Parallel Wave Vector Research Articles

Highly Precise Magnetic Field Measurement Based RF-Assisted Sagnac Interferometer With Tapered Fiber

A radio frequency (RF)-assisted Sagnac interferometer based on a dual-loop optoelectronic oscillator (OEO) is experimentally demonstrated for high-precision magnetic field measurement, in which the tapered fiber covered with the magnetic fluid (MF) as the magnetic field sensing head is embedded in the Sagnac interferometer. The evanescent field of the tapered fiber can interact with the MF under the external magnetic field to cause the birefringence variation of the fundamental mode, leading to the change in the free spectral range (FSR) of the interferometer, which can be mapped to the oscillation frequency shift of the OEO in the microwave domain. By the above converting, the magnetic field measurement with high interrogation speed and resolution can be realized. In addition, the designed device shows a certain measurement directionality of the magnetic field due to two orthogonally polarized fundamental modes asymmetric to the magnetic field, obtaining a good conformity with the constructed theoretical models. The experimental results show the maximum magnetic field sensitivities of 159.4 Hz/mT in the range of 8.48-27.83 mT, and 350.8 Hz/mT in the range of 0-5.14 mT, corresponding to the light wave vector parallel and perpendicular to the magnetic field, respectively.

Ripple patterns over oblique Ar+ sputtered SiC/Si(1 1 1) surfaces: Role of preferential sputtering

Controlled surface structuring of amorphous silicon carbide (a-SiC) thin films as grown on silicon substrates Si(111) by Radio Frequency (RF) sputtering and irradiation of obliquely incident 80 keV Ar+ ion beam has been investigated. Sub-wavelength ripple patterns with wave-vector parallel to the ion beam projection are found to evolve on argon sputtered surfaces. Studies reveal that the temporal parameters such as ripple wavelength and amplitude, ordering and homogeneity of these patterns vary non-linearly with argon ion fluence. The formation of such surface structures is attributed to the preferential sputtering of silicon in comparison to carbon. Our results reveal that ion irradiation can be exploited to fabricate controlled surface nanostructures by optimizing the irradiation parameters.

Anomalous Reflection with Omnidirectional Active Metasurfaces Operating in Free Space

Metasurfaces that manipulate the reflection of impinging sound have recently received significant attention, but virtually all past designs are passive structures with high temporal and spatial dispersion and thus have narrow bandwidths and are unidirectional. In this work, we introduce a method to design omnidirectional and broadband anomalous acoustic reflectors composed of nonresonant active scatterers arranged in a periodic lattice of subwavelength periodicity. The method relies on the manipulation of the wave-vector component parallel to the reflector's surface (i.e., transverse component) in a broadband manner and for arbitrary impinging sound directions. To illustrate our method experimentally, we design and measure an anomalous reflecting metasurface operating in free space for which the transverse wave-vector components of the incident and reflected waves differ by a prescribed constant. We show that the effect does not depend on the direction of incidence and has a bandwidth an order of magnitude larger than passive anomalous reflectors. This work shows that the active scatterers behave similarly to the atoms of a natural material rather than the unit cells of gratings or phononic crystals in that rearranging the active scatterers changes the metasurface geometry while preserving the device's desired functionality.

Two-dimensional internal gravity wave beam instability. Linear theory and subcritical instability

The instability of propagating internal gravity waves (IGWs) is of long-standing interest in geophysical fluid dynamics since breaking IGWs exchange energy and momentum with the large-scale flow and hence they support the large-scale circulation. In this study a low-order IGW beam model is used to delineate both linear and so called non-modal transient instability. In the first part of the study, linear normal mode instability of a wave beam consisting of two finite-amplitude plane monochromatic IGWs with the same frequency and parallel wave vectors of different magnitude is investigated using the Galerkin method. It is concluded that the wave beam is linearly more unstable than its constituent plane waves, taken separately. The degree of instability increases with the separation of the constituent waves in the wave number space, that is, with the wave beam concentration in the physical space. The narrower a wave beam is, the more linearly unstable it is. In its turn, transient instability typically occurs for linearly stable flows or before linear instability can set in (subcritical instability) if the governing system matrix is non-normal. In the second part of the paper, first the non-normality of the linear system matrix of the wave beam model is examined by computing measures like the Henrici number, the pseudospectrum, and the range of the matrix. Subsequently, the robustness of the transient growth is studied when the initial condition for optimal growth is randomly perturbed. It is concluded that for full randomisation, in particular, shallow wave beams can show subcritical growth when entering a turbulent background field. Such growing and eventually breaking wave beams might add turbulence to existing background turbulence that originates from other sources of instability. However, the robustness of transient growth for wave beam perturbations depends strongly on the strength of randomisation of the initial conditions, the beam angle and the perturbation wavelength.

