Antisymmetric Profile Research Articles

Non-Hermitian mode management in periodically modulated waveguide amplifiers

We propose an efficient mode management scheme in active nonlinear multimode fibers based on non-Hermitian potentials in the longitudinal direction with an antisymmetric transverse profile. The proposal takes advantage of the nonlinear saturation toward particular mode configurations, which can be tuned by the non-Hermitian potential. We demonstrate flexible control of the beam profile within the parameter space of the applied potential with various possibilities, such as improving the beam quality by condensing photons to the lowest order Hermite mode, exciting higher order modes, or engineering a desired mode profile as a combination of modes. The effect is also analytically predicted using a mode-expansion approach, showing good agreement with the full model calculations based on the complex Ginzburg–Landau equation. This study was performed for 1D planar guiding structures, yet the results can be extended to 2D fibers and could be very useful in applications of fiber amplifiers and lasers.

Chorus Wave Generation Modulated by Field Line Resonance and Mirror‐Mode ULF Waves

AbstractEarth's inner magnetosphere is a zoo of plasma waves where electromagnetic and electrostatic emissions with distinct frequencies coexist and interact. Spacecraft observations have shown that whistler‐mode chorus waves, one of the key components in the magnetospheric dynamics, are often modulated by ultralow frequency (ULF) waves. Here, we investigate the effects of two typical ULF wave modes (i.e., field line resonance and mirror mode) on the nonlinear generation process of chorus waves. We report for the first time periodic excitations of lower‐ and upper‐band chorus waves near ULF wave crests and troughs, respectively. Their anticorrelated occurrence is explained by the nonlinear theory, in which the threshold amplitude of nonlinear wave growth is modified by the ULF wave field configuration and the modulated electron distributions. In this framework, the newly observed feature of chorus wave occurrence near the ULF wave crests is attributed to the antisymmetric field profiles of mirror‐mode ULF waves, which periodically modulate the threshold amplitude by modifying the second‐order derivative of the background dipole field. In addition to the second‐order derivative, the first‐order derivative of the background magnetic field is also modulated by the ULF waves to regulate the size of the chorus wave source region.

Numerical studies on wake and turbulence characteristics of aquaculture nets

This paper aims to understand the drag coefficient discrepancy between the equivalent-twine and twisted-twine nets based on their wake and turbulence characteristics. To that end, we conduct unsteady Reynolds-averaged Navier-Stokes (URANS) and the second-moment (Reynolds stress, RSM) simulations at a Reynolds number,Re=4.5×103, based on the effective diameter of the net twine, which corresponds to the subcritical flow regime. Then, the vortex structures and the turbulence statistics are assessed at AOA=90°. The results highlight that the wake interactions for the twisted-twine net are relatively strong compared to the equivalent-twine net, due to the disturbance of the helixes on the twisted twines. In comparison to the classical Karman vortex, the overall vortex shedding of these two nets is well organized. Symmetric vortices form behind the equivalent-twine net, while single vortices form behind the twisted-twine net. Moreover, the Reynolds normal and shear stresses show symmetric and anti-symmetric profiles. The addition of helixes to smooth circular cylinders changes the flow development, leading to a decrease of turbulence kinetic energy. With this understanding, engineers need to be carefully select the net type for preliminary design of marine aquaculture cages to avoid over- or underestimation of the drag forces.

Second-order elastic topological insulator with valley-selective corner states

Second-order elastic topological insulators (SETIs) with topologically protected corner states offer new routes for the realization of the robust manipulation of elastic waves in lower dimensions, providing the unprecedented ways for integrated ultrasound sensors and energy harvesting devices. However, traditional SETIs lack the flexibility of turning on and off the corner states on demand. Herein, we proposed a type of SETIs possessing the valley-selective topological corner states, endowing the capability of activating corner states by engineering the valley positions. An extra type of corner states with anti-symmetric displacement profile is also observed in the proposed SETIs, which is attributed to the enhanced long-range effect. The discrete mechanical model is firstly investigated and then expanded into the PC plate, in which the theory of Wannier center can well elucidate the corner states with valley-selectivity. The experimental results validate the existing corner states with valley-selectivity and anti-symmetric displacement profile. The proposed discrete model acts as a versatile platform presenting the relevant topological effects. The achieved valley-selectivity and multi-polarization features hosted in the designed SETI may contribute to their practicality and reconfigurability in elastic energy trapping, detecting, and harvesting device.

