Low-frequency Regime Research Articles

Mapping (Pseudo)Capacitive Charge Storage Dynamics in Titanium Carbide MXene Electrodes in Aqueous Electrolytes Using 3D Bode Analysis

Pseudocapacitive materials offer high charge storage capacities at high rates with charging time scales of tens of seconds to a few minutes. Voltammetry methods are extensively employed in understanding complex charge storage processes in pseudocapacitive materials. In this study, three-dimensional (3D) Bode analysis is employed in investigating charge storage dynamics of two-dimensional (2D) titanium carbides, Ti3C2Tx and Ti2CTx MXenes. Ti3C2Tx is used as a model system due to its high metallic conductivity and electrolyte-dependent capacitive/redox charge storage properties to better understand the charge storage dynamics in acidic and neutral electrolytes. In electrochemical impedance measurements, Ti3C2Tx/acidic electrolyte interface shows a high real capacitance accompanied by a low phase angle at specific potentials in the capacitive domain of frequencies (10 mHz to 1 Hz), demonstrating pseudocapacitive behavior. On the other hand, Ti3C2Tx/neutral electrolyte interface exhibits invariant low real capacitance with high phase angle values in the low frequency regime, indicating a double-layer charge storage mechanism. The charge storage dynamics of MXene electrodes determined using 3D Bode analysis corroborates well with the kinetic analyses using voltammetry methods.

Inverse design of structured materials for broadband sound absorption

This paper discusses the design of structured materials for broadband waterborne sound absorption. The structured materials are composed of a rubber matrix embedded periodically with cavities. To find the optimal distribution of cavities, an inverse design method based on topology optimization is proposed. Structured materials with novel hybrid cavities are thus designed. Efficient absorption over a wide frequency range between 600 Hz and 8000 Hz is achieved. The underlying mechanism behind the broadband absorption performance is revealed. Both the bending motion of the structured material and the translational motion of its steel backing affect the absorption in the low-frequency regime. Coherent coupling of local resonant modes together with the multiple scattering effects among cavities contribute to sound absorption in the mid-to-high frequency range. Moreover, a comparison with a conventionally structured material demonstrates the advantages of our design. Finally, an inverse design process with a different rubber matrix is performed. The results show that these cavities still exhibit similar topological features when the shear wave velocity of the rubber matrix is varied.

Longitudinal and transverse low frequency viscoelastic characterization of wood pulp fibers at different relative humidity

The viscoelastic behavior of wood pulp fibers plays a fundamental role in the performance of paper and paper products. Wood pulp fibers are hierarchical composites consisting of different cell wall layers and have anisotropic properties. Since accessing the individual fibers is challenging, no measurement technique has been able to characterize the viscoelastic properties in both – the longitudinal and transverse – fiber direction yet. Here, an atomic force microscopy (AFM)-based method is applied to investigate the viscoelastic properties of wood pulp fibers at varying relative humidity (RH) in both fiber directions. Experimental creep tests have been performed to investigate the material's low frequency regime response. A spring-dashpot model has been applied to characterize the viscoelastic behavior. The results indicate surprisingly small differences of the properties between the fiber directions. Transferring the results into a spectral representation explains an opposing trend of the viscosity that is connected to the long-term behavior.

Dynamical Freezing and Scar Points in Strongly Driven Floquet Matter: Resonance vs Emergent Conservation Laws

We consider a clean quantum system subject to strong periodic driving. The existence of a dominant energy scale, $h_D^x$, can generate considerable structure in an effective description of a system which, in the absence of the drive, is non-integrable, interacting, and does not host localization. In particular, we uncover points of freezing in the space of drive parameters (frequency and amplitude). At those points, the dynamics is severely constrained due to the emergence of an almost exact local conserved quantity, which scars the {\it entire} Floquet spectrum by preventing the system from heating up ergodically, starting from any generic state, even though it delocalizes over an appropriate subspace. At large drive frequencies, where a na\"ive Magnus expansion would predict a vanishing effective (average) drive, we devise instead a strong-drive Magnus expansion in a moving frame. There, the emergent conservation law is reflected in the appearance of an `integrability' of an effective Hamiltonian. These results hold for a wide variety of Hamiltonians, including the Ising model in a transverse field in {\it any dimension} and for {\it any form of Ising interactions}. The phenomenon is also shown to be robust in the presence of {\it two-body Heisenberg interactions with any arbitrary choice of couplings}. Further, we construct a real-time perturbation theory which captures resonance phenomena where the conservation breaks down, giving way to unbounded heating. This opens a window on the low-frequency regime where the Magnus expansion fails.

