Multiple Oscillator Model Research Articles

Electrochemical and optical studies on photoactive BiVO4-TiO2/poly 3,4-ethylenedioxythiophene assemblies in gel electrolyte: Role of inorganic/organic interfaces in surface functionalization

Inorganic/organic interface assemblies were created from poly 3,4-ethylenedioxythio­phene (PEDOT) interfaced with amorphous BiVO4 and with BiVO4-TiO2. Electrochemical cells-based thermoplastic gel electrolytes containing KI/I2 were used to study the photoelectrochemical behavior of the Inorganic/organic interface electrodes. Optical studies show that doping BiVO4 with TiO2 narrowed the optical band gap to allow more absorption in the visible region and increases solar energy conversion. Evidence for both direct and indirect band gaps was observed. Refractive index data indicates that BiVO4 and BiVO4/TiO2 obey the anomalous dispassion multiple-oscillator model. Chronoampero­metry of these assemblies shows the phenomena of dark current, which correlates to the presence of random electron/hole generation in the depletion layer. PEDOT enhances the photoactivity of BiVO4 only. Electrochemical impedance spectroscopy studies indicated that both kinetic and diffusional control at high and low frequencies, respectively. Furthermore, studies show that as frequency increases, the conductivity increases due to dispersion and charge carrier hopping. All photoactivity outcomes were reproducible.

Optical Characterization of Al Island Films: A Round Robin Test

The determination of the effective optical constants of metal island films is an essential step towards the practical incorporation of this kind of films in optical coatings. In this work, the optical properties of aluminium island films deposited by electron beam evaporation on quartz substrates are investigated using different approaches employed by three research groups. The effective optical constants of the island films are inferred from optical measurements (spectrophotometry and spectroscopic ellipsometry) using: (i) a parameter-free dispersion model, (ii) a multiple oscillator model based on Gaussian line-shapes and (iii) the β distributed oscillator model. All the used approaches provide similar physical insights, i.e., an increase in the effective thickness of the metal island film, a red-shift and broadening of the plasmon resonance and an enhancement of the infrared absorption as the amount of deposited material increases. However, the optimal values of the effective optical constants and thickness significantly depend on the employed model and the experimental data used for data fitting.

Investigation of Electrochromic, Combinatorial TiO2-SnO2 Mixed Layers by Spectroscopic Ellipsometry Using Different Optical Models.

We determined the optimal composition of reactive magnetron-sputtered mixed layers of Titanium oxide and Tin oxide (TiO2-SnO2) for electrochromic purposes. We determined and mapped the composition and optical parameters using Spectroscopic Ellipsometry (SE). Ti and Sn targets were put separately from each other, and the Si-wafers on a glass substrate (30 cm × 30 cm) were moved under the two separated targets (Ti and Sn) in a reactive Argon-Oxygen (Ar-O2) gas mixture. Different optical models, such as the Bruggeman Effective Medium Approximation (BEMA) or the 2-Tauc-Lorentz multiple oscillator model (2T-L), were used to obtain the thickness and composition maps of the sample. Scanning Electron Microscopy (SEM) with Energy-Dispersive X-ray Spectroscopy (EDS) has been used to check the SE results. The performance of diverse optical models has been compared. We show that in the case of molecular-level mixed layers, 2T-L is better than EMA. The electrochromic effectiveness (the change of light absorption for the same electric charge) of mixed metal oxides (TiO2-SnO2) that are deposited by reactive sputtering has been mapped too.

