characteristics is the combination of the transition metal spin-orbit coupling and the soft odd-parity phonons arising from the ferroelectric fluctuation; it gives rise to the fluctuating Rashba effect, which can mediate the pairing interaction for time-reversal invariant topological superconductivity. As the PE / FE phase transition can be driven by applying strain on the heterostructure, this system provides a tunable electron-phonon coupling. Through the first-principle calculations on the (001) [BaOsO3][BaTiO3]4, we find such electron-phonon coupling to be strong over a wide range of applied tensile bi-axial strain in the monolayer BaOsO3 sandwiched between the (001) BaTiO3, hence qualifying it as a good candidate material. Furthermore, the stability of topological superconductivity in this material is enhanced by its orbital physics that gives rise to the anisotropic dispersion.

Limitations to the ‘basic’ HOD model and beyond

ABSTRACT We use the IllustrisTNG cosmological, hydrodynamical simulations to test fundamental assumptions of the mass-based halo occupation distribution (HOD) approach to modelling the galaxy–halo connection. By comparing the clustering of galaxies measured in the 300 Mpc TNG box (TNG300) with that predicted by the standard (basic) HOD model, we find that, on average, the ‘basic’ HOD model underpredicts the real-space correlation function in the TNG300 box by ∼15 per cent on scales of $1 \,\,\lt\,\, r \,\,\lt\,\, 20 \ {\rm Mpc}\, h^{-1}$, which is well beyond the target precision demanded of next-generation galaxy redshift surveys. We perform several tests to establish the robustness of our findings to systematic effects, including the effect of finite box size and the choice of halo finder. In our exploration of ‘secondary’ parameters with which to augment the ‘basic’ HOD, we find that the local environment of the halo, the velocity dispersion anisotropy, β, and the product of the half-mass radius and the velocity dispersion, σ2Rhalfmass, are the three most effective measures of assembly bias that help reconcile the ‘basic’ HOD-predicted clustering with that in TNG300. In addition, we test other halo properties such as halo spin, formation epoch, and halo concentration. We also find that at fixed halo mass, galaxies in one type of environment cluster differently from galaxies in another. We demonstrate that a more complete model of the galaxy–halo connection can be constructed if we combine both mass and local environment information about the halo.

Metasurface-Enabled Advanced Short Backfire Antenna

Metasurfaces with independently tunable anisotropic dispersion characteristics have been designed to significantly improve the radiation efficiency of a short backfire antenna (SBFA) when placed within the antenna cavity walls. This advanced SBFA (A-SBFA) features a hexagonal aperture, which typically results in much lower aperture efficiency and increased cross polarization. However, the addition of metasurfaces leads to a higher aperture efficiency than is achievable with a conventional circular SBFA. Through careful design and optimization of the metasurface dispersion, the A-SBFA achieves high aperture efficiency across two discrete frequency bands while maintaining a relatively low height and weight. A detailed design and optimization methodology is presented here, along with the simulation studies that explore the design space and provide insight into how the antenna performance is affected as the metasurface parameters are adjusted. The A-SBFA is shown to be a promising candidate for future satellite-based antenna arrays due to its high aperture efficiency, hexagonal cavity, low weight, and low profile.

Electronic properties and polaronic dynamics of semi-Dirac system within Ladder approximation

We investigate the electronic properties of the semi-Dirac system and its polaronic dynamics when coupled with a Fermi bath with quadratic dispersion. The electronic anisotropic transport properties and the semiclassical dynamics of the semi-Dirac system are studied, including the density-of-states, conductivity, transport relaxation rate, specific heat, electrical current density, and free energy. The attractive polaron formed as the semi-Dirac impurity dressed with the particle–hole excitations in a two-dimensional system are studied both analytically and numerically. The pair propagator, self-energy, spectral function are being detailly calculated and discussed. The method of medium T-matrix approximation (nonself-consistent), which equivalent to the partially dressed interaction vertex by summing over all ladder diagrams, is applied, and compared with some other methods (for many-body problem), like the leading-order 1/N expansion (GW approximation), Hartree–Fock theory, and the Nozieres–Schmitt–Rink theory. Since we focus on the weak-coupling region, the mean-field approximation is also applicable. The polaron properties is related to the anisotropic effective masses of the semi-Dirac system. That’s in contrast to the polarons formed in the surface of normal Dirac systems which has an isotropic dispersion, since the anisotropic dispersion of the semi-Dirac systems results in anisotropic effective mass and anisotropic charge carrier transport. Besides, the symmetry between electron and hole is also broken since the effective masses of the electron and hole are different, which are affected by the polaronic effect when the semi-Dirac material is deposited on a polar substrate, like hBN. The self-localization, short-range potential, and experimental methods as well as the possible formation of the Bose polaron on the surface of semi-Dirac system are also discussed in the end. Our results are useful also for the investigation of two/three-dimensional bosonic polaron as well as the polarons in other solid state systems, like the magnetic matter or the topological systems.

