Low Mode Research Articles

Controllable phase transition of two-dimensional ferromagnetic chromium telluride thin films grown by molecular beam epitaxy

Two-dimensional (2D) Cr(1+δ)Te2 materials exhibit strong magnetic ordering and high Curie temperatures, making them attractive for various applications. It is crucial to achieve controllable synthesis for their successful integration into device technologies. In this study, we present the synthesis of phase-controllable 2D Cr(1+δ)Te2 films on the Si (111) substrate via molecular beam epitaxy. The composition and phase transition of the as-grown Cr(1+δ)Te2 films are characterized by using in-situ reflection high-energy electron diffraction, scanning tunneling microscopy, ex-situ X-ray photoelectron spectroscopy, X-ray diffraction, and theoretical calculations. At low growth temperatures, by carefully adjusting the film thickness from 2 to more than 3 layers, we achieve precise control over the phase of Cr(1+δ)Te2, from CrTe2 to Cr intercalated Cr2Te3. At a relatively elevated growth temperature, it is demonstrated that the Cr(1+δ)Te2 phase is independent of the film thickness, only Cr2Te3 forms and its growth mode is thickness-dependent. These phase transitions at low growth temperatures and growth mode changes at elevated growth temperatures are attributed to interfacial effects and the phase stability of Cr(1+δ)Te2 compounds. Additionally, we utilize scanning tunneling spectroscopy and computations to gain insights into the electronic properties of Cr2Te3. The magnetic measurements reveal that the 30-nm Cr2Te3 film exhibits ferromagnetic behavior with a Curie temperature of about 180 K. Our work offers a robust method for the controllable growth of high-quality 2D Cr(1+δ)Te2 films on Si substrates, providing an ideal platform for investigating their intrinsic properties and advancing the development of 2D magnet-based spintronics devices.

Magnetic Structures and Spin-Wave Excitations in Rare-Earth Iron Garnets Near the Compensation Temperature

We introduce a simple model for the ferrimagnetic non-collinear “magnetic umbrella” states of rare-earth iron garnets (REIG), common when the rare-earth moments have non-zero orbital angular momentum. The spin-wave excitations are calculated within linear spin wave theory and temperature effects via mean-field theory. This could be used to determine the magnetic polarization of each mode and thereby the spin currents generated by thermal excitations including the effects of mixed chirality. The spectra reproduce essential features seen in more complete models, with hybridization between the rare earth crystal field excitations and the propagating mode on the iron moments. By the symmetry of the model, only one rare earth mode hybridizes, inducing a gap at zero wave number and level repulsion at finite frequency. At the compensation point, the hybridization gap closes and finally, as we approach the Néel temperature, the hybridization gap appears to reopen. The chirality of the lowest mode changes its sign around the frequency at which the level repulsion occurs. This is important to estimate the spin current generation in REIGs.

Vision transformers motivating superior OAM mode recognition in optical communications.

Orbital angular momentum (OAM) has recently obtained tremendous research interest in free-space optical communications (FSO). During signal transmission within the free-space link, atmospheric turbulence (AT) poses a significant challenge as it diminishes the signal strength and introduce intermodal crosstalk, significantly reducing OAM mode detection accuracy. This issue directly impacts the performance of OAM-based communication systems and leads to a reduction in received information. To address this critical bottleneck of low mode recognition accuracy in OAM-based FSO-communications, a deep learning method based on vision transformers (ViT) is proposed for what we believe is for the first time. Designed carefully by numerous experts, the advanced self-attention mechanism of ViT captures more global information from the input image. To train the model, pretraining on a large dataset, named IMAGENET is conducted. Subsequently, we performed fine-tuning on our specific dataset, consisting of OAM beams that have undergone varying AT strengths. The computer simulation shows that based on ViT method, the multiple OAM modes can be recognized with a high accuracy (nearly 100%) under weak-to-moderate turbulence and with almost 98% accuracy even under long transmission distance with strong turbulence (C N2=1×10-14). Our findings highlight that leveraging ViT enables robust detection of complex OAM beams, mitigating the adverse effects caused by atmospheric turbulence.