High-Performance Refractive Index-Based Sensor Using Ellipsoid Ag–Au Nanoparticles

The nanoparticles’ localized surface plasmon resonance (LSPR) is sensitive to the surrounding medium refractive index change. Therefore, biosensing devices commonly use it as a refractive index-based sensor. In this work, an improvement strategy for the sensing performance of gold (Au) and silver (Ag) ellipsoid nanoparticles (NP-ELs) through controlling the size, aspect ratio, and core@shell structure was proposed. The investigation was started by constructing a multiple oscillator model to derive the optical permittivity function and compute the absorption of nanoparticles via discrete dipole approximation (DDA). The change in position and width of the LSPR peak concerning the refractive index of the medium was calculated to determine the sensing performance. Sensitivity and figure of merit (FOM) were used to describe sensing performance. As a result, the aspect ratio parameter of nanoparticles was found to be more dominant in increasing sensitivity and figure of merit (FOM) compared to particle size. In the core@shell structure, the absorption spectrum is also affected by the shell thickness, particle size, and aspect ratio. Therefore, those parameters can tune the sensing performance of the Ag@Au NP-ELs. The maximum sensitivity obtained for Ag@Au NP-ELs was 597.7 nm/RIU with a FOM of 34 RIU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Therefore, the high performance refractive index-based sensor proposed in this study opens a new gate for developing biosensors in future.

Learning about reward identities and time.

We discuss three empirical findings that we think any theory attempting to integrate interval timing with associative learning concepts will need to address. These empirical phenomena all come from studies that combine peak timing procedures with reinforcer devaluation or conditional discrimination tasks commonly employed, respectively, in interval timing or associative learning research traditions. The three phenomena we discuss include: (1) the observation that disruptions in reward identity encoding have little to no impact on the encoding of reward time in the peak procedure (Delamateret al., 2018), (2) the findings that organisms tend to average their time estimates when presented with a stimulus compound consisting of separately learned stimuli indicating short or long reward times but that such temporal averaging, itself, is sensitive to post-conditioning selective reward devaluation, and (3) that rats can learn a temporal patterning task in which two stimuli presented independently indicate one time to reward availability while their compound indicates another. We review our prior results and present new findings illustrating these three phenomena and we discuss the special challenges they pose for cascade theories of timing, for multiple-oscillator models, and for any approach that attempts to integrate interval timing and associative models. We close by illustrating some ways in which multi-layer connectionist network models might begin to address some of our key findings. We believe this will require an approach that includes separate mechanisms that code for reward identity and time, but that does so in a way that permits for integration between the two systems.

Investigation of Combinatorial WO3-MoO3 Mixed Layers by Spectroscopic Ellipsometry Using Different Optical Models.

Reactive (Ar-O2 plasma) magnetron sputtered WO3-MoO3 (nanometer scaled) mixed layers were investigated and mapped by Spectroscopic Ellipsometry (SE). The W- and Mo-targets were placed separately, and 30 × 30 cm glass substrates were slowly moved under the two (W and Mo) separated targets. We used different (oscillator- and Effective Medium Approximation, EMA-based) optical models to obtain the thickness and composition maps of the sample layer relatively quickly and in a cost-effective and contactless way. In addition, we used Rutherford Backscattering Spectrometry to check the SE results. Herein, we compare the “goodness” of different optical models depending upon the sample preparation conditions, for instance, the speed and cycle number of the substrate motion. Finally, we can choose between appropriate optical models (2-Tauc-Lorentz oscillator model vs. the Bruggeman Effective Medium Approximation, BEMA) depending on the process parameters. If one has more than one “molecular layer” in the “sublayers”, BEMA can be used. If one has an atomic mixture, the multiple oscillator model is better (more precise) for this type of layer structure.

Optical and surface energy probe of Hamaker constant in copper oxide thin films for NEMS and MEMS stiction control applications

