Strong Frequency Dependence Research Articles

Influence of the silicon layer’s properties on the capacitance parameters of MIS/SOS structures

The influence of the silicon epitaxial layer’s peculiarities and the silicon–sapphire interface in the silicon-on-sapphire (SOS) structure on the capacitance parameters of MIS-structures formed on SOS with submicron layers of silicon has been studied. The presence of a strong frequency dependence of the capacitance characteristics in test MIS-structures formed on such SOS is revealed in the studies. It is shown experimentally that the C–V-characteristics of the test structures to a significant degree are determined by the size and configuration of the silicon layer of MIS-structures and the concentration of deep compensating levels in the boundary layer at the Si–sapphire interface. In particular, it is shown that substantial changes in the frequency–capacitance dependences by transferring from the test formed on the standard SOS to the tests on SOS with a recrystallized silicon layer are associated with the presence of a great concentration of deep compensating levels in the boundary layer at the silicon–sapphire interface for standard SOS-structures.

Gain-enhanced high-k transmission through metal-semiconductor hyperbolic metamaterials

We analyze the steady-state transmission of high-momentum (high-$k$) electromagnetic waves through metal-semiconductor multilayer systems with loss and gain in the near-infrared (NIR). Using a semi-classical optical gain model in conjunction with the scattering matrix method (SMM), we study indium gallium arsenide phosphide (InGaAsP) quantum wells as the active semiconductor, in combination with the metals, aluminum-doped zinc oxide (AZO) and silver (Ag). Under moderate external pumping levels, we find that NIR transmission through Ag/InGaAsP systems may be enhanced by several orders of magnitude relative to the unpumped case, over a large angular and frequency bandwidth. Conversely, transmission enhancement through AZO/InGaAsP systems is orders of magnitude smaller, and has a strong frequency dependence. We discuss the relative importance of Purcell enhancement on our results and validate analytical calculations based on the SMM with numerical finite-difference time domain simulations.

Strong frequency dependence of vibrational relaxation in bulk and surface water reveals sub-picosecond structural heterogeneity.

Because of strong hydrogen bonding in liquid water, intermolecular interactions between water molecules are highly delocalized. Previous two-dimensional infrared spectroscopy experiments have indicated that this delocalization smears out the structural heterogeneity of neat H2O. Here we report on a systematic investigation of the ultrafast vibrational relaxation of bulk and interfacial water using time-resolved infrared and sum-frequency generation spectroscopies. These experiments reveal a remarkably strong dependence of the vibrational relaxation time on the frequency of the OH stretching vibration of liquid water in the bulk and at the air/water interface. For bulk water, the vibrational relaxation time increases continuously from 250 to 550 fs when the frequency is increased from 3,100 to 3,700 cm−1. For hydrogen-bonded water at the air/water interface, the frequency dependence is even stronger. These results directly demonstrate that liquid water possesses substantial structural heterogeneity, both in the bulk and at the surface.

Mott physics and spin fluctuations: A unified framework

We present a formalism for strongly correlated electrons systems which consists in a local approximation of the dynamical three-leg interaction vertex. This vertex is self-consistently computed with a quantum impurity model with dynamical interactions in the charge and spin channels, similar to dynamical mean field theory (DMFT) approaches. The electronic self-energy and the polarization are both frequency and momentum dependent. The method interpolates between the spin-fluctuation or GW approximations at weak coupling and the atomic limit at strong coupling. We apply the formalism to the Hubbard model on a two-dimensional square lattice and show that as interactions are increased towards the Mott insulating state, the local vertex acquires a strong frequency dependence, driving the system to a Mott transition, while at low enough temperatures the momentum-dependence of the self-energy is enhanced due to large spin fluctuations. Upon doping, we find a Fermi arc in the one-particle spectral function, which is one signature of the pseudo-gap state.

