18,483 research outputs found
On some geometric features of the Kramer interior solution for a rotating perfect fluid
Geometric features (including convexity properties) of an exact interior
gravitational field due to a self-gravitating axisymmetric body of perfect
fluid in stationary, rigid rotation are studied. In spite of the seemingly
non-Newtonian features of the bounding surface for some rotation rates, we
show, by means of a detailed analysis of the three-dimensional spatial
geodesics, that the standard Newtonian convexity properties do hold. A central
role is played by a family of geodesics that are introduced here, and provide a
generalization of the Newtonian straight lines parallel to the axis of
rotation.Comment: LaTeX, 15 pages with 4 Poscript figures. To be published in Classical
and Quantum Gravit
Bias-free Measurement of Giant Molecular Cloud Properties
(abridged) We review methods for measuring the sizes, line widths, and
luminosities of giant molecular clouds (GMCs) in molecular-line data cubes with
low resolution and sensitivity. We find that moment methods are robust and
sensitive -- making full use of both position and intensity information -- and
we recommend a standard method to measure the position angle, major and minor
axis sizes, line width, and luminosity using moment methods. Without
corrections for the effects of beam convolution and sensitivity to GMC
properties, the resulting properties may be severely biased. This is
particularly true for extragalactic observations, where resolution and
sensitivity effects often bias measured values by 40% or more. We correct for
finite spatial and spectral resolutions with a simple deconvolution and we
correct for sensitivity biases by extrapolating properties of a GMC to those we
would expect to measure with perfect sensitivity. The resulting method recovers
the properties of a GMC to within 10% over a large range of resolutions and
sensitivities, provided the clouds are marginally resolved with a peak
signal-to-noise ratio greater than 10. We note that interferometers
systematically underestimate cloud properties, particularly the flux from a
cloud. The degree of bias depends on the sensitivity of the observations and
the (u,v) coverage of the observations. In the Appendix to the paper we present
a conservative, new decomposition algorithm for identifying GMCs in
molecular-line observations. This algorithm treats the data in physical rather
than observational units, does not produce spurious clouds in the presence of
noise, and is sensitive to a range of morphologies. As a result, the output of
this decomposition should be directly comparable among disparate data sets.Comment: Accepted to PASP (19 pgs., 12 figures). The submission describes an
IDL software package available from
http://cfa-www.harvard.edu/~erosolow/cprops
Current-voltage characteristics of quasi-one-dimensional superconductors: An S-curve in the constant voltage regime
Applying a constant voltage to superconducting nanowires we find that its
IV-characteristic exhibits an unusual S-behavior. This behavior is the direct
consequence of the dynamics of the superconducting condensate and of the
existence of two different critical currents: j_{c2} at which the pure
superconducting state becomes unstable and j_{c1}<j_{c2} at which the phase
slip state is realized in the system.Comment: 4 pages, 5 figures, replaced with minor change
Stress condensation in crushed elastic manifolds
We discuss an M-dimensional phantom elastic manifold of linear size L crushed
into a small sphere of radius R << L in N-dimensional space. We investigate the
low elastic energy states of 2-sheets (M=2) and 3-sheets (M=3) using analytic
methods and lattice simulations. When N \geq 2M the curvature energy is
uniformly distributed in the sheet and the strain energy is negligible. But
when N=M+1 and M>1, both energies appear to be condensed into a network of
narrow M-1 dimensional ridges. The ridges appear straight over distances
comparable to the confining radius R.Comment: 4 pages, RevTeX + epsf, 4 figures, Submitted to Phys. Rev. Let
Information Content in Decays and the Angular Moments Method
The time-dependent angular distributions of decays of neutral mesons into
two vector mesons contain information about the lifetimes, mass differences,
strong and weak phases, form factors, and CP violating quantities. A
statistical analysis of the information content is performed by giving the
``information'' a quantitative meaning. It is shown that for some parameters of
interest, the information content in time and angular measurements combined may
be orders of magnitude more than the information from time measurements alone
and hence the angular measurements are highly recommended. The method of
angular moments is compared with the (maximum) likelihood method to find that
it works almost as well in the region of interest for the one-angle
distribution. For the complete three-angle distribution, an estimate of
