Frequency Dependence Research Articles

Frequency dependence of reflections on radar landmarks

Purpose. Reducing the dispersion of radar reflections from local objects, with multi-frequency sensing, to solve the problem of orientation by radar reflections from objects. Methodology. Reflections from local objects in the entire frequency range of the radar station (RS) at the same radio engineering position were measured by three independent radars of the same type on different days and at different antenna elevations. The deviation of the radar stations in the position did not exceed 500 meters. Coordinates (azimuth, range) of reflections of several separate local objects were allocated for each radar. The average values of reflections from local objects and their dispersion in the frequency range were calculated. Using various algorithms, individual frequencies were sampled and the reflected signals were averaged at these frequencies. The decrease in the dispersion of the reflected signal from the number of frequencies at which reflections were measured and from the algorithm for selecting these frequencies was investigated. Findings. Averaging the values of reflections from local objects for several frequencies leads to a decrease in dispersion and, as a result, to a more accurate correspondence of the reflected signal level to the geometric size of the local object. The variance decreases most rapidly for a small number of frequencies selected for averaging when selecting frequencies located in an interval of at least 1% relative to each other. Originality. To solve the problem of orientation based on radar reflections from local objects, it is necessary to identify the landmarks selected on a digital terrain model. Due to the fact that local objects (hills) are a collection of many reflectors falling into the allowed volume of the radar, with different levels of reflections and random phases, there may not be radar reflection from a local object at a certain frequency, or it may be very small. In order to unambiguously identify all landmarks, measurements must be carried out at several frequencies. The work has established how many frequencies measurements should be performed at and on what principle these frequencies should be selected. Practical value. The advent of digital terrain models made it possible to solve the problem of terrain orientation by comparing radar reflections from local objects with reflection models based on digital terrain maps. Radar reflection models use mathematical expectations of reflection values, unlike real reflections, which have random deviations in signal levels depending on the operating frequency. Reducing the variance of these deviations increases the accuracy of identifying characteristic local objects (landmarks) used to orient the radar in the absence of data from satellite navigation systems.

Melting and solidification in periodically modulated thermal convection

Melting and solidification in periodically time-modulated thermal convection are relevant for numerous natural and engineering systems, for example, glacial melting under periodic sun radiation and latent thermal energy storage under periodically pulsating heating. It is highly relevant for the estimation of melt rate and melt efficiency management. However, even the dynamics of a solid–liquid interface shape subjected to a simple sinusoidal heating has not yet been investigated in detail. In this paper, we offer a better understanding of the modulation frequency dependence of the melting and solidification front. We numerically investigate periodic melting and solidification in turbulent convective flow with the solid above and the melted liquid below, and sinusoidal heating at the bottom plate with the mean temperature equal to the melting temperature. We investigate how the periodic heating can prevent the full solidification, and the resulting flow structures and the quasi-equilibrium interface height. We further study the dependence on the heating modulation frequency. As the frequency decreases, we found two distinct regimes, which are ‘partially solid’ and ‘fully solid’. In the fully solid regime, the liquid freezes completely, and the effect of the modulation is limited. In the partially solid regime, the solid partially melts, and a steady or unsteady solid–liquid interface forms depending on the frequency. The interface height can be derived based on the energy balance through the interface. In the partially solid regime, the interface height oscillates periodically, following the frequency of modulation. Here, we propose a perturbation approach that can predict the dependency of the oscillation amplitude on the modulation frequency.