Investigation of a low frequency coherent mode in Wendelstein 7-X with island divertor

During the island divertor operation of W7-X, especially in standard magnetic configuration (with 5/5 island chain in the scrape-off-layer (SOL)), quasi-periodic electromagnetic oscillation is observed. It appears in the frequency range of 1 kHz–2 kHz, therefore it is called the low frequency mode (LFM) within this paper. It is observed by multiple diagnostics, amongst them, a poloidal correlation reflectometer allowing radial localization of the LFM. The LFM is localized in the SOL and shows an obvious modulation effect on the plasma perpendicular velocity ( V⊥ ). Furthermore, broadband turbulence is observed in the fluctuation spectra of the electron density and the magnetic field and the perpendicular correlation length of turbulence eddies are modulated. The calculation of the poloidal flow velocity and its oscillation allows us to study the effect of the LFM on the flow. Cross correlation analysis shows that the perpendicular flow oscillation and the turbulence modulation are closely correlated. The application of external control coils to adjust the island size and position result in a strong modification of the magnetic topology at the plasma edge which affects the appearance, amplitude and frequency of the LFM. Bi-coherence analysis indicates that nonlinear interactions among turbulence components is a possible mechanism for the generation of the LFM.

Expansion of the functional capacities of electrostatic mirror analyzers for electron spectroscopy

Electron spectroscopy methods are widely used in scientific research and for technological purposes. The main element of spectrometers is an analyzer of charged particle beams. Electrostatic mirror systems are widely used due to their simpler practical realization. At their development two purposes were set: to improve the quality of spatial focusing of charged particles or to increase the value of linear energy dispersion. The objects of the study are electrostatic systems characterized by small sizes, simplicity of stabilization and localization of the working field and its shielding from the external electromagnetic disturbances. From all the known types of energy analyzers, suitable for the analysis of solid surfaces, preference is given to those that have good electron-optical properties, are simple in manufacture and operation. Therefore, spherical and cylindrical mirrors, which have become a basic tool for firms producing spectrometers, have been chosen as the object of study. The work solves the problem of expansion of the functional capacities of these systems by, firstly, combining several research methods in one device; and secondly, by solving specific narrower problems. A photoelectron or Auger spectrometer with an increased scanning area is proposed, where the initial angular opening of the beam 4° after passing a cylindrical mirror is increased to 10°, and the image smearing is reduced to 0.05 %. An Auger-electron spectrometer for analysis of rough surface has been developed, which allows to increase the probing depths by more than 5 times. A double filter type energy analyzer is calculated. Energy resolution was improved to 1.37 % by eliminating potential barrier smearing in low energy filter mode. Previously, the energy resolution was limited to 10 % due to this drawback

Fracture toughness of colored slurry seal at low temperatures and different loading Modes: Comparative study with warm and hot mix asphalt materials

Sustainable pavement, including colored slurry seal, was treated by crack growth under different traffic loads and temperatures. The current research examined colored slurry seals’ fracture toughness at various temperatures under mixed modes I/II and I/III. Three-point bend fixtures were used to prepare edge-notched disk bend (ENDB) disk specimens with edge cracks along one side of the specimens. Then, different temperatures and complete range mode mixity, including I/II and I/III, were used to determine fracture toughness data as well as the critical stress-intensity factor. Results indicated that the samples’ fracture resistance initially declined followed by an increment by an increment in the shear load in I/II mixed mode. From pure mode I to III, colored mixture resistances decreased with crack growth by raising mode III fraction at the crack tip. Moreover, lower temperatures led to better fracture performance between mixtures, especially at −20 °C. Mixtures’ fracture performance was positively influenced by temperature reduction due to bitumen strength characteristics. Comparison of fracture envelopes of cold mix asphalts (CMAs) in I/II and I/III mode mixities with other types of asphalt concrete, such as warm and hot mix asphalt (WMA and HMA), showed higher susceptibility of CMA mixture to cracking failure mode, making it noticeably weaker than WMA and HMA mixtures.