The moment generating function of pairwise velocity in the context of redshift space distortion

We study the connections between the pairwise velocity moment generating function G(k ∥, r) and redshift space distortion (RSD) modeling. Here k ∥ is the Fourier wavevector parallel to the line of sight, and r is the pair separation vector. We demonstrate its usage by two examples. (1) Besides the known relations between G and the RSD power spectrum (and the correlation function), we propose a new RSD statistics Ps (k ∥,r ⊥) whose connection to G is convenient to evaluate numerically. (2) We then develop a fast method to numerically evaluate G, and apply it to a high resolution N-body simulation. We find that G (ln G) shows complicated dependence on k ∥ beyond the linear and quadratic dependencies. This not only shows inaccuracy in some existing models and identifies sources of inaccuracy but also provides possible ways of improving the RSD modeling. Consequently, more comprehensive investigations on G are needed to fully explore the usage of G in RSD modeling.

Graphene-Based One-Dimensional Terahertz Phononic Crystal: Band Structures and Surface Modes.

In this paper, we provide a theoretical and numerical study of the acoustic properties of infinite and semi-infinite superlattices made out of graphene-semiconductor bilayers. In addition to the band structure, we emphasize the existence and behavior of localized and resonant acoustic modes associated with the free surface of such structures. These modes are polarized in the sagittal plane, defined by the incident wavevector and the normal to the layers. The surface modes are obtained from the peaks of the density of states, either inside the bulk bands or inside the minigaps of the superlattice. In these structures, the two directions of vibrations (longitudinal and transverse) are coupled giving rise to two bulk bands associated with the two polarizations of the waves. The creation of the free surface of the superlattice induces true surface localized modes inside the terahertz acoustic forbidden gaps, but also pseudo-surface modes which appear as well-defined resonances inside the allowed bands of the superlattice. Despite the low thickness of the graphene layer, and though graphene is a gapless material, when it is inserted periodically in a semiconductor, it allows the opening of wide gaps for all values of the wave vector (parallel to the interfaces). Numerical illustrations of the band structures and surface modes are given for graphene-Si superlattices, and the surface layer can be either Si or graphene. These surface acoustic modes can be used to realize liquid or bio-sensors graphene-based phononic crystal operating in the THz frequency domain.

Asymmetric surface wave excitation through metasurface-edge diffraction.

The selective excitation of localized surface wave modes remains a challenge in the design of both leaky-wave and bound-wave devices. In this Letter, we show how the truncation of a metasurface can play an important role in breaking the spatial inversion symmetry in the excitation of surface waves supported by the structure. This is done by combining a large anisotropy in the dispersion relation and the presence of an edge that also serves as a coupling mechanism between the plane wave excitation and the induced surface waves. By resorting to the exact solution to the scattering problem based on a discrete Wiener-Hopf technique, we show that by inverting the component of the impinging wavevector parallel to the truncation, two distinct surface waves are excited.

Influence of anisotropy, tilt and pairing of Weyl nodes: the Weyl semimetals TaAs, TaP, NbAs and NbP

By means of ab initio band structure methods and model Hamiltonians we investigate the electronic, spin and topological properties of four monopnictides crystallizing in bct structure. We show that the Weyl bands around a WP W1 or W2 possess a strong anisotropy and tilt of the accompanying Dirac cones. These effects are larger for W2 nodes than for W1 ones. The node tilts and positions in energy space significantly influence the DOS of single-particle Weyl excitations. The node anisotropies destroy the conventional picture of (anti)parallel spin and wave vector of a Weyl fermion. This also holds for the Berry curvature around a node, while the monopole charges are independent as integrated quantities. The pairing of the nodes strongly modifies the spin texture and the Berry curvature for wave vectors in between the two nodes. Spin components may change their orientation. Integrals over planes perpendicular to the connection line yield finite Zak phases and winding numbers for planes between the two nodes, thereby indicating the topological character.Graphical abstract

