3,591 research outputs found
The phase diagram of 2D polar condensates in a magnetic field
Spin one condensates in the polar (antiferromagnetic) phase in two dimensions
are shown to undergo a transition of the Ising type, in addition to the
expected Kosterlitz--Thouless (KT) transition of half vortices, due to the
quadratic Zeeman effect. We establish the phase diagram in terms of temperature
and the strength of the Zeeman effect using Monte Carlo simulations. When the
Zeeman effect is sufficiently strong the Ising and KT transitions merge. For
very strong Zeeman field the remaining transition is of the familiar integer KT
type.Comment: 4 pages, 7 figure
Superconducting phase transitions in frustrated Josephson-junction arrays on a dice lattice
Transport measurements are carried out on dice Josephson-junction arrays with
the frustration index and 1/2 which possess, within the limit of the
model, an accidental degeneracy of the ground states as a consequence of
the formation of zero-energy domain walls. The measurements demonstrate that
both the systems undergo a phase transition to a superconducting vortex-ordered
state at considerably high temperatures. The experimental findings are in
apparent contradiction with the theoretical expectation that frustration
effects in the system are particularly strong enough to suppress a
vortex-ordering transition down to near zero temperature. The data for
are more consistent with theoretical evaluations. The agreement between the
experiments and the Monte Carlo simulations of a model for
suggests that the order-from-disorder mechanism for the removal of an
accidental degeneracy may still be effective in the system. The
transport data also reveal that the dice arrays with zero-energy domain walls
experience a much slower critical relaxation than other frustrated arrays only
with finite-energy walls.Comment: 4 pages, 4 figure
Comment on "Probing vortex unbinding via dipole fluctuations"
We demonstrate that the method suggested by Fertig and Straley [Phys. Rev. B
66, 201402 (2002)] for the identification of different phases in
two-dimensional XY models does not allow to make any unambiguous conclusions
and make a tentative proposal of another approach to this problem.Comment: The final version - as published in Physical Review B (2 pages). Two
paragraphs have been added with a proposal of a new approach to
distinguishing phases with bound and unbound vortice
A cluster algorithm for resistively shunted Josephson junctions
We present a cluster algorithm for resistively shunted Josephson junctions
and similar physical systems, which dramatically improves sampling efficiency.
The algorithm combines local updates in Fourier space with rejection-free
cluster updates which exploit the symmetries of the Josephson coupling energy.
As an application, we consider the localization transition of a single junction
at intermediate Josephson coupling and determine the temperature dependence of
the zero bias resistance as a function of dissipation strength.Comment: 4 page
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