Copper oxide films hold substantial promise as anti-stiction coatings in micro-electromechanical (MEMS) devices and with shrinking dimensions on the nanometre scale on nano electromechanical (NEMS) devices. The Hamaker constant will play a very significant role in understanding stiction and tribology in these devices. We used an approximate but sufficiently accurate form of the Lifshitz theory using the multiple oscillator model to calculate the Hamakers constant of symmetric copper oxide thin films based on experimentally obtained dielectric data in the wavelength range 190-850 nm using spectroscopic ellipsometry. We also used the Tabor–Winterton approximation (TWA) and Surface energy measurements to determine the Hamaker constant. There was better agreement in the Hamaker constant values obtained by the limited Lifshitz theory and TWA approach than with the Surface energy approach. The difference is explained through the influence of surface roughness on the surface energy using extensions of the stochastic KPZ growth model and the Family-Vicsek scaling relation and rigorous treatment of the Cassie-Baxter and Wenzel models as optimisations of a surface free energy functional linking roughness and surface tension. The dominance of the Cu2O phase in the films and of the London dispersion force on the surface of the films was previously confirmed by FTIR Cu(I)–O vibrational mode observation and XPS Cu 2p3/2 binding energy peak and its fitted satellites. The use of the limited Lifshitz theory and ellipsometry data would seem to provide a suitable best first approximation for determining the Hamaker constant of predominantly dispersive anti-stiction coatings in technologically important MEMS/NEMS devices.

Seismic damage identification by fitting the nonlinear and hysteretic dynamic response of monitored buildings

The identification of hysteretic degrading systems exposed to nonstationary loading is a paramount research topic, especially in the case of structures subjected to ground motion excitations. In this paper, the data recorded by a masonry building are used to detect the presence of seismic damage. To this aim, a parametric nonlinear identification is performed by adopting a Bouc–Wen-type multiple oscillator model. Starting from the results of the identification process, a damage index based on the degrading stiffness matrix is defined. The damage index provides preliminary information to be used in post-event management. Finally, the identification procedure is applied to a monitored masonry structure, the Town Hall of Pizzoli (L’Aquila), which is part of the Italian Seismic Observatory of Structures (OSS). The records measured by accelerometers permanently installed on the building are used for the detection of damage occurred during a seismic event.

氧化镁单晶在太赫兹波段的介电特性

The dielectric properties of single-crystal MgO were studied by terahertz time-domain spectroscopy in the frequency range extending from 0.5 to 9.5 THz. The refractive index, power absorption, and the complex dielectric function were extracted from experimental data. At low terahertz frequencies band (2 THz), the absorption coefficient was extremely low and increased with increasing frequency. Meanwhile the corresponding refractive index had low dispersion, increasing from 3.12 to 3.15. Two prominent resonances were observed at 3.16 THz and 8.11 THz and were well-described by a multiple-oscillator model through theoretical fitting. The interaction of incident photons and the transverse optical (TO) phonons in the crystal were studied and it gives good evidence to further applications in developing broadband terahertz components and terahertz spectroscopy.

A theoretical approach to the mechanism of jet lag using multiple-oscillator models

Jet-lag symptoms result from temporal mismatch between the internal circadian clock and external solar time. Recent experiments on mice indicate that jet lag is due to complex clock cell dynamics in the brain including desynchronization within clock cells. We consider some models possessing the hierarchical structure of clock cells in the brain. We focus primarily on the effects of light stimuli undergoing daily variation and mutual coupling between clock cells. The results reveal that jet lag corresponding to eastbound long-distance trips strongly disturbs oscillator dynamics and causes slow re-entrainment.

Multiple oscillator models for the optical constants of polycrystalline zinc oxide thin films over a wide wavelength range

Zinc oxide (ZnO) films were prepared on Si(1 1 1) and quartz substrates using RF-magnetron sputtering in N2 plasma at room temperature. From the X-ray diffraction observations, it was found that all films are polycrystalline with a preferred orientation of (1 0 1). X ray photoelectron spectroscopy was used to analyze the chemical composition of the films by observing the behavior of the Zn2p3, O1s, N1s, and C1s lines. The thicknesses and optical constants of the ZnO thin films were determined using variable angle spectroscopic ellipsometry through the Genosc™ Herzinger–Johs parameterized semiconductor oscillator functions and multiple Gaussian oscillator models. Combining multiple oscillator types provided a very flexible approach to fitting optical constants over a wavelength range 190–1400 nm while simultaneously enforcing Kramers–Kronig consistency in the fitted ellipsometric parameters. Refractive indices of the films were determined to be in the range 1.68–1.93 and extinction coefficients in the range 4.56 × 10−6–0.23. A direct bandgap of 3.38 ± 0.03 eV was calculated from the extinction coefficient. Low temperature photoluminescence studies of the films exhibited one prominent peak at 3.41 eV. The equality of the ZnO thin films was obtained through the depolarization measurements.