Universality of Lower Hybrid Waves at Earth's Magnetopause

AbstractWaves around the lower hybrid frequency are frequently observed at Earth's magnetopause and readily reach very large amplitudes. Determining the properties of lower hybrid waves is crucial because they are thought to contribute to electron and ion heating, cross‐field particle diffusion, anomalous resistivity, and energy transfer between electrons and ions. All these processes could play an important role in magnetic reconnection at the magnetopause and the evolution of the boundary layer. In this paper, the properties of lower hybrid waves at Earth's magnetopause are investigated using the Magnetospheric Multiscale mission. For the first time, the properties of the waves are investigated using fields and direct particle measurements. The highest‐resolution electron moments resolve the velocity and density fluctuations of lower hybrid waves, confirming that electrons remain approximately frozen in at lower hybrid wave frequencies. Using fields and particle moments, the dispersion relation is constructed and the wave‐normal angle is estimated to be close to 90° to the background magnetic field. The waves are shown to have a finite parallel wave vector, suggesting that they can interact with parallel propagating electrons. The observed wave properties are shown to agree with theoretical predictions, the previously used single‐spacecraft method, and four‐spacecraft timing analyses. These results show that single‐spacecraft methods can accurately determine lower hybrid wave properties.

Correlation-function structure in square-gradient models of the liquid-gas interface: Exact results and reliable approximations.

In a recent article, we described how the microscopic structure of density-density correlations in the fluid interfacial region, for systems with short-ranged forces, can be understood by considering the resonances of the local structure factor occurring at specific parallel wave vectors q [Nat. Phys. 15, 287 (2019)1745-247310.1038/s41567-018-0361-z]. Here we investigate this further by comparing approximations for the local structure factor and pair correlation function against three new examples of analytically solvable models within square-gradient theory. Our analysis further demonstrates that these approximations describe the pair correlation function and structure factor across the whole spectrum of wave vectors, encapsulating the crossover from the Goldstone mode divergence (at small q) to bulklike behavior (at larger q). As shown, these approximations are exact for some square-gradient model potentials and never more than a few percent inaccurate for the others. Additionally, we show that they describe very accurately the correlation function structure for a model describing an interface near a tricritical point. In this case, there are no analytical solutions for the correlation functions, but the approximations are nearly indistinguishable from the numerical solutions of the Ornstein-Zernike equation.

BO: A unified tool for plasma waves and instabilities analysis

A unified numerically solvable framework for dispersion relations with an arbitrary number of species drifting at arbitrary directions and with Krook collision is derived in linear uniform/homogenous kinetic plasma model, which greatly extends the standard one given by Stix (1992). The purpose of this work is to provide a kinetic plasma dispersion relation tool not only in the physical model but also the numerical approach as general/powerful as possible. As a general application, we give the dispersion relations which assume the equilibrium distribution function to be bi-Maxwellian including parallel drift, two directions of perpendicular drift (i.e., drift across magnetic field), ring beam and loss-cone. Both electromagnetic and electrostatic versions are provided, with also the Darwin (a.k.a., magnetoinductive or magnetostatic) version. The species can be treated either magnetized or unmagnetized. Later, the equations are transformed to the matrix form that can be solved by using the powerful matrix algorithm (Xie and Xiao, 2016), which is the first approach can give all the important solutions of a linear kinetic plasma system without requiring initial guess for root finding. To the best of our knowledge, the present model is the most comprehensive model in literature for the distribution function constructed from Maxwellian distributions, which thus can be applied widely for studying waves and instabilities in space, astrophysics, fusion and laser plasmas. We limit the present work to non-relativistic case. Program summaryProgram Title: BOProgram Files doi:http://dx.doi.org/10.17632/cvbrftzfy5.1Licensing provisions: BSD 3-clauseProgramming language: MatlabNature of problem: The linear fluid and kinetic waves and instabilities in plasma can be described by dispersion relations. The challenges are to provide a dispersion relation as general as possible and to obtain all the solutions of it, which is the goal of BO tool. The kinetic version of BO tool provides a unified numerically solvable framework for kinetic dispersion relations, which greatly extends the standard one (Stix, 1992), with an arbitrary number of species. It contains many new features, including anisotropic temperature/loss cone/drift in arbitrary direction/ring beam/collision, unmagnetized/magnetized, electrostatic/electromagnetic/Darwin, can support parallel wave vector kz≤0, etc.Solution method: Approximating the plasma dispersion function in the kinetic dispersion relation with J-pole Padé expansion. Transforming the dispersion relation to an equivalent matrix eigenvalue problem and find all the solutions using standard matrix eigenvalue library function.Additional comments including restrictions and unusual features: Kinetic relativistic effects are not included in the present version yet.

