24,604 research outputs found
Parametric oscillator tuning curve from observations of total parametric fluorescence
Measurements of total emitted parametric fluorescence power are presented and used to fix one point on the predicted tuning curve of a parametric oscillator. The method is particularly useful for predicting the tuning curve of infrared pumped parametric oscillators. Experimental results, which verify the usefulness of the technique in a 1.06-μ-pumped oscillator, are presented
Frequency analysis via the method of moment functionals
Several variants are presented of a linear-in-parameters least squares formulation for determining the transfer function of a stable linear system at specified frequencies given a finite set of Fourier series coefficients calculated from transient nonstationary input-output data. The basis of the technique is Shinbrot's classical method of moment functionals using complex Fourier based modulating functions to convert a differential equation model on a finite time interval into an algebraic equation which depends linearly on frequency-related parameters
X-type and Y-type junction stability in domain wall networks
We develop an analytic formalism that allows one to quantify the stability
properties of X-type and Y-type junctions in domain wall networks in two
dimensions. A similar approach might be applicable to more general defect
systems involving junctions that appear in a range of physical situations, for
example, in the context of F- and D-type strings in string theory. We apply
this formalism to a particular field theory, Carter's pentavac model, where the
strength of the symmetry breaking is governed by the parameter .
We find that for low values of the symmetry breaking parameter X-type junctions
will be stable, whereas for higher values an X-type junction will separate into
two Y-type junctions. The critical angle separating the two regimes is given by
\alpha_c = 293^{\circ}\sqrt{|\epsilon|} and this is confirmed using simple
numerical experiments. We go on to simulate the pentavac model from random
initial conditions and we find that the dominant junction is of \ytype for
|\epsilon| \geq 0.02 and is of \xtype for |\epsilon| \leq 0.02\epsilon\qsubrm{N}{dw}\propto
t^{-1}\epsilont^{-1}$ lore.Comment: 24 pages, 13 figures; typos fixe
Aerodynamic parameter estimation via Fourier modulating function techniques
Parameter estimation algorithms are developed in the frequency domain for systems modeled by input/output ordinary differential equations. The approach is based on Shinbrot's method of moment functionals utilizing Fourier based modulating functions. Assuming white measurement noises for linear multivariable system models, an adaptive weighted least squares algorithm is developed which approximates a maximum likelihood estimate and cannot be biased by unknown initial or boundary conditions in the data owing to a special property attending Shinbrot-type modulating functions. Application is made to perturbation equation modeling of the longitudinal and lateral dynamics of a high performance aircraft using flight-test data. Comparative studies are included which demonstrate potential advantages of the algorithm relative to some well established techniques for parameter identification. Deterministic least squares extensions of the approach are made to the frequency transfer function identification problem for linear systems and to the parameter identification problem for a class of nonlinear-time-varying differential system models
Suppression of spin-pumping by a MgO tunnel-barrier
Spin-pumping generates pure spin currents in normal metals at the ferromagnet
(F)/normal metal (N) interface. The efficiency of spin-pumping is given by the
spin mixing conductance, which depends on N and the F/N interface. We directly
study the spin-pumping through an MgO tunnel-barrier using the inverse spin
Hall effect, which couples spin and charge currents and provides a direct
electrical detection of spin currents in the normal metal. We find that
spin-pumping is suppressed by the tunnel-barrier, which is contrary to recent
studies that suggest that the spin mixing conductance can be enhanced by a
tunnel-barrier inserted at the interface
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