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Results for “FREQUENCY DEPENDENCE”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Dynamics of electronic transitions and frequency dependence of negative capacitance in semiconductor diodes under high forward bias

We observed qualitatively dissimilar frequency dependence of negative capacitance under high charge injection in two sets of functionally different junction diodes: III-V based light emitting and Si-based non-light emitting diodes. Using an advanced approach based on bias activated differential capacitance, we developed a generalized understanding of negative capacitance phenomenon which can be extended to any diode based device structure. We explained the observations as the mutual competition of fast and slow electronic transition rates which are different in different devices. This study can be useful in understanding the interfacial effects in semiconductor heterostructures and may lead to superior device functionality.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

First-principles wave-vector- and frequency-dependent exchange-correlation kernel for jellium at all densities

Here we propose a spatially and temporally nonlocal exchange correlation (XC) kernel for the spin-unpolarized fluid phase of ground-state jellium for use in time-dependent density functional and linear response calculations. The kernel is constructed to satisfy known properties of the exact XC kernel to accurately describe the correlation energies of bulk jellium and to satisfy frequency-moment sum rules at a wide range of bulk jellium densities, including those low densities that display strong correlation and symmetry breaking. These effects are easier to understand in the simple jellium model than in real systems. All exact constraints satisfied by the recent MCP07 kernel are maintained in the revised MCP07 (rMCP07) kernel, while others are added. The revision $f^{rMCP07}_{XC}$ (q, ω) differs from MCP07 only for nonzero frequencies ω. Only at densities much lower than those of real bulk metals is the frequency dependence of the kernel important for the correlation energy of jellium. As the wave vector q tends to zero, the kernel has a -4$πα(ω)/q^2$ divergence whose frequency-dependent ultranonlocality coefficient $α(ω)$ vanishes in jellium, and is predicted by rMCP07 to be extremely small for the real metals Al and Na.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Self-consistent modeling of terahertz waveguide and cavity with frequency-dependent conductivity

The surface resistance of metals, and hence the Ohmic dissipation per unit area, scales with the square root of the frequency of an incident electromagnetic wave. As is well recognized, this can lead to excessive wall losses at terahertz (THz) frequencies. On the other hand, high-frequency oscillatory motion of conduction electrons tends to mitigate the collisional damping. As a result, the classical theory predicts that metals behave more like a transparent medium at frequencies above the ultraviolet. Such a behavior difference is inherent in the AC conductivity, a frequency-dependent complex quantity commonly used to treat electromagnetics of metals at optical frequencies. The THz region falls in the gap between microwave and optical frequencies. However, metals are still commonly modeled by the DC conductivity in currently active vacuum electronics research aimed at the development of high-power THz sources (notably the gyrotron), although a small reduction of the DC conductivity due to surface roughness is sometimes included. In this study, we present a self-consistent modeling of the gyrotron interaction structures (a metallic waveguide or cavity) with the AC conductivity. The resulting waveguide attenuation constants and cavity quality factors are compared with those of the DC-conductivity model. The reduction in Ohmic losses under the AC-conductivity model is shown to be increasingly significant as the frequency reaches deeper into the THz region. Such effects are of considerable importance to THz gyrotrons for which the minimization of Ohmic losses constitutes a major design consideration.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2019

The NANOGrav 12.5 yr Data Set: The Frequency Dependence of Pulse Jitter in Precision Millisecond Pulsars

Low-frequency gravitational-wave experiments require the highest timing precision from an array of the moststable millisecond pulsars. Several known sources of noise on short timescales in single radio pulsar observationsare well described by a simple model of three components: template fitting from a finite signal-to-noise ratio, pulsephase/amplitude jitter from single-pulse stochasticity, and scintillation errors from short-timescale interstellarscattering variations. Currently template-fitting errors dominate, but as radio telescopes push toward higher signalto-noise ratios, jitter becomes the next dominant term for most millisecond pulsars. Understanding the statistics ofjitter becomes crucial for properly characterizing arrival time uncertainties. We characterize the radio frequencydependence of jitter using data on 48 pulsars in the North American Nanohertz Observatory for GravitationalWaves timing program. We detect significant jitter in 43 of the pulsars and test several functional forms for itsfrequency dependence; we find significant frequency dependence for 30 pulsars. We find moderate correlations ofrms jitter with pulse width (R = 0.62) and number of profile components (R = 0.40); the single-pulse rms jitter istypically 1% of pulse phase. The average frequency dependence for all pulsars using a power-law model hasindex 0.42. We investigate the jitter variations for the interpulse of PSR B1937+21 and find no significantdeviations from the main pulse rms jitter. We also test the time variation of jitter in two pulsars and find thatsystematics likely bias the results for high-precision pulsars. Pulsar timing array analyses must properly modeljitter as a significant component of the noise within the detector.