Extraction of two-magnon scattering and detection of inverse spin Hall effect up to 40 GHz inlow-damping Fe65Co35/Pt bilayer thin film

Abstract We report an investigation of the inverse spin Hall effect (ISHE), as a measure of the pure spin current (JS), induced by spin pumping in a Fe65Co35/Pt bilayer using coplanar waveguide ferromagnetic resonance (CPW-FMR) in the in-plane (IP) geometry over the broadband microwave frequency range of 8-40 GHz. From the IP-FMR measurements, the defect-induced two-magnon scattering (TMS) contribution is evaluated by analyzing the frequency dependence of FMR linewidth using Arias and Mills approach. Here, we have determined a very low Gilbert damping constant (αG) for Fe65Co35 bare film around 1.3 x 10−3, which is essential for the high spin current (> 107 A/m2) generation. The enhanced value of αG ≈ 3.2 x 10−3 obtained for Fe65Co35/Pt bilayer film is due to spin pumping damping, αSP = 1.9 x 10−3, confirmed by the ISHE induced DC voltage (VDC) signal measured across the edges of Pt layer. Out-of-plane (OP) angular ISHE measurements have been performed to disentangle spin pumping from spin rectification effects in the VDC signal. Further, we evaluated the effective spin-mixing conductance geff ↑↓ = 3.33 x 1019 m-2, which is higher than the values reported in FeCo-alloy-based bilayer systems. The present study aims to detect the JS injected by low-damping Fe65Co35 thin films, leading to the realization of energy-efficient spintronic devices.

Characteristic of temporal light modulation reference meter and standard light source for calibration and verification

Abstract The measurement of the temporal light modulation (TLM) of a light source is gaining increasing importance owing to its detrimental effects on human vision and health. To establish traceability in TLM measurements, the National Institute of Metrology, China (NIM), established a TLM reference meter (TLMM) and standard TLM light source (STLS) for calibration and verification services. The TLMM and STLS employ detector- and source-based approaches, respectively, to define the standards for TLM quantities. The TLMM rapidly acquires light waveform data from the TLM light sources and calculates corresponding TLM quantities. Frequency calibration result is with an expanded uncertainty of 1 × 10−5, frequency response function is almost flat in range of 1 kHz–25 kHz, and response time is less than 8 × 10−6 s. The electrical noise was evaluated at approximately 1000:1–5000:1. The STLS generates an arbitrary TLM waveform by converting the virtual waveform into an actual LED output light waveform. For the frequency response function, the relative amplitude decreased by 1% with a frequency of 15 kHz, and response time was less than 5 × 10−6 s. The linearity of the frequency dependency was studied with ratio varied by approximately 0.19% when the periodic triangle excitation in frequency band from 1 Hz to 1 kHz. Experimental comparisons demonstrated that TLM quantities (frequency, percentage flicker, flicker index, short-term flicker severity P st LM , and stroboscopic effect visibility measure ( SVM )) are consistent between the TLMM and STLS devices on 14 typical TLM waveforms. For P st LM , most of the deviations were in the range of −0.019–0.079 (except for one with 0.182). For SVM , most of the deviations were in the range of −0.001–0.003 (except for one with 0.014).

A primer of community ecology using the R language

AbstractCommunity ecology beginners often struggle to understand theories expressed in complex mathematical formulas and to master computer programming. To remedy this situation, this article provides a practical, R‐based introduction to community ecology by illustrating core concepts (vital rates, carrying capacity, density dependence) and models that can be used to explore the patterns of species abundance and diversity. The structure of this article consists of three modeling exercises, each asking a general question that can be answered by a combination of theory and R programming: (1) what determines the abundance of species, and what makes a population persist and go extinct?; (2) what determines the distribution of species and species diversity?; (3) what determines the relative abundance of species and what allows species to coexist? Through the exercises, I discuss the following five concepts and ideas that provide valuable insights into the questions: (i) the tragedy of the commons, (ii) the theory of island biogeography, (iii) competitive exclusion, (iv) the neutral theory of biodiversity, and (v) frequency dependence. These materials are thus designed to guide the reader in developing an intuition for ecological thinking that will help capture the essence of the global environmental and biodiversity crisis. Although this article does not delineate the scope and depth of the vast field of community ecology, I hope that it will motivate the reader to step up to a more formal introduction to community ecology.