Dispersion property of electromagnetic wave in 1D magnetized ferrites photonic crystals for TE mode in longitudinal magnetization configuration

In this article, properties of dispersion characteristics and phase index of magnetized one dimensional ferrite photonic crystals are investigated for transverse electric mode when external magnetic field is along the propagation direction. The dispersion relation and phase index equation for electromagnetic wave propagating in the considered structure is obtained by using transfer matrix method. It is found that in this configuration, two Eigen modes viz. LCP and RCP waves are obtained. The effect of external magnetic field, filling factor and normalized parallel wave vector β on dispersion and phase index are investigated. It is found that filling factor, external magnetic field and β have strong influence on the dispersion and phase index for LCP wave. For RCP wave, dispersion and phase index do not show external field dependence. With increase in external magnetic fields, the first order PBGs is shifted towards high frequency domain for LCP wave whereas for RCP wave there is no effect of external magnetic field on dispersion. With increase of β at constant external magnetic field and filling factor, thickness of PBGs increases for both LCP & RCP waves. PBGs occur in the form of lobes and the number of lobes decreases with increase in β values in the same frequency domain. For same f-value, the magnitude of ɸ is lower for LCP wave as compared to RCP wave.

Spin-polarized electronic surface states of Re(0001): An ab-initio investigation

We study the electronic structure of the Re(0001) surface by means of ab-initio techniques based on the Fully Relativistic (FR) Density Functional Theory (DFT) and the Projector Augmented-Wave (PAW) method. We identify the main surface states and resonances and study in detail their energy dispersion along the main symmetry lines of the SBZ. Moreover, we discuss the effect of spin-orbit coupling on the energy splittings and the spin-polarization of the main surface states and resonances. Whenever possible, we compare the results with previously studied heavy metals surfaces. We find empty resonances, located below a gap similar to the L-gap of the (111) fcc surfaces, that have a downward dispersion and cross the Fermi level, similarly to the recently studied Os(0001) surface. Their spin polarization at the Fermi level is similar to that predicted by the Rashba model, but the usual level crossing at Γ¯ is not found with our slab thickness. Moreover, for selected states, we follow the spin polarization along the high symmetry lines, discussing its behavior with respect to k∥, the wave-vector parallel to the surface.

Nonlocal effects in Weibel instability and two stream instability in a plasma

A nonlocal theory of Weibel and two stream instabilities of a relativistic electron beam of finite width propagating through a plasma, is developed. The beam pushes a fraction of the plasma electrons out of the beam region and imparts the residual electrons return velocity. The relative drift between the beam and plasma electrons drives an electromagnetic perturbation with quantized transverse wave vector, causing Weibel instability. The system is also unstable to electrostatic perturbation of finite parallel wave vector (parallel to beam velocity). For typical parameters the growth rate of the instability acquires values comparable to a fraction of the plasma frequency.

Dispersion properties of one-dimensional magnetized ferrite photonic crystals in transverse magnetization configuration for transverse magnetic modes

The dispersion characteristics of electromagnetic wave in one-dimensional magnetized ferrite photonic crystals containing yttrium iron garnets and nickel ferrite in a unit cell is investigated for transverse magnetic mode when external magnetic field is perpendicular to the plane of wave propagation. The dispersion relation is derived using transfer matrix method based on the continuity conditions of tangential electric and magnetic field components. It is observed that dispersion properties and phase index are strongly dependent on the normalized parallel wave vector and the filling factor. At the Brewster angle, width of photonic band gap shrinks to zero. Also, the considered structure indicates its dielectric behavior for electromagnetic wave propagation.

A k · p treatment of edge states in narrow 2D topological insulators, with standard boundary conditions for the wave function and its derivative

For 2D topological insulators with strong electron–hole hybridization, such as HgTe/CdTe quantum wells, the widely used 4 × 4 k · p Hamiltonian based on the first electron and heavy hole sub-bands yields an equal number of physical and spurious solutions, for both the bulk states and the edge states. For symmetric bands and zero wave vector parallel to the sample edge, the mid-gap bulk solutions are identical to the edge solutions. In all cases, the physical edge solution is exponentially localized to the boundary and has been shown previously to satisfy standard boundary conditions for the wave function and its derivative, even in the limit of an infinite wall potential. The same treatment is now extended to the case of narrow sample widths, where for each spin direction, a gap appears in the edge state dispersions. For widths greater than 200 nm, this gap is less than half of the value reported for open boundary conditions, which are called into question because they include a spurious wave function component. The gap in the edge state dispersions is also calculated for weakly hybridized quantum wells such as InAs/GaSb/AlSb. In contrast to the strongly hybridized case, the edge states at the zone center only have pure exponential character when the bands are symmetric and when the sample has certain characteristic width values.