Anti-symmetric Compton scattering in LiNiPO 4: Towards a direct probe of the magneto-electric multipole moment.

Magnetoelectric multipoles, which break both space-inversion and time-reversal symmetries, play an important role in the magnetoelectric response of a material. Motivated by uncovering the underlying fundamental physics of the magnetoelectric multipoles and the possible technological applications of magnetoelectric materials, understanding as well as detecting such magnetoelectric multipoles has become an active area of research in condensed matter physics. Here we employ the well-established Compton scattering effect as a possible probe for the magnetoelectric toroidal moments in LiNiPO 4. We employ combined theoretical and experimental techniques to compute as well as detect the antisymmetric Compton profile in LiNiPO 4. For the theoretical investigation we use density functional theory to compute the anti-symmetric part of the Compton profile for the magnetic and structural ground state of LiNiPO 4. For the experimental verification, we measure the Compton signals for a single magnetoelectric domain sample of LiNiPO 4, and then again for the same sample with its magnetoelectric domain reversed. We then take the difference between these two measured signals to extract the antisymmetric Compton profile in LiNiPO 4. Our theoretical calculations indicate an antisymmetric Compton profile in the direction of the t y toroidal moment in momentum space, with the computed antisymmetric profile around four orders of magnitude smaller than the total profile. The difference signal that we measure is consistent with the computed profile, but of the same order of magnitude as the statistical errors and systematic uncertainties of the experiment. While the weak difference signal in the measurements prevents an unambiguous determination of the antisymmetric Compton profile in LiNiPO 4, our results motivate further theoretical work to understand the factors that influence the size of the antisymmetric Compton profile, and to identify materials exhibiting larger effects.

Anti-symmetric Compton scattering in LiNiPO4: Towards a direct probe of the magneto-electric multipole moment

Background: Magnetoelectric multipoles, which break both space-inversion and time-reversal symmetries, play an important role in the magnetoelectric response of a material. Motivated by uncovering the underlying fundamental physics of the magnetoelectric multipoles and the possible technological applications of magnetoelectric materials, understanding as well as detecting such magnetoelectric multipoles has become an active area of research in condensed matter physics. Here we employ the well-established Compton scattering effect as a possible probe for the magnetoelectric toroidal moments in LiNiPO4. Methods: We employ combined theoretical and experimental techniques to compute as well as detect the antisymmetric Compton profile in LiNiPO4. For the theoretical investigation we use density functional theory to compute the anti-symmetric part of the Compton profile for the magnetic and structural ground state of LiNiPO4. For the experimental verification, we measure the Compton signals for a single magnetoelectric domain sample of LiNiPO4, and then again for the same sample with its magnetoelectric domain reversed. We then take the difference between these two measured signals to extract the antisymmetric Compton profile in LiNiPO4. Results: Our theoretical calculations indicate an antisymmetric Compton profile in the direction of the ty toroidal moment in momentum space, with the computed antisymmetric profile around four orders of magnitude smaller than the total profile. The difference signal that we measure is consistent with the computed profile, but of the same order of magnitude as the statistical errors and systematic uncertainties of the experiment. Conclusions: While the weak difference signal in the measurements prevents an unambiguous determination of the antisymmetric Compton profile in LiNiPO4, our results motivate further theoretical work to understand the factors that influence the size of the antisymmetric Compton profile, and to identify materials exhibiting larger effects.

Revealing hidden magnetoelectric multipoles using Compton scattering

Magneto-electric multipoles, which are odd under both space-inversion $\cal I$ and time-reversal $\cal T$ symmetries, are fundamental in understanding and characterizing magneto-electric materials. However, the detection of these magneto-electric multipoles is often not straightforward as they remain "hidden" in conventional experiments in part since many magneto-electrics exhibit combined $\cal IT$ symmetry. In the present work, we show that the anti-symmetric Compton profile is a unique signature for all the magneto-electric multipoles, since the asymmetric magnetization density of the magneto-electric multipoles couples to space via spin-orbit coupling, resulting in an anti-symmetric Compton profile. We develop the key physics of the anti-symmetric Compton scattering using symmetry analysis and demonstrate it using explicit first-principles calculations for two well-known representative materials with magneto-electric multipoles, insulating LiNiPO$_4$ and metallic Mn$_2$Au. Our work emphasizes the crucial roles of the orientation of the spin moments, the spin-orbit coupling, and the band structure in generating the anti-symmetric Compton profile in magneto-electric materials.