Terahertz Crystal-Field Transitions and Quasi Ferromagnetic Magnon Excitations in a Noncollinear Magnet for Hybrid Spin-Wave Computation.

The complexity of interactions between the crystal-field and unusual noncollinear spin arrangement in nontrivial magnets demands novel tools to unravel the mystery underneath. In this work, we study such interaction dynamics of crystal-field excitations (CFE) and low-energy magnetic excitations in orthochromite TmCrO3 with controls of temperature and magnetic field using high-resolution magneto-terahertz (THz) time-domain spectroscopy. The THz energy spectrum spanning 0.5-10 meV possesses a low-frequency spin-excitation (magnon) mode and a multitude of CFE modes at 10 K, all of which uniquely embody a range of phenomena. For the magnon mode, a temperature dependence of peak frequency is induced by magnetic interactions between Tm and Cr subsystems. While a change from blue to red shift of peak frequency of this mode marks the magnetization reversal transition, the spin-reorientation temperature and change of magnetic anisotropy are depicted by different features of field and temperature dependent peak frequency dynamics. The modes corresponding to CFE are robust and laden with a multitude of submodes, which are attributes of nontrivial interactions across different transitions. These modes are suppressed only upon substitution of Tb3+ at the Tm3+ site, which suggests a dominant role of single-ion anisotropy in controlling entire THz excitation spectra. Overall, this remarkable range of phenomena seen through the unique lens of all-optical THz tools provides deeper insights into the origin of magnetic phases in systems with complex interactions between rare-earth and transition metal ions and provides a multitude of novel combinations of closely spaced modes for emerging hybrid spin-wave computation.

First TPymT-Ln complexes (TPymT = 2,4,6-Tris(2-pyrimidyl)-1,3,5-triazine; Ln = Eu, Gd, Tb, Dy): Solvothermal synthesis, structure, magnetic and photoluminescent properties

Four novel mononuclear lanthanide complexes, [Ln(TPymT)(NO3)3(H2O)2] (Ln: Eu3+ (1), Gd3+ (2), Tb3+ (3) and Dy3+ (4)), have been solvothermally synthesized using multidentate 2,4,6-tris(2-pyrimidyl)-1,3,5-triazine (TPymT). Compounds 1–3 have been characterized by means of elemental analysis, FT-IR, and single-crystal/powder X-ray diffraction analysis. Compound 4 was structurally characterized. The Ln3+ ion in 1–4 is ten-coordinated, where TPymT serves as an N3 donor in an isostructural series. The alternating-current magnetic studies showed frequency dependence below ca. 10 K for 3, as an indication of a single-ion magnet. The activation energy for the magnetization reorientation was estimated as Ueff/kB = 95(9) K after applying a dc bias field 2000 Oe for 3. The photoluminescent studies clarified that 1 and 3 behaved as red and green light emitters with quantum yields as high as 17 and 39 %, respectively. The TD-DFT calculation supports the energy level scheme of ground and excited states of TPymT together with the reference compound 4′-phenyl-2,2’:6′,2″-terpyridine. The present work suggests a broad chance and motivation to apply TPymT to the field of 4f coordination chemistry.

Phase transition investigation of ferroelectric and dielectric properties of (NH 2 CH 2 COOH)3 H2SO4 crystal using green function method and two sublattice PLCM model Hamiltonian

The Greens function technique and extended two sublattice pseudospin lattice coupled-mode (PLCM) model Hamiltonian have been applied to study the phase transition mechanism in (NH2CH2COOH)3H2SO4 compound. The two sublattice PLCM model Hamiltonian is modified by considering anharmonic phonon interactions, extra spin-lattice interactions, and an external applied electric field term. By using the double-time thermal-dependent Green function technique, the theoretical equations for the total shift, total width, normal mode frequency, transition temperature, relative permittivity, dielectric tangent loss, acoustic attenuation and spontaneous polarization are derived. By fitting the distinct model parameters in the theoretical equations, we calculated the temperature dependence of normal mode frequency, relative permittivity, tangent loss, acoustic attenuation and spontaneous polarization. The numerically obtained results are hold well and correlated with experimental data.

