550 research outputs found
Evaluation of the synoptic and mesoscale predictive capabilities of a mesoscale atmospheric simulation system
The overall performance characteristics of a limited area, hydrostatic, fine (52 km) mesh, primitive equation, numerical weather prediction model are determined in anticipation of satellite data assimilations with the model. The synoptic and mesoscale predictive capabilities of version 2.0 of this model, the Mesoscale Atmospheric Simulation System (MASS 2.0), were evaluated. The two part study is based on a sample of approximately thirty 12h and 24h forecasts of atmospheric flow patterns during spring and early summer. The synoptic scale evaluation results benchmark the performance of MASS 2.0 against that of an operational, synoptic scale weather prediction model, the Limited area Fine Mesh (LFM). The large sample allows for the calculation of statistically significant measures of forecast accuracy and the determination of systematic model errors. The synoptic scale benchmark is required before unsmoothed mesoscale forecast fields can be seriously considered
Multi-photon transitions between energy levels in a current-biased Josephson tunnel junction
The escape of a small current-biased Josephson tunnel junction from the zero
voltage state in the presence of weak microwave radiation is investigated
experimentally at low temperatures. The measurements of the junction switching
current distribution indicate the macroscopic quantum tunneling of the phase
below a cross-over temperature of . At
temperatures below we observe both single-photon and
\emph{multi-photon} transitions between the junction energy levels by applying
microwave radiation in the frequency range between and to the junction. These observations reflect the anharmonicity of the
junction potential containing only a small number of levels.Comment: 4 pages, 5 figure
Multiply-connected Bose-Einstein condensed alkali gases: Current-carrying states and their decay
The ability to support metastable current-carrying states in
multiply-connected settings is one of the prime signatures of superfluidity.
Such states are investigated theoretically for the case of trapped Bose
condensed alkali gases, particularly with regard to the rate at which they
decay via thermal fluctuations. The lifetimes of metastable currents can be
either longer or shorter than experimental time-scales. A scheme for the
experimental detection of metastable states is sketched.Comment: 4 pages, including 1 figure (REVTEX
Mode-Locking in Quantum-Hall-Effect Point Contacts
We study the effect of an ac drive on the current-voltage (I-V)
characteristics of a tunnel junction between two fractional Quantum Hall fluids
at filling an odd integer. Within the chiral Luttinger liquid model
of edge states, the point contact dynamics is described by a driven damped
quantum mechanical pendulum. In a semi-classical limit which ignores electron
tunnelling, this model exhibits mode-locking, which corresponds to current
plateaus in the I-V curve at integer multiples of , with
the ac drive angular frequency. By analyzing the full quantum model at
non-zero using perturbative and exact methods, we study the effect of
quantum fluctuation on the mode-locked plateaus. For quantum
fluctuations smear completely the plateaus, leaving no trace of the ac drive.
For smeared plateaus remain in the I-V curve, but are not
centered at the currents . For rounded plateaus
centered around the quantized current values are found. The possibility of
using mode locking in FQHE point contacts as a current-to-frequency standard is
discussed.Comment: 12 pages, 8 figures, minor change
Novel Bose-Einstein Interference in the Passage of a Fast Particle in a Dense Medium
When an energetic particle collides coherently with many medium particles at
high energies, the Bose-Einstein symmetry with respect to the interchange of
the exchanged virtual bosons leads to a destructive interference of the Feynman
amplitudes in most regions of the phase space but a constructive interference
in some other regions of the phase space. As a consequence, the recoiling
medium particles have a tendency to come out collectively along the direction
of the incident fast particle, each carrying a substantial fraction of the
incident longitudinal momentum. Such an interference appearing as collective
recoils of scatterers along the incident particle direction may have been
observed in angular correlations of hadrons associated with a high-
trigger in high-energy AuAu collisions at RHIC.Comment: 10 pages, 2 figures, invited talk presented at the 35th Symposium on
Nuclear Physics, Cocoyoc, Mexico, January 3, 2012, to be published in IOP
Conference Serie
Voltage rectification by a SQUID ratchet
We argue that the phase across an asymmetric dc SQUID threaded by a magnetic
flux can experience an effective ratchet (periodic and asymmetric) potential.