High-harmonic-driven inverse Raman scattering.

Spectral analysis of high-order harmonics generated by ultrashort mid-infrared pulses in molecular nitrogen reveals well-resolved signatures of inverse Raman scattering, showing up near the frequencies of prominent vibrational transitions of nitrogen molecules. When tuned on a resonance with the v'=0→v''=0 pathway within the B3Πg→C3Πu second positive system of molecular nitrogen, the eleventh harmonic of a 3.9 µm, 80 fs driver is shown to acquire a distinctive antisymmetric spectral profile with red-shifted bright and blue-shifted dark features as indicators of stimulated Raman gain and loss. This high-harmonic setting extends the inverse Raman effect to a vast class of strong-field light-matter interaction scenarios.

Multiple solutions and hysteresis in the flows driven by surface with antisymmetric velocity profile* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11902043 and 11772065), and the Science Challenge Project (Grant No. TZ2016001).

Multiple steady solutions and hysteresis phenomenon in the square cavity flows driven by the surface with antisymmetric velocity profile are investigated by numerical simulation and bifurcation analysis. A high order spectral element method with the matrix-free pseudo-arclength technique is used for the steady-state solution and numerical continuation. The complex flow patterns beyond the symmetry-breaking at Re ≃ 320 are presented by a bifurcation diagram for Re < 2500. The results of stable symmetric and asymmetric solutions are consistent with those reported in literature, and a new unstable asymmetric branch is obtained besides the stable branches. A novel hysteresis phenomenon is observed in the range of 2208 < Re < 2262, where two pairs of stable and two pairs of unstable asymmetric steady solutions beyond the stable symmetric state coexist. The vortices near the sidewall appear when the Reynolds number increases, which correspond to the bifurcation of topology structure, but not the bifurcation of Navier–Stokes equations. The hysteresis is proposed to be the result of the combined mechanisms of the competition and coalescence of secondary vortices.

Strain Distribution Across an Individual Shear Band in Real and Simulated Metallic Glasses.

Because of the fast dynamics of shear band formation and propagation along with the small size and transient character of the shear transformation zones (STZs), the elementary units of plasticity in metallic glasses, the description of the nanoscale mechanism of shear banding often relies on molecular dynamics (MD) simulations. However, the unrealistic parameters used in the simulations related to time constraints may raise questions about whether quantitative comparison between results from experimental and computational analyses is possible. Here, we have experimentally analyzed the strain field arising across an individual shear band by nanobeam X-ray diffraction and compared the results with the strain characterizing a shear band generated by MD simulations. Despite their largely different spatiotemporal scales, the characteristic features of real and simulated shear bands are strikingly similar: the magnitude of the strain across the shear band is discontinuous in both cases and the direction of the principal strain axes exhibits the same antisymmetric profile. This behavior can be explained by considering the mechanism of STZ activation and percolation at the nanoscale, indicating that the nanoscale effects of shear banding are not limited to the area within the band but they extend well into the surrounding elastic matrix. These findings not only demonstrate the reliability of MD simulations for explaining (also quantitatively) experimental observations of shear banding but also suggest that designed experiments can be used the other way around to verify numerical predictions of the atomic rearrangements occurring within a band.

Redirection and splitting of sound waves by a periodic chain of thin perforated cylindrical shells

A line of perforated cylindrical shells in air is practically transparent for sound since each individual unit is a weak scatterer. However, strong scattering occurs due to the coupling to the acoustic eigenmodes of the chain. Here we develop an analytical theory of sound transmission and scattering at a linear chain of perforated shells and predict strong anomalous effect for oblique incidence. The chain eigenmodes are weakly decaying, with symmetric profile and anomalous dispersion, or with antisymmetric profile and normal dispersion, and their excitation leads to deep minima in the transmission and 90̊-redirection of the external sound. At normal incidence, only the symmetric eigenmode can be excited, otherwise both modes are excited at close frequencies. Moreover, the wave which resonates with the normal-dispersion mode is redirected along the “right” direction, whereas the wave resonating with the anomalous-dispersion mode is redirected in the “wrong” direction. Thus, a periodic chain of perforated shells may serve not only as a 90̊-redirecting antenna but also as a splitter of sound waves with close frequencies. For example, an acoustic signal containing two frequencies around 3 kHz can be split into monochromatic components propagating in opposite directions along the chain, if the beat frequency is ≈500 Hz.