Comparison of two techniques for reliable characterization of thin metal–dielectric films

In the present study we determine the optical parameters of thin metal-dielectric films using two different characterization techniques based on nonparametric and multiple oscillator models. We consider four series of thin metal-dielectric films produced under various deposition conditions with different optical properties. We compare characterization results obtained by nonparametric and multiple oscillator techniques and demonstrate that the results are consistent. The consistency of the results proves their reliability.

Terahertz spectroscopy studies of far-infrared optical and dielectric signatures of melamine

We report on the far–infrared optical properties of melamine characterized by terahertz time–domain spectroscopy (THz–TDS). Transmission measurement reveals three low–frequency vibration modes at 1.99, 2.25, and 2.61 THz, which may provide fingerprints for direct melamine detection. By combining THz–TDS with Fourier transform infrared (FTIR) spectroscopy, an overall low–frequency optical response of melamine is presented in an extended spectral range of 0.2–6.2 THz. In this range, the low-frequency vibration mode at 3.96 THz is recorded via FTIR. The measured THz spectra are well fit by the multiple–oscillator model, thereby demonstrating that the low–frequency THz response of melamine is a consequence of the lowest four vibration modes.

Sensory Feedback, Error Correction, and Remapping in a Multiple Oscillator Model of Place-Cell Activity

Mammals navigate by integrating self-motion signals (“path integration”) and occasionally fixing on familiar environmental landmarks. The rat hippocampus is a model system of spatial representation in which place cells are thought to integrate both sensory and spatial information from entorhinal cortex. The localized firing fields of hippocampal place cells and entorhinal grid-cells demonstrate a phase relationship with the local theta (6–10 Hz) rhythm that may be a temporal signature of path integration. However, encoding self-motion in the phase of theta oscillations requires high temporal precision and is susceptible to idiothetic noise, neuronal variability, and a changing environment. We present a model based on oscillatory interference theory, previously studied in the context of grid cells, in which transient temporal synchronization among a pool of path-integrating theta oscillators produces hippocampal-like place fields. We hypothesize that a spatiotemporally extended sensory interaction with external cues modulates feedback to the theta oscillators. We implement a form of this cue-driven feedback and show that it can retrieve fixed points in the phase code of position. A single cue can smoothly reset oscillator phases to correct for both systematic errors and continuous noise in path integration. Further, simulations in which local and global cues are rotated against each other reveal a phase-code mechanism in which conflicting cue arrangements can reproduce experimentally observed distributions of “partial remapping” responses. This abstract model demonstrates that phase-code feedback can provide stability to the temporal coding of position during navigation and may contribute to the context-dependence of hippocampal spatial representations. While the anatomical substrates of these processes have not been fully characterized, our findings suggest several signatures that can be evaluated in future experiments.

Cue-based feedback enables remapping in a multiple oscillator model of place cell activity

Cue-based feedback enables remapping in a multiple oscillator model of place cell activity

Terahertz Dielectric Properties of MgO Nanocrystals

Terahertz properties of chemically synthesized MgO nanocrystals are studied by terahertz time-domain spectroscopy in the frequency range extending up to 3.7 THz. The experimentally determined optical and dielectric properties of the nanocrystalline MgO are compared with those of a bulk MgO single crystal. Three prominent resonance modes, which are essentially responsible for the dielectric properties of the nanostructured MgO in the terahertz regime, are characterized at ω1/(2π) = 4.26 THz, ω2/(2π) = 7.68 THz, and ω3/(2π) = 12.18 THz and are well-described by a multiple-oscillator model through theoretical fitting. The MgO nanocrystals may find intriguing applications in developing terahertz quasi-optic components and terahertz spectroscopy.