Noise control of outdoor unit of split type air-conditioner using periodic scatterers made with array of Helmholtz resonators

One of the reason for which the split type air-conditioner (AC) is preferred over window mounted air-conditioner is low noise. However, the outdoor unit of split type AC produces significant noise predominantly in low frequency regime and typically below 200 Hz. The sonic crystal, which is found suitable for outdoor noise control, is endeavored for attenuating the noise of outdoor unit of split type ACs. The challenge of designing a finite sonic crystal with scatterers having low diameter and minimum number of rows to attenuate the low frequency noise is addressed in present research by suggesting a tuneable multi resonant scatterer. The scatterer is made with multiple hollow compartments in the direction of height and having perforations which is tuned using finite element analyses. At last, the sonic crystal is fabricated and acoustically characterized in an anechoic chamber. The experimental investigations are conducted by deploying the sonic crystal near outdoor unit of three split type ACs and the noise measurements exhibit that the overall noise levels reduce by ≈7 dB.

Intramolecular relaxation of ring polymers in dilute solutions

The intramolecular relaxation dynamics of unconcatenated ring polymers in dilute solutions is theoretically investigated within the framework of the Rouse–Zimm theory. The excluded volume interactions (EVIs) between the nonbonded monomers are modeled by a harmonic potential, where the interaction parameter is evaluated from Flory’s mean-field approach. The hydrodynamic interactions (HIs) between the pairs of monomers are approximated by a preaveraged Oseen tensor. The mechanical moduli are dominated by the smaller relaxation rates corresponding to the collective relaxation modes in the low frequency regime, while they are governed by the higher relaxation rates corresponding to the local relaxation modes in the high frequency regime. EVI decreases the relaxation rates of the normal modes where the decrease for the collective modes is larger than that for the local modes, which consequently expands the width of the relaxation spectrum. The characteristic overall relaxation time is evaluated from the inverse of the crossover frequency, which is the same for rings of various sizes with HI and with both HI and EVI, while it shifts to lower frequencies with increasing ring size for the Rouse rings.

Study of coulomb drag with the effect of local field correction and dielectric medium

We present a numerical calculation for the drag resistivity of charge carriers interaction in coupled layer system separated by a barrier. Analytical expressions of non-linear susceptibility function and effective interlayer Coulomb interaction have been shown for non homogeneous dielectric environment at low temperature and low frequency regime. The system is studied within the model of random phase approximation (RPA) and taking into account the static local field correction (LFC). We employ Hubbard approximations (HA) and self consistent STLS approximations based LFC for obtaining the results, where LFC includes the exchange and correlation effects. Exchange and correlation effects enhanced the drag resistivity and the net screening. Another important quantity is the local form factors (LFF), which is the function of the bare inter- and intra-layer potential of the dielectric media. LFF for a multi-layer system is derived from the Poisson equation solution.

Noise and ergodic properties of Brownian motion in an optical tweezer: Looking at regime crossovers in an Ornstein-Uhlenbeck process.

We characterize throughout the spectral range of an optical trap the nature of the noise that drives the Brownian motion of an overdamped trapped single microsphere and its ergodicity, comparing experimental, analytical, and simulated data. We carefully analyze noise and ergodic properties (i) using the Allan variance for characterizing the noise and (ii) exploiting a test of ergodicity tailored for experiments done over finite times. We derive these two estimators in the Ornstein-Uhlenbeck low-frequency trapped-diffusion regime and study analytically their evolution toward the high-frequency Wiener-like free-diffusion regime, in very good agreement with simulated and experimental results. This study is performed comprehensively from the free-diffusion to the trapped-diffusion regimes. It also carefully looks at the specific signatures of the estimators at the crossover between the two regimes. This analysis is important to conduct when exploiting optical traps in a metrology context.

Theoretical investigation on the acoustic performance of Helmholtz resonator integrated microperforated panel absorber

Broadband sound absorption at low frequency with minimal space consumption and material cost is a trending research problem in engineering acoustics. Conventional sound absorbers like porous materials and microperforated panels (MPP) are not good enough to absorb sound in the low frequency regime. To enhance the acoustic performance, Helmholtz resonator with inserted neck (HRIN) is integrated with MPP in this investigation. The sound absorption characteristics of series and parallel arrangements of both MPP and multiple HRINs are theoretically modelled using electro-acoustic analogy and the results are compared with full field finite element simulations. Most importantly, it is observed that the parallel arrangement of MPP and multiple HRINs considerably reduced the thickness of the absorber. In addition, a single unit comprising four parallel Helmholtz resonators when arranged in conjunction with MPP absorbed more than 65% sound over a band width of 318 Hz to 880 Hz whereas the successive arrangement of the same unit with MPP is affected with high anti-resonance effect. It is demonstrated that, the proposed modelling strategy very well predicts the acoustic performance of HR integrated MPP absorbers on par with the computational intensive numerical simulations. Moreover, these configurations exhibited efficient low frequency sound absorption capability even with reduced absorber thickness.

