178 research outputs found
Kinks in the Presence of Rapidly Varying Perturbations
Dynamics of sine-Gordon kinks in the presence of rapidly varying periodic
perturbations of different physical origins is described analytically and
numerically. The analytical approach is based on asymptotic expansions, and it
allows to derive, in a rigorous way, an effective nonlinear equation for the
slowly varying field component in any order of the asymptotic procedure as
expansions in the small parameter , being the frequency
of the rapidly varying ac driving force. Three physically important examples of
such a dynamics, {\em i.e.}, kinks driven by a direct or parametric ac force,
and kinks on rotating and oscillating background, are analysed in detail. It is
shown that in the main order of the asymptotic procedure the effective equation
for the slowly varying field component is {\em a renormalized sine-Gordon
equation} in the case of the direct driving force or rotating (but phase-locked
to an external ac force) background, and it is {\em the double sine-Gordon
equation} for the parametric driving force. The properties of the kinks
described by the renormalized nonlinear equations are analysed, and it is
demonstrated analytically and numerically which kinds of physical phenomena may
be expected in dealing with the renormalized, rather than the unrenormalized,
nonlinear dynamics. In particular, we predict several qualitatively new effects
which include, {\em e.g.}, the perturbation-inducedComment: New copy of the paper of the above title to replace the previous one,
lost in the midst of the bulletin board. RevTeX 3.
A classical statistical model for distributions of escape events in swept-bias Josephson junctions
We have developed a model for experiments in which the bias current applied
to a Josephson junction is slowly increased from zero until the junction
switches from its superconducting zero-voltage state, and the bias value at
which this occurs is recorded. Repetition of such measurements yields
experimentally determined probability distributions for the bias current at the
moment of escape. Our model provides an explanation for available data on the
temperature dependence of these escape peaks. When applied microwaves are
included we observe an additional peak in the escape distributions and
demonstrate that this peak matches experimental observations. The results
suggest that experimentally observed switching distributions, with and without
applied microwaves, can be understood within classical mechanics and may not
exhibit phenomena that demand an exclusively quantum mechanical interpretation.Comment: Eight pages, eight figure
Investigation of resonant and transient phenomena in Josephson junction flux qubits
We present an analytical and computational study of resonances and transient
responses in a classical Josephson junction system. A theoretical basis for
resonances in a superconducting loop with three junctions is presented,
outlining both the direct relationship between the dynamics of single- and
multi-junction systems, and the direct relationships between observations of
the classical counterparts to Rabi oscillations, Ramsey fringes, and spin echo
oscillations in this class of systems. We show simulations data along with
analytical analyses of the classical model, and the results are related to
previously reported experiments conducted on three junction loops. We further
investigate the effect of off-resonant microwave perturbations to, e.g., the
Rabi-type response of the Josephson system, and we relate this response back to
the nonlinear and multi-valued resonance behavior previously reported for a
single Josephson junction. The close relationships between single and
multi-junction behavior demonstrates the underlying dynamical mechanism for a
whole class of classical counterparts to expected quantum mechanical
observations in a variety of systems; namely the resonant and transient
behavior of a particle in an anharmonic potential well with subsequent escape.Comment: 11 pages, seven figure
Phase-Locking of Vortex Lattices Interacting with Periodic Pinning
We examine Shapiro steps for vortex lattices interacting with periodic
pinning arrays driven by AC and DC currents. The vortex flow occurs by the
motion of the interstitial vortices through the periodic potential generated by
the vortices that remain pinned at the pinning sites. Shapiro steps are
observed for fields B_{\phi} < B < 2.25B_{\phi} with the most pronouced steps
occuring for fields where the interstitial vortex lattice has a high degree of
symmetry. The widths of the phase-locked current steps as a function of the
magnitude of the AC driving are found to follow a Bessel function in agreement
with theory.Comment: 5 pages 5 postscript figure
Charge Transport Transitions and Scaling in Disordered Arrays of Metallic Dots
We examine the charge transport through disordered arrays of metallic dots
using numerical simulations. We find power law scaling in the current-voltage
curves for arrays containing no voids, while for void-filled arrays charge
bottlenecks form and a single scaling is absent, in agreement with recent
experiments. In the void-free case we also show that the scaling exponent
depends on the effective dimensionality of the system. For increasing applied
drives we find a transition from 2D disordered filamentary flow near threshold
to a 1D smectic flow which can be identified experimentally using
characteristics in the transport curves and conduction noise.Comment: 4 pages, 4 postscript figure
First-principles study of the energetics of charge and cation mixing in U_{1-x} Ce_x O_2
The formalism of electronic density-functional-theory, with Hubbard-U
corrections (DFT+U), is employed in a computational study of the energetics of
U_{1-x} Ce_x O_2 mixtures. The computational approach makes use of a procedure
which facilitates convergence of the calculations to multiple self-consistent
DFT+U solutions for a given cation arrangement, corresponding to different
charge states for the U and Ce ions in several prototypical cation
arrangements. Results indicate a significant dependence of the structural and
energetic properties on the nature of both charge and cation ordering. With the
effective Hubbard-U parameters that reproduce well the measured
oxidation-reduction energies for urania and ceria, we find that charge transfer
between U(IV) and Ce(IV) ions, leading to the formation of U(V) and Ce(III),
gives rise to an increase in the mixing energy in the range of 4-14 kJ/mol of
formula unit, depending on the nature of the cation ordering. The results
suggest that although charge transfer between uranium and cerium ions is
disfavored energetically, it is likely to be entropically stabilized at the
high temperatures relevant to the processing and service of urania-based solid
solutions.Comment: 8 pages, 6 figure
Effective temperature in driven vortex lattices with random pinning
We study numerically correlation and response functions in non-equilibrium
driven vortex lattices with random pinning. From a generalized
fluctuation-dissipation relation we calculate an effective transverse
temperature in the fluid moving phase. We find that the effective temperature
decreases with increasing driving force and becomes equal to the equilibrium
melting temperature when the dynamic transverse freezing occurs. We also
discuss how the effective temperature can be measured experimentally from a
generalized Kubo formula.Comment: 4 pages, 4 figure
- âŚ