Frequency Dispersion Research Articles

Trap-assisted enhancement of the responsivity in asymmetric planar GaN-based nanodiodes at low temperature

The microwave detection capability of GaN-based asymmetric planar nanodiodes (so-called Self-Switching Diode, SSD, due to its non-linearity) has been characterized in a wide temperature range, from 70 K up to 300 K. At low temperature, microwave measurements reveal an enhancement of the responsivity at frequencies below 1 GHz, which, together with a pronounced hysteresis in the DC curves, indicate a significant influence of the surface states. This leads to a significant variability and non-repeatability which needs to be reduced since it degrades the accuracy of the detection. For this sake, the RF characterization was repeated after applying a positive/negative voltage able to fill/empty the surface states in order to have a well-established preconditioned state. As a consequence of the positive pre-soak bias, a significant enhancement of the measured responsivity, with a × 10 increase at low temperature. The RF detection measurements after such preconditioning contains a time dependence induced by the slow discharge mechanism of the traps, so that the improved responsivity remains even after 100s of seconds. On the other hand, a negative voltage pre-soak benefits the discharge process, thus suppressing the low frequency dispersion and the important variability of the detection without the pre-conditioning step. We also show that the relation between the voltage and current responsivities in each case allows to explain the impact of the surface charges in terms of the device impedance.

Dodecyltrimethylammonium bromide-styrene microemulsion dielectric investigation in aqueous media

In this paper, we report the complex permittivity of aqueous microemulsions of N-dodecyltrimethylammonium bromide and styrene. The studies were carried out at 298.15 K while varying the styrene to surfactant concentration ratio, S0 (0.16 ≤ S0 ≤ 0.71) and the surfactant concentration, c (0.1032 ≤ c (mol·dm−3) ≤ 0.7806). The frequencies ranged from 100 MHz to 89 GHz. An analysis of a particular solution is conducted over the temperature range of 278.15–328.15 K to calculate the energy of the activation parameters. At 298.15 K, the spectra are fitted to a total of five Debye processes for concentration dependency series and three Debye processes for temperature dependence series. Br− surface and bulk diffusion surrounding the micelles best describes the two low-frequency dispersions, at 100 MHz and 0.8 GHz, respectively. These mechanisms were examined using Grosse theory. The Maxwell–Wagner relaxation process is equivalent to high-frequency micelle dispersion at 0.8 GHz. The thickness of the conducting shell of the micelle is determined by using the electrical conductivity of the particles and parameters taken from the Grosse theory, and the results are comparable to those from an examination of the Pauly and Schwan model. The Grosse model and solvent dispersion analysis were used to calculate the volume fractions of micelles. Both approaches were in agreement. The surfactant head group non-rotationally hydrates almost ∼7 ± 1 water molecules. Styrene addition allows for the non-rotationally bound accommodation of an additional ∼7 ± 1 water molecules.

Calorimetric measurement of nuclear spin-lattice relaxation rate in metals

The quasiparticle density of states in correlated and quantum-critical metals directly probes the effect of electronic correlations on the Fermi surface. Measurements of the nuclear spin-lattice relaxation rate provide one such experimental probe of quasiparticle mass through the electronic density of states. By far the most common way of accessing the spin-lattice relaxation rate is via nuclear magnetic resonance and nuclear quadrupole resonance experiments, which require resonant excitation of nuclear spin transitions. Here we report non-resonant access to spin-lattice relaxation dynamics in AC-calorimetric measurements. The nuclear spin-lattice relaxation rate is inferred in our measurements from its effect on the frequency dispersion of the thermal response of the calorimeter-sample assembly. We use fast, lithographically-defined nanocalorimeters to access the nuclear spin-lattice relaxation times in metallic indium from 0.3~K to 7~K and in magnetic fields up to 35~T.

Exchange of Carbapenem-Resistant Escherichia coli Sequence Type 38 Intercontinentally and among Wild Bird, Human, and Environmental Niches.