Inverse design of asymmetric Y-junctions for ultra-compact, broadband, and low crosstalk mode (de)multiplexers.

Asymmetric Y-junctions, compared with mode coupling-based devices, possess considerably smaller wavelength dependence and thus are more promising for ultra-broadband mode (de)multiplexing in integrated optics. However, these devices also feature relatively high mode crosstalk and insertion loss. Here, we show that the mode crosstalk and loss of an asymmetric Y-junction can be significantly reduced by optimizing the waveguide shape of the Y-junction using an adjoint-based inverse design. Based on such inverse-designed asymmetric Y-junctions, we realize ultra-compact, broadband, and low crosstalk silicon photonic TE00 & TE1 and TE0 & TE2 mode (de)multiplexers with sizes of only 4.5 × 1.2 µm2 and 6 × 1.4 µm2, respectively. From simulations it is shown that the TE0 & TE1 and TE0 & TE2 mode (de)multiplexers contain wide bandwidths of 160 nm (1460-1620 nm) and 140 nm (1460-1600 nm), respectively, over which the mode crosstalks are below about -20 dB, and the losses are <0.41 dB and <0.88 dB, respectively. The experimental results show that in the corresponding TE0 & TE1 and TE0 & TE2 mode division multiplexing systems, the crosstalks are less than -15.5 dB and -15 dB over the spectral ranges of 1453-1580 nm and 1460-1566 nm, respectively, and the losses are <1.7 dB at 1520 nm and <8.24 dB over the entire measured wavelength range.

Multiband antenna design with enhanced radiations using characteristic mode analysis

In this research, a multiband patch antenna design based on the superposition of multiple modes at the same resonant frequency is presented. The concept of the contribution of lower order modes (LOMs) with the higher order dominant modes (HODMs) is investigated using characteristic mode analysis (CMA). In order to provide similar broadside radiations in three bands, the radiating capability of the LOMs is enhanced in the resonant frequencies of HODMs. These HODMs when excited alone provide null in the broadside radiations of the antenna. Using a single feed, enhancement in the broadside radiations of the antenna is achieved with the superposition of multiple modes at the same resonant frequency. Based on the proposed concept two antennas have been designed and fabricated. The antenna provides stable and enhanced radiations. simulated and measured results are in good agreement. These antennas find application in many applications including communication systems, base station antennas, 5G communications, satellite communication etc.

Active Vibration Control Using Loudspeaker-Based Inertial Actuator with Integrated Piezoelectric Sensor

With the evolution of the aerospace industry, structures have become larger and more complex. These structures exhibit significant characteristics such as extensive flexibility, low natural frequencies, numerous modes, and minimal structural damping. Without implementing vibration control measures, the risk of premature structural fatigue failure becomes imminent. In present times, the installation of inertial actuators and control signal acquisition units typically requires independent setups, which can be cumbersome for practical engineering purposes. To address this issue, this study introduces a novel approach: an independent control unit combining a loudspeaker-based inertial actuator (LBIA) with an integrated piezoelectric ceramic sensor. This unit enables autonomous vibration control, offering the advantages of ease of use, low cost, and lightweight construction. Experimental verification was performed to assess the mechanical properties of the LBIA. Additionally, a mathematical model for the LBIA with an integrated piezoelectric ceramic sensor was developed, and its efficacy as a control unit for thin plate structure vibration control was experimentally validated, showing close agreement with numerical results. Furthermore, the LBIA’s benefits as an actuator for low-frequency mode control were verified through experiments using external sensors. To further enhance control effectiveness, a mathematical model of the strain differential feedback controller based on multi-bandpass filtering velocity improvement was established and validated through experiments on the clamp–clamp thin plate structure. The experimental results demonstrate that the designed LBIA effectively reduces vibration in low-frequency bands, achieving vibration energy suppression of up to 12.3 dB and 23.6 dB for the first and second modes, respectively. Moreover, the LBIA completely suppresses the vibration of the fourth mode. Additionally, the improved control algorithm, employing bandpass filtering, enhances the effectiveness of the LBIA-integrated sensor, enabling accurate multimodal damping control of the structure’s vibrations for specified modes.

