12,058 research outputs found
Introduction to Systems Approach
Main aspects of systems theory are outlined. Emphasis is on the interface of between time and systems - natural or artificial
Precise time and time interval (PTTI), an overview
Present applications of precise time and frequency (T/F) technology can be grouped as follows: (1) Communications systems which require T/F for time division multiplexing and for using spread spectrum techniques. (2) Navigation systems which need T/F for position fixing using a timed signal. (3) Scientific-Metrological applications which use T/F as the most precisely reproducible standard of measurement. (4) Astronomical-Space applications which cover a variety of the most demanding applications such as pulsar research, Very Long Baseline Interferometry (VLBI) and laser/radar ranging. In particular, pulsar time-of-arrival measurements require submicrosecond precision over a period of one-half year referred to an extraterrestrial inertial system, and constitute the most stringent requirements for uniform timekeeping to date
Semiflexible polymers under external fields confined to two dimensions
The non-equilibrium structural and dynamical properties of semiflexible
polymers confined to two dimensions are investigated by molecular dynamics
simulations. Three different scenarios are considered: The force-extension
relation of tethered polymers, the relaxation of an initially stretched
semiflexible polymer, and semiflexible polymers under shear flow. We find
quantitative agreement with theoretical predictions for the force-extension
relation and the time dependence of the entropically contracting polymer. The
semiflexible polymers under shear flow exhibit significant conformational
changes at large shear rates, where less stiff polymers are extended by the
flow, whereas rather stiff polymers are contracted. In addition, the polymers
are aligned by the flow, thereby the two-dimensional semiflexible polymers
behave similarly to flexible polymers in three dimensions. The tumbling times
display a power-law dependence at high shear rate rates with an exponent
comparable to the one of flexible polymers in three-dimensional systems.Comment: Accepted for publication in J. Chem. Phy
Dynamic regimes of fluids simulated by multiparticle-collision dynamics
We investigate the hydrodynamic properties of a fluid simulated with a
mesoscopic solvent model. Two distinct regimes are identified, the `particle
regime' in which the dynamics is gas-like, and the `collective regime' where
the dynamics is fluid-like. This behavior can be characterized by the Schmidt
number, which measures the ratio between viscous and diffusive transport.
Analytical expressions for the tracer diffusion coefficient, which have been
derived on the basis of a molecular-chaos assumption, are found to describe the
simulation data very well in the particle regime, but important deviations are
found in the collective regime. These deviations are due to hydrodynamic
correlations. The model is then extended in order to investigate self-diffusion
in colloidal dispersions. We study first the transport properties of heavy
point-like particles in the mesoscopic solvent, as a function of their mass and
number density. Second, we introduce excluded-volume interactions among the
colloidal particles and determine the dependence of the diffusion coefficient
on the colloidal volume fraction for different solvent mean-free paths. In the
collective regime, the results are found to be in good agreement with previous
theoretical predictions based on Stokes hydrodynamics and the Smoluchowski
equation.Comment: 15 pages, 15 figure
Sum rules for an atomic hyperfine structure in a magnetic field
Sum rules for the energy levels of a hyperfine multiplet in a constant
uniform magnetic field is presented. It is found that for any values of the
electron angular moment and the nuclear spin there are certain linear
combinations of energy levels which do not depend on the magnetic field and can
be used to determine the unperturbated hfs separation in the presence of
perturbing magnetic field. It is also demonstrated that there are other linear
combinations which are linear with the external magnetic field and hence can be
used to determine bound values of the electron and nuclear magnetic moments.
The accuracy of the approximation within which the result is valid is also
discussed
Spin Density Matrix of Spin-3/2 Hole Systems
For hole systems with an effective spin j=3/2, we present an invariant
decomposition of the spin density matrix that can be interpreted as a multipole
expansion. The charge density corresponds to the monopole moment and the spin
polarization due to a magnetic field corresponds to a dipole moment while heavy
hole-light hole splitting can be interpreted as a quadrupole moment. For quasi
two-dimensional hole systems in the presence of an in-plane magnetic field B
the spin polarization is a higher-order effect that is typically much smaller
than one even if the minority spin subband is completely depopulated. On the
other hand, the field B can induce a substantial octupole moment which is a
unique feature of j=3/2 hole systems.Comment: 8 pages, 1 figure, 3 table
Generation of spin currents and spin densities in systems with reduced symmetry
We show that the spin-current response of a semiconductor crystal to an
external electric field is considerably more complex than previously assumed.
While in systems of high symmetry only the spin-Hall components are allowed, in
systems of lower symmetry other non-spin-Hall components may be present. We
argue that, when spin-orbit interactions are present only in the band
structure, the distinction between intrinsic and extrinsic contributions to the
spin current is not useful. We show that the generation of spin currents and
that of spin densities in an electric field are closely related, and that our
general theory provides a systematic way to distinguish between them in
experiment. We discuss also the meaning of vertex corrections in systems with
spin-orbit interactions.Comment: 4 page
Tuning the scattering length with an optically induced Feshbach resonance
We demonstrate optical tuning of the scattering length in a Bose-Einstein
condensate as predicted by Fedichev {\em et al.} [Phys. Rev. Lett. {\bf 77},
2913 (1996)]. In our experiment atoms in a Rb condensate are exposed to
laser light which is tuned close to the transition frequency to an excited
molecular state. By controlling the power and detuning of the laser beam we can
change the atomic scattering length over a wide range. In view of laser-driven
atomic losses we use Bragg spectroscopy as a fast method to measure the
scattering length of the atoms.Comment: submitted to PRL, 5 pages, 5 figure
Quantum Hamiltonian for gravitational collapse
Using a Hamiltonian formulation of the spherically symmetric gravity-scalar
field theory adapted to flat spatial slicing, we give a construction of the
reduced Hamiltonian operator. This Hamiltonian, together with the null
expansion operators presented in an earlier work, form a framework for studying
gravitational collapse in quantum gravity. We describe a setting for its
numerical implementation, and discuss some conceptual issues associated with
quantum dynamics in a partial gauge fixing.Comment: 17 pages, published version (minor changes
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