Mixed-mode coupling in the red clum. I. Standard single star models

The investigation of global, resonant oscillation modes in red giant stars offers valuable insights into their internal structures. In this study, we investigate in detail the information we can recover on the structural properties of core-helium burning (CHeB) stars by examining how the coupling between gravity- and pressure-mode cavities depends on several stellar properties, including mass, chemical composition, and evolutionary state. Using the structure of models computed with the stellar evolution code we calculate the coupling coefficient implementing analytical expressions, which are appropriate for the strong coupling regime and the structure of the evanescent region in CHeB stars. Our analysis reveals a notable anti-correlation between the coupling coefficient and both the mass and metallicity of stars in the regime $M in agreement with data. We attribute this correlation primarily to variations in the density contrast between the stellar envelope and core. The strongest coupling is expected thus for red-horizontal branch stars, partially stripped stars, and stars in the higher-mass range exhibiting solar-like oscillations ($M While our investigation emphasises some limitations of current analytical expressions, it also presents promising avenues. The frequency dependence of the coupling coefficient emerges as a potential tool for reconstructing the detailed stratification of the evanescent region.

Feasibility of loop-gain tuning for general measurement systems inspired by quantum locking for DECIGO

Abstract A series of quantum locking theories have been proposed to enhance the quantum-noise-limited target sensitivity of the DECi-hertz Interferometer Gravitational wave Observatory. The quantum locking that uses a square completion optimizes the sensitivity across all frequencies. However, a substantial amount of data-series must be post-processed since the square completion is a form of signal processing technique. This paper approaches the optimal sensitivity across all frequencies from an alternative perspective: by optimizing the frequency dependence of a servo gain in a feedback loop. The optimal servo gain is formulated by comparing the alternative method with the square completion method for the same optical setup. This will be shown in general noise issues extending the framework of the quantum locking. We find that the optimal servo gain forms a non-feasible filter but has certain characteristics. We also find that the noise of the measurement signal deteriorates proportionally to the noise measured in the feedback loop when the servo gain is slightly imperfect.

Mercury-probe measurement of electron mobility in β-Ga2O3 using junction moderated dielectric relaxation

Abstract We demonstrate the junction-moderated dielectric relaxation method to measure the in-plane electron mobility in β-Ga2O3 epitaxial layers. Unlike the Hall technique and channel mobility measurement in field-effect transistors, this method does not require deposition of permanent metal contacts. Rather, it measures the bias voltage and frequency dependence of the equivalent capacitance of the mercury/β-Ga2O3/mercury structure consisting of a Schottky contact, a quasi-neutral thin film semiconductor, and an Ohmic contact connected in series. The intrinsic dielectric relaxation of the bulk -Ga2O3 semiconductor typically occurs at ~ 1012 s-1, but when moderated by the mercury/β-Ga2O3 Schottky junction, it manifests itself as an inflection in the capacitance-frequency characteristics at a much lower frequency ~ 106 s-1 within the range of most capacitance measuring instruments. Using carrier density and layer thickness determined from capacitance-voltage measurement, we extract the electron mobility of β-Ga2O3 from the junction-moderated dielectric relaxation frequency.

A frequency-dependent model for bone remodeling using a micromorphic porous medium subjected to harmonic mechanical loading

In this paper, the bone tissue was modeled as a linear viscoelastic material saturated with interstitial fluid. We considered a specific case of harmonic loading and related the mechanical stimuli to the loading frequency. In this way, we could include the inertial effect in the model while not having to deal with the perturbation during each loading period. Two types of mechanical signals were considered: strain energy and dissipation energy. A parametric study revealed the dependency of the two signals on loading frequency and material property. The evolution of the apparent mass density supported the parametric study’s findings. Under the three different frequency loadings, the strain energy-stimulated samples experienced identical remodeling scenarios. The samples stimulated with dissipation energy, on the other hand, exhibited a strong frequency dependence. An additional study was performed to investigate the effect of long-term variations in the loading frequency on the remodeling process. This demonstrated the model’s capabilities in designing and evaluating load regimes for rehabilitation following a bone injury or bone reconstruction.