-symmetry in Rydberg atoms

We propose a scheme to realize parity-time ()-symmetry in an ensemble of strongly interacting Rydberg atoms, which act as superatoms due to the dipole blockade mechanism. We show that Rydberg-dressed 87Rb atoms in a four-level inverted Y-type configuration is highly efficient to generate the refractive index for a probe field, with a symmetric (antisymmetric) profile spatially in the corresponding real (imaginary) part. Comparing with earlier investigations, the present scheme provides a versatile platform to control the system from -symmetry to non--symmetry via different external parameters, i.e., coupling field detuning, probe field intensity and control field intensity.

On the possibility of using X-ray Compton scattering to study magnetoelectrical properties of crystals.

This paper discusses the possibility of using Compton scattering--an inelastic X-ray scattering process that yields a projection of the electron momentum density--to probe magnetoelectrical properties. It is shown that an antisymmetric component of the momentum density is a unique fingerprint of such time- and parity-odd physics. It is argued that polar ferromagnets are ideal candidates to demonstrate this phenomenon and the first experimental results are shown, on a single-domain crystal of GaFeO3. The measured antisymmetric Compton profile is very small (≃ 10(-5) of the symmetric part) and of the same order of magnitude as the statistical errors. Relativistic first-principles simulations of the antisymmetric Compton profile are presented and it is shown that, while the effect is indeed predicted by theory, and scales with the size of the valence spin-orbit interaction, its magnitude is significantly overestimated. The paper outlines some important constraints on the properties of the antisymmetric Compton profile arising from the underlying crystallographic symmetry of the sample.

Snake states and their symmetries in graphene

Snake states are open trajectories for charged particles propagating in two dimensions under the influence of a spatially varying perpendicular magnetic field. In the quantum limit they are protected edge modes that separate topologically inequivalent ground states and can also occur when the particle density rather than the field is made nonuniform. We examine the correspondence of snake trajectories in single-layer graphene in the quantum limit for two families of domain walls: (a) a uniform doped carrier density in an antisymmetric field profile and (b) antisymmetric carrier distribution in a uniform field. These families support different internal symmetries but the same pattern of boundary and interface currents. We demonstrate that these physically different situations are gauge equivalent when rewritten in a Nambu doubled formulation of the two limiting problems. Using gauge transformations in particle-hole space to connect these problems, we map the protected interfacial modes to the Bogoliubov quasiparticles of an interfacial one-dimensional p-wave paired state. A variational model is introduced to interpret the interfacial solutions of both domain wall problems.

Spectral separation of optical spin based on antisymmetric Fano resonances.

We propose a route to the spectral separation of optical spin angular momentum based on spin-dependent Fano resonances with antisymmetric spectral profiles. By developing a spin-form coupled mode theory for chiral materials, the origin of antisymmetric Fano spectra is clarified in terms of the opposite temporal phase shift for each spin, which is the result of counter-rotating spin eigenvectors. An analytical expression of a spin-density Fano parameter is derived to enable quantitative analysis of the Fano-induced spin separation in the spectral domain. As an application, we demonstrate optical spin switching utilizing the extreme spectral sensitivity of the spin-density reversal. Our result paves a path toward the conservative spectral separation of spins without any need of the magneto-optical effect or circular dichroism, achieving excellent purity in spin density superior to conventional approaches based on circular dichroism.

Unbreakable PT symmetry of solitons supported by inhomogeneous defocusing nonlinearity.