A discrete chemo-dynamical model of M87’s globular clusters: Kinematics extending to ∼400 kpc

ABSTRACT We study the mass distribution and kinematics of the giant elliptical galaxy M87 (NGC 4486) using discrete chemo-dynamical, axisymmetric Jeans equation modelling. Our catalogue comprises 894 globular clusters (GCs) extending to a projected radius of ∼430 kpc with line-of-sight velocities and colours, and Multi Unit Spectroscopic Explorer integral field unit data within the central 2.4 kpc of the main galaxy. The gravitational potential for our models is a combination of a luminous matter potential with a varying mass-to-light ratio for the main galaxy, a supermassive black hole and a dark matter (DM) potential with a cusped or cored DM halo. The best-fitting models with either a cusped or a cored DM halo show no significant differences and both are acceptable. We obtain a total mass of $(2.16 \pm 0.38) \times 10^{13} \, \mathrm{M}_{\odot }$ within ∼400 kpc. By including the stellar mass-to-light ratio gradient, the DM fraction increases from ∼26 per cent (with no gradient) to ∼73 per cent within $1\, R_e^{\rm maj}$ (major axis of half-light isophote, 14.2 kpc), and from ∼84 per cent to ∼94 per cent within $5\, R_e^{\rm maj}$ (71.2 kpc). Red GCs have moderate rotation with Vmax/σ ∼ 0.4, and blue GCs have weak rotation with Vmax/σ ∼ 0.1. Red GCs have tangential velocity dispersion anisotropy, while blue GCs are consistent with being nearly isotropic. Our results suggest that red GCs are more likely to be born in situ, while blue GCs are more likely to be accreted.

Finite dimensional global attractor for a damped fractional anisotropic Schrödinger type equation with harmonic potential

We study the long time behaviour of the solutions for a class of nonlinear damped fractional Schrödinger type equation with anisotropic dispersion and in presence of a quadratic potential in a two dimensional unbounded domain. We prove that this behaviour is characterized by the existence of regular compact global attractor with finite fractal dimension.

Fingering regimes in unstable miscible displacements

We study the life-cycle of miscible fingering, from the early fingering initiation, through their growth and nonlinear interactions to their decay to a single finger at late times. Dimensionless analysis is used to relate the number of fingers, the nature of their nonlinear interactions (spreading, coalescence, tip splitting), and their eventual decay to the viscosity ratio, transverse Peclet number, and anisotropic dispersion. We show that the initial number of fingers that grow is approximately half that predicted by analytical solutions that neglect the impact of longitudinal diffusion smearing the interface between the injected solvent and the displaced fluid. The growth rates of these fingers are also approximately one quarter that predicted by these analyses. Nonetheless, we find that the dynamics of finger interactions over time can be scaled using the most dangerous wavenumber and associated growth rate determined from linear stability analysis. This subsequently allows us to provide a relationship that can be used to estimate when predict when the late time, single finger regime will occur.

Barrier tunneling of quasiparticles in double-Weyl semimetals

We investigate barrier tunneling of quasiparticles in double-Weyl semimetals. Being the anisotropic dispersion, it exhibits different scattering properties for different barrier orientations. We discuss two cases. In the first case where the barrier is perpendicular to linear momentum direction, we show that the incoming quasiparticles can fully pass through the barrier at normal incidence. This is known as Klein tunneling, a familiar phenomenon in the Graphene. The full transmission also occurs for some other special angles determined by the resonance condition between the barrier width and the wave vector in the barrier. In the second case where the barrier is perpendicular to quadratic momentum direction, the incoming quasiparticles can be fully reflected by the barrier at normal incidence, which is known as anti-Klein tunneling. As the energy of incident quasiparticles changes, it induces a transition from Klein tunneling to anti-Klein tunneling. When the band gap is opened, it has impact on the chirality and Berry phase. So the transmission coefficient is also changed.