Estimation of attenuation characteristics of Aswan reservoir region, Egypt

The attenuation characteristics of Aswan reservoir region in Egypt was estimated using microearthquakes records. Frequency-dependent attenuation characteristic of S wave was estimated using coda normalization method. The evaluated quality factor (Q s ) shows strong frequency dependence with its value increasing with increasing frequency. For Aswan region, the average frequency-dependent relation is given by Q s = (34.2 ± 2.9)f(1.023 ± 0.107). The low values of Q s lie within those observed for seismically active regions of the world, although the study area is seismically moderately active and known to be a region where reservoir-induced seismicity occurs. The low value of Q s would mean that the medium is highly heterogeneous. Separation of intrinsic and scattering Q was carried out using estimated Q s values and values of coda Q (Q c ) estimated earlier by other workers from the same data set. It is observed from this that S and coda wave attenuation is primarily controlled by intrinsic attenuation at 1-Hz frequency. At higher frequencies, intrinsic and scattering attenuations contribute almost equally as seismic albedo at 1-Hz frequency is 0.04, and at higher frequencies, it varies between 0.34 and 0.55. Extinction length varies between 16.4 and 23.6 km. The frequency-dependent relationship for scattering Q (Q sc ) and intrinsic Q (Q i ) are Q sc = (699.3 ± 334.5)f (0.13 ± 0.06) and Q i = (52.9 ± 5.6)f (1.09 ± 0.03).

Electron–Phonon Coupling in CdSe/CdS Core/Shell Quantum Dots

Resonance Raman spectra and excitation profiles have been measured and semiquantitatively modeled for core/shell quantum dots consisting of 2.7 nm diameter zincblende CdSe cores and thin (0.5 nm) or thick (1.6 nm) CdS shells. The Raman spectra show previously reported trends of increased peak frequency for both the CdSe and the CdS longitudinal optical (LO) phonons with increasing shell thickness. We also find a strong dependence of the peak CdS frequency on excitation energy and a large discrepancy between the experimental frequency of the CdSe + CdS combination band and the sum of the corresponding fundamental frequencies. This suggests that the dominant transitions at high excitation energies are localized on either the CdSe core or the CdS shell and thereby cannot enhance combination band transitions between core and shell. The CdS to CdSe Raman intensity ratios at high excitation energies further support this picture. The electron-phonon coupling for the CdSe LO phonon in the lowest excitonic transition is slightly weaker in the core/shell structures than in pure CdSe quantum dots, contrary to expectations for the Fröhlich coupling mechanism. Possible explanations for this discrepancy are discussed.

Reciprocity failure in piezoelectric polymer composite

Reciprocity principles, which entail equivalent outcome on exchange of cause and effect, are widely used and accepted. We present a piezoelectric composite system designed so that reciprocity does not hold; sensitivity is substantially enhanced. Reciprocity failure is observed in which the piezoelectric direct effect (stress causes polarization) sensitivity is unequal to the converse effect (electric field causes deformation) sensitivity . The piezoelectric polymer PVDF under isothermal conditions on a polymer substrate obeys reciprocity. Reciprocity failure occurs when a bumpy contact condition causes stress gradients. Reciprocity failure with strong frequency dependence occurs in the presence of thermal flux that is modulated by force: a non-equilibrium condition. Non-reciprocal effects give rise to a maximum enhancement of a factor of five in sensitivity.

X-ray Irradiation Effect on the Dielectric Charge Response in the Dimer–Mott Insulator κ-(BEDT-TTF)2Cu2(CN)3

The x-ray irradiation effect on the resistivity and dielectric constant of the dimer–Mott insulator κ-(BEDT-TTF)2Cu2(CN)3 has been investigated. The x-ray irradiation, producing defects and random potentials mainly in anion molecular layers, reduces the resistivity. We observed two anomalous peaks in the temperature dependence of the dielectric constant with an increasing irradiation time. One of the peaks, which is originally observable showing a Curie–Weiss behavior with a strong frequency dependence, shifts to lower temperatures and decreases in intensity. We found an additional peak that emerges after irradiation of ≈250 h and becomes dominant with irradiation. The qualitative differences in the change in the anomalies suggest that the additional anomaly is directly due to the anion defects and that the change in the original anomaly is due to the segmentation of local polar regions and the decrease in the size of domains. These results are explained by the charge degrees of freedom in the BEDT-TTF dimer.

Spin-glass like behaviour in strongly interacting nanocrystalline Ni embedded in SiO2

Ni nanoparticles embedded in the SiO2 host are prepared by sol–gel route. Studies on X-ray diffraction and transmission electron microscopy confirm chemically single phase, good crystallinity and formation of nearly spherical nanoparticles with average size of 18nm. Temperature dependence of zero field cooled (ZFC) magnetization shows a broad maximum around 230K (Tmax), below which memory effect is observed in the field cooled (FC) and zero field cooled (ZFC) modes. A strong frequency dependence of ac susceptibility is observed around Tmax and the shift of peak position with frequency satisfies the dynamical scaling law with parameters close to that observed for atomic spin-glasses. At low temperature signature of spin-glass (SG) like behaviour is confirmed from the observed memory effect in the ZFC and isothermal remanent magnetization studies. The SG like behaviour is also apparent from the analysis of the relaxation mechanism.