Dispersion of axisymmetric longitudinal waves propagating in an inhomogeneous prestressed hollow cylinder immersed in an inviscid compressible fluid

The paper investigates the dispersion of axisymmetric waves in a cylinder with inhomogeneous initial stresses caused by the hydrostatic pressure of the compressible inviscid fluid in which this cylinder is immersed. In the context of this study, the motion of the cylinder is described by the three-dimensional linearized equations for elastic waves in bodies with initial stresses, but the flow of the fluid is described by the linearized Navier-Stokes equations for compressible (barotropic) inviscid fluids. The initial stresses in the cylinder, due to the action of hydrostatic pressure on the outer surface of the cylinder, are determined according to the well-known Lame formulas of classical linear elasticity theory. The corresponding eigenvalue problem for the dispersion of axisymmetric waves propagating in the hydro-elastic system under consideration is solved using the discrete-analytic method. The dispersion equation is obtained and solved numerically. Concrete numerical investigations are carried out for three pairs of materials: Soft Rubber + Glycerin, Lucite + Water and Steel + Water. The dispersion curves obtained for these pairs of materials are presented and discussed for different values of the intensity of the external pressure. In particular, it is found that the inhomogeneous initial stresses in the cylinder cause zero group velocity (ZGV) points to appear on the dispersion curves of the lowest modes. However, the ZGV points (or modes) appear on the dispersion curves of the higher modes as a result of the contact of the hollow cylinder with the fluid.

Ultra-narrow linewidth and a low-noise cascading Brillouin random fiber laser with a dual pump.

A cascading Brillouin random fiber laser with a dual pump (DP-CBRFL) is proposed and demonstrated. The DP-CBRFL can improve the Brillouin gain significantly to achieve an ultra-narrow linewidth (95.5 Hz) with 200 mW pump power, due to two cascading Brillouin gain fibers with an identical Brillouin frequency shift. Compared with the general Brillouin random fiber laser, the proposed random fiber laser has a more stable Brillouin gain spectrum and a lower mode density, which makes it have a lower intensity noise and frequency noise, especially in the low-frequency range. Meanwhile, it exhibits a high slope efficiency of 28% even if the pump power has reached 1.1 W due to the strong suppression ability of the high-order Stokes light.

A Linear Operator Method to Compute the Normal Modes with Rotation of any Asymmetric 3D Planet with Pure Vector Spherical Harmonics

In order to compute the free core nutation of the terrestrial planets, such as Earth and Mars, the Moon and lower degree normal modes of the Jovian planets, we propose a linear operator method (LOM). Generalized surface spherical harmonics (GSSHs) are usually applied to the elliptical models with a stress tensor, which cannot be expressed in vector spherical harmonics explicitly. However, GSSHs involve complicated math. LOM is an alternative to GSSHs, whereas it only deals with the coupling fields of the same azimuthal order m, as is the case when a planet model is axially symmetric and rotates about that symmetry axis. We extend LOM to any asymmetric 3D model. The lower degree spheroidal modes of the Earth are computed to validate our method, and the results agree very well with what is observed. We also compute the normal modes of a two-layer Saturn model as a simple application.

Impact of RF stress on different topologies of 100 nm X-band robust GaN LNA

In this article, we study the robustness of 3 versions of a single stage LNA configured according to different modes of detectivity or robustness against electromagnetic jamming signals. Four successive sequences of RF step stress at 10 GHz are applied to each of the 3 LNAs under study. These robust MMIC LNAs have been designed using the D01GH GaN process from OMMIC technology, to switch from a nominal low noise mode to a high linearity mode. This DC-bias switch allows increasing the power input 1 dB compression point by 8 dB. This study focuses on the robustness of these LNAs (LNA#A for agile) when they operate under nominal low-noise mode (featuring lower IP1dB) or under nominal high-linearity mode (at the price of a degraded noise figure NF50). This original LNA#A is compared to a robust conventional design using a larger sized device (LNA#R for robust). The step-stresses are operated at 10 GHz, which is the center frequency band of these LNAs. All modes of operation are shown to exhibit fairly reproducible step stress plots, although thermal or nonlinear effects can be differentiated between low-noise and high-linearity operating conditions, and compared with the robust design LNA#R. We demonstrate the relevance of an alternative approach to conventional LNA circuit design strategies in order to achieve natural electronic protection, without a limiter placed before the LNA#A or LNA#R or without turning-off the DC-biasing: this protection option benefits from maintaining the LNA in an operational detection situation for longer when the incident input signal increases, without any degradation in electrical performance (DC and RF) or noise (NF50), even after many sequences of RF step stress.

