Additional Degrees Of Freedom Research Articles

Control of second-harmonic generation in all-dielectric intersubband metasurfaces by controlling the polarity of χ(2).

All-dielectric metasurfaces have recently led to a paradigm shift in nonlinear optics as they allow for circumventing the phase matching constraints of bulk crystals and offer high nonlinear conversion efficiencies when normalized by the light-matter interaction volume. Unlike bulk crystals, in all-dielectric metasurfaces nonlinear conversion efficiencies primarily rely on the material nonlinearity, field enhancements, and the modal overlaps, therefore most efforts to date have only focused on utilizing these degrees of freedom. In this work, we demonstrate that for second-harmonic generation in all-dielectric metasurfaces, an additional degree of freedom is the control of the polarity of the nonlinear susceptibility. We demonstrate that semiconductor heterostructures that support resonant nonlinearities based on quantum-engineered intersubband transitions provide this new degree of freedom. We can flip and control the polarity of the nonlinear susceptibility of the dielectric medium along the growth direction and couple it to the Mie-type photonic modes. Here we demonstrate that engineering the χ(2) polarity in the meta-atom enables the control of the second-harmonic radiation pattern and conversion efficiency. Our results therefore open a new direction for engineering and optimizing second-harmonic generation using all-dielectric intersubband nonlinear metasurfaces.

Optical pulling and pushing forces via Bloch surface waves

For flexible tailoring of optical forces, as well as for extraordinary optomechanical effects, additional degrees of freedom should be introduced into a system. Here, we demonstrate that photonic crystals are a versatile platform for optical manipulation due to both Bloch surface waves (BSWs) and the complex character of the reflection coefficient paving a way for controlled optomechanical interactions. We demonstrate enhanced pulling and pushing transversal optical forces acting on a single dipolar bead above a one-dimensional photonic crystal due to directional excitation of BSWs. Our results demonstrate angle- or wavelength-assisted switching between BSW-induced optical pulling and pushing forces. Easy to fabricate for any desired spectral range, photonic crystals are shown to be prospective for precise optical sorting of nanoparticles, which are difficult to sort with conventional optomechanical methods. Our approach opens opportunities for novel, to the best of our knowledge, optical manipulation schemes and platforms, and enhanced light-matter interaction in optical trapping setups.

Dynamics of Middle East Respiratory Syndrome Coronavirus (MERS-CoV) involving fractional derivative with Mittag-Leffler kernel

Since 2012, the Middle East has seen a steady rise in the Middle East Respiratory Syndrome Coronavirus (MERS-CoV). A fractional derivative of the non-singular Mittag-Leffler type is used in this research to conduct a mathematical analysis of the dynamics of MERS-CoV infection transmission. The dynamics of such a disease with an additional degree of freedom and non-singular behavior are discovered through the use of the aforementioned fractional operator, and this is one of the important components of our prepared paper. Using the concept of fixed point theory, the existence and uniqueness of solutions are demonstrated. The stability analysis is also tested with the help of the Ulam–Hyers approach, respectively. The numerical solution has been conducted by using the fractional Adams–Bashforth scheme. In the numerical simulation, all classes are demonstrated through the graphical presentation regarding the changing values of fractional-order at time t. The results at various fractional-order laying between (0,1] are drawn with the help of Matlab. We also provide a comparison of the proposed approach with that of the Caputo operator. The outcomes that were achieved illustrate that the considered scheme is highly methodical and very efficient compared to the Caputo fractional operator.

Structure formation in dark matter particle production cosmology

We investigate a cosmological scenario in which the dark matter particles can be created during the evolution of the Universe. By regarding the Universe as an open thermodynamic system and using non-equilibrium thermodynamics, we examine the mechanism of gravitational particle production. In this setup, we study the large-scale structure (LSS) formation of the Universe in the Newtonian regime of perturbations and derive the equations governing the evolution of the dark matter overdensities. Then, we implement the cosmological data from Planck 2018 CMB measurements, SNe Ia and BAO observations, as well as the Riess et al. (2019) local measurement for H0 to provide some cosmological constraints for the parameters of our model. We see that the best case of our scenario (χtot2=3834.40) fits the observational data better than the baseline ΛCDM model (χtot2=3838.00) at the background level. We moreover estimate the growth factor of linear perturbations and show that the best case of our model (χfσ82=39.85) fits the LSS data significantly better than the ΛCDM model (χfσ82=45.29). Consequently, our model also makes a better performance at the level of the linear perturbations compared to the standard cosmological model. Although the improvement in χtot for Model 2 compared to ΛCDM at the background level may arise somewhat from the additional degree of freedom, the better consistency of this model at the level of linear perturbations results mainly from the impact of particle production on the sound speed at low redshifts.

