552 research outputs found
Anti-phase locking in a two-dimensional Josephson junction array
We consider theoretically phase locking in a simple two-dimensional Josephson
junction array consisting of two loops coupled via a joint line transverse to
the bias current. Ring inductances are supposed to be small, and special
emphasis is taken on the influence of external flux. Is is shown, that in the
stable oscillation regime both cells oscillate with a phase shift equal to
(i.e. anti-phase). This result may explain the low radiation output
obtained so far in two-dimensional Josephson junction arrays experimentally.Comment: 11 pages, REVTeX, 1 Postscript figure, Subm. to Appl. Phys. Let
Dynamic surface critical behavior of isotropic Heisenberg ferromagnets: boundary conditions, renormalized field theory, and computer simulation results
The dynamic critical behavior of isotropic Heisenberg ferromagnets with a
planar free surface is investigated by means of field-theoretic renormalization
group techniques and high-precision computer simulations. An appropriate
semi-infinite extension of the stochastic model J is constructed. The relevant
boundary terms of the action of the associated dynamic field theory are
identified, the implied boundary conditions are derived, and the
renormalization of the model in bulk dimensions is clarified. Two
distinct renormalization schemes are utilized. The first is a massless one
based on minimal subtraction of dimensional poles and the dimensionality
expansion about . To overcome its problems in going below
dimensions, a massive one for fixed dimensions is constructed. The
resulting renormalization group (or Callan Symanzik) equations are exploited to
obtain the scaling forms of surface quantities like the dynamic structure
factor. In conjunction with boundary operator expansions scaling relations
follow that relate the critical indices of the dynamic and static infrared
singularities of surface quantities to familiar \emph{static} bulk and surface
exponents. To test the predicted scaling forms and scaling-law expressions for
the critical exponents involved, accurate computer-simulation data are
presented for the dynamic surface structure factor. These are in conformity
with our predictions.Comment: Revtex4-file with 4 figures included as eps-files, 21 pages in
print-format, typos corrected, to appear in Phys. Rev. B, July
Critical Casimir Effect in 3He-4He films
Universal aspects of the thermodynamic Casimir effect in wetting films of
3He-4He mixtures near their bulk tricritical point are studied within suitable
models serving as representatives of the corresponding universality class. The
effective forces between the boundaries of such films arising from the
confinement are calculated along isotherms at several fixed concentrations of
3He. Nonsymmetric boundary conditions impose nontrivial concentration profiles
leading to repulsive Casimir forces which exhibit a rich behavior of the
crossover between the tricritical point and the line of critical points. The
theoretical results agree with published experimental data and emphasize the
importance of logarithmic corrections.Comment: 12 pages, 4 figures, submitted to the Phys. Rev. Let
Theory of phase-locking in generalized hybrid Josephson junction arrays
A recently proposed scheme for the analytical treatment of the dynamics of
two-dimensional hybrid Josephson junction arrays is extended to a class of
generalized hybrid arrays with ''horizontal'' shunts involving a capacitive as
well as an inductive component. This class of arrays is of special interest,
because the internal cell coupling has been shown numerically to favor in-phase
synchronization for certain parameter values. As a result, we derive limits on
the circuit design parameters for realizing this state. In addition, we obtain
formulas for the flux-dependent frequency including flux-induced switching
processes between the in-phase and anti-phase oscillation regime. The treatment
covers unloaded arrays as well as arrays shunted via an external load.Comment: 24 pages, REVTeX, 5 Postscript figures, Subm. to Phys. Rev.
Electron Refrigeration in the Tunneling Approach
The qualities of electron refrigeration by means of tunnel junctions between
superconducting and normal--metal electrodes are studied theoretically. A
suitable approximation of the basic expression for the heat current across
those tunnel junctions allows the investigation of several features of the
device such as its optimal bias voltage, its maximal heat current, its optimal
working point, and the maximally gained temperature reduction. Fortunately, the
obtained results can be compared with those of a recent experiment.Comment: 4 pages, 4 Postscript figures, uses eps
Monte Carlo simulation results for critical Casimir forces
The confinement of critical fluctuations in soft media induces critical
Casimir forces acting on the confining surfaces. The temperature and geometry
dependences of such forces are characterized by universal scaling functions. A
novel approach is presented to determine them for films via Monte Carlo
simulations of lattice models. The method is based on an integration scheme of
free energy differences. Our results for the Ising and the XY universality
class compare favourably with corresponding experimental results for wetting
layers of classical binary liquid mixtures and of 4He, respectively.Comment: 14 pages, 5 figure
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