Carbapenem-resistant Enterobacteriaceae (CRE) are a global threat to human health and are increasingly being isolated from nonclinical settings. OXA-48-producing Escherichia coli sequence type 38 (ST38) is the most frequently reported CRE type in wild birds and has been detected in gulls or storks in North America, Europe, Asia, and Africa. The epidemiology and evolution of CRE in wildlife and human niches, however, remains unclear. We compared wild bird origin E. coli ST38 genome sequences generated by our research group and publicly available genomic data derived from other hosts and environments to (i) understand the frequency of intercontinental dispersal of E. coli ST38 clones isolated from wild birds, (ii) more thoroughly measure the genomic relatedness of carbapenem-resistant isolates from gulls sampled in Turkey and Alaska, USA, using long-read whole-genome sequencing and assess the spatial dissemination of this clone among different hosts, and (iii) determine whether ST38 isolates from humans, environmental water, and wild birds have different core or accessory genomes (e.g., antimicrobial resistance genes, virulence genes, plasmids) which might elucidate bacterial or gene exchange among niches. Our results suggest that E. coli ST38 strains, including those resistant to carbapenems, are exchanged between humans and wild birds, rather than separately maintained populations within each niche. Furthermore, despite close genetic similarity among OXA-48-producing E. coli ST38 clones from gulls in Alaska and Turkey, intercontinental dispersal of ST38 clones among wild birds is uncommon. Interventions to mitigate the dissemination of antimicrobial resistance throughout the environment (e.g., as exemplified by the acquisition of carbapenem resistance by birds) may be warranted. IMPORTANCE Carbapenem-resistant bacteria are a threat to public health globally and have been found in the environment as well as the clinic. Some bacterial clones are associated with carbapenem resistance genes, such as Escherichia coli sequence type 38 (ST38) and the carbapenemase gene blaOXA-48. This is the most frequently reported carbapenem-resistant clone in wild birds, though it was unclear if it circulated within wild bird populations or was exchanged among other niches. The results from this study suggest that E. coli ST38 strains, including those resistant to carbapenems, are frequently exchanged among wild birds, humans, and the environment. Carbapenem-resistant E. coli ST38 clones in wild birds are likely acquired from the local environment and do not constitute an independent dissemination pathway within wild bird populations. Management actions aimed at preventing the environmental dissemination and acquisition of antimicrobial resistance by wild birds may be warranted.

An overview of the monkeypox

Smallpox-like symptoms are caused by the human monkeypox orthopoxvirus, a zoonotic orthopoxvirus. The zoonotic human monkeypox was detected in 1970, twelve years after the monkeypox virus was found in a Danish lab in 1958. It has lately been documented in cases outside of Africa, and it has spread to other regions of Africa. (mainly West and Central Africa). The genesis, epidemiology, and ecology of the disease are still largely unknown, which has caused a rise in the frequency and geographic dispersion of human monkeypox cases in recent years. 8 days on average were needed for incubation. (range 4–14 days). A multi-nation outbreak of human monkeypox infections, involving transmission from person to person, was documented in Europe and North America in May 2022. Monitoring and epidemiological analysis need to be more precise in order to calculate the effect on public health and develop policies to reduce the risk of a larger spread of disease. Small viral epidemics with mortality rates around 10% and rates of secondary human-to-human transmission around the same percentage are frequent in tropical Central and West Africa. On July 31, 2022, the first case of monkeypox mortality was reported in India; the victim was a 22-year-old man who had recently returned from the United Arab Emirates. In light of the continuous outbreaks throughout the world, we provide current information on monkeypox for medical professionals in this overview. There is a zoonotic orthopoxvirus that produces human monkeypox. However, ST 246 has been found to be effective in vivo and in vitro in infected animals, and experiments were safely carried out on non-infected humans. ST 246 has not been tested for its effectiveness on monkey pox or orthopox-infected individuals.

White-light channeled imaging polarimeter using Savart plates and a polarization Sagnac interferometer.

A Stokes white-light channeled imaging polarimeter using Savart plates and a polarization Sagnac interferometer (IPSPPSI) is presented, which provides an effective solution to the problem of channel aliasing in broadband polarimeters. The expression for the light intensity distribution and a method to reconstruct polarization information are derived, and an example design for an IPSPPSI is given. The results reveal that a complete measurement of the Stokes parameters in broad band can be achieved with a snapshot on a single detector. The use of dispersive elements like gratings suppresses broadband carrier frequency dispersion so the channels in the frequency domain do not affect each other, ensuring the integrity of information coupled across the channels. Furthermore, the IPSPPSI has a compact structure and does not employ moving parts or require image registration. It shows great application potential in remote sensing, biological detection, and other fields.