Optimization of Fractional Order PI Controller by PSO Algorithm Applied to a Grid-Connected Photovoltaic System

With the increasing integration of renewable energies into power grids, their control and power quality are becoming the main focus of many research efforts. In a grid-connected photovoltaic system, the control strategy is necessary to efficiently use the solar energy as well as to ensure high power quality. This paper presents a study on the robustness of a Fractional Order PI controller based on the Particle Swarm Optimization algorithm (PSO-FOPI) in a grid-connected PV system. The controller used was integrated into the inverter to apply voltage-oriented control (VOC). Fractional order controllers have an additional degree of freedom, so that a wider range of parameters is available to provide better control. Parameter optimization of the FOPI and classical PI controllers are performed using the PSO algorithm. The performance of the FOPI controller is compared with that of the classical PI controller. A complete study of the behavior of the grid connected PV system is tested using MATLAB/Simulink. The simulation results show the performance and efficiency of the PSO-FOPI controller compared to the classical PI controller in terms of rapidity, stability and precision, as well as the THD reduction of the current injected to the grid for any variation of solar irradiance.

Explaining excesses in four-leptons at the LHC with a double peak from a CP violating Two Higgs Doublet Model

Extended scalar sectors with additional degrees of freedom appear in many scenarios beyond the Standard Model. Heavy scalar resonances that interact with the neutral current could be discovered via broad resonances in the tails of the four-lepton invariant mass spectrum, where the Standard Model background is small and well understood. In this article we consider a recent ATLAS measurement of four-lepton final states, where the data is in excess over the background for invariant masses above 500 GeV. We discuss the possibility that this excess could be interpreted as a “double peak” from the two extra heavy neutral scalars of a CP violating Two Higgs Doublet Model, both coupling to the Z boson. We apply an iterative fitting procedure to find viable model parameters that can match the excess, resulting in a benchmark point where the observed four-lepton invariant mass spectrum can be explained by two scalar particles H2 and H3, with masses of 544 GeV and 629 GeV, respectively, being admixtures of the CP eigenstates. Our explanation predicts additional production processes for toverline{t} , W+W−, 4b and γγ, some of which have cross sections close to the current experimental limits. Our results further imply that the electric dipole moment of the electron should be close to the present bounds.

Color Palette Tactic for Multicolor Electrochromics Enabled by Electrolyte Thermochromic Engineering

AbstractElectrochromism is a reversible color change triggered by the external electrical field, which can be utilized in smart windows, displays, etc. The ability to display multiple colors in a single electrochromic device is highly beneficial for the versatile applications of the electrochromic technique. Yet, achieving the ability to adopt multiple colors in one electrochromic device has proven to be a challenging task by merely using the conventional chemical structure engineering of the electrochromic materials. In this study, a thermochromic electrolyte is introduced. A multicolor electrochromic prototype device is produced by integrating the thermochromic engineered electrolyte together with electrochromic material in which both can independently contribute to the coloring of the device. This “color palette” strategy gives an additional degree of freedom and extensive flexibility to the design of multicolor electrochromic devices. It introduces a new pathway for producing multicolor electrochromic devices and removes a bottleneck imposed by conventional strategies merely depending on the electrochromic material structure engineering. Moreover, it inspires further development of electrolytes, that share the responsibility of tuning the optical properties of electrochromic devices together with the electrochromic materials, thereby accelerating and diversifying the applications of electrochromics in industry.

Characterization of Tensile Stress-Dependent Directional Magnetic Incremental Permeability in Iron-Cobalt Magnetic Sheet: Towards Internal Stress Estimation through Non-Destructive Testing.