We consider bright solitons supported by a symmetric inhomogeneous defocusing nonlinearity growing rapidly enough toward the periphery of the medium, combined with an antisymmetric gain-loss profile. Despite the absence of any symmetric modulation of the linear refractive index, which is usually required to establish a parity-time (PT) symmetry in the form of a purely real spectrum of modes, we show that the PT symmetry is never broken in the present system, and that the system always supports stable bright solitons, i.e., fundamental and multi-pole ones. This fact is connected to the nonlinearizability of the underlying evolution equation. The increase of the gain-loss strength results, in lieu of the PT symmetry breaking, in merger of pairs of different soliton branches, such as fundamental and dipole, or tripole and quadrupole ones. The fundamental and dipole solitons remain stable at arbitrarily large values of the gain-loss coefficient.

Out-of-plane shear flow effects on fast magnetic reconnection in a two-dimensional hybrid simulation model

The effects of out-of-plane shear flows on fast magnetic reconnection are numerically investigated by a two-dimensional (2D) hybrid model in an initial Harris sheet equilibrium with flows. The equilibrium and driven shear flows out of the 2D reconnection plane with symmetric and antisymmetric profiles respectively are used in the simulation. It is found that the out-of-plane flows with shears in-plane can change the quadrupolar structure of the out-of-plane magnetic field and, therefore, modify the growth rate of magnetic reconnection. Furthermore, the driven flow varying along the anti-parallel magnetic field can either enhance or reduce the reconnection rate as the direction of flow changes. Secondary islands are also generated in the process with converting the initial X-point into an O-point.

Anomalous Nernst effect in strained graphene coupled to a substrate inducing a time-reversal symmetry breaking

A novel anomalous Nernst effect in graphene engineered by a strain and coupled to an environment with a broken time-reversal breaking is predicted. We consider the Haldane model (1988 Phys. Rev. Lett. 61 2015) including a uniaxial strain and analyze explicitly the time-reversal symmetry behavior. We find in this case that the total Nernst coefficient contributed by the two Dirac cones is no longer zero. For further insight we study the interplay of the stagger-field-induced gap of the substrate and the time-reversal symmetry-breaking-induced gap. The former preserves the opposite signs of the Berry curvatures, whereas in the latter the Berry curvatures possess the same sign. For a weak time reversal breaking (TRB), the total Nernst signal experiences a sign change as the Fermi level increases. When TRB is large, the total Nernst coefficient exhibits a two-peak structure varying with the Fermi level. Another feature is that the anomalous Nernst signal has an anti-symmetric profile with respect to the neutral Dirac point instead of the symmetric behavior known for the case of an external magnetic field. We further find a robust six-leaf modulation of the total Nernst coefficient as the direction of the strain is varied.

Stability of Double Tearing Mode in the Presence of Shear Flows

The stability of two eigen states of double tearing mode (DTM) with symmetric or antisymmetric islands in the presence of shear flows is numerically simulated based on a reduced MHD model in slab geometry. For given antisymmetric flow profile, a degenerated state is observed at a critical flow amplitude vc. Below vc, the shear flow stabilizes the DTM with antisymmetric islands and destabilizes the other one through distorting the magnetic flux mainly governed by the global effect of flow profile. Above vc, the degenerated state bifurcates into two eigen states with the same growth rate but opposed propagating direction. These two eigen modes show

Optimized boundary driven flows for dynamos in a sphere

We perform numerical optimization of the axisymmetric flows in a sphere to minimize the critical magnetic Reynolds number Rmcr required for dynamo onset. The optimization is done for the class of laminar incompressible flows of von Kármán type satisfying the steady-state Navier-Stokes equation. Such flows are determined by equatorially antisymmetric profiles of driving azimuthal (toroidal) velocity specified at the spherical boundary. The model is relevant to the Madison plasma dynamo experiment, whose spherical boundary is capable of differential driving of plasma in the azimuthal direction. We show that the dynamo onset in this system depends strongly on details of the driving velocity profile and the fluid Reynolds number Re. It is found that the overall lowest Rmcr≈200 is achieved at Re≈240 for the flow, which is hydrodynamically marginally stable. We also show that the optimized flows can sustain dynamos only in the range Rmcr&amp;lt;Rm&amp;lt;Rmcr2, where Rmcr2 is the second critical magnetic Reynolds number, above which the dynamo is quenched. Samples of the optimized flows and the corresponding dynamo fields are presented.