A novel approach for modeling of the ultrasonic signal backscattered in immersed multilayer structures at normal incidence: Time, Frequency, and Velocity dispersion representation

This study proposes a theoretical procedure for modeling ultrasonic signal backscattered in normal incidence by a multilayer structure (MS) immersed in water. The propagation of a longitudinal ultrasonic wave in a MS is studied analytically by using the transfer matrix method (TMM), in which each layer is established as a quadrupole formalism combining the stresses and velocities. The transfer matrices allow us to determine the reflection coefficient of the multilayer structure. The MS solid/liquid/solid case is studied in both insonation configurations (direct and reverse). The modeled ultrasonic response is represented in both time and frequency domains and validated by comparison with experimental data published previously. The time–frequency methods such as spectrogram (SP) and the smoothed pseudo-Wigner–Ville (SPWV) are also used to investigate the character and the value of the non-dispersive of the longitudinal wave. Moreover, the TMM method is used under different conditions (different layer thicknesses, non-uniform distribution of layers, and any frequency range), which are among the causes of MS dispersion. TMM method is able to show this behavior and is exploited to produce (Distance versus Time) and (Distance versus Frequency) representations of wave propagation for each studied MS. The perfect agreement of the comparison of TMM dispersion results with the results is achieved by using time–frequency methods in the case of a 5 MHz central frequency.

Zone-Folded Quasi-bound State Metasurfaces with Customized, Symmetry-Protected Energy-Momentum Relations

Energy-momentum (dispersion) relations are a foundational framework for photonic device design, but our incomplete command thereof still limits key technologies. For instance, narrowband optical combiners for augmented reality should ideally reflect with unitary efficiency selected wavelengths that encode the artificial information over a large range of oblique incident angles, yet they are conventionally severely limited in field of view by dispersion. Here, we engineer nonlocal metasurfaces to support quasi-bound states in continuum with zero first-order resonant frequency dispersion at any desired quasi-momentum by exploiting a zone-folding technique that leverages a change in the lattice family. Our platform thereby advances photonic devices by enabling unprecedented control over the energy-momentum properties of nonlocal states, rationally controlled through a perturbation scheme that produces minimal distortion to non-resonant light.

Giant and broadband THz and IR emission in drift-biased graphene-based hyperbolic nanostructures

We demonstrate that Cherenkov radiation can be manipulated in terms of operation frequency, bandwidth, and efficiency by simultaneously controlling the properties of drifting electrons and the photonic states supported by their surrounding media. We analytically show that the radiation rate strongly depends on the momentum of the excited photonic state, in terms of magnitude, frequency dispersion, and its variation vs the properties of the drifting carriers. This approach is applied to design and realize miniaturized, broadband, tunable, and efficient terahertz and far-infrared sources by manipulating and boosting the coupling between drifting electrons and engineered hyperbolic modes in graphene-based nanostructures. The broadband, dispersive, and confined nature of hyperbolic modes relax momentum matching issues, avoid using electron beams, and drastically enhance the radiation rate—allowing that over 90% of drifting electrons emit photons. Our findings open an exciting paradigm for the development of solid-state terahertz and infrared sources.

On the propagation of gravity waves in the lower solar atmosphere in different magnetic configurations

Gravity waves are generated by turbulent subsurface convection overshooting or penetrating locally into a stably stratified medium. While propagating energy upwards, their characteristic negative phase shift over height is a well-recognized observational signature. Since their first detailed observational detection and estimates of energy content, a number of studies have explored their propagation characteristics and interaction with magnetic fields and other waves modes in the solar atmosphere. Here, we present a study of the atmospheric gravity wave dispersion diagrams utilizing intensity observations that cover photospheric to chromospheric heights over different magnetic configurations of quiet-Sun (magnetic network regions), a plage, and a sunspot as well as velocity observations within the photospheric layer over a quiet and a sunspot region. In order to investigate the propagation characteristics, we construct two–height intensity - intensity and velocity- velocity cross-spectra and study phase and coherence signals in the wavenumber - frequency dispersion diagrams and their association with background magnetic fields. We find signatures of association between magnetic fields and much reduced coherence and phase shifts over height from intensity-intensity and velocity-velocity phase and coherence diagrams, both indicating suppression/scattering of gravity waves by the magnetic fields. Our results are consistent with the earlier numerical simulations, which indicate that gravity waves are suppressed or scattered and reflected back into the lower solar atmosphere in the presence of magnetic fields.