Iron-Cobalt ferromagnetic alloys are promoted for electrical energy conversion in aeronautic applications, but their high magnetostrictive coefficients may result in undesired behaviors. Internal stresses can be tuned to limit magnetostriction but must be adequately assessed in a non-destructive way during production. For this, directional magnetic incremental permeability is proposed in this work. For academic purposes, internal stresses have been replaced by homogenous external stress, which is easier to control using traction/compression testbench and results in similar effects. Tests have been limited to tensile stress stimuli, the worst-case scenario for magnetic stress observation on positive magnetostriction coefficient materials. Hysteresis cycles have been reconstructed from the incremental permeability measurement for stability and reproducibility of the measured quantities. The directionality of the sensor provides an additional degree of freedom in the magnetic response observation. The study reveals that an angle of π/2 between the DC (Hsurf DC) and the AC (Hsurf AC) magnetic excitations with a flux density Ba at Hsurf DC = 10 kA·m−1 constitute the ideal experimental situation and the highest correlated parameter to a homogeneous imposed tensile stress. Magnetic incremental permeability is linked to the magnetic domain wall bulging magnetization mechanism; this study thus provides insights for understanding such a mechanism.

Theoretical modeling and investigations of lossy mode resonance prism sensor based on TiO2 film.

The optical sensor based on lossy mode resonance can overcome the limitations of traditional surface plasmon resonance sensors and work under TE and TM polarized light. In this paper, an LMR sensor theoretical model with the configuration of prism/matching layer/lossy layer/sensing layer is proposed, which is based on the principle of attenuated total reflection. By using TiO2 film as the lossy layer and LiF film as the matching layer, the resonance signal under angle interrogation is effectively improved. One of the advantages of the proposed sensor is that the detection range and detection accuracy are dynamically adjustable, which provides additional degrees of freedom in the design and use of the device. The structural parameters (film thickness, layer refractive index) affecting the resonance signal have been investigated based on the electric field distribution at resonance and the coupled mode theory. The LMR signal under TE and TM polarization can be switched by changing the thickness ratio of the matching layer and the lossy layer. All possible combinations of film thicknesses are given as a reference for the design of the LMR prism sensor based on TiO2 film. Under proper thickness combination, the proposed sensor is capable of detecting the medium with refractive index ranging from 1.32 ∼ 1.47, with a sensitivity range of 34 ∼ 148 °/RIU under angle interrogation and a maximum value of 192 RIU-1 for FOM under TM polarization. We hope these investigations can prove the advantages of LMR prism sensors and provide guidance for the experimental implementation of LMR prism sensors in the future.

1600 Gbps PAM-4 FSO link enabled using OFCL-based WDM and OAM-multiplexing techniques

The twisted light beams carry orbital angular momentum (OAM), which acts as additional degrees of freedom. In this work, we have proposed a free space optics (FSO) communication system employing wavelength division multiplexing (WDM), OAM multiplexing, and four-level pulse amplitude modulation (PAM-4) techniques. A 1600 Gbps data has been successfully transmitted over free space in different environmental scenarios such as clear weather, weak turbulence, moderate turbulence, and strong turbulence. The effects of atmospheric turbulence on the PAM-4 signals carrying OAM modes are examined theoretically and simulated for the first time. Optical frequency comb lines are generated by employing a single laser source and used as a broadband laser source. A sufficient low power penalty of 1.489 dB at a BER value ∼ 10-9 along with clear eye diagrams has been achieved for the proposed system. Thus, the proposed 1600 Gbps PAM-4, OAM, and WDM-based system can be a promising solution for the next generation of long-reach and high-data-rate free-space communication.

Hints of early dark energy in Planck , SPT, and ACT data: New physics or systematics?