Lam, M. T.↗
DOE OSTI2015

Memory-Efficient Simulation of Frequency-Dependent Q

Memory-variable methods have been widely applied to approximate frequency-independent quality factor Q in numerical simulation of wave propagation. The frequency-independent model is often appropriate for frequencies up to about 1 Hz but at higher frequencies is inconsistent with some regional studies of seismic attenuation. We apply the memory-variable approach to frequency-dependent Q models that are constant below, and follow a power-law above, a chosen transition frequency. We present numerical results for the corresponding memory-variable relaxation times and weights, obtained by nonnegative least-squares fitting of the Q(f) function, for a range of exponent values; these times and weights can be scaled to an arbitrary transition frequency and a power-law prefactor, respectively. The resulting memory-variable formulation can be used with numerical wave-propagation solvers based on methods such as finite differences (FDs) or spectral elements and may be implemented in either conventional or coarse-grained form. In the coarse-grained approach, we fit effective Q for low-Q values (<200) using a nonlinear inversion technique and use an interpolation formula to find the corresponding weighting coefficients for arbitrary Q. A 3D staggered-grid FD implementation closely approximates the frequency–wavenumber solution to both a half-space and a layered model with a shallow dislocation source for Q as low as 20 over a bandwidth of two decades. We compare the effects of different power-law exponents using a finite-fault source model of the 2008 Mw 5.4 Chino Hills, California, earthquake and find that Q(f) models generally better fit the strong-motion data than the constant Q models for frequencies above 1 Hz.

Geochemistry & Geophysics↗
DOE OSTI2016

FREQUENCY-DEPENDENT DISPERSION MEASURES AND IMPLICATIONS FOR PULSAR TIMING

The dispersion measure (DM), the column density of free electrons to a pulsar, is shown to be frequency dependent because of multipath scattering from small-scale electron-density fluctuations. DMs vary between propagation paths whose transverse extent varies strongly with frequency, yielding arrival times that deviate from the high-frequency scaling expected for a cold, uniform, unmagnetized plasma (1/frequency{sup 2}). Scaling laws for thin phase screens are verified with simulations; extended media are also analyzed. The rms DM difference across an octave band near 1.5 GHz is ∼ 4 × 10{sup −5} pc cm{sup −3} for pulsars at ∼1 kpc distance. The corresponding arrival-time variations are a few to hundreds of nanoseconds for DM ≲ 30 pc cm{sup −3} but increase rapidly to microseconds or more for larger DMs and wider frequency ranges. Chromatic DMs introduce correlated noise into timing residuals with a power spectrum of “low pass” form. The correlation time is roughly the geometric mean of the refraction times for the highest and lowest radio frequencies used, ranging from days to years, depending on the pulsar. We discuss implications for methodologies that use large frequency separations or wide bandwidth receivers for timing measurements. Chromatic DMs are partially mitigable by including an additional chromatic term in arrival time models. Without mitigation, an additional term in the noise model for pulsar timing is implied. In combination with measurement errors from radiometer noise, an arbitrarily large increase in total frequency range (or bandwidth) will yield diminishing benefits and may be detrimental to overall timing precision.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

An angular frequency dependence on the Aharonov–Casher geometric phase

A quantum effect characterized by a dependence of the angular frequency associated with the confinement of a neutral particle to a quantum ring on the quantum numbers of the system and the Aharonov–Casher geometric phase is discussed. Then, it is shown that persistent spin currents can arise in a two-dimensional quantum ring in the presence of a Coulomb-type potential. A particular contribution to the persistent spin currents arises from the dependence of the angular frequency on the geometric quantum phase.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Benchmarking Correlation-Consistent Basis Sets for Frequency-Dependent Polarizabilities with Multiresolution Analysis