Microwave response of type-II superconductors at weak pinning

Theory of linear microwave response of thin films of type-II superconductors in the mixed state is developed taking into account random spatial fluctuations of the parameters of the system, such as the order parameter, diffusion coefficient, or film thickness. In the regime of collective pinning the microwave response of the system exhibits strong frequency dispersion, arising from nonequilibrium vortex core quasiparticles. The corresponding contribution to the ac conductivity is controlled by the inelastic relaxation time, and may exceed the usual Bardeen-Stephen conductivity. It is caused by the Debye-type inelastic relaxation. Debye mechanism of microwave losses may be responsible for strong effects of electromagnetic noise upon dc conductivity in the mixed state at low temperatures.

Efficient Recursive Convolution Schemes for the Accurate FDTD Modeling of Graphene Interband Contribution via Complex Conjugate Pairs

The accurate time-domain numerical modeling of graphene surface conductivity due to the interband contribution is presented in this paper. Initially, the lower frequency limit for the interband conductivity inclusion is studied, highlighting that even in the far-infrared regime it should not be ignored. Then, a precise vector-fitting technique is utilized to decompose the analytical conductivity function into complex conjugate pairs. Every pair is connected to a complex surface current, while the frequency dispersion is evaluated via a proper recursive convolution scheme. Finally, a straightforward algebraic manipulation is conducted to eliminate the complex terms from the update equations of the electromagnetic components, concerning the finite-difference time-domain (FDTD) algorithm. The precision and efficiency of the proposed methodology are thoroughly validated by comparing the numerically extracted surface wave propagation characteristics to those obtained in terms of analytical expressions.

Ab initio study of phononic thermal conduction in ScAgC half-Heusler

We present a first-principles lattice calculations to comprehend the thermal expansion $\alpha(T)$ and lattice thermal conductivity $\kappa_{ph}$ of ScAgC. The obtained positive frequencies of phonon dispersion shows the dynamical stability of ScAgC in FCC structure. The estimated $\alpha(T)$ from quasi-harmonic approximation (QHA) at 300(1200) K is $\sim$4(4.6)$\times$$10^{-6}$ K$^{-1}$. The predicted value of total $\kappa_{ph}$ from phonon-phonon interaction (PPI) at 300(1200) K is $\sim$7.4(1.8) Wm$^{-1}$K$^{-1}$. The highest group velocity for acoustic $\&$ optical branches (AB $\&$ OB) is $\sim$6.7 and $\sim$3.5 km/s, respectively. The predicted average phonon lifetime ($\tau_{\lambda}$) for AB(OB) is $\sim$2.5(1.65) ps at 300 K, whereas it is $\sim$0.6(0.4) ps at 1200 K. The estimated highest heat capacity ($C_{\lambda}$) at 200 K for AB(OB) is $\sim$23.5 (19.5) meV/K. We fitted the equation $A_{\kappa}$T$^{-x_{\kappa}}$($A_{\tau}$T$^{-x_{\tau}}$) in the $\kappa_{ph}$($\tau_{\lambda}$) curve to gain a thorough understanding of temperature-dependent $\kappa_{ph}$ trend. The $x_{\kappa}$ for total branches is calculated to be $\sim$1.02. The $x_{\tau}$ value due to total AB(OB) is estimated to be $\sim$1.04(1.02), while it is $\sim$1.03 for total branches. The calculated $x_{\kappa}$ for total AB(OB) is $\sim$1.04(0.95), implying that AB contributes more to the total $\kappa_{ph}$. This research could be important for enhancing the properties of ScAgC regarding thermoelectric and photovoltaic applications.

Study on the effect of seafloor sound speed dispersion on sound propagation

As one of the boundaries of ocean waveguide, the acoustic properties of seafloor sediments have been a hot research problem in ocean acoustic propagation and acoustic field modeling. The existing research has shown that the sound velocity in seafloor sediment is frequency dependent. In order to solve the problem that the sedimentary sound speed dispersing often be ignored in actual applications, this article uses the parabolic Newton to iterate to solve the dispersion equation to obtain the characteristic value of the normal mode. And the impact of frequency dispersion of sediment sound speed through simulation are studied. The results show that the sound wave in the time domain will appear more obvious broadening phenomenon, and the propagation loss will be reduced after considering the sound velocity dispersion of the sediment. Also, with the increase of frequency, the more obvious the effect of sound velocity dispersion on the sound propagation in the frequency range of sound velocity dispersion.