Electrically controllable cyclotron resonance

Cyclotron resonance (CR) is considered one of the fundamental phenomena in conducting systems. In this paper, we study CR in a gated two-dimensional (2D) electron system (ES). Namely, we analyze the absorption of electromagnetic radiation incident normal to the gated 2DES, where a standard dielectric substrate separates the 2D electron sheet and the metallic steering electrode ("gate"); the whole system is placed in the perpendicular magnetic field. Our analysis reveals the redshift of the absorption peak frequency compared to the electron cyclotron frequency. The redshift appears in low-frequency regime, when the resonant frequency is much less than the frequency of Fabry-Perot modes in natural resonator "2D electron sheet - substrate - gate". Moreover, we find this shift to be dependent on the electron density of 2DES. Therefore, it can be controlled by varying the gate voltage. We predict that the shift can be large in realistic gated or back-gated 2DESs. The obtained controllability of CR in gated 2DES opens the door for exploring new physics and applications of this phenomenon.

Level statistics and Anderson delocalization in two-dimensional granular materials

Contrary to the theoretical predictions that all waves in two-dimensional disordered materials are localized, Anderson localization is observed only for sufficiently high frequencies in an isotropically jammed two-dimensional disordered granular packing of photoelastic disks. More specifically, we have performed an experiment in analyzing the level statistics of normal mode vibrations. We observe delocalized modes in the low-frequency boson-peak regime and localized modes in the high frequency regime with the crossover frequency just below the Debye frequency. We find that the level-distance distribution obeys Gaussian-Orthogonal-Ensemble (GOE) statistics, i.e. Wigner-Dyson distribution, in the boson-peak regime, whereas those in the high-frequency regime Poisson statistics is observed. The scenario is found to coincide with that of harmonic vibrational excitations in three-dimensional disordered solids.

High-bandwidth beam balance for vacuum-weight experiment and Newtonian noise subtraction

We report the experimental results of a prototype balance for the Archimedes experiment, devoted to measure the interaction between quantum vacuum energy and gravity. The prototype is a beam balance working at room temperature which shares with the final balance several mechanical and optical components. The balance sensitivity has been tested at the site of the Virgo gravitational wave detector in order to benefit from its quiet environment and control facilities. This allowed also the test of the coherence of the balance data with the Virgo interferometer signal and with the environmental data. In the low-frequency regime, the balance has shown a sensitivity of about 8times 10^{-12} {text {Nm}}/sqrt{text {Hz}}, which is among the best in the world, and it is very promising toward the final Archimedes measurement. In the high-frequency region, above a few Hz, relying on the behavior of the balance as a rotational sensor, the ground tilt has been measured in view of the next work devoted to Newtonian noise subtraction (NNS) in Virgo. The measured ground tilt reaches a minimum of about 8times 10^{-11} {text {rad}}/sqrt{{text {Hz}}} in the few Hz region and ranges from 10^{-10} to 10^{-9} {text {rad}}/sqrt{{text {Hz}}} in the 10–20 Hz region, where a very interesting coherence, at some frequencies, with the Virgo interferometer signal is shown.

GPR wave dispersion for material characterization

This paper studies the dispersion of GPR wave’s phase velocity at different wideband frequencies in plywood and concrete with varying moisture content. This study makes use of two GPR antennas with 2 GHz centre frequency operating in wide angle reflection and refraction (WARR) mode and with computation of spectral analysis of the surface wave (SASW). Computation of phase velocities is based on the acquisition of the cross-power spectrum and phase unwrap of two distorted ground waves at positions closer to and farther away from the transmitting antenna. The velocities of the ground waves are found to experience greater dispersion in low frequency regimes within the effective frequency bandwidths determined and thresholded by time–frequency analysis (TFA) and coherence plotting of the ground waves. This study validates not only the methodology, but also identifies the optimal distance between the first (Rx1) and second (Rx2) receivers as λ/2, which is based on a fixed transmitter (Tx) minus the first receiver (Rx1) distance. It serves as an indication of changeable separation distance when other lower frequency GPR is used because the distances of Tx-Rx1 and Rx1-Rx2 are wavelength dependent and thus also frequency dependent. Effects of moisture contents and chloride contamination in concrete were also characterized according to the dispersion plots that wave traelling in lower frequencies in GPR wave is much decelerated than that in higher frequencies. This research also contributes to the building of the “GPR-WARR machine” suggested in Annan and Jackson (2017) [1], within which the effects of wave dispersion on phase velocity can be inversely modelled to characterize variations in the material properties of infrastructure as a means of detecting surface damage.