Under an external ac current, a rocking ratchet mechanism operates whereby one
sign of the time derivative of the phase is favored. We show that there exists
a range of parameters in which a fixed sign (and, in a narrower range, even a
fixed value) of the average voltage across the ring occurs, regardless of the
sign of the external current dc component.Comment: 4 pages, 4 EPS figures, uses psfig.sty. Revised version, to appear in
Physical Review Letters (26 August 1996
Effect of granularity on the insulator-superconductor transition in ultrathin Bi films
We have studied the insulator-superconductor transition (IST) by tuning the
thickness in quench-condensed films. The resistive transitions of the
superconducting films are smooth and can be considered to represent
"homogeneous" films. The observation of an IST very close to the quantum
resistance for pairs, on several substrates supports
this idea. The relevant length scales here are the localization length, and the
coherence length. However, at the transition, the localization length is much
higher than the superconducting coherence length, contrary to expectation for a
"homogeneous" transition. This suggests the invalidity of a purely fermionic
model for the transition. Furthermore, the current-voltage characteristics of
the superconducting films are hysteretic, and show the films to be granular.
The relevant energy scales here are the Josephson coupling energy and the
charging energy. However, Josephson coupling energies () and the charging
energies () at the IST, they are found to obey the relation .
This is again contrary to expectation, for the IST in a granular or
inhomogeneous, system. Hence, a purely bosonic picture of the transition is
also inconsistent with our observations. We conclude that the IST observed in
our experiments may be either an intermediate case between the fermioinc and
bosonic mechanisms, or in a regime of charge and vortex dynamics for which a
quantitative analysis has not yet been done.Comment: accepted in Physical Review
Single-vortex-induced voltage steps in Josephson-junction arrays
We have numerically and analytically studied ac+dc driven Josephson-junction
arrays with a single vortex or with a single vortex-antivortex pair present. We
find single-vortex steps in the voltage versus current characteristics (I-V) of
the array. They correspond microscopically to a single vortex phase-locked to
move a fixed number of plaquettes per period of the ac driving current. In
underdamped arrays we find vortex motion period doubling on the steps. We
observe subharmonic steps in both underdamped and overdamped arrays. We
successfully compare these results with a phenomenological model of vortex
motion with a nonlinear viscosity. The I-V of an array with a vortex-antivortex
pair displays fractional voltage steps. A possible connection of these results
to present day experiments is also discussed.Comment: 10 pages double sided with figures included in the text. To appear in
Journal of Physics, Condensed Matte
Diffusion controlled initial recombination
This work addresses nucleation rates in systems with strong initial
recombination. Initial (or `geminate') recombination is a process where a
dissociated structure (anion, vortex, kink etc.) recombines with its twin
brother (cation, anti-vortex, anti-kink) generated in the same nucleation
event. Initial recombination is important if there is an asymptotically
vanishing interaction force instead of a generic saddle-type activation
barrier. At low temperatures, initial recombination strongly dominates
homogeneous recombination. In a first part, we discuss the effect in one-,
two-, and three-dimensional diffusion controlled systems with spherical
symmetry. Since there is no well-defined saddle, we introduce a threshold which
is to some extent arbitrary but which is restricted by physically reasonable
conditions. We show that the dependence of the nucleation rate on the specific
choice of this threshold is strongest for one-dimensional systems and decreases
in higher dimensions. We discuss also the influence of a weak driving force and
show that the transport current is directly determined by the imbalance of the
activation rate in the direction of the field and the rate against this
direction. In a second part, we apply the results to the overdamped sine-Gordon
system at equilibrium. It turns out that diffusive initial recombination is the
essential mechanism which governs the equilibrium kink nucleation rate. We
emphasize analogies between the single particle problem with initial
recombination and the multi-dimensional kink-antikink nucleation problem.Comment: LaTeX, 11 pages, 1 ps-figures Extended versio
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