Increment of dielectric constant in binary composite based on PVDF/clay system

Binary composite films, with 10 wt% of barium doped porous clay heterostructure (Ba-PCH) in polyvinylidene difluoride, were fabricated by using mechanical mixing (Brabender) and hot pressing methods. The grain morphology of the composite films, as investigated by using the field emission scanning electron microscope, exhibits some agglomeration of Ba-PCH particles. The dielectric properties, as functions of frequency and temperature, were measured using a network analyzer. The dielectric constant of the composite film exhibits strong frequency and temperature dependence. The sharp drop of the dielectric constant is resulted, when temperature is decreased below the glass transition temperature of the polyvinylidene difluoride. The relaxation behaviors are analyzed carefully using the Arrhenius law and the other possible mechanisms and the study indicated that the dielectric behavior in these composite films is highly dominated by the electronic polarizability contributions.

Engineering of the frequency dependence of the ferroelectric properties of thin film Pt/Ba0.5Sr0.5TiO3/Pt structures

Engineering of a frequency dependent permittivity can be of interest for various sensor application. In this work a strong modification of the frequency dependence of the ferroelectric properties is achieved via a controlled diffusion of the metal electrode (Pt) into the ferroelectric layer ((Ba,Sr)TiO3). For this purpose a series of Ba0.5Sr0.5TiO3 layers is deposited onto Pt coated sapphire at various temperatures range from 660°C to 760°C. Using an additional Pt top electrode, the electronic properties of the resulting capacitors are investigated via frequency dependent cryoelectronic measurements. The structure and stoichiometry of the layers are analyzed via X-ray and Rutherford backscattering spectrometry, respectively. The analysis of the permittivity and loss tangent shows a strong frequency dependence of the permittivity in a small region of the deposition temperatures (680–710°C), i.e. the permittivity changes from εeff≃600 at low frequency to εeff≃20 at high frequency. This behavior is caused by a partial diffusion of Pt into the ferroelectric layer and can be explained by the Maxwell–Wagner model.

Positively Frequency-Dependent Interference Competition Maintains Diversity and Pervades a Natural Population of Cooperative Microbes

Positively frequency-dependent selection is predicted from theory to promote diversity in patchily structured populations and communities, but empirical support for this prediction has been lacking. Here,we investigate frequency-dependent selection among isolates from a local natural population of the highly social bacterium Myxococcus xanthus. Upon starvation, closely related cells of M.xanthus cooperate to construct multicellular fruiting bodies, yet recently diverged genotypes co-residing in a local soil population often antagonize one another during fruiting-body development in mixed groups. In the experiments reported here, both fitness per se and strong forms of interference competition exhibit pervasive and strong positive frequency dependence (PFD) among many isolates from a centimeter-scale soil population of M.xanthus. All strains that compete poorly at intermediate frequency are shown to be competitively dominant at high frequency in most genotype pairings during both growth and development, and strongly so. Interference competition is often lethal and appears to be contact dependent rather than mediated by diffusible compounds. Finally, we experimentally demonstrate that positively frequency-dependent selection maintains diversity when genotype frequencies vary patchily in structured populations. These results suggest that PFD contributes to the high levels of local diversity found among M.xanthus social groups in natural soil populations by reinforcing social barriers to cross-territory invasion and thereby also promotes high within-group relatedness. More broadly, our results suggest that potential roles of PFD in maintaining patchily distributed diversity should be investigated more extensively in other species.