Estimation of added effects and their frequency dependence in various fluid–structure interaction problems

This paper focuses on a fluid–structure interaction topic—the determination of added effects caused by fluid forces acting on a body, considering the standard linear equation of motion. We present various problems that assume small-displacement oscillations of single and multiple bodies in inviscid irrotational (potential) flow or viscous incompressible flow in both closed domain and external flow. For inviscid flow, effects of geometric parameters on the added effects were studied. The presented results extend results known from the literature. For viscous flow, frequency dependence of the added effects was studied for a wide range of frequency. The added effects were computed from data from numerical simulations of fluid flow, where the body oscillations were modeled using the dynamic mesh approach. Effects of the phase shift caused by the dynamic mesh were addressed. The added mass was compared with the corresponding value determined for inviscid flow where applicable. The results show strong dependence of the added effects on many parameters, making their proper computation challenging even for simplified cases.

Study on frequency dependent convolution methods for sound event detection

Sound event detection (SED) aims to classify target sound events from a given audio clip with the corresponding onset and offset time localization. SED, a core research field in machine listening, has been rapidly growing by adopting the methods by applying deep learning methods in image recognition fields. One representative example is 2D convolution which is applied to 2D time-frequency audio features. However, 2D audio data exhibits physical discrepancies from 2D image data, one of which is that frequency dimension of 2D audio data is translational-variant while position dimension of 2D image data is translational-invariant. Therefore, to make 2D convolution more consistent to 2D audio data, we need to make 2D convolution dependent to frequency dimension. Previously, to address this problem, Frequency Dynamic Convolution (FDYConv) was proposed to replaces 2D convolution. FDYConv is dependent on frequency dimensions since it applies different kernels on frequency dimension. While it has proven its outstanding performance on SED, there are several more alternative frequency dependent convolution methods which are yet to be explored, In this work, we propose other simpler frequency dependent convolution methods and compare them with FDYConv in order to further investigate the effect of frequency dependence of 2D convolution methods.

Effective models of metasurface/metainterface made of three-dimensional Helmholtz resonators

We study a reflective metasurface made of 3D subwavelength Helmholtz resonators. We provide an effective model in the time domain obtained by using a three-scale asymptotic approach. The metasurface is reduced to an homogenized boundary condition involving a resonant, frequency dependant impedance. Our effective model takes into account boundary layer effects near the neck opening, and we provide a precise method to calculate the " added length ". The study is extended to meta-interface in transmission, where the set of effective boundary conditions are replaced by effective jump conditions, and the results are discussed in frequency domain with comparison with direct numerical simulations.

On the excess loss in FeSiAl soft magnetic composites

Aiming at the issue of inconsistent theoretical expression of excess loss in soft magnetic composites, the irrational linear frequency dependence of excess loss W ex is analyzed. By comparing and analyzing the experimental data of magnetic loss of FeSiAl core under different transverse magnetic fields with the theoretical results, it is determined that the excess loss of FeSiAl composite is proportional to the 1/2th power of the frequency. Meanwhile, it is experimentally discovered that there is a positive correlation between the hysteresis loss and the excess loss of FeSiAl magnetic core, which might provide an effective approach to reduce the excess loss of FeSiAl magnetic core.

