27 research outputs found
Evidence for directed percolation universality at the onset of spatiotemporal intermittency in coupled circle maps
We consider a lattice of coupled circle maps, a model arising naturally in
descriptions of solid state phenomena such as Josephson junction arrays. We
find that the onset of spatiotemporal intermittency (STI) in this system is
analogous to directed percolation (DP), with the transition being to an unique
absorbing state for low nonlinearities, and to weakly chaotic absorbing states
for high nonlinearities. We find that the complete set of static exponents and
spreading exponents at all critical points match those of DP very convincingly.
Further, hyperscaling relations are fulfilled, leading to independent controls
and consistency checks of the values of all the critical exponents. These
results lend strong support to the conjecture that the onset of STI in
deterministic models belongs to the DP universality class.Comment: Submitted to Physical Review
Magnetic-field dependence of dynamical vortex response in two-dimensional Josephson junction arrays and superconducting films
The dynamical vortex response of a two-dimensional array of the resistively
shunted Josephson junctions in a perpendicular magnetic field is inferred from
simulations. It is found that, as the magnetic field is increased at a fixed
temperature, the response crosses over from normal to anomalous, and that this
crossover can be characterized by a single dimensionless parameter. It is
described how this crossover should be reflected in measurements of the complex
impedance for Josephson junction arrays and superconducting films.Comment: 4 pages including 5 figures in two columns, final versio
Breakdown of universality in transitions to spatiotemporal chaos
We show that the transition from laminar to active behavior in extended chaotic systems can vary from a continuous transition in the universality class of directed percolation with infinitely many absorbing states to what appears as a first-order transition. The latter occurs when finite lifetime nonchaotic structures, called "solitons," dominate the dynamics. We illustrate this scenario in an extension of the deterministic Chaté-Manneville coupled map lattice model and in a soliton including variant of the stochastic Domany-Kinzel cellular automaton
Flux-noise spectra around the Kosterlitz-Thouless transition for two-dimensional superconductors
The flux-noise spectra around the Kosterlitz-Thouless transition are obtained
from simulations of the two-dimensional resistively shunted junction model. In
particular the dependence on the distance between the pick-up coil and the
sample is investigated. The typical experimental situation corresponds to the
large- limit and a simple relation valid in this limit between the complex
impedance and the noise spectra is clarified. Features, which distinguish
between the large- and small- limit, are identified and the possibility of
observing these features in experiments is discussed.Comment: 12 pages including 8 figures, submitted to Phys. Rev.
Vortex Fluctuations in High-Tc Films: Flux Noise Spectrum and Complex Impedance
The flux noise spectrum and complex impedance for a 500 {\AA} thick YBCO film
are measured and compared with predictions for two dimensional vortex
fluctuations. It is verified that the complex impedance and the flux noise
spectra are proportional to each other, that the logarithm of the flux noise
spectra for different temperatures has a common tangent with slope , and that the amplitude of the noise decreases as , where is
the height above the film at which the magnetic flux is measured. A crossover
from normal to anomalous vortex diffusion is indicated by the measurements and
is discussed in terms of a two-dimensional decoupling.Comment: 5 pages including 4 figures in two columns, to appear in Phys. Rev.
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Dynamic Scaling of Magnetic Flux Noise Near the KTB Transition in Overdamped Josephson Junction Arrays
We have used a dc Superconducting QUantum Interference Device to measure the
magnetic flux noise generated by the equilibrium vortex density fluctuations
associated with the Kosterlitz-Thouless-Berezinskii (KTB) transition in an
overdamped Josephson junction array. At temperatures slightly above the KTB
transition temperature, the noise is white for and scales as
for . Here , where is the correlation
length and is the dynamic exponent. Moreover, when all frequencies are
scaled by , data for different temperatures and frequencies collapse on
to a single curve. In addition, we have extracted the dynamic exponent and
found .Comment: 5 pages, LaTeX (REVTeX) format, requires epsfig and amstex style
files. 3 figures included. Tentatively scheduled for publication in Physical
Review Letters, 18 March, 199
Fluctuation-dissipation theorem and flux noise in overdamped Josephson junction arrays
The form of the fluctuation-dissipation theorem for a resistively shunted
Josephson juction array is derived with the help of the method which
explicitely takes into acoount screening effects. This result is used to
express the flux noise power spectrum in terms of frequency dependent sheet
impedance of the array. The relation between noise amplitude and parameters of
the detection coil is analysed for the simplest case of a single-loop coil.Comment: ReVTeX, 8 page
Langevin Simulations of Two Dimensional Vortex Fluctuations: Anomalous Dynamics and a New -exponent
The dynamics of two dimensional (2D) vortex fluctuations are investigated
through simulations of the 2D Coulomb gas model in which vortices are
represented by soft disks with logarithmic interactions. The simulations
trongly support a recent suggestion that 2D vortex fluctuations obey an
intrinsic anomalous dynamics manifested in a long range 1/t-tail in the vortex
correlations. A new non-linear IV-exponent a, which is different from the
commonly used AHNS exponent, a_AHNS and is given by a = 2a_AHNS - 3, is
confirmed by the simulations. The results are discussed in the context of
earlier simulations, experiments and a phenomenological description.Comment: Submitted to PRB, RevTeX format, 28 pages and 13 figures, figures in
postscript format are available at http://www.tp.umu.se/~holmlund/papers.htm
Flux noise in high-temperature superconductors
Spontaneously created vortex-antivortex pairs are the predominant source of
flux noise in high-temperature superconductors. In principle, flux noise
measurements allow to check theoretical predictions for both the distribution
of vortex-pair sizes and for the vortex diffusivity. In this paper the
flux-noise power spectrum is calculated for the highly anisotropic
high-temperature superconductor Bi-2212, both for bulk crystals and for
ultra-thin films. The spectrum is basically given by the Fourier transform of
the temporal magnetic-field correlation function. We start from a
Berezinskii-Kosterlitz-Thouless type theory and incorporate vortex diffusion,
intra-pair vortex interaction, and annihilation of pairs by means of a
Fokker-Planck equation to determine the noise spectrum below and above the
superconducting transition temperature. We find white noise at low frequencies
omega and a spectrum proportional to 1/omega^(3/2) at high frequencies. The
cross-over frequency between these regimes strongly depends on temperature. The
results are compared with earlier results of computer simulations.Comment: 9 pages, 4 PostScript figures, to be published in Phys. Rev.
Vortex dynamics for two-dimensional XY models
Two-dimensional XY models with resistively shunted junction (RSJ) dynamics
and time dependent Ginzburg-Landau (TDGL) dynamics are simulated and it is
verified that the vortex response is well described by the Minnhagen
phenomenology for both types of dynamics. Evidence is presented supporting that
the dynamical critical exponent in the low-temperature phase is given by
the scaling prediction (expressed in terms of the Coulomb gas temperature
and the vortex renormalization given by the dielectric constant
) both for RSJ and TDGL
and that the nonlinear IV exponent a is given by a=z+1 in the low-temperature
phase. The results are discussed and compared with the results of other recent
papers and the importance of the boundary conditions is emphasized.Comment: 21 pages including 15 figures, final versio