404 research outputs found
Fast Algorithms For Josephson Junction Arrays : Bus--bars and Defects
We critically review the fast algorithms for the numerical study of
two--dimensional Josephson junction arrays and develop the analogy of such
systems with electrostatics. We extend these procedures to arrays with
bus--bars and defects in the form of missing bonds. The role of boundaries and
of the guage choice in determing the Green's function of the system is
clarified. The extension of the Green's function approach to other situations
is also discussed.Comment: Uuencoded 1 Revtex file (11 Pages), 3 Figures : Postscript Uuencode
Structure and Strength of Dislocation Junctions: An Atomic Level Analysis
The quasicontinuum method is used to simulate three-dimensional
Lomer-Cottrell junctions both in the absence and in the presence of an applied
stress. The simulations show that this type of junction is destroyed by an
unzipping mechanism in which the dislocations that form the junction are
gradually pulled apart along the junction segment. The calculated critical
stress needed for breaking the junction is comparable to that predicted by line
tension models. The simulations also demonstrate a strong influence of the
initial dislocation line directions on the breaking mechanism, an effect that
is neglected in the macroscopic treatment of the hardening effect of junctions.Comment: 4 pages, 3 figure
Two-fluid dynamics for a Bose-Einstein condensate out of local equilibrium with the non-condensate
We extend our recent work on the two-fluid hydrodynamics of a Bose-condensed
gas by including collisions involving both condensate and non-condensate atoms.
These collisions are essential for establishing a state of local thermodynamic
equilibrium between the condensate and non-condensate. Our theory is more
general than the usual Landau two-fluid theory, to which it reduces in the
appropriate limit, in that it allows one to describe situations in which a
state of complete local equilibrium between the two components has not been
reached. The exchange of atoms between the condensate and non-condensate is
associated with a new relaxational mode of the gas.Comment: 4 pages, revtex, 1 postscript figure, Fig.1 has been correcte
Finite Sized Atomistic Simulations of Screw Dislocations
The interaction of screw dislocations with an applied stress is studied using
atomistic simulations in conjunction with a continuum treatment of the role
played by the far field boundary condition. A finite cell of atoms is used to
consider the response of dislocations to an applied stress and this introduces
an additional force on the dislocation due to the presence of the boundary.
Continuum mechanics is used to calculate the boundary force which is
subsequently accounted for in the equilibrium condition for the dislocation.
Using this formulation, the lattice resistance curve and the associated Peierls
stress are calculated for screw dislocations in several close packed metals. As
a concrete example of the boundary force method, we compute the bow out of a
pinned screw dislocation; the line-tension of the dislocation is calculated
from the results of the atomistic simulations using a variational principle
that explicitly accounts for the boundary force.Comment: LaTex, 20 pages, 11 figure
Dynamics of An Underdamped Josephson Junction Ladder
We show analytically that the dynamical equations for an underdamped ladder
of coupled small Josephson junctions can be approximately reduced to the
discrete sine-Gordon equation. As numerical confirmation, we solve the coupled
Josephson equations for such a ladder in a magnetic field. We obtain
discrete-sine-Gordon-like IV characteristics, including a flux flow and a
``whirling'' regime at low and high currents, and voltage steps which represent
a lock-in between the vortex motion and linear ``phasons'', and which are
quantitatively predicted by a simple formula. At sufficiently high anisotropy,
the fluxons on the steps propagate ballistically.Comment: 11pages, latex, no figure
Charge-Vortex Duality in Double-Layered Josephson Junction Arrays
A system of two parallel Josephson junction arrays coupled by interlayer
capacitances is considered in the situation where one layer is in the
vortex-dominated and the other in the charge-dominated regime. This system
shows a symmetry (duality) of the relevant degrees of freedom, i.e. the
vortices in one layer and the charges in the other. In contrast to single-layer
arrays both contribute to the kinetic energy. The charges feel the magnetic
field created by vortices, and, vice versa, the vortices feel a gauge field
created by charges. For long-range interaction of the charges the system
exhibits two Berezinskii-Kosterlitz-Thouless transitions, one for vortices and
another one for charges. The interlayer capacitance suppresses both transition
temperatures. The charge-unbinding transition is suppressed already for
relatively weak coupling, while the vortex-unbinding transition is more robust.