Dielectric and photocatalytic characteristics of novel CaCu3Ti4O12 modified Ba0.5Sr0.5TiO3-based heterojunction synthesized by wet-chemistry method

The study presents a comprehensive investigation into the synthesis, characterization, and application of barium strontium titanate-calcium copper titanate (BST-CCTO) heterojunction, offering insights into their potential for high-energy-density capacitors and photocatalytic applications. It discusses the synthesis methods for BST, CCTO nanoparticles and their heterojunction. It delves into the structural analysis of prepared heterojunctions using techniques like X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The photocatalytic performance of heterojunctions, particularly in the degradation of organic dyes and pharmaceutical waste under visible light, is explored. Results demonstrate the superior photocatalytic efficiency of BST-CCTO heterojunction, i.e., B2 (99.5 % for RhB) compared to individual BST (78.4 %), suggesting potential applications in environmental remediation. This study also explores the development and properties of dielectric materials, particularly focusing on polyvinylidene fluoride (PVDF) and its composites with high-dielectric ceramics such as BST and CCTO. Various aspects of dielectric behavior, including polarization mechanisms and frequency dependence, are also discussed. Dielectric studies reveal the enhanced dielectric constant (50.93 at 1 kHz for 3 wt% loading of B3) of PBCT-3 composite as compared to pure PVDF composite (10 at 1 kHz), attributed to factors such as interface engineering, complementary electrical properties, and grain boundary effects. The mechanisms underlying the interaction between fillers and PVDF matrix are elucidated, highlighting the role of interface engineering and polarization enhancement in optimizing dielectric performance.

Testing secondary sex ratio bias hypotheses in white-tailed deer in Mississippi, USA.

Natural selection favors individuals with the highest inclusive fitness (i.e., total number of descendants). In cases where one sex is more productive, one or both parents may maximize their inclusive fitness by investing in the offspring of the more prolific sex. Such preferential production can lead to skewed sex ratios at various life history stages, including at birth, resulting in secondary sex ratio bias. Several competing hypotheses have been proposed to explain observed variation in secondary sex ratios including Fisher's frequency dependence and two hypotheses related to maternal condition: Trivers-Willard and the local resource hypotheses. Although it has been shown that maternal condition can influence the number of offspring produced in white-tailed deer, there is no consensus as to which of the hypotheses drives sex ratio bias in wild populations. Using a spatiotemporally extensive dataset of pregnant white-tailed deer from Mississippi, USA, we examined fetal sex ratio in relation to the Fisherian frequency-dependence hypothesis and hypotheses related to maternal condition. While there was a male-sex ratio bias in pregnant females that reduced in intensity with the number of offspring, there was no support for condition-related hypotheses. Instead, secondary sex ratios for white-tailed deer in Mississippi were nearly consistent with Fisherian frequency dependence. Our findings add to the body of literature on secondary sex biases in white-tailed deer and help inform sex bias ratios for a southern population of a cervid of management importance in the US.

Frequency Dependence and Propagation Mechanisms of Path Loss in Indoor Environments

Frequency Dependence and Propagation Mechanisms of Path Loss in Indoor Environments

Influence of acoustic modes on resonance properties of a quartz tuning fork immersed in superfluid 4He and liquid mixtures 3He–4He

The oscillating quartz tuning fork method has been used to study resonance phenomena in experimental cells of different sizes filled with superfluid 4He and concentrated liquid mixtures of 3He–4He. An analysis of the temperature dependence of the resonance frequencies of the tuning forks showed that in a number of cases, the incompressible fluid model is not sufficient to interpret the experimental results and that acoustic processes in the cell should be taken into account. The frequencies of the resonances of the first sound in cylindrical geometry are estimated and their influence on the resonant frequencies of the tuning fork is shown, which can lead to a distortion of the shape of the resonant line. A comparison is made between experimental results for superfluid 4He and mixtures of 3He-4He with light isotope concentrations of 5% and 15%. It is shown that, in contrast to pure helium, the model of a viscous incompressible fluid cannot be applied to mixtures, even in the absence of first acoustic resonances. This can be explained by the fact that, when studying concentrated solutions, the excitation of the second sound along with the first can have a noticeable effect on the resonance characteristics of the tuning fork.