1,005 research outputs found

### 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

### Degeneracy and ordering of the non-coplanar phase of the classical bilinear-biquadratic Heisenberg model on the triangular lattice

We investigate the zero-temperature behavior of the classical Heisenberg
model on the triangular lattice in which the competition between exchange
interactions of different orders favors a relative angle between neighboring
spins in the interval (0,2pi/3). In this situation, the ground states are
noncoplanar and have an infinite discrete degeneracy. In the generic case, the
set of the ground states is in one to one correspondence (up to a global
rotation) with the non-crossing loop coverings of the three equivalent
honeycomb sublattices into which the bonds of the triangular lattice can be
partitioned. This allows one to identify the order parameter space as an
infinite Cayley tree with coordination number 3. Building on the duality
between a similar loop model and the ferromagnetic O(3) model on the honeycomb
lattice, we argue that a typical ground state should have long-range order in
terms of spin orientation. This conclusion is further supported by the
comparison with the four-state antiferromagnetic Potts model [describing the
case when the angle between neighboring spins is equal to arccos(-1/3)], which
at zero temperature is critical and in terms of the solid-on-solid
representation is located exactly at the point of roughening transition. At
other values of the angle between neighboring spins an additional constraint
appears, whose presence drives the system into an ordered phase (unless this
angle is equal to pi/2, when another constraint is removed and the model
becomes trivially exactly solvable).Comment: 10 pages, 5 figure

### Universal scaling behavior of the single electron box in the strong tunneling limit

We perform a numerical analysis of recently proposed scaling functions for
the single electron box. Specifically, we study the ``magnetic'' susceptibility
as a function of tunneling conductance and gate charge, and the effective
charging energy at zero gate charge as a function of tunneling conductance in
the strong tunneling limit. Our Monte Carlo results confirm the accuracy of the
theoretical predictions.Comment: Published versio

### Phase diagram of the fully frustrated transverse-field Ising model on the honeycomb lattice

Motivated by the current interest in the quantum dimer model on the
triangular lattice, we investigate the phase diagram of the closely related
fully-frustrated transverse field Ising model on the honeycomb lattice using
classical and semi-classical approximations. We show that, in addition to the
fully polarized phase at large field, the classical model possesses a multitude
of phases that break the translational symmetry which in the dimer language,
correspond to a plaquette phase and a columnar phase separated by an infinite
cascade of mixed phases. The modification of the phase diagram by quantum
fluctuations has been investigated in the context of linear spin-wave theory.
The extrapolation of the semiclassical energies suggests that the plaquette
phase extends down to zero field for spin 1/2, in agreement with the
$\sqrt{12}\times\sqrt{12}$ phase of the quantum dimer model on the triangular
lattice with only kinetic energy.Comment: 15 Pages, 11 Figures, Accepted for PR

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