Virial models and anisotropy of velocity dispersion in E-galaxies

A tensor virial approach is used to construct detailed models of 22 flattened ellipticals. The models are combined with the new observational data: extended surface brightness distributions, the profiles of isophotes flattening and their twisting, the rotation curves and the velocity dispersion profiles. A key feature of these models is a rigorous consideration of the influence of the spatial shape of galaxies (oblate spheroid or triaxial ellipsoid) on the dynamics, as well as the structure of the internal density layers in these systems. For each galaxy the ratio of the rotational energy to the potential energy was found. Comparing this ratio with the observed it is concluded that the majority of these systems cannot have isotropic velocity dispersion tensors. The anisotropy parameter limited to the interval 0.0-0.224. A correlation was found between and flattening of the outer regions for giant E-galaxies. Our results are compared with the results of other researchers. It was found that for small fast rotators our values \b{eta} are in good agreement with those, obtained in the ATLAS project. However, for giant E-galaxies, our models provide better agreement with observations, than axisymmetric JAM models. In addition, our velocity dispersion anisotropy results are in satisfactory agreement with the results of high resolution cosmological simulations in the Illustris project

Regional variations in ex-vivo diffusion tensor anisotropy are associated with cardiomyocyte remodeling in rats after left ventricular pressure overload

BackgroundPressure overload left ventricular (LV) hypertrophy is characterized by increased cardiomyocyte width and ventricle wall thickness, however the regional variation of this remodeling is unclear. Cardiovascular magnetic resonance (CMR) diffusion tensor imaging (DTI) may provide a non-invasive, comprehensive, and geometrically accurate method to detect regional differences in structural remodeling in hypertrophy. We hypothesized that DTI parameters, such as fractional and planar anisotropy, would reflect myocyte remodeling due to pressure overload in a regionally-dependent manner.MethodsWe investigated the regional distributions of myocyte remodeling in rats with or without transverse aortic constriction (TAC) via direct measurement of myocyte dimensions with confocal imaging of thick tissue sections, and correlated myocyte cross-sectional area and other geometric features with parameters of diffusivity from ex-vivo DTI in the same regions of the same hearts.ResultsWe observed regional differences in several parameters from DTI between TAC hearts and SHAM controls. Consistent with previous studies, helix angles from DTI correlated strongly with those measured directly from histological sections (p < 0.001, R2 = 0.71). There was a transmural gradient in myocyte cross-sectional area in SHAM hearts that was diminished in the TAC group. We also found several regions of significantly altered DTI parameters in TAC LV compared to SHAM, especially in myocyte sheet angle dispersion and planar anisotropy. Among others, these parameters correlated significantly with directly measured myocyte aspect ratios.ConclusionsThese results show that structural remodeling in pressure overload LV hypertrophy is regionally heterogeneous, especially transmurally, with a greater degree of remodeling in the sub-endocardium compared to the sub-epicardium. Additionally, several parameters derived from DTI correlated significantly with measurements of myocyte geometry from direct measurement in histological sections. We suggest that DTI may provide a non-invasive, comprehensive method to detect regional structural myocyte LV remodeling during disease.

Numerical study of the conductive liquid metal elastomeric composites

Flexible composites of liquid metal droplets dispersed in compliant elastomers display multifunctional properties that could be leveraged for applications in soft robotics and electronics. One function of these liquid composites is the unique, tunable thermal-mechanical properties. While thermal conductivity in these systems has been studied experimentally and with effective medium theories, robust computational models remain underdeveloped. Computational models could shed light on the effect of microstructure on material properties, therefore guiding material design. In this work, a fast Fourier transform (FFT) micromechanical computational methodology was developed for quick and efficient analysis of the effective thermal properties of these soft matter composites. The work presents the formulation and implementation of the methods needed for this purpose. Computationally predicted thermal properties are compared against experiments to gauge the validity of the modeling method. The critical effect of anisotropic liquid metal dispersion observed in these systems on the effective thermal conductivity of the composite is also presented. The study wraps up with some concluding remarks and suggestions for future work.

THE GLOBAL CAUCHY PROBLEM FOR THE NLS WITH HIGHER ORDER ANISOTROPIC DISPERSION

AbstractWe use a method developed by Strauss to obtain global well-posedness results in the mild sense and existence of asymptotic states for the small data Cauchy problem in modulation spaces${M}^s_{p,q}(\mathbb{R}^d)$, whereq= 1 and$s\geq0$or$q\in(1,\infty]$and$s&gt;\frac{d}{q'}$for a nonlinear Schrödinger equation with higher order anisotropic dispersion and algebraic nonlinearities.