We investigate constraints on early dark energy (EDE) using ACT DR4, SPT-3G 2018, Planck polarization, and restricted Planck temperature data (at $\ell<650$), finding a $3.3\sigma$ preference ($\Delta\chi^2=-16.2$ for 3 additional degrees of freedom) for EDE over $\Lambda$CDM. The EDE contributes a maximum fractional energy density of $f_{\rm EDE}(z_c)=0.163^{+0.047}_{-0.04}$ at a redshift $z_c=3357\pm200$ and leads to a CMB inferred value of the Hubble constant $H_0=74.2^{+1.9}_{-2.1}$ km/s/Mpc. We find that Planck and ACT DR4 data provide the majority of the improvement in $\chi^2$, and that the inclusion of SPT-3G pulls the posterior of $f_{\rm EDE}(z_c)$ away from $\Lambda$CDM. This is the first time that a moderate preference for EDE has been reported for these three combined CMB data sets. We find that including measurements of supernovae luminosity distances and the baryon acoustic oscillation standard ruler only minimally affects the preference ($3.0\sigma$), while measurements that probe the clustering of matter at late times - the lensing potential power spectrum from Planck and $f \sigma_8$ from BOSS - decrease the significance of the preference to 2.6$\sigma$. Conversely, adding a prior on the $H_0$ value as reported by the SH0ES collaboration increases the preference to the $4-5\sigma$ level. In the absence of this prior, the inclusion of Planck TT data at $\ell>1300$ reduces the preference from $3.0\sigma$ to $2.3\sigma$ and the constraint on $f_{\rm EDE}(z_c)$ becomes compatible with $\Lambda$CDM at $1\sigma$. We explore whether systematic errors in the Planck polarization data may affect our conclusions and find that changing the TE polarization efficiencies significantly reduces the Planck preference for EDE. More work will be necessary to establish whether these hints for EDE within CMB data alone are the sole results of systematic errors or an opening to new physics.

Multitension strings in high-resolution U(1)×U(1) simulations

Topological defects are a fossil relic of early Universe phase transitions, with cosmic strings being the best motivated example. While in most cases one studies Nambu-Goto or Abelian-Higgs strings, one also expects that cosmologically realistic strings should have additional degrees of freedom in their worldsheets, one specific example being superstrings from Type IIB superstring theory. Here we continue the scientific exploitation of our recently developed multi-GPU field theory cosmic strings code to study the evolution of U(1)$\times$U(1) multitension networks, which are a numerically convenient proxy: these contain two lowest-tension strings networks able to interact and form bound states, providing a convenient first approximation to the behaviour expected from cosmic superstrings. (...) We rely on the largest field theory simulations of this model so far, specifically $4096^3$, $\Delta x = 0.5$ boxes. We present robust evidence of scaling for the lightest strings, measured through a complete and self-consistent set of correlation length and velocity diagnostics. We also find a linearly growing average length of the bound state segments, consistent with a scaling behaviour. (In previously reported lower-resolution simulations, such behaviour had only been identified with carefully engineered initial conditions, rich in those segments.) Finally, while we see no evidence of a large population of bound states forming at early stages of the network evolution, we do present tentative evidence for an asymptotic constant value of the fraction of bound states, with this value being different in the radiation and the matter eras. Our work demonstrates that our GPU-accelerated field theory code can by successfully extended beyond the simple Abelian-Higgs approximation, and enables future detailed studies of realistic string networks and of their observational signatures.

Interlayer Coupling: An Additional Degree of Freedom in Two-Dimensional Materials.

Due to their layered nature, two-dimensional nanomaterials can stack into artificial material systems, with van der Waals interaction between the adjacent constituent layers. In such heterostructures, the physical properties are largely affected by the interlayer coupling and can thus be effectively tuned by a number of means. In this Perspective, we highlight four such experimental approaches: stacking order, electric field, intercalation, and pressure, and we discuss challenges and opportunities in future studies for van der Waals heterostructures.

Matrix and vectorial generalized Calogero–Moser models

Many-body one-dimensional systems with 1/x2 interactions are known as Calogero–Moser systems. The so called ordinary system with a common coupling constant for all interacting pairs can be generalized into models with couplings which evolve as additional degrees of freedom. This can be done via unitary reduction of a linear matrix model or, alternatively, by assigning a vectorial degree of freedom to each particle. We briefly review the matrix and vector formulations in the context of Hamiltonian mechanics. Various models of this type were investigated separately in several previous studies. In this paper we focus on relations among them. In particular we prove that the seemingly two-particle variables governing the repulsion of each pair are in fact functions of one-particle variables. Identification of the ordinary Calogero–Moser system as an isolated stationary point in the space of generalized models is a starting point to classifying the generalized models according to the rank of the matrix of couplings or, equivalently, the effective number of independent non-spacial degrees of freedom. Finally, we present two new examples of generalized Calogero–Moser models. The first one combines the matrix and vector degrees of freedom and results in additional 1/x interaction terms. The second one stems from a Hamilton function which mimics the Hamiltonian of a quantum model with spin exchange terms and leads to matrix anticommutator equations of motion.

