44,086 research outputs found
Mechanical detection of nuclear spin relaxation in a micron-size crystal
A room temperature nuclear magnetic resonance force microscope (MRFM), fitted
in a Tesla electromagnet, is used to measure the nuclear spin relaxation of
H in a micron-size (70ng) crystal of ammonium sulfate. NMR sequences,
combining both pulsed and continuous wave r.f. fields, have allowed us to
measure mechanically and , the transverse and longitudinal spin
relaxation times. Because two spin species with different values are
measured in our thick crystal, magnetic resonance imaging of
their spatial distribution inside the sample section are performed. To
understand quantitatively the measured signal, we carefully study the influence
of the spin-lattice relaxation and the non-adiabaticity of the c.w. sequence on
the intensity and time dependence of the detected signal.Comment: latex drafteps.tex, 13 files, 12 pages [SPEC-S00/010], submitted to
Eur. Phys. J.
Applications of model structure determination to flight test data
Several statistical and information criteria need to be considered when selecting an adequate model. Incorrect stability and control derivates result from inadequate aerodynamic model structure. Stepwise regression is used to determine the structure for an adequate model. Flight data which covers a nonlinear aerodynamic model range may be analyzed as a single data set or partitioned into several distinct sets. Stepwise regression for model structure detemination and parameter estimation was successfully applied to three aircraft types (single engine general aviation, unaugmented modern jet fighter, jet transport)
Theory of Raman scattering from Leggett's collective mode in a multiband superconductor: Application to MgB
In 1966 Leggett used a two-band superconductor to show that a new collective
mode could exist at low temperatures, corresponding to a counter-flow of the
superconducting condensates in each band. Here, the theory of electronic Raman
scattering in a superconductor by Klein and Dierker (1984) is extended to a
multiband superconductor. Raman scattering creates particle/hole pairs. In the
relevant \ symmetry, the attraction that produces pairing necessarily
couples excitations of superconducting pairs to these p/h excitations. In the
Appendix it is shown that for zero wave vector transfer this coupling
modifies the Raman response and makes the long-range Coulomb correction null.
The 2-band result is applied to MgB where this coupling activates
Leggett's collective mode. His simple limiting case is obtained when the
interband attractive potential is decreased to a value well below that given by
LDA theory. The peak from Leggett's mode is studied as the potential is
increased through the theoretical value: With realistic MgB\ parameters,
the peak broadens through decay into the continuum above the smaller (
band) superconducting gap. Finite effects are also taken into account,
yielding a Raman peak that agrees well in energy with the experimental result
by Blumberg \textit{et el.} (2007). This approach is also applied to the ,
2-band model of the Fe-pnictides considered by Chubukov \textit{et al.}(2009).Comment: 10 pages, 3 figures. To appear in Physical Review
Compatibility check of measured aircraft responses using kinematic equations and extended Kalman filter
An extended Kalman filter smoother and a fixed point smoother were used for estimation of the state variables in the six degree of freedom kinematic equations relating measured aircraft responses and for estimation of unknown constant bias and scale factor errors in measured data. The computing algorithm includes an analysis of residuals which can improve the filter performance and provide estimates of measurement noise characteristics for some aircraft output variables. The technique developed was demonstrated using simulated and real flight test data. Improved accuracy of measured data was obtained when the data were corrected for estimated bias errors
Determination of airplane model structure from flight data using splines and stepwise regression
A procedure for the determination of airplane model structure from flight data is presented. The model is based on a polynomial spline representation of the aerodynamic coefficients, and the procedure is implemented by use of a stepwise regression. First, a form of the aerodynamic force and moment coefficients amenable to the utilization of splines is developed. Next, expressions for the splines in one and two variables are introduced. Then the steps in the determination of an aerodynamic model structure and the estimation of parameters are discussed briefly. The focus is on the application to flight data of the techniques developed
Metastability in stochastic dynamics of disordered mean-field models
We study a class of Markov chains that describe reversible stochastic
dynamics of a large class of disordered mean field models at low temperatures.
Our main purpose is to give a precise relation between the metastable time
scales in the problem to the properties of the rate functions of the
corresponding Gibbs measures. We derive the analog of the Wentzell-Freidlin
theory in this case, showing that any transition can be decomposed, with
probability exponentially close to one, into a deterministic sequence of
``admissible transitions''. For these admissible transitions we give upper and
lower bounds on the expected transition times that differ only by a constant.
The distribution rescaled transition times are shown to converge to the
exponential distribution. We exemplify our results in the context of the random
field Curie-Weiss model.Comment: 73pp, AMSTE
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