Heuristic inverse design of integrated mode converter by directly reshaping silicon waveguide

Compact photonic devices for mode manipulation in silicon waveguide are of importance as fundamental building blocks in on-chip photonic integrated circuits. In this paper, we present a heuristic inverse design scheme based on directly reshaping the silicon waveguide for TE-polarized mode conversions. No additional complicated internal nanoscale structures are introduced to the waveguide. The boundary deformations of the 6 μm-long functional region are described by Bernstein polynomials and the optimal design is searched and determined by the gradient-based method of moving asymptotes. The TE0-to-TE1 (TE1-to-TE2) mode converter can be efficiently obtained after 19 (52) iterations from a straight waveguide. Quasi-3D simulations show the high mode purity (>0.99) at center wavelength of 1550 nm, while the lowest mode purity keeps higher than 0.97 within the operational bandwidth of 60 nm for all designs. Relatively high transmission efficiency is maintained as well. 3D simulations with SOI configuration validate the functionality and the satisfactory performances. For both 340 nm and 220 nm waveguide thicknesses, the conversion efficiencies respectively keep higher than 0.98 and 0.90 for TE0-to-TE1 and TE1-to-TE2 mode converters at center wavelength. By simply cascading the two proposed mode converters, reciprocal TE0-to-TE2 mode conversion can be achieved. Moreover, we demonstrate the robustness of proposed designs by adding ±10 nm geometric deviations.

A grid of 200 000 models of young δ Scuti stars using mesa and GYRE

ABSTRACT The rapidly increasing number of delta Scuti stars with regular patterns among their pulsation frequencies necessitates modelling tools to better understand the observations. Further, with a dozen identified modes per star, there is potential to make meaningful inferences on stellar structure using these young δ Sct stars. We compute and describe a grid of &amp;gt;200 000 stellar models from the early pre-main sequence (pre-MS) to roughly one-third of the MS lifetime, and calculate their pulsation frequencies. From these, we also calculate asteroseismic parameters and explore how those parameters change with mass, age, and metal mass fraction. We show that the large frequency separation, Δν, is insensitive to mass at the zero-age main sequence. In the frequency regime observed, the Δν we measure (from modes with n ∼ 5–9) differs from the solar scaling relation by ∼13 per cent. We find that the lowest radial order is often poorly modelled, perhaps indicating that the lower order pressure modes contain further untapped potential for revealing the physics of the stellar interior. We also show that different nuclear reaction networks available in mesa can affect the pulsation frequencies of young δ Sct stars by as much as 5 per cent. We apply the grid to five newly modelled stars, including two pre-MS stars each with 15+ modes identified, and we make the grid available as a community resource.

Estimation of propagation characteristics of l1 series of LP modes of few-mode W-type fibers using numerical and analytical approach

In this work, single and two parameter variational fields for the l1 series of linearly polarised (LPl1) modes of a graded index few-mode W-type fibers have been proposed. The propagation characteristics, such as, the propagation constant, the normalised dispersion parameter and the normalised group delay have been computed for lower order LPl1 modes of such fibers using these variational fields. The exact results were derived by numerically solving the scalar wave equation for the LPl1 modes of W-type fibers. Further, the cut-off frequencies for the first few lower order modes have been computed numerically. Accuracies of the estimated propagation parameters have been investigated by comparing it with the numerical results. Compared to single-parameter variational fields, the two-parameter variational fields provide superior accuracy. Additionally, it has been found that the accuracy increases for lower order LPl1 modes. Thus, few-mode graded-index W-type fiber based photonic devices may utilise these proposed trial fields.