Modeling of the Controllability of Frequency Dispersion of Waves in Reconfigurable 3D Gratings of Ferromagnetic Nanowires in the Millimeter Wave Range

Modeling of the Controllability of Frequency Dispersion of Waves in Reconfigurable 3D Gratings of Ferromagnetic Nanowires in the Millimeter Wave Range

Effect of DC bias on dipolar response of yttrium iron garnet

Yttrium iron garnet (YIG) is a prominent magnetic (ferrimagnetic) insulator and an excellent material for wide ranging microwave and millimeter-wave applications. In polycrystalline form it exhibits high dielectric loss due to contributions coming from grain & grain boundaries, and Fe heterovalency. We have investigated the effect of dc bias voltage on the dielectric properties of YIG in a wide range of temperature (100 - 600 K) and frequencies (100 Hz - 1 MHz). Consistent with previous observations, without bias a dominant frequency dispersion due to hopping of the Fe2＋ & Fe3＋ is observed in low temperature region and a wider frequency dispersion is observed at higher temperatures arising due to grain-grain boundary contributions. Application of a small dc bias voltage increases the dielectric constant and reduces the dielectric loss significantly at higher frequencies. The DC bias-induced improvements are much more dramatic at higher temperatures, where grain boundary contributions dominate. Our detailed analysis of dc and ac conductivity data show that the dc bias suppresses the grain boundary contributions significantly and at the same time it boosts the polarizability due to the Fe2＋/Fe3＋ hopping.

High-Gain Traveling-Wave Parametric Amplifier Based on Three-Wave Mixing

We extend the theory for a Josephson-junction traveling-wave parametric amplifier (TWPA) operating in the three-wave-mixing regime and we propose a scheme for achieving high gain. The continuous three-mode model [P.K. Tien, J. Appl. Phys. 29, 1347 (1958)] is, on one hand, extended to describe a discrete chain of Josephson junctions at high frequencies close to the spectral cutoff where there is no up-conversion. On the other hand, we also develop a continuous multimode theory for the small frequency limit where the frequency dispersion is close to linear. We find that in both cases the gain is significantly reduced compared to the prediction by the continuous three-mode model as the result of increasingly strong dispersion at the high frequencies and generation of up-converted modes at the small frequencies. The developed theory is in quantitative agreement with simulations of the full solution of the dynamical equations. To recover the high gain, we propose to engineer a chain with dispersive features to form a two-band frequency spectrum and to place the pump frequency within the upper band close to the spectral cutoff. We prove that there exists a sweet spot, where the signal and the pump are phase matched, while the up-conversion is inhibited. This results in a high gain, which grows exponentially with the length of the TWPA.

Transition metals based metalides TM-Janus-TM (where TM=Sc–Zn and Janus=F6C6H6); A theoretical study of nonconventional metalides with excellent static and dynamic nonlinear optical properties

Continuous progress is being made to bring forward the novel strategies for designing materials with excellent stability and exceptional nonlinear optical response. For the first time, 3d-transition series (Sc–Zn) is doped as source of excess electrons to design novel class of TM-JN-TM (TM = Sc–Zn) metalides based upon Janus-type all-cis1,2,3,4,5,6-hexafluorocyclohexane (JN) molecule. DFT calculations are performed at M06-2X/6-31+G(d,p) level of theory to explore the electronic and NLO properties of these complexes. The obtained novel metalides show excellent thermal stabilities than those of conventional alkalides and their interaction energies ranging from −3.50 to −88.33 kcal mol−1. Under the large facial polarization of JN, the valence electrons of transition metals (TM) on fluorine face serve as source of excess electron to TM metals on hydrogen face. With the excellent electronic stabilities, these complexes show significant decreased HOMO-LUMO gaps and, the performed NBO analysis also reveals their metalide character where the negative charge is observed at metal (TMH) on hydrogen face of complex C, D, G, H, and I. Among the designed metalides (TM-JN-TM), the significant first and second hyperpolarizability values of 2.43 × 106 au and 1.72 × 109 au are observed for Fe-JN-Fe complex. Simple excess electron complexes have better NLO response than that of true metalides. The observed dynamic first hyperpolarizabilities β(ω) values are much pronounced at smaller dispersion frequency (1064 nm). The frequency-dependent second hyperpolarizability γ(ω) value increased up to 5.75 × 1010 au for V-JN-V. At 1300 and 1900 nm, significant dynamic second hyperpolarizability γ(ω) values are observed. The scattering hyperpolarizability (βHRS) show significant value (2.0 × 107 au) for Fe-JN-Fe complex. We hope this work could open up new possibilities for NLO material design by using transition metals as source excess electron and, on the other hand, encourage greater experimental efforts in the laboratory to synthesize such stable compounds.