Nutation dynamics and multifrequency resonance in a many-body seesaw

The multifrequency resonance has been widely explored in the context of single-particle models, of which the modulating Rabi model has been the most widely investigated. It has been found that with diagonal periodic modulation, steady dynamics can be realized in some well-defined discrete frequencies. These frequencies are independent of off-diagonal couplings. In this work, we generalize this physics to the many-body seesaw realized using the tilted Bose–Hubbard model. We find that the wave function will recover to its initial condition when the modulation frequency is commensurate with the initial energy level spacing between the ground and the first excited levels. The period is determined by the driving frequency and commensurate ratio. In this case, the wave function will be almost exclusively restricted to the lowest two instantaneous energy levels. By projecting the wave function to these two relevant states, the dynamics is exactly the same as that for the spin precession dynamics and nutation dynamics around an oscillating axis. We map out the corresponding phase diagram, and show that, in the low-frequency regime, the state is thermalized, and in the strong modulation limit, the dynamics is determined by the effective Floquet Hamiltonian. The measurement of these dynamics from the mean position and mean momentum in phase space are also discussed. Our results provide new insights into multifrequency resonance in the many-body system.

Structural and AC Electrical Properties of Tantalum Disulfide Embedded Polyaniline Composites

Employing in situ chemical polymerization technique, polyaniline-tantalum disulfide (PANI/TaS2) composites in different compositions have been synthesized. Poor crystallinity of the composites has been ascertained by XRD spectra. FTIR studies confirmed that the interaction in composites occurred at interfacial level. TGA results indicated an improvement in the stability of composites. The results of field emission scanning electron microscopy, transmission electron microscopy and x-ray photoelectron spectroscopy have confirmed the incorporation of TaS2 in PANI matrix as the embedded structures. Conductivity in the composite was found to increase by one order of magnitude compared to pristine PANI, and impedance spectra revealed that the conductivity is of electronic in nature. Dielectric behavior of the composites is attributed to the Maxwell–Wagner polarization. As the observed tangent loss peaks are in the low-frequency regime (103 Hz), the present composites may be explored for designing of low or medium frequency devices. Various parameters such as power law exponent, critical frequency and relaxation time have been estimated and discussed.

Static Structure Factor and Viscoelastic Properties of Dendrimer Grafted Nanoparticles in Solution.

The theory for the dynamics of multiscale branched polymeric structures is applied to understand the dendrimer-grafted nanoparticles in a dilute solution. The multiscale nature of dendrimer-grafted nanoparticles arises due to larger beads for the nanoparticles and the smaller beads for the polymeric structure connected through the harmonic springs. The multiscale generalized Gaussian structure approach allows us to study several viscoelastic properties: (i) storage and loss moduli and (ii) intrinsic viscosity. The influence of nanoparticles in the dendrimer structure is reflected in low and intermediate frequency regimes of the viscoelastic relaxation moduli. The increase in the size and the number fraction of nanoparticle shows an anomalous enhancement in the relaxation moduli. The increase in number fraction of nanoparticle in dendrimer-grafted nanoparticles decreases the transition frequency between solid- and liquid-like viscoelastic region. The intrinsic viscosity of dendrimer-grafted nanoparticles increases with increasing the size of nanoparticle. The inclusion of hydrodynamic interactions facilitates the dynamics of dendrimer-grafted nanoparticles. The Kratky plot of the static structure factor of all conformation of dendrimer-grafted nanoparticles is also analyzed as a function of number fraction and the size of the nanoparticles. At low wavenumbers, all conformations of dendrimer-grafted nanoparticles show a universal behavior. The compactness of dendrimer-grafted nanoparticles increases with the increase in number fraction and the size of the nanoparticles.