The effect of the ground motion frequency content on seismic performance and fragility curve of special concentric braced frames

Several articles have investigated the effect of various seismic parameters on the response of the structure. However, there has been limited discussion on the impact of seismic frequency content on the brace failure modes and the collapse mechanism of this system. This study investigated the effect of this parameter, considering three special concentric bracing frames of 2, 4, and 6 stories. For the frame dynamic analysis, the open system for earthquake engineering simulation software (OpenSees) is used. To evaluate the effect of frequency content, 60 earthquake records with high, medium and low frequency content are selected from the pacific earthquake engineering research center ground motion database, considering the peak ground acceleration divided by the peak ground velocity ratio. Incremental dynamic analysis is conducted to draw fragility curves and examine the response of structures.The results indicated that the collapse capacity of structures decreased as the frequency content was reduced. The percentage of reduction in collapse capacity under earthquakes with high to the low frequency content in 2, 4 and 6 story structures were 68, 67 and 71, respectively. In fact, reducing the frequency content increased the probability of occurrence of low cycle fatigue mode in the brace.

Consistent assembly method for elemental damping

This study proposes a new assembly method to address the problems of using the conventional direct assembly method for assembling elemental damping matrices for a global damping matrix. These problems include the challenges in calibrating an elemental damping ratio for a desired global damping ratio, significantly low and high global damping ratios associated with lower and higher modes, respectively, and unintended huge coupling effects between modes. The new assembly method is based on the recently proposed bell-shaped damping model. It gives a global damping ratio consistent with the elemental damping ratio used in a structure. It maintains modal orthogonality such that all global modes are uncoupled if the same damping ratio is used for all the elements. It also allows the damping ratio to be different from element to element or even from material to material, facilitating a broader damping theory that relates global damping to elemental damping, material damping, and internal state variables other than stiffness. Examples of response history analysis have been carried out to showcase the performance of the proposed assembly method compared to the direct assembly method.

Three- and four-wave resonances in the nonlinear quadratic Kelvin lattice

In this paper we investigate analytically and numerically the nonlinear Kelvin lattice, namely a chain of masses and nonlinear springs, as in the α-Fermi-Pasta–Ulam-Tsingou (FPUT) chain, where, in addition, each mass is connected to a nonlinear resonator, i.e., a second mass free to oscillate. Both nonlinearities are quadratic in the equations of motion. This setup represents the simplest prototype of nonlinear wave propagation on a nonlinear metamaterial. In the linear case, we diagonalize the system, and the two branches of the dispersion relation can be found. Using this result, we derive in the nonlinear case the equations of motion for the normal variables in Fourier space, obtaining a system governed by triad interactions among the two branches of the dispersion relation. We find that the transfer of energy between these two branches is ruled by three- and four-wave resonant interactions. We perform numerical simulations of the primitive equations of motion and highlight the role of resonances as an efficient mechanism for transferring energy. Moreover, as predicted by the theory, we provide direct evidence that four-wave resonances appear on a time scale that is longer than the time scale for three-wave resonances. We also assess the recurrence behaviour (usual in the FPUT system) for the nonlinear Kelvin lattice, and we show that, while recurrence is observed if all the energy is placed, at time t=0, in the lowest mode of the acoustical branch, a non-recurrent behaviour is observed if the initial energy is located in the optical branch.

Dissipation wavenumber and regularity for electron magnetohydrodynamics

We consider the electron magnetohydrodynamics (MHD) with static background ion flow. A special situation of B(x,y,t)=∇×(ae→z)+be→z with scalar-valued functions a(x,y,t) and b(x,y,t) was studied numerically in the physics paper [8]. The authors concluded from numerical simulations that there is no evidence of dissipation cutoff for the electron MHD. In this paper we show the existence of determining wavenumber for the electron MHD, and establish a regularity condition only on the low modes of the solution. Our results suggest that the conclusion of the physics paper on the dissipation cutoff for the electron MHD is debatable.