Investigation on electrical transport and dielectric relaxation mechanism in TbCrO3 perovskite orthochromite

Herein, the TbCrO3 (TCO) chromite was synthesized by the sol-gel auto-combustion route and sintered at 900 °C. The Rietveld analysis of the room temperature powder X-ray diffraction profile confirmed the single-phase formation of the material crystallized in the orthorhombic phase of the perovskite structure with Pbnm space group symmetry. The surface morphology, the elemental composition, and the chemical states of the compound were investigated by field emission scanning electron microscopy, energy dispersive X-ray analysis, and X-ray photoelectron spectroscopy, respectively. The electronic density of states, band structure, and electron density were analyzed by Density Functional Theory. The electrical transport and dielectric relaxation mechanism were investigated using complex impedance spectroscopy over the experimental frequency and temperature ranging from 40-1.2x106Hz and 138−273 K, respectively. Two semicircular arcs in the impedance complex plane plots were modeled to an equivalent circuit configured by RG∥QG+RGB∥QGB which were attributed to the contributions of the highly resistive grain boundaries and relatively conducting grains in the material. The electrical conductivity as a function of frequency followed Jonscher’s universal power law behavior and an increasing nature of the temperature-dependent frequency exponent suggested small polaron hopping as the electrical conduction mechanism of the material. The least capacitive response of the TCO system was investigated by the complex electrical modulus formalism and the frequency-dependent imaginary modulus spectra were modeled by the Kohlrausch–Williams–Watts function, which resolved the thermally activated peaks of bulk and interface effects to the dielectric relaxation mechanism. The dielectric constant exhibited Maxwell-Wagner interfacial polarization effects and the dielectric loss factor showed frequency dispersion at all temperatures.

Dispersive instantaneous frequency imaging

Amplitude and phase characteristics are widely utilized in Lamb wave imaging. Nevertheless, the aforementioned methods are mostly dependent on the baseline (i.e. the health response from an undamaged specimen) and are more or less affected by artifacts from the undesired mode. Except the amplitude and phase features, the instantaneous frequency is also a crucial damage information source. However, it is often neglected. If using it, owing to the negligible change in the instantaneous frequency of the damage wave packets before and after baseline subtraction, the baseline would no longer be necessary. Owing to the distinction in frequency between the compensated A0 and S0 mode responses, suppression of the undesired mode is possible. Therefore, to diminish the influence of the undesired S0 mode and eliminate the baseline dependence, a dispersive instantaneous frequency imaging algorithm is proposed for Lamb waves. Profiting from the dispersive pre-compensated focusing strategy, the damage echo is compensated to be consistent with the excitation, which implies that its instantaneous frequency is identical to the center frequency of the excitation. By quantitatively comparing their consistency, an imaging index (i.e. the pixel value of an image) is assigned for every scanning point to generate C-scan images. As a simple extension of the instantaneous phase, the instantaneous frequency is obtained by taking its derivative. Therefore, frequency-based images not only preserve the merits of phase images without significantly increasing the calculation cost, but also incorporate baseline independence and undesired S0 mode suppression. Moreover, because the amplitude, phase and frequency features of the defects can be acquired successively for one path, a compound imaging method is presented. Experiments were conducted on an isotropic aluminum plate with diverse defects and array forms. In comparison with the amplitude-based and phase-based images, the S0 mode artifact in the frequency-based images is alleviated and the correctly identified defects are more conspicuous without using baseline.