Carbon nanotube array as a van der Waals two-dimensional hyperbolic material

We use an ab-initio approach to design and study a novel two-dimensional material - a planar array of carbon nanotubes separated by an optimal distance defined by the van der Waals interaction. We show that the energy spectrum for an array of quasi-metallic nanotubes is described by a strongly anisotropic hyperbolic dispersion and formulate a model low-energy Hamiltonian for its semi-analytical treatment. Periodic-potential-induced lifting of the valley degeneracy for an array of zigzag narrow-gap nanotubes leads to the band gap collapse. In contrast, the band gap is opened in an array of gapless armchair tubes. These unusual spectra, marked by pronounced van Hove singularities in the low-energy density of states, open the opportunity for interesting physical effects and prospective optoelectronic applications.

Inverse-perovskites A3BO (A = Sr, Ca, Eu/B = Pb, Sn): A platform for control of Dirac and Weyl Fermions

Bulk Dirac electron systems have attracted strong interest for their unique magnetoelectric properties as well as their close relation to topological (crystalline) insulators. Recently, the focus has been shifting toward the role of magnetism in stabilizing Weyl fermions as well as chiral surface states in such materials. While a number of nonmagnetic systems are well known, experimental realizations of magnetic analogs are a key focus of current studies. Here, we report on the physical properties of a large family of inverse perovskites A3BO (A = Sr, Ca, Eu/B = Pb, Sn) in which we are able to not only stabilize 3D Dirac electrons at the Fermi energy but also chemically control their properties. In particular, it is possible to introduce a controllable Dirac gap, change the Fermi velocity, tune the anisotropy of the Dirac dispersion, and—crucially—introduce complex magnetism into the system. This family of compounds therefore opens up unique possibilities for the chemical control and systematic investigation of the fascinating properties of such topological semimetals.

Underwater acoustic surface waves on a periodically perforated metal plate.

Acoustic surface waves are supported at the surface of appropriately structured elastic materials. Here the excitation and propagation of the lowest-order surface mode supported by a square array of open-ended cavities on a metal plate submerged in water is demonstrated. This mode, which has a half-wavelength character in the cavity, arises due to inter-cavity interaction by evanescent diffraction of the pressure field, and forms a band from zero-frequency to an asymptotic limit frequency. The authors perform an acoustic characterization of the pressure field close to the surface of the perforated plate in the 60-100 kHz frequency range; sound is pulsed from a fixed point-like acoustic source, and the evolution of the acoustic field across the sample surface is measured as a function of time and space with a traversing detector. Using Fourier analysis, the dispersion is imaged between points of high-symmetry (Γ,X,M) and at planes in momentum-space at fixed frequencies. Beaming of acoustic energy on the surface over a narrow frequency band was observed, caused by the anisotropic mode dispersion of the acoustic surface wave on the square lattice. The measured dispersion shows good agreement with the predictions of a numerical model.

Anisotropy and Inhomogeneity in Drifter Dispersion

AbstractOcean flows are known to be locally anisotropic and inhomogeneous. Nonetheless, the ocean's statistical dispersion properties are traditionally assumed to be isotropic and homogeneous. Here, we investigate the effect of local anisotropy and inhomogeneity on dispersion statistics, using a unique data set of roughly 300 near‐surface drifters that were launched within 10 days in the summer of 2012. The unique launch strategy based on nested triplets resulted in an unusually large number of nearly colocated drifter pairs. Thus, this data set is ideally suited for an estimate of the directional bias and inhomogeneity effects inherent in drifter pair statistics.Several metrics are proposed to assess the time evolution of anisotropy and inhomogeneity effects at multiple initial separation scales (100 m, 200 m, 500 m, 1 km, and 10 km). Locally, statistically significant anisotropy and inhomogeneity are observed at all scales, although anisotropy is noticeably less at 10 km, suggesting that the energetic processes driving anisotropic dispersion operate primarily at smaller scales. Moreover, averaged over a sufficient variety of different flow regimes, the signature of both anisotropy and inhomogeneity in dispersion metrics lessens. These trends hold generally across all scales, with longer time scales associated with larger spatial scales. The results indicate that oceanic dispersion is statistically isotropic and homogeneous over large swaths, but for an application in a specific location, local anisotropy and inhomogeneity matter. What size swath is large enough is situation dependent: For this specific data set, statistics had to be evaluated over multiple deployments, giving a required area greater than 150 km2.