The shift of the transition temperature for vortices is calculated in the
quasi-classical approximation for arbitrary relations between the capacitances
(both weak and strong coupling).Comment: 12 pages, Revtex 3.
Kinetic Theory of Collective Excitations and Damping in Bose-Einstein Condensed Gases
We calculate the frequencies and damping rates of the low-lying collective
modes of a Bose-Einstein condensed gas at nonzero temperature. We use a complex
nonlinear Schr\"odinger equation to determine the dynamics of the condensate
atoms, and couple it to a Boltzmann equation for the noncondensate atoms. In
this manner we take into account both collisions between
noncondensate-noncondensate and condensate-noncondensate atoms. We solve the
linear response of these equations, using a time-dependent gaussian trial
function for the condensate wave function and a truncated power expansion for
the deviation function of the thermal cloud. As a result, our calculation turns
out to be characterized by two dimensionless parameters proportional to the
noncondensate-noncondensate and condensate-noncondensate mean collision times.
We find in general quite good agreement with experiment, both for the
frequencies and damping of the collective modes.Comment: 10 pages, 8 figure
Dislocations and the critical endpoint of the melting line of vortex line lattices
We develop a theory for dislocation-mediated structural transitions in the
vortex lattice which allows for a unified description of phase transitions
between the three phases, the elastic vortex glass, the amorphous vortex glass,
and the vortex liquid, in terms of a free energy functional for the dislocation
density. The origin of a critical endpoint of the melting line at high magnetic
fields, which has been recently observed experimentally, is explained.Comment: 4 pages, 1 figur
The Arabian Sea as a high-nutrient, low-chlorophyll region during the late Southwest Monsoon
© The Authors, 2010. This article is distributed under the terms of the Creative Commons Attribution 3.0 License. The definitive version was published in Biogeosciences 7 (2010): 2091-2100, doi:10.5194/bg-7-2091-2010.Extensive observations were made during the late Southwest Monsoon of 2004 over the Indian and Omani shelves, and along a transect that extended from the southern coast of Oman to the central west coast of India, tracking the southern leg of the US JGOFS expedition (1994–1995) in the west. The data are used, in conjunction with satellite-derived data, to investigate long-term trends in chlorophyll and sea surface temperature, indicators of upwelling intensity, and to understand factors that control primary production (PP) in the Arabian Sea, focussing on the role of iron. Our results do not support an intensification of upwelling in the western Arabian Sea, reported to have been caused by the decline in the winter/spring Eurasian snow cover since 1997. We also noticed, for the first time, an unexpected development of high-nutrient, low-chlorophyll condition off the southern Omani coast. This feature, coupled with other characteristics of the system, such as a narrow shelf and relatively low iron concentrations in surface waters, suggest a close similarity between the Omani upwelling system and the Peruvian and California upwelling systems, where PP is limited by iron. Iron limitation of PP may complicate simple relationship between upwelling and PP assumed by previous workers, and contribute to the anomalous offshore occurrence of the most severe oxygen (O2) depletion in the region. Over the much wider Indian shelf, which experiences large-scale bottom water O2-depletion in summer, adequate iron supply from reducing bottom-waters and sediments seems to support moderately high PP; however, such production is restricted to the thin, oxygenated surface layer, probably because of the unsuitability of the O2-depleted environment for the growth of oxygenic photosynthesizers.Financial support was provided
by CSIR through the Network Project CMM0009 to SWAN and
by NSF through OCE-0327227S to JWM
Modelling avalanches in martensites
Solids subject to continuous changes of temperature or mechanical load often
exhibit discontinuous avalanche-like responses. For instance, avalanche
dynamics have been observed during plastic deformation, fracture, domain
switching in ferroic materials or martensitic transformations. The statistical
analysis of avalanches reveals a very complex scenario with a distinctive lack
of characteristic scales. Much effort has been devoted in the last decades to
understand the origin and ubiquity of scale-free behaviour in solids and many
other systems. This chapter reviews some efforts to understand the
characteristics of avalanches in martensites through mathematical modelling.Comment: Chapter in the book "Avalanches in Functional Materials and
Geophysics", edited by E. K. H. Salje, A. Saxena, and A. Planes. The final
publication is available at Springer via
http://dx.doi.org/10.1007/978-3-319-45612-6_
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