This paper presents the first converged frequency-dependent HF polarizability results for general molecules, on a set of 89 closed-shell atoms and molecules. The solver employs multiresolution analysis (MRA) in a multiwavelet basis to compute both ground and response states to a guaranteed precision, which are validated against independent numerical grid calculations on atoms and linear molecules. The MRA ground-state energies and response properties are used to evaluate results in correlation-consistent basis sets up to 5Z augmented with either single or double diffuse functions and core-polarization functions. Systematic trends are revealed through consideration of chemical composition as well as the use of machine learning to cluster convergence trends, the latter suggesting the possibility of learning and correcting basis-set error.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Features of the Frequency Dependence of Capacitance–Voltage Characteristics of a Semiconductor Structure of a Photoelectric Converter Based on a p–n Junction with an Antireflective Film of Porous Silicon

The frequency dependence of capacitance–voltage characteristics of a semiconductor structure with an antireflective film of porous silicon, which was formed by electrochemical etching above a p–n junction, is studied. Photoluminescence spectra of layers of porous silicon of the experimental samples are also examined. It is demonstrated that the capacitance–voltage curves are shaped by competing influences of capacitances of the p–n junction and the surface structure forming in a porous Si film due to its inhomogeneity. A structural model of layers of the studied semiconductor structure and a capacitance equivalent circuit are proposed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1988

Frequency-dependent dielectric analysis - Monitoring the chemistry and rheology of thermosets during cure

Frequency-dependent dielectric measurements in a curing resin sample were performed using an impedance analyzer and a permittivity sensor, to separate and measure the ionic (sigma) and dipolar (tau) mobilities, used as molecular probes of the cure reaction in the resin. The resin samples were tetraglycidyl 4,4-prime-diaminodiphenylmethane epoxy resins which included catalyzed (Hercules 3501-6) and uncatalyzed l3502 resin samples. The values of sigma and tau were used to quantitatively determine the viscosity, the degree of cure, and the T(g) of the resin. It is shown that the values of the Arrhenius and WLF constants during the cure process reflect the buildup of the crosslink network.

Kranbuehl, D.↗
DOE OSTI2018

A Magnus approximation approach to harmonic systems with time-dependent frequencies

We use a Magnus approximation at the level of the equations of motion for a harmonic system with a time-dependent frequency, to find an expansion for its in–out effective action, and a unitary expansion for the Bogoliubov transformation between in and out states. The dissipative effects derived therefrom are compared with the ones obtained from perturbation theory in powers of the time-dependent piece in the frequency, and with those derived using multiple scale analysis in systems with parametric resonance. We also apply the Magnus expansion to the in–in effective action, to construct reality and causal equations of motion for the external system. We show that the nonlocal equations of motion can be written in terms of a “retarded Fourier transform” evaluated at the resonant frequency.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1991

Finite element modeling of truss structures with frequency-dependent material damping

A physically motivated modelling technique for structural dynamic analysis that accommodates frequency dependent material damping was developed. Key features of the technique are the introduction of augmenting thermodynamic fields (AFT) to interact with the usual mechanical displacement field, and the treatment of the resulting coupled governing equations using finite element analysis methods. The AFT method is fully compatible with current structural finite element analysis techniques. The method is demonstrated in the dynamic analysis of a 10-bay planar truss structure, a structure representative of those contemplated for use in future space systems.

Lesieutre, George A.↗
DOE OSTI2015

Frequency-dependent photothermal measurement of transverse thermal diffusivity of organic semiconductors

We have used a photothermal technique, in which chopped light heats the front surface of a small (∼1 mm{sup 2}) sample and the chopping frequency dependence of thermal radiation from the back surface is measured with a liquid-nitrogen-cooled infrared detector. In our system, the sample is placed directly in front of the detector within its dewar. Because the detector is also sensitive to some of the incident light, which leaks around or through the sample, measurements are made for the detector signal that is in quadrature with the chopped light. Results are presented for layered crystals of semiconducting 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pn) and for papers of cellulose nanofibrils coated with semiconducting poly(3,4-ethylene-dioxythiophene):poly(styrene-sulfonate) (NFC-PEDOT). For NFC-PEDOT, we have found that the transverse diffusivity, smaller than the in-plane value, varies inversely with thickness, suggesting that texturing of the papers varies with thickness. For TIPS-pn, we have found that the interlayer diffusivity is an order of magnitude larger than the in-plane value, consistent with previous estimates, suggesting that low-frequency optical phonons, presumably associated with librations in the TIPS side groups, carry most of the heat.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Improving Planck calibration by including frequency-dependent relativistic corrections