A simplified view of blazars: contribution to the X-ray and γ-ray extragalactic backgrounds

The "blazar simplified view" is a new paradigm that explains well the diverse statistical properties of blazars observed over the entire electromagnetic spectrum on the basis of minimal assumptions on blazars' physical and geometrical properties. In this paper, the fourth in a series, we extend the predictions of this paradigm below the sensitivity of existing surveys and estimate the contribution of blazars to the X-ray and gamma-ray extragalactic backgrounds. We find that the integrated light from blazars can explain up to 100% of the cosmic background at energies larger than ~10 GeV, and contribute ~40 to 70% of the gamma-ray diffuse radiation between 100 MeV and 10 GeV. The contribution of blazars to the X-ray background, between 1 and 50 keV, is approximately constant and of the order of 4-5%. On the basis of an interpolation between the estimated flux at X-ray and gamma-ray energies we can expect that the contribution of blazars raises to ~10% at 100 keV, and continues to increase with energy until it becomes the dominant component at a few MeV. Finally, we show that a strong dependence of the synchrotron peak frequency on luminosity, as postulated by the "blazar sequence", is ruled out by the observational data as it predicts a gamma-ray background above a few GeV that is far in excess of the observed value.

Non-ferroelectric nature of the conductance hysteresis in CH3NH3PbI3 perovskite-based photovoltaic devices

We present measurements of conductance hysteresis on CH3NH3PbI3 perovskite thin films, performed using the double-wave method, in order to investigate the possibility of a ferroelectric response. A strong frequency dependence of the hysteresis is observed in the range of 0.1 Hz to 150 Hz, with a hysteretic charge density in excess of 1000 μC cm−2 at frequencies below 0.4 Hz—a behaviour uncharacteristic of a ferroelectric response. We show that the observed hysteretic conductance, as well as the presence of a double arc in the impedance spectroscopy, can be fully explained by the migration of mobile ions under bias on a timescale of seconds. Our measurements place an upper limit of ≈1 μC cm−2 on any intrinsic frequency-independent polarisation, ruling out ferroelectricity as the main cause of current-voltage hysteresis and providing further evidence of the importance of ionic migration in modifying the efficiency of CH3NH3PbI3 devices.

Structural and electrical properties of Bi2WO6 piezoceramics prepared by solid state reaction method

Stoichiometric Bi2WO6 (BWO) piezoelectric ceramic, with round-shaped grains and a relative density of ∼96%, was prepared by solid state reaction method. The dielectric, resistivity, ferroelectric and piezoelectric properties of BWO were investigated. The dielectric constant and loss show strong frequency dependence with increasing temperature in BWO, which can be attributed to the contribution from the defect dipoles. These defect dipoles have significant effects on the electrical resistivity and polarization–electric (P–E) hysteresis loops of unpoled and poled BWO, and also thermal depoling behavior of poled BWO.

INFLUENCE OF SILICON LAYER PROPERTIES ON CAPACITANCE PARAMETERS OF MIS/SOS-STRUCTURES

The influence of the epitaxial layer features and so silicon-sapphire interface in SOS-structures on capacitance parameters of MIS-structures formed on SOS with submicron silicon layers were investigated in this article. Both standard SOS-structures with 0.3 μk silicon n-type layer and analog SOS-structures with the blastic silicon layer amorphizated by the oxygen ion implantation with following high-temperature annealing. The two different types of test MIS-structures with 30 nm gate oxide, polysilicon gate and metallic contacts were fabricated. The strong difference in the capacitance characteristics for standard SOS and SOS with recrystallization silicon layer was disclosed in depletion region. It was revealed the strong frequency dependence of capacitance characteristics for the test MIS-structures formed on almost all SOS-structures. It was shown that the character of the test structures C — V -curves were defined by dimension and configuration of the silicon layer as well as the deep compensate levels concentration in layer near silicon-sapphire interface. Particularly it was shown that big changes of the frequency-capacitance dependences at transition from the tests formed on standard SOS to tests on SOS with blastic silicon layer were determined by great concentration of deep compensate centers in layer near interface for standard SOS. It was shown that the recrystallization of silicon layer by oxygen ion implantation may lead to appearance of the centers, formed donor levels into silicon band gap and appearance of the frequency-capacitance dependences in depletion region of such SOS-structures.

