12,672 research outputs found
Signal processing technique for non-invasive real-time estimation of cardiac output by inductance cardiography (thoracocardiography)
Inductance cardiography (thoracocardiography) non-invasively monitors changes in stroke volume by recording ventricular volume curves with an inductive plethysmographic transducer encircling the chest at the level of the heart. Clinical application of this method has been hampered, as data analysis has not been feasible in real time. Therefore a novel, real-time signal processing technique for inductance cardiography has been developed. Its essential concept consists in performance of multiple tasks by several, logically linked signal processing modules that have access to common databases. Based on these principles, a software application was designed that performs acquisition, display, filtering and ECG-triggered ensemble averaging of inductance signals and separates cardiogenic waveforms from noise related to respiration and other sources. The resulting ventricular volume curves are automatically analysed. Performance of the technique for monitoring cardiac output in real time was compared with thermodilution in four patients in an intensive care unit. The bias (mean difference) among 76 paired thoracocardiographic and thermodilution derived changes in cardiac output was 0%; limits of agreement (±2 SD of the bias) were ±25%. It is concluded that the proposed signal processing technique for inductance cardiography holds promise for non-invasive, real-time estimation of changes in cardiac outpu
Nonequilibrium perturbation theory of the spinless Falicov-Kimball model
We perform a perturbative analysis for the nonequilibrium Green functions of
the spinless Falicov-Kimball model in the presence of an arbitrary external
time-dependent but spatially uniform electric field. The conduction electron
self-energy is found from a strictly truncated second-order perturbative
expansion in the local electron-electron repulsion U. We examine the current at
half-filling, and compare to both the semiclassical Boltzmann equation and
exact numerical solutions for the contour-ordered Green functions from a
transient-response formalism (in infinite dimensions) on the
Kadanoff-Baym-Keldysh contour. We find a strictly truncated perturbation theory
in the two-time formalism cannot reach the long-time limit of the steady state;
instead it illustrates pathological behavior for times larger than
approximately 2/U
A variational problem on Stiefel manifolds
In their paper on discrete analogues of some classical systems such as the
rigid body and the geodesic flow on an ellipsoid, Moser and Veselov introduced
their analysis in the general context of flows on Stiefel manifolds. We
consider here a general class of continuous time, quadratic cost, optimal
control problems on Stiefel manifolds, which in the extreme dimensions again
yield these classical physical geodesic flows. We have already shown that this
optimal control setting gives a new symmetric representation of the rigid body
flow and in this paper we extend this representation to the geodesic flow on
the ellipsoid and the more general Stiefel manifold case. The metric we choose
on the Stiefel manifolds is the same as that used in the symmetric
representation of the rigid body flow and that used by Moser and Veselov. In
the extreme cases of the ellipsoid and the rigid body, the geodesic flows are
known to be integrable. We obtain the extremal flows using both variational and
optimal control approaches and elucidate the structure of the flows on general
Stiefel manifolds.Comment: 30 page
Steady-state nonequilibrium dynamical mean-field theory and the quantum Boltzman
We derive the formalism for steady state nonequilibrium dynamical mean-field
theory in a real-time formalism along the Kadanoff-Baym contour. The resulting
equations of motion are first transformed to Wigner coordinates (average and
relative time), and then re-expressed in terms of differential operators.
Finally, we perform a Fourier transform with respect to the relative time, and
take the first-order limit in the electric field to produce the quantum
Boltzmann equation for dynamical mean-field theory. We next discuss the
structure of the equations and their solutions, describing how these equations
reduce to the Drude result in the limit of a constant relaxation time. We also
explicitly demonstrate the equivalence between the Kubo and nonequilibrium
approaches to linear response. There are a number of interesting modifications
of the conventional quantum Boltzmann equation that arise due to the underlying
bandstructure of the lattice.Comment: (14 pages, proceedings of the Workshop on Progress in Nonequilibrium
Green's Functions III, Kiel Germany
Luther-Emery Phase and Atomic-Density Waves in a Trapped Fermion Gas
The Luther-Emery liquid is a state of matter that is predicted to occur in
one-dimensional systems of interacting fermions and is characterized by a
gapless charge spectrum and a gapped spin spectrum. In this Letter we discuss a
realization of the Luther-Emery phase in a trapped cold-atom gas. We study by
means of the density-matrix renormalization-group technique a two-component
atomic Fermi gas with attractive interactions subject to parabolic trapping
inside an optical lattice. We demonstrate how this system exhibits compound
phases characterized by the coexistence of spin pairing and atomic-density
waves. A smooth crossover occurs with increasing magnitude of the atom-atom
attraction to a state in which tightly bound spin-singlet dimers occupy the
center of the trap. The existence of atomic-density waves could be detected in
the elastic contribution to the light-scattering diffraction pattern.Comment: 10 pages, 3 figures, 1 Table, submitted to Phys. Rev. on July 25th
200
On the lowest energy excitations of one-dimensional strongly correlated electrons
It is proven that the lowest excitations of one-dimensional
half-integer spin generalized Heisenberg models and half-filled extended
Hubbard models are -periodic functions. For Hubbard models at fractional
fillings , where , and is
the number of electrons per unit cell. Moreover, if one of the ground states of
the system is magnetic in the thermodynamic limit, then for
any , so the spectrum is gapless at any wave vector. The last statement is
true for any integer or half-integer value of the spin.Comment: 6 Pages, Revtex, final versio
Spin Susceptibility of an Ultra-Low Density Two Dimensional Electron System
We determine the spin susceptibility in a two dimensional electron system in
GaAs/AlGaAs over a wide range of low densities from 2cm to
4cm. Our data can be fitted to an equation that describes
the density dependence as well as the polarization dependence of the spin
susceptibility. It can account for the anomalous g-factors reported recently in
GaAs electron and hole systems. The paramagnetic spin susceptibility increases
with decreasing density as expected from theoretical calculations.Comment: 5 pages, 2 eps figures, to appear in PR
When physics helps mathematics: calculation of the sophisticated multiple integral
There exists a remarkable connection between the quantum mechanical
Landau-Zener problem and purely classical-mechanical problem of a ball rolling
on a Cornu spiral. This correspondence allows us to calculate a complicated
multiple integral, a kind of multi-dimensional generalization of Fresnel
integrals. A direct method of calculation is also considered but found to be
successful only in some low-dimensional cases. As a byproduct of this direct
method, an interesting new integral representation for is obtained.Comment: 13 pages, no figure
Measuring the temporal coherence of an atom laser beam
We report on the measurement of the temporal coherence of an atom laser beam
extracted from a Rb Bose-Einstein condensate. Reflecting the beam from a
potential barrier creates a standing matter wave structure. From the contrast
of this interference pattern, observed by magnetic resonance imaging, we have
deduced an energy width of the atom laser beam which is Fourier limited by the
duration of output coupling. This gives an upper limit for temporal phase
fluctuations in the Bose-Einstein condensate.Comment: 4 pages, 3 figure
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