59,620 research outputs found
molecular ions can exist in strong magnetic fields
Using the variational method it is shown that for magnetic fields G there can exist a molecular ion .Comment: LaTeX, 7 pp, 1 table, 4 figures. Title modified. Consideration of the
longitudinal size of the system was adde
Nonlinear dynamics of quantum dot nuclear spins
We report manifestly nonlinear dependence of quantum dot nuclear spin
polarization on applied magnetic fields. Resonant absorption and emission of
circularly polarized radiation pumps the resident quantum dot electron spin,
which in turn leads to nuclear spin polarization due to hyperfine interaction.
We observe that the resulting Overhauser field exhibits hysteresis as a
function of the external magnetic field. This hysteresis is a consequence of
the feedback of the Overhauser field on the nuclear spin cooling rate. A
semi-classical model describing the coupled nuclear and electron spin dynamics
successfully explains the observed hysteresis but leaves open questions for the
low field behaviour of the nuclear spin polarization.Comment: 7 pages, 4 figure
Orbital Decay of the PSR J0045-7319/B Star Binary System: Age of Radio Pulsar and Initial Spin of Neutron Star
Recent timing observations of PSR J0045-7319 reveal that the neutron star/B
star binary orbit is decaying on a time scale of |\Porb/\dot\Porb|=0.5 Myr,
shorter than the characteristic age ( Myr) of the pulsar (Kaspi et
al.~1996a). We study mechanisms for the orbital decay. The standard weak
friction theory based on static tide requires far too short a viscous time to
explain the observed \dot\Porb. We show that dynamical tidal excitation of
g-modes in the B star can be responsible for the orbital decay. However, to
explain the observed short decay timescale, the B star must have some
significant retrograde rotation with respect to the orbit --- The retrograde
rotation brings lower-order g-modes, which couple much more strongly to the
tidal potential, into closer ``resonances'' with the orbital motion, thus
significantly enhancing the dynamical tide. A much less likely possibility is
that the g-mode damping time is much shorter than the ordinary radiative
damping time. The observed orbital decay timescale combined with a generic
orbital evolution model based on dynamical tide can be used as a ``timer'',
giving an upper limit of Myr for the age of the binary system since the
neutron star formation. Thus the characteristic age of the pulsar is not a good
age indicator. Assuming standard magnetic dipole braking for the pulsar and no
significant magnetic field decay on a timescale \lo 1 Myr, the upper limit
for the age implies that the initial spin of the neutron star at birth was
close to its current value.Comment: AASTeX, 9 pages, 3 ps figures. ApJ Letters, in pres
On the Implementation of Efficient Channel Filters for Wideband Receivers by Optimizing Common Subexpression Elimination Methods
No abstract availabl
Radiative transitions of the helium atom in highly magnetized neutron star atmospheres
Recent observations of thermally emitting isolated neutron stars revealed
spectral features that could be interpreted as radiative transitions of He in a
magnetized neutron star atmosphere. We present Hartree-Fock calculations of the
polarization-dependent photoionization cross sections of the He atom in strong
magnetic fields ranging from 10^12 G to 10^14 G. Convenient fitting formulae
for the cross sections are given as well as related oscillator strengths for
various bound-bound transitions. The effects of finite nucleus mass on the
radiative absorption cross sections are examined using perturbation theory.Comment: 14 pages, 7 figures. Minor changes. MNRAS in pres
Minimum Restraint Functions for unbounded dynamics: general and control-polynomial systems
We consider an exit-time minimum problem with a running cost, and
unbounded controls. The occurrence of points where can be regarded as a
transversality loss. Furthermore, since controls range over unbounded sets, the
family of admissible trajectories may lack important compactness properties. In
the first part of the paper we show that the existence of a -minimum
restraint function provides not only global asymptotic controllability (despite
non-transversality) but also a state-dependent upper bound for the value
function (provided ). This extends to unbounded dynamics a former result
which heavily relied on the compactness of the control set.
In the second part of the paper we apply the general result to the case when
the system is polynomial in the control variable. Some elementary, algebraic,
properties of the convex hull of vector-valued polynomials' ranges allow some
simplifications of the main result, in terms of either near-affine-control
systems or reduction to weak subsystems for the original dynamics.Comment: arXiv admin note: text overlap with arXiv:1503.0344
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