564 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
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
Decay of Superflow Confined in Thin Torus: A Realization of Tunneling Quantum Fields
The quantum nucleation of phase slips in neutral superfluids confined in a
thin torus is investigated by means of the collective coordinate method. We
have devised, with numerical justification, a certain collective coordinate to
describe the quantum nucleation process of a phase slip. Considering the
quantum fluctuation around the local minimum of the action, we calculate the
effective mass of the phase slip. Due to the coherence of the condensate
throughout the torus, the effective mass is proportional to the circumference L
of the torus, and the decay rate has a strong exponential L-dependence.Comment: 4 pages, 2 figures, REVTe
Critical currents in Josephson junctions with macroscopic defects
The critical currents in Josephson junctions of conventional superconductors
with macroscopic defects are calculated for different defect critical current
densities as a function of the magnetic field. We also study the evolution of
the different modes with the defect position, at zero external field. We study
the stability of the solutions and derive simple arguments, that could help the
defect characterization. In most cases a reentrant behavior is seen, where both
a maximum and a minimum current exist.Comment: 17 pages with 16 figures, submitted to Supercond. Sci. Techno
Finite Size and Current Effects on IV Characteristics of Josephson Junction Arrays
The effects of finite size and of finite current on the current-voltage
characteristics of Josephson junction arrays is studied both theoretically and
by numerical simulations. The cross-over from non-linear to linear behavior at
low temperature is shown to be a finite size effect and the non-linear behavior
at higher temperature, , is shown to be a finite current effect.
These are argued to result from competition between the three length scales
characterizing the system. The importance of boundary effects is discussed and
it is shown that these may dominate the behavior in small arrays.Comment: 5 pages, figures included, to appear in PR
Microscopic self-consistent theory of Josephson junctions including dynamical electron correlations
We formulate a fully self-consistent, microscopic model to study the
retardation and correlation effects of the barrier within a Josephson junction.
The junction is described by a series of planes, with electronic correlation
included through a local self energy for each plane. We calculate current-phase
relationships for various junctions, which include non-magnetic impurities in
the barrier region, or an interfacial scattering potential. Our results
indicate that the linear response of the supercurrent to phase across the
barrier region is a good, but not exact indicator of the critical current. Our
calculations of the local density of states show the current-carrying Andreev
bound states and their energy evolution with the phase difference across the
junction.
We calculate the figure of merit for a Josephson junction, which is the
product of the critical current, Ic, and the normal state resistance, R(N), for
junctions with different barrier materials. The normal state resistance is
calculated using the Kubo formula, for a system with zero current flow and no
superconducting order. Semiclassical calculations would predict that these two
quantities are determined by the transmission probabilities of electrons in
such a way that the product is constant for a given superconductor at fixed
temperature. Our self-consistent solutions for different types of barrier
indicate that this is not the case. We suggest some forms of barrier which
could increase the Ic.R(N) product, and hence improve the frequency response of
a Josephson device.Comment: 46 pages, 21 figure
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
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
Fate of the Josephson effect in thin-film superconductors
The dc Josephson effect refers to the dissipationless electrical current --
the supercurrent -- that can be sustained across a weak link connecting two
bulk superconductors. This effect is a probe of the fundamental nature of the
superconducting state. Here, we analyze the case of two superconducting thin
films connected by a point contact. Remarkably, the Josephson effect is absent
at nonzero temperature, and the resistance across the contact is nonzero.
Moreover, the point contact resistance is found to vary with temperature in a
nearly activated fashion, with a UNIVERSAL energy barrier determined only by
the superfluid stiffness characterizing the films, an angle characterizing the
geometry, and whether or not the Coulomb interaction between Cooper pairs is
screened. This behavior reflects the subtle nature of the superconductivity in
two-dimensional thin films, and should be testable in detail by future
experiments.Comment: 16 + 8 pages. 1 figure, 1 tabl
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