The Planck satellite detectors are calibrated in the 2015 release using the 'orbital dipole', which is the time-dependent dipole generated by the Doppler effect due to the motion of the satellite around the Sun. Such an effect has also relativistic time-dependent corrections of relative magnitude 10{sup −3}, due to coupling with the 'solar dipole' (the motion of the Sun compared to the CMB rest frame), which are included in the data calibration by the Planck collaboration. We point out that such corrections are subject to a frequency-dependent multiplicative factor. This factor differs from unity especially at the highest frequencies, relevant for the HFI instrument. Since currently Planck calibration errors are dominated by systematics, to the point that polarization data is currently unreliable at large scales, such a correction can in principle be highly relevant for future data releases.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Frequency-dependent ferroelectric behavior of BaMn3Ti4O14.25 at room temperature

We report the activation field and selective frequency-dependent ferroelectric behavior of BaMn3Ti4O14.25 (BMT-134) at room temperature. BMT-134, a recently discovered multiferroic complex oxide, exhibits antiferromagnetic and ferroelectric behavior and belongs to the hollandite crystal class. The microstructure can be manipulated through processing conditions to prepare a nanocrystalline textured tablet. We measured polarization-electric field (P-E) hysteresis loops and strain-electric field butterfly loops as a function of frequency in order to investigate the AC dynamics of domain switching and strain behavior. Under an electric field loading condition, a clear hysteresis loop of the electric field-displacement curve is obtained at 50 Hz, indicating that room temperature ferroelectricity is attainable under the right processing conditions. When the frequency is increased to 500 Hz, the coercive field also increases, until the frequency reaches 5 kHz, at which point the electric field versus electric displacement becomes linear indicating the limit of domain switching at high frequency.

Hossain, Muhammad E.↗
DOE OSTI2015

FREQUENCY DEPENDENCE OF POLARIZATION OF ZEBRA PATTERN IN TYPE-IV SOLAR RADIO BURSTS

We investigated the polarization characteristics of a zebra pattern (ZP) in a type-IV solar radio burst observed with AMATERAS on 2011 June 21 for the purpose of evaluating the generation processes of ZPs. Analyzing highly resolved spectral and polarization data revealed the frequency dependence of the degree of circular polarization and the delay between two polarized components for the first time. The degree of circular polarization was 50%–70% right-handed and it varied little as a function of frequency. Cross-correlation analysis determined that the left-handed circularly polarized component was delayed by 50–70 ms relative to the right-handed component over the entire frequency range of the ZP and this delay increased with the frequency. We examined the obtained polarization characteristics by using pre-existing ZP models and concluded that the ZP was generated by the double-plasma-resonance process. Our results suggest that the ZP emission was originally generated in a completely polarized state in the O-mode and was partly converted into the X-mode near the source. Subsequently, the difference between the group velocities of the O-mode and X-mode caused the temporal delay.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Spin dynamics and frequency dependence of magnetic damping study in soft ferromagnetic FeTaC film with a stripe domain structure

Perpendicular magnetic anisotropy (PMA) and low magnetic damping are the key factors for the free layer magnetization switching by spin transfer torque technique in magnetic tunnel junction devices. The magnetization precessional dynamics in soft ferromagnetic FeTaC thin film with a stripe domain structure was explored in broad band frequency range by employing micro-strip ferromagnetic resonance technique. The polar angle variation of resonance field and linewidth at different frequencies have been analyzed numerically using Landau-Lifshitz-Gilbert equation by taking into account the total free energy density of the film. The numerically estimated parameters Landé g-factor, PMA constant, and effective magnetization are found to be 2.1, 2 × 10{sup 5} erg/cm{sup 3} and 7145 Oe, respectively. The frequency dependence of Gilbert damping parameter (α) is evaluated by considering both intrinsic and extrinsic effects into the total linewidth analysis. The value of α is found to be 0.006 at 10 GHz and it increases monotonically with decreasing precessional frequency.

36 MATERIALS SCIENCE↗