possible statistical errors expected on the observables of interest is
obtained. It indicates that the three-angle distribution, unraveled by the
method of angular moments, would be able to nail down many quantities of
interest and will help in pointing unambiguously to new physics.Comment: LaTeX, 34 pages with 9 figure
Variation of the density of states in amorphous GdSi at the metal-insulator transition
We performed detailed conductivity and tunneling mesurements on the
amorphous, magnetically doped material -GdSi (GdSi), which
can be driven through the metal-insulator transition by the application of an
external magnetic field. Conductivity increases linearly with field near the
transition and slightly slower on the metallic side. The tunneling conductance,
proportional to the density of states , undergoes a gradual change with
increasing field, from insulating, showing a soft gap at low bias, with a
slightly weaker than parabolic energy dependence, i.e. , , towards metallic behavior, with , energy
dependence. The density of states at the Fermi level appears to be zero at low
fields, as in an insulator, while the sample shows already small, but
metal-like conductivity. We suggest a possible explanation to the observed
effect.Comment: 6 pages, 6 figure
Dynamic transitions between metastable states in a superconducting ring
Applying the time-dependent Ginzburg-Landau equations, transitions between
metastable states of a superconducting ring are investigated in the presence of
an external magnetic field. It is shown that if the ring exhibits several
metastable states at a particular magnetic field, the transition from one
metastable state to another one is governed by both the relaxation time of the
absolute value of the order parameter tau_{|psi|} and the relaxation time of
the phase of the order parameter tau_{phi}. We found that the larger the ratio
tau_{|psi|}tau_{phi} the closer the final state will be to the absolute minimum
of the free energy, i.e. the thermodynamic equilibrium. The transition to the
final state occurs through a subsequent set of single phase slips at a
particular point along the ring.Comment: 7 pages, 6 figures, Revtex 4.0 styl
Thermodynamic inequalities in superfluid
We investigate general thermodynamic stability conditions for the superfluid.
This analysis is performed in an extended space of thermodynamic variables
containing (along with the usual thermodynamic coordinates such as pressure and
temperature) superfluid velocity and momentum density. The stability conditions
lead to thermodynamic inequalities which replace the Landau superfluidity
criterion at finite temperatures.Comment: 7 pages, 1 figur
Negative differential conductance in quantum dots in theory and experiment
Experimental results for sequential transport through a lateral quantum dot
in the regime of spin blockade induced by spin dependent tunneling are compared
with theoretical results obtained by solving a master equation for independent
electrons. Orbital and spin effects in electron tunneling in the presence of a
perpendicular magnetic field are identified and discussed in terms of the
Fock-Darwin spectrum with spin. In the nonlinear regime, a regular pattern of
negative differential conductances is observed. Electrical asymmetries in
tunnel rates and capacitances must be introduced in order to account for the
experimental findings. Fast relaxation of the excited states in the quantum dot
have to be assumed, in order to explain the absence of certain structures in
the transport spectra.Comment: 4 pages, 4 figure
Efficient simulation of quantum evolution using dynamical coarse-graining
A novel scheme to simulate the evolution of a restricted set of observables
of a quantum system is proposed. The set comprises the spectrum-generating
algebra of the Hamiltonian. The idea is to consider a certain open-system
evolution, which can be interpreted as a process of weak measurement of the
distinguished observables performed on the evolving system of interest. Given
that the observables are "classical" and the Hamiltonian is moderately
nonlinear, the open system dynamics displays a large time-scales separation
between the dephasing of the observables and the decoherence of the evolving
state in the basis of the generalized coherent states (GCS), associated with
the spectrum-generating algebra. The time scale separation allows the unitary
dynamics of the observables to be efficiently simulated by the open-system
dynamics on the intermediate time-scale.The simulation employs unraveling of
the corresponding master equations into pure state evolutions, governed by the
stochastic nonlinear Schroedinger equantion (sNLSE). It is proved that GCS are
globally stable solutions of the sNLSE, if the Hamilonian is linear in the
algebra elements.Comment: The version submitted to Phys. Rev. A, 28 pages, 3 figures, comments
are very welcom
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