1,528 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
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
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
Quantum phase-slips in Josephson junction rings
We study quantum phase-slip (QPS) processes in a superconducting ring
containing N Josephson junctions and threaded by an external static magnetic
flux. In a such system, a QPS consists of a quantum tunneling event connecting
two distinct classical states of the phases with different persistent currents
[K. A. Matveev et al., Phys. Rev. Lett. 89, 096802 (2002)]. When the Josephson
coupling energy EJ of the junctions is larger than the charging energy EC =
e2/2C where C is the junction capacitance, the quantum amplitude for the QPS
process is exponentially small in the ratio EJ/EC. At given magnetic flux each
QPS can be described as the tunneling of the phase difference of a single
junction of almost 2pi, accompanied by a small harmonic displacement of the
phase difference of the other N-1 junctions. As a consequence the total QPS
amplitude nu is a global property of the ring. Here we study the dependence of
nu on the ring size N taking into account the effect of a finite capacitance C0
to ground which leads to the appearance of low-frequency dispersive modes.
Josephson and charging effects compete and lead to a nonmonotonic dependence of
the ring critical current on N. For N=infty, the system converges either
towards a superconducting or an insulating state, depending on the ratio
between the charging energy E0 = e2/2C0 and the Josephson coupling energy EJ.Comment: (19 pages, 12 figures) The final version deviated from the original
version. One of the author was removed from the lis
Fluctuations and vortex pattern ordering in fully frustrated XY model with honeycomb lattice
The accidental degeneracy of various ground states in a fully frustrated XY
model with a honeycomb lattice is shown to survive even when the free energy of
the harmonic fluctuations is taken into account. The reason for that consists
in the existence of a hidden gauge symmetry between the Hamiltonians describing
the harmonic fluctuations in all these ground states. A particular vortex
pattern is selected only when anharmonic fluctuations are taken into account.
However, the observation of vortex ordering requires relatively large system
size L>>100000.Comment: 4 pages, 2 figures, RevTeX4, a different method is used to find which
state is selected by anharmonic fluctuations, the last third of the text is
completly rewritte
Sequence of phase transitions induced in an array of Josephson junctions by their crossover to pi-state
We show that the transition of Josephson junctions between the conventional
and pi states caused by the decrease in temperature induces in a regular
two-dimensional array of such junctions not just a single phase transition
between two phases with different ordering but a sequence of two, three or four
phase transitions. The corresponding phase diagrams are constructed for the
cases of bipartite (square or honeycomb) and triangular lattices.Comment: 5 pages, v2: as published in EP
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