Properties of dispersion and phase index in magnetized one dimensional ferrite photonic crystals in longitudinal configuration for TM mode

The band structure and phase index in magnetized one dimensional ferrite photonic crystals are computed for TM mode in longitudinal configuration using transfer matrix method. The parametric studies are performed to investigate the effect of filling factor, incident angle and external magnetic fields on dispersion and phase index of the considered structure. The variation of frequency and filling factor at fixed normalized parallel wave vector shows that photonic band gaps occur in the form of lobes. The variation of frequency with external magnetic fields at different normalized parallel wave vector shows that effect of the external magnetic fields have strong influence on second and higher order photonic band gaps. The phase index is also found to be sensitive function of filling factor and normalized parallel wave vector.

Effect of curvature in the magnetic shear profile on micro-tearing modes in the tokamak pedestal

The pedestal region of a tokamak plasma differs from the core in several respects. This work emphasizes the magnetic shear profile in the pedestal which is distinctive for two reasons: the pedestal is close to a magnetic-field separatrix so that the q-profile is intrinsically steep, and the steep pedestal gradients cause a peak in the bootstrap current with obvious effects on q(r). In fact, the pedestal q-profile has a complicated structure yielding a curved magnetic shear profile. Global linear simulations suggest that such a q-profile could substantially affect the nature of micro-instabilities. This paper analytically investigates this special feature of the pedestal, after appropriately modifying the slab-model field equations to allow for more complicated variation of the parallel wave vector. Surprisingly, the departure from a locally linear q profile (flat magnetic shear), even when small, supports a qualitatively novel mode whose width is fixed by curvature in the profile. The mode is unstable; its substantial growth rate is robust in the sense of persisting even when the q-profile modification is assumed to be small. However, this mode, originating from details of the q-profile, will not be found in a typical local ballooning gyrokinetic simulation.

Multifunctional metasurface: from extraordinary optical transmission to extraordinary optical diffraction in a single structure

We show that, a metasurface composed of subwavelength metallic slit array embedded in an asymmetric environment can exhibit either extraordinary optical transmission (EOT) or extraordinary optical diffraction (EOD). By employing an analytical model expansion method and the diffraction order chart in k-vector space, we found that the resonance decaying pathway of the local slit cavity mode can be tuned to either 0th or -1st diffraction order by changing the parallel wavevector, which gives rise to enhanced 0th transmission (EOT) of the structure for small incident angles, and enhanced -1st diffraction (EOD) for large incident angles. Based on this appealing feature, a multifunctional metasurface that can switch its functionality between transmission filter, mirror and off-axis lens is demonstrated. Our findings provide a convenient way to construct multifunctional integrated optical devices on a single planar device.

Global gyrokinetic simulations of intrinsic rotation in ASDEX Upgrade Ohmic L-mode plasmas

Non-linear, radially global, turbulence simulations of ASDEX Upgrade (AUG) plasmas are performed and the nonlinear generated intrinsic flow shows agreement with the intrinsic flow gradients measured in the core of Ohmic L-mode plasmas at nominal parameters. Simulations utilising the kinetic electron model show hollow intrinsic flow profiles as seen in a predominant number of experiments performed at similar plasma parameters. In addition, significantly larger flow gradients are seen than in a previous flux-tube analysis (Hornsby et al 2017 Nucl. Fusion 57 046008). Adiabatic electron model simulations can show a flow profile with opposing sign in the gradient with respect to a kinetic electron simulation, implying a reversal in the sign of the residual stress due to kinetic electrons. The shaping of the intrinsic flow is strongly determined by the density gradient profile. The sensitivity of the residual stress to variations in density profile curvature is calculated and seen to be significantly stronger than to neoclassical flows (Hornsby et al 2017 Nucl. Fusion 57 046008). This variation is strong enough on its own to explain the large variations in the intrinsic flow gradients seen in some AUG experiments. Analysis of the symmetry breaking properties of the turbulence shows that profile shearing is the dominant mechanism in producing a finite parallel wave-number, with turbulence gradient effects contributing a smaller portion of the parallel wave-vector.