1,462 research outputs found
Topological Defects from First Order Gauge Theory Phase Transitions
We investigate the mechanism by which topological defects form in first order
phase transitions with a charged order parameter. We show how thick
superconductor vortices and heavy cosmic strings form by trapping of magnetic
flux. In an external magnetic field, intermediate objects such as strips and
membranes of magnetic flux and chains of single winding defects are produced.
At non-zero temperature, a variety of spontaneous defects of different winding
numbers arise. In cosmology, our results mean that the magnetic flux thermal
fluctuations get trapped in a primordial multi-tension string network. The
mechanism may also apply to the production of cosmic-like strings in brane
collisions. In a thin type-I superconductor film, flux strips are found to be
meta-stable while thick vortices are stable up to some critical value of the
winding number which increases with the thickness of the film. In addition, a
non-dissipative Josephson-like current is obtained across the strips of
quantized magnetic flux.Comment: Corrections made on sections 4,5. Higher quality figures in published
versio
Superconducting states and depinning transitions of Josephson ladders
We present analytical and numerical studies of pinned superconducting states
of open-ended Josephson ladder arrays, neglecting inductances but taking edge
effects into account. Treating the edge effects perturbatively, we find
analytical approximations for three of these superconducting states -- the
no-vortex, fully-frustrated and single-vortex states -- as functions of the dc
bias current and the frustration . Bifurcation theory is used to derive
formulas for the depinning currents and critical frustrations at which the
superconducting states disappear or lose dynamical stability as and are
varied. These results are combined to yield a zero-temperature stability
diagram of the system with respect to and . To highlight the effects of
the edges, we compare this dynamical stability diagram to the thermodynamic
phase diagram for the infinite system where edges have been neglected. We
briefly indicate how to extend our methods to include self-inductances.Comment: RevTeX, 22 pages, 17 figures included; Errata added, 1 page, 1
corrected figur
Superconductors with Topological Order and their Realization in Josephson Junction Arrays
We will describe a new superconductivity mechanism, proposed by the authors
in [1], which is based on a topologically ordered ground state rather than on
the usual Landau mechanism of spontaneous symmetry breaking. Contrary to anyon
superconductivity it works in any dimension and it preserves P-and
T-invariance. In particular we will discuss the low-energy effective field
theory, what would be the Landau-Ginzburg formulation for conventional
superconductors.Comment: invited review, to appear in "Superconductivity Research Advances",
Nova Publishers, 32 page
Interaction of two systems with saddle-node bifurcations on invariant circles. I. Foundations and the mutualistic case
The saddle-node bifurcation on an invariant circle (SNIC) is one of the
codimension-one routes to creation or destruction of a periodic orbit in a
continuous-time dynamical system. It governs the transition from resting
behaviour to periodic spiking in many class I neurons, for example. Here, as a
first step towards theory of networks of such units the effect of weak coupling
between two systems with a SNIC is analysed. Two crucial parameters of the
coupling are identified, which we call \delta_1 and \delta_2. Global
bifurcation diagrams are obtained here for the "mutualistic" case \delta_1
\delta_2 > 0. According to the parameter regime, there may coexist resting and
periodic attractors, and there can be quasiperiodic attractors of torus or
cantorus type, making the behaviour of even such a simple system quite
non-trivial. In a second paper we will analyse the mixed case \delta_1 \delta_2
< 0 and summarise the conclusions of this study.Comment: 37 pages, 27 figure
Invariant submanifold for series arrays of Josephson junctions
We study the nonlinear dynamics of series arrays of Josephson junctions in
the large-N limit, where N is the number of junctions in the array. The
junctions are assumed to be identical, overdamped, driven by a constant bias
current and globally coupled through a common load. Previous simulations of
such arrays revealed that their dynamics are remarkably simple, hinting at the
presence of some hidden symmetry or other structure. These observations were
later explained by the discovery of (N - 3) constants of motion, each choice of
which confines the resulting flow in phase space to a low-dimensional invariant
manifold. Here we show that the dimensionality can be reduced further by
restricting attention to a special family of states recently identified by Ott
and Antonsen. In geometric terms, the Ott-Antonsen ansatz corresponds to an
invariant submanifold of dimension one less than that found earlier. We derive
and analyze the flow on this submanifold for two special cases: an array with
purely resistive loading and another with resistive-inductive-capacitive
loading. Our results recover (and in some instances improve) earlier findings
based on linearization arguments.Comment: 10 pages, 6 figure
Synchronized Switching in a Josephson Junction Crystal
We consider a superconducting coplanar waveguide resonator where the central
conductor is interrupted by a series of uniformly spaced Josephson junctions.
The device forms an extended medium that is optically nonlinear on the single
photon level with normal modes that inherit the full nonlinearity of the
junctions but are nonetheless accessible via the resonator ports. For specific
plasma frequencies of the junctions a set of normal modes clusters in a narrow
band and eventually become entirely degenerate. Upon increasing the intensity
of a red detuned drive on these modes, we observe a sharp and synchronized
switching from low occupation quantum states to high occupation classical
fields, accompanied by a pronounced jump from low to high output intensity.Comment: 13 pages, 5 figure
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