Control and tunability of magnetic bubble states in multilayers with strong perpendicular magnetic anisotropy at ambient conditions

The reversal of magnetic bubble helicity through topologically trivial transient states provides an additional degree of freedom that promises the development of multidimensional magnetic memories. A key requirement for this concept is the stabilization of bubble states at ambient conditions on application-compatible substrates. In the present work, we demonstrate a stabilization routine for remanent bubble states in high perpendicular magnetic anisotropy ${\text{[Co(0.44 nm)/Pt(0.7 nm)]}}_{X},$ $X=48$, 100, and 150 multilayers on $\mathrm{Si}/{\mathrm{SiO}}_{2}$ substrates by exploring the effect of external magnetic fields $({H}_{\mathrm{m}})$ of different strength and angles $(\ensuremath{\theta})$ with respect to the film surface normal. By systematic variation of these two parameters, we demonstrate that remanent bubble density and mean bubble diameter can be carefully tuned and optimized for each sample. Our protocol based on magnetometry only reveals the densest remanent bubble states at ${H}_{\mathrm{m}}=0.87{H}_{\mathrm{s}}$ (${H}_{\mathrm{s}}$ is the magnetic saturation field) and $\ensuremath{\theta}={60}^{\ensuremath{\circ}}$--${75}^{\ensuremath{\circ}}$ for all $X$ with a maximum of 3700 domains/100 $\ensuremath{\mu}{\mathrm{m}}^{2}$ for the $X=48$ sample. The experimental observations are supported by micromagnetic simulations, taking into account the nanoscale lateral grain structure of multilayers synthesized by magnetron sputter deposition, and thus helping to understand the different densities of the bubble states found in these systems.

Removing membrane locking in quadratic NURBS-based discretizations of linear plane Kirchhoff rods: CAS elements

NURBS-based discretizations of the Galerkin method suffer from membrane locking when applied to primal formulations of curved thin-walled structures. We consider linear plane curved Kirchhoff rods as a model problem to study how to remove membrane locking from NURBS-based discretizations. In this work, we propose continuous-assumed-strain (CAS) elements, an assumed strain treatment that removes membrane locking from quadratic NURBS for an ample range of slenderness ratios. CAS elements take advantage of the C1 inter-element continuity of the displacement vector given by quadratic NURBS to interpolate the membrane strain using linear Lagrange polynomials while preserving the C0 inter-element continuity of the membrane strain. To the authors’ knowledge, CAS elements are the first NURBS-based element type able to remove membrane locking for a broad range of slenderness ratios that combines the following distinctive characteristics: (1) No additional degrees of freedom are added, (2) No additional systems of algebraic equations need to be solved, and (3) The nonzero pattern of the stiffness matrix is preserved. Since the only additional computations required by the proposed element type are to evaluate the derivatives of the basis functions and the unit tangent vector at the knots, the proposed scheme barely increases the computational cost with respect to the locking-prone NURBS-based discretization of the primal formulation. The benchmark problems show that the convergence of CAS elements is independent of the slenderness ratio up to 104 while the convergence of quadratic NURBS elements with full and reduced integration, local B̄ elements, and local ANS elements depends heavily on the slenderness ratio and the error can even increase as the mesh is refined. The numerical examples also show how CAS elements remove the spurious oscillations in stress resultants caused by membrane locking while quadratic NURBS elements with full and reduced integration, local B̄ elements, and local ANS elements suffer from large-amplitude spurious oscillations in stress resultants. In short, CAS elements are an accurate, robust, and computationally efficient numerical scheme to overcome membrane locking in quadratic NURBS-based discretizations.

Nonlocal Manipulation of Magnetism in an Itinerant Two-Dimensional Ferromagnet.

Two-dimensional (2D) magnets are crucial in the construction of 2D magnetic and spintronic devices. Many devices, including spin valves and multiple tunneling junctions, have been developed by vertically stacking 2D magnets with other functional blocks. However, owing to limited local interactions at the interfaces, the device structures are typically extremely complex. To solve this problem, the nonlocal manipulation of magnetism may be a good solution. In this study, we use the magneto-optical Kerr effect technique to demonstrate the nonlocal manipulation of magnetism in an itinerant 2D ferromagnet, Fe3GeTe2 (FGT), whose magnetism can be manipulated via an antiferromagnet/ferromagnet interface or a current-induced spin-orbital torque placed distant from the local site. It is discovered that the coupling of a small piece of MnPS3 (∼40 μm2) with FGT can significantly enhance the coercive field and emergence of exchange bias in the entire FGT flake (∼2000 μm2). Moreover, FGT flakes with different thicknesses have the same coercive field at low temperatures if they are coupled together. Our study provides an understanding of the basic magnetism of 2D itinerant ferromagnets as well as opportunities for engineering magnetism with an additional degree of freedom.