Continuum Flexural Metamaterial for Broadband Low-Frequency Band Gap

Despite enormous efforts being invested in the elastic wave's band gap, achieving a broadband low-frequency band gap is still a great challenge. Previous attempts to realize a broadband low-frequency band gap have focused on specific cases, such as linkage connections, the piezoelectric effect, or elastic foundations, which cannot be extended to other meaningful advances. Herein, we propose a way to achieve a broadband low-frequency band gap without any specific conditions, but with only the continuum metamaterial itself. Our proposed idea consists of a hollow cylindrical configuration and a bow-tie-shaped part, which can be easily extended to any other vibrational systems. To explain the idea, the extended mass-spring system is analytically investigated, and the idea is explained in detail based on the mass, inertia, bending, and shear-stiffness values. The proposed idea is supported by various numerical simulations. In addition, experimental realization of the broadband low-frequency band gap is carried out. It is expected that the proposed idea could be utilized to realize various vibration systems, such as a low-frequency cavity, vibration shielding, or filtering devices, which can extend vibration physics at the low-frequency regime.

Dielectric relaxation and small polaron hopping transport in sol-gel-derived NiCr2O4 spinel chromite

Herein, we have synthesized NiCr2O4 chromite by the sol-gel auto-combustion route. The refined X-ray powder diffraction pattern confirmed the single-phase formation of the material, which crystallized in a cubic normal spinel structure with space group Fd3m. The complex impedance study indicated the existence of relaxation phenomenon and predicted the semiconducting nature of the material. The frequency-dependent Nyquist plot modeled by (RGBCGB) (RGQG) equivalent circuit exhibited the effects of grain and grain boundary to the electrical response of the compound over the measured temperature range of 263-393 K. The experimental AC conductivity data followed Jonscher’s double power-law behavior, and the frequency exponent profile suggested small polaron hopping as the most probable transport mechanism for the sample. The complex modulus analysis showed the non-Debye type behavior of the material. In contrast, the dielectric constant exhibited dispersion in the low-frequency regime with a large tangent loss directly related to temperature.

Chemically-Tuned Solid-State Nanopores for Single-Molecule Biophyics

Solid-state nanopores (SSNs) are nanofluidic conduits fabricated through synthetic materials such as silicon nitride, silicon dioxide, graphene, etc. Intrapore variations, temporal fluctuations in open-pore current and analyte sticking are legacy limitations associated with SSNs which have challenged its footprint in single molecule science. We report a modification to the Controlled Dielectric Breakdown (CDB) method of nanopore fabrication on silicon nitride (SiNx) membranes where a blend of electrolyte and sodium hypochlorite is used as the conducting solution. These chemically tuned SSNs are devoid of the fundamental challenges associated with SSNs. The change in surface chemistry, with ∼3 orders of magnitude weaker Ka compared to conventional CDB nanopores, is thought to be inextricably linked with the Ohmic nature, extremely stable open-pore current during analyte translocation (hours), minimum analyte sticking with self-correction of current signal (if analyte sticking causes current deviation) and higher analyte responsiveness. The transport phenomena through these chemically modified nanopores are successfully tested by translocating 1 kb dsDNA and the human serum transferrin (hSTf) protein, with the highlight being the yield of an excess of 200 thousand DNA translocation events. Moreover, a comprehensive noise analysis done for the low frequency regime of this new class of SSNs revealed interesting characteristics which are distinct from the conventionally fabricated SSNs.

Vibroacoustic performance assessment of aircraft panels in low, mid and high frequency regimes

In the present work, vibroacoustic (VA) characteristics, namely sound transmission loss (STL), overall sound pressure levels (OASPLs) of aircraft panels made up of aluminum, composites and fiber metal laminates (FML) are studied. The investigation involves modeling of aircraft panels using finite element method (FEM) for low frequency, Boundary Element Method (BEM) for mid-frequency and statistical energy analysis (SEA) in high-frequency bands. To obtain the VA characteristics of the panels, twin chambers, namely source and receiver are numerically modeled, and the panels are placed in between them. The VA performances of the considered panels under low, mid and high-frequency sound environments are numerically simulated using an infinite duct model. The sound source in low, mid and high frequency is modeled in the source chamber as sound pressure, plane wave and diffusive acoustic field, respectively and the receiver chamber is idealized as an anechoic chamber at the ends. This numerical study helps in understanding the VA behavior of aircraft materials and also minimizes the cost and time involved in conducting experiments. The results are presented in the form of STL and OASPL. From this study, it was found that the aluminum and fiber metal laminates have good VA characteristics when compared to composites in all the three frequency regimes and composite panels behaving better in the mid-frequency region of 200 Hz to 300 Hz than aluminum and fiber metal laminates. An optimization approach for aircraft panels is also presented.