Magnetic and magnetodielectric coupling anomalies in the Haldane spin-chain system Nd2BaNiO5

We report the magnetic, heat-capacity, dielectric and magnetodielectric (MDE) behaviour of a Haldane spin-chain compound containing light rare-earth ion, Nd2BaNiO5, in detail, as a function of temperature (T) and magnetic field (H) down to 2 K. In addition to the well-known long range antiferromagnetic order setting in at (TN = ) 48 K as indicated in dc magnetization (M), we have observed another magnetic transition near 10 K; this transition appears to be of a glassy-type which vanishes with a marginal application of external magnetic field (even H = 100 Oe). There are corresponding anomalies in dielectric constant (ε′) as well with variation of T. The isothermal M(H) curves at 2 and 5 K reveal the existence of a magnetic-field induced transition around 90 kOe; the isothermal ε′(H) also tracks such a metamagnetic transition. These results illustrate the MDE coupling in this compound. Additionally, we observe a strong frequency dependence of a step in ε′(T) with this feature appearing around 25-30 K for the lowest frequency of 1 kHz, far below TN. This is attributed to interplay between crystal-field effect and exchange interaction between Nd and Ni, which establishes the sensitivity of dielectric measurements to detect such effects. Interestingly enough, the observed dispersions of the ε′(T) curves is essentially H-independent in the entire T-range of measurement, despite the existence of MDE coupling, which is in sharp contrast with other heavy rare-earth members in this series.

Micromachined rectangular-spiral-coil actuator for radio-controlled cantilever-like actuation

This paper reports of a micromachined shape-memory-alloy (SMA) actuator in the shape of a rectangular spiral coil that is designed to perform cantilever-like actuation. The SMA-coil actuator itself forms a passive resonant circuit that functions as a wireless heat source activated using external radio-frequency (RF) electromagnetic fields for frequency-selective control of the actuation. The SiO2 stress layers are selectively patterned on the actuator structure of nickel–titanium SMA, or Nitinol, to manipulate the cantilever profile at the nominal cold state. RF radiation with varying field frequencies shows strong frequency dependence of wireless heating, actuation displacement, and force generation by microfabricated actuators with resonant frequencies of 170–245MHz. The actuators excited at resonance exhibit maximum out-of-plane displacement and force of 215μm and 71mN, respectively. The developed wireless SMA actuator not only provides >2× the force and 2× more power-efficient response compared to the preceding design with similar self-heating mechanism but also offers broader potential applications brought by its commonly adopted cantilever-based actuation.

The potential and electric field in the cochlear outer hair cell membrane.

Outer hair cell electromechanics, critically important to mammalian active hearing, is driven by the cell membrane potential. The membrane protein prestin is a crucial component of the active outer hair cell's motor. The focus of the paper is the analysis of the local membrane potential and electric field resulting from the interaction of electric charges involved. Here the relevant charges are the ions inside and outside the cell, lipid bilayer charges, and prestin-associated charges (mobile-transferred by the protein under the action of the applied field, and stationary-relatively unmoved by the field). The electric potentials across and along the membrane are computed for the case of an applied DC-field. The local amplitudes and phases of the potential under different frequencies are analyzed for the case of a DC + AC-field. We found that the effect of the system of charges alters the electric potential and internal field, which deviate significantly from their traditional linear and constant distributions. Under DC + AC conditions, the strong frequency dependence of the prestin mobile charge has a relatively small effect on the amplitude and phase of the resulting potential. The obtained results can help in a better understanding and experimental verification of the mechanism of prestin performance.

Effects of processing methods on dielectric properties of BaTiO3/poly(vinylidene fluoride) nanocomposites

Effects of different liquid phase processes on microstructure and dielectric properties of BaTiO3/poly(vinylidene fluoride) (BT/PVDF) nanocomposites fabricated by using spherical BT nanoparticles (<100nm) with different volume fractions (fBT=0–0.4) were investigated. The dielectric permittivity (ε′) of BT/PVDF prepared by a liquid-phase assisted dispersion (LAD) method was nearly independent of frequency in the range of 102–106Hz. In contrast, the strong frequency dependence of ε′ in a low-frequency range was observed in the nanocomposites that were prepared by a solution processing (SP) method, especially for high-loading BT nanoparticles. ε′ values at 103Hz of the BT/PVDF nanocomposites with fBT=0.4 prepared by these two methods were found to be 61.9 and 623.3, respectively, while the loss tangent values were 0.218 and 0.887, respectively. Uniformly dispersed BT/PVDF nanocomposites were obtained by a LAD method. For a SP method, a strong agglomeration of BT nanoparticles resulted in the formation of large spherical clusters with sizes of ∼10–30μm. The different dielectric responses were described based on the strong dipole polarization in the large clusters of agglomerated BT nanoparticles and the interfacial polarization at the large interface area of PVDF thin layer between BT clusters.