Temporal Memory of Interfood Interval Distributions with the Same Mean and Variance

The time of occurrence of reinforced lever responses of rats depends on the characteristics of the distribution of times from food to the next available food. Two groups of 10 rats were trained, in counterbalanced order, on two variable-interval schedules of reinforcement that were equated for the mean, standard deviation, and range of the intervals from food to the next available food, but which differed in shape. The differences in the shape of the interfood interval distributions resulted in differences in the distribution of interfood intervals, of postreinforcement pauses, the function relating response rate to time since food, and the power spectra of times of response. Quantitative timing models, such as scalar timing theory and a multiple-oscillator model, differ in their assumptions about the nature of the internal clock and the representation of time in memory. The multiple-oscillator model and scalar timing theory accounted for different features of the data.

Dissociative sticking of diatomic molecules on cold, non-rigid surfaces: comparison of quantal and semiclassical surface oscillator models

The influence of a moving substrate on the dissociative sticking probability of diatomic molecules is investigated with the help of time-dependent quantum dynamical methods. The dissociating molecule is treated as a wave packet, whereas the moving surface is idealized as a set of quantal and classical generalized Langevin-oscillators, respectively. By comparing quantal with classical single-Einstein oscillator models, a systematic estimate of the errors introduced when the surface motion is approximated by classical mechanics becomes possible. Further, by comparing classical single- to multiple-oscillator models, the validity of the popular single-oscillator treatment can be addressed. It is argued that, at least in the present model, it seems more important to include many surface oscillators approximately, rather than treating a single oscillator “exactly”.

Comparisons of Hamaker Constants for Ceramic Systems with Intervening Vacuum or Water: From Force Laws and Physical Properties

Van der Waals dispersive forces produce attractive interactions between bodies, playing an important role in many material systems influencing colloidal and emulsion stability, wetting behavior, and intergranular forces in glass–ceramic systems. It is of technological importance to accurately quantify these interactions, conveniently represented by the Hamaker constant,A. To set the current level of accuracy for determiningA, they were calculated from Lifshitz theory using full spectral data for muscovite mica, Al2O3, SiO2, Si3N4, and rutile TiO2, separated by vacuum or water. These were compared to Hamaker constants calculated from physical properties using the Tabor–Winterton approximation, a single oscillator model, a multiple oscillator model, andA’s calculated using force vs separation data from surface force apparatus and atomic force microscope studies. For materials with refractive indices between 1.4 and 1.8 separated by vacuum, all methods produce similar values, but for indices larger than 1.8 separated by vacuum, and any of these materials separated by water, results span a broader range. The present level of accuracy for the determination of Hamaker constants, here taken to be represented by the level of agreement between various methods, ranges from about 10% for the case of SiO2/vacuum/SiO2and TiO2/water/TiO2to a factor of approximately 7 for mica/water/mica.

Preferred rates of repetitive tapping and categorical time production.

In a constrained finger-tapping task, in which a subject attempts to match the rate of tapping responses to the rate of a pacer stimulus, interresponse interval (IRI) was a nonlinear function of interstimulus interval (ISI), in agreement with the results of Collyer, Broadbent, and Church (1992). In an unconstrained task, the subjects were not given an ISI to match, but were instructed to tap at their preferred rate, one that seemed not too fast or too slow for comfortable production. The distribution of preferred IRIs was bimodal rather than unimodal, with modes at 272 and 450 msec. Preferred IRIs also tended to become shorter over successive sessions. Time intervals that were preferred in the unconstrained task tended to be intervals that were overproduced (IRI > ISI) when they were used as ISIs in the constrained task. A multiple-oscillator model of timing developed by Church and Broadbent (1990) was used to simulate the two tasks. The nonlinearity in constrained tapping, termed the oscillator signature, and the bimodal distribution in unconstrained tapping were both exhibited by the model. The nature of the experimental results and the success of the simulation in capturing them both provide further support for a multiple-oscillator view of timing.