Deterministic Terahertz Wave Control in Scattering Media.

Scattering-assisted synthesis of broadband optical pulses is recognized to have a cross-disciplinary fundamental and application importance. Achieving full-waveform synthesis generally requires means for assessing the instantaneous electric field, i.e., the absolute electromagnetic phase. These are generally not accessible to established methodologies for scattering-assisted pulse envelope and phase shaping. The lack of field sensitivity also results in complex indirect approaches to evaluate the scattering space–time properties. The terahertz frequency domain potentially offers some distinctive new possibilities, thanks to the availability of methods to perform absolute measurements of the scattered electric field, as opposed to optical intensity-based diagnostics. An interesting conceptual question is whether this additional degree of freedom can lead to different types of methodologies toward wave shaping and direct field-waveform control. In this work, we theoretically investigate a deterministic scheme to achieve broadband, spatiotemporal waveform control of terahertz fields mediated by a scattering medium. Direct field access via time-domain spectroscopy enables a process in which the field and scattering matrix of the medium are assessed with minimal experimental efforts. Then, illumination conditions for an arbitrary targeted output field waveform are deterministically retrieved through numerical inversion. In addition, complete field knowledge enables reconstructing field distributions with complex phase profiles, as in the case of phase-only masks and optical vortices, a significantly challenging task for traditional implementations at optical frequencies based on intensity measurements aided with interferometric techniques.

Photonic Integrated Circuit for Rapidly Tunable Orbital Angular Momentum Generation Using Sb2Se3 Ultra‐Low‐Loss Phase Change Material

AbstractThe generation of rapidly tunable optical vortex (OV) beams is one of the most demanding research areas of the present era as they possess orbital angular momentum (OAM) with additional degrees of freedom that can be exploited to enhance signal‐carrying capacity by using mode division multiplexing and information encoding in optical communication. Particularly, rapidly tunable OAM devices at a fixed wavelength in the telecom band stir extensive interest among researchers for both classical and quantum applications. This article demonstrates the realistic design of a Si‐integrated photonic device for rapidly tunable OAM wave generation at a 1550‐nm wavelength by using an ultra‐low‐loss phase change material (PCM) embedded with a Si‐ring resonator with angular gratings. Different OAM modes are achieved by tuning the effective refractive index using rapid electrical switching of Sb2Se3 film from amorphous to crystalline states and vice versa. The generation of OAM waves relies on a traveling wave modulation of the refractive index of the micro‐ring, which breaks the degeneracy of oppositely oriented whispering gallery modes. The proposed device is capable of producing rapidly tunable OV beams, carrying different OAM modes by using electrically controllable switching of ultra‐low‐loss PCM Sb2Se3.

Pseudo-Hermitian magnon-polariton system with a three-dimensional exceptional surface

We introduce a non-Hermitian platform allowing us to obtain a three-dimensional exceptional surface in a magnon-polariton coupled system. A two-tone driving system gives rise to a complex effective coupling, leading to a pseudo-Hermitian Hamiltonian. Independent tuning of the real and imaginary components of the coupling provides additional degrees of freedom, allowing us to form a four-dimensional parameter space in which a three-dimensional exceptional surface is proposed. We show that this pseudo-Hermitian Hamiltonian has the most general form in cavity spintronics with $CPT$ symmetry. Our exceptional surface allows us to form a $\mathcal{PT}$-like circle where the $\mathcal{PT}$ and anti-$\mathcal{PT}$-symmetric exceptional points are unified. Moreover, we show that the $CPT$ symmetry in our system can be reduced to $\mathcal{PT}$ symmetry. Our proposal makes the cavity magnon polariton a rich platform for realizing intriguing phenomena of exceptional points regardless of fabrication imperfections or exact detuning of the system frequencies.