398 research outputs found
Vortex Pull by an External Current
In the context of a dynamical Ginzburg-Landau model it is shown numerically
that under the influence of a homogeneous external current J the vortex drifts
against the current with velocity in agreement to earlier analytical
predictions. In the presence of dissipation the vortex undergoes skew
deflection at an angle with respect to the
external current. It is shown analytically and verified numerically that the
angle and the speed of the vortex are linked through a simple
mathematical relation.Comment: 19 pages, LATEX, 6 Postscript figures included in separate compressed
fil
Solitary Waves of Planar Ferromagnets and the Breakdown of the Spin-Polarized Quantum Hall Effect
A branch of uniformly-propagating solitary waves of planar ferromagnets is
identified. The energy dispersion and structures of the solitary waves are
determined for an isotropic ferromagnet as functions of a conserved momentum.
With increasing momentum, their structure undergoes a transition from a form
ressembling a droplet of spin-waves to a Skyrmion/anti-Skyrmion pair. An
instability to the formation of these solitary waves is shown to provide a
mechanism for the electric field-induced breakdown of the spin-polarized
quantum Hall effect.Comment: 5 pages, 3 eps-figures, revtex with epsf.tex and multicol.st
Enhanced statistical stability in coherent interferometric imaging
http://iopscience.iop.org/0266-5611/International audienc
Search for the Nondimerized Quantum Nematic Phase in the Spin-1 Chain
Chubukov's proposal concerning the possibility of a nondimerized quantum
nematic phase in the ground-state phase diagram of the bilinear-biquadratic
spin-1 chain is studied numerically. Our results do not support the existence
of this phase, but they rather indicate a direct transition from the
ferromagnetic into the dimerized phase.Comment: REVTEX, 14 pages +8 PostScript figure
``Smoke Rings'' in Ferromagnets
It is shown that bulk ferromagnets support propagating non-linear modes that
are analogous to the vortex rings, or ``smoke rings'', of fluid dynamics. These
are circular loops of {\it magnetic} vorticity which travel at constant
velocity parallel to their axis of symmetry. The topological structure of the
continuum theory has important consequences for the properties of these
magnetic vortex rings. One finds that there exists a sequence of magnetic
vortex rings that are distinguished by a topological invariant (the Hopf
invariant). We present analytical and numerical results for the energies,
velocities and structures of propagating magnetic vortex rings in ferromagnetic
materials.Comment: 4 pages, 3 eps-figures, revtex with epsf.tex and multicol.sty. To
appear in Physical Review Letters. (Postscript problem fixed.
Dynamical Toroidal Hopfions in a Ferromagnet with Easy-Axis Anisotropy
Three-dimensional toroidal precession solitons with a nonzero Hopf index,
which uniformly move along the anisotropy axis in a uniaxial ferromagnet, have
been found. The structure and existence region of the solitons have been
numerically determined by solving the Landau-Lifshitz equation.Comment: 6 pages, 4 figure
Dynamical properties of Au from tight-binding molecular-dynamics simulations
We studied the dynamical properties of Au using our previously developed
tight-binding method. Phonon-dispersion and density-of-states curves at T=0 K
were determined by computing the dynamical-matrix using a supercell approach.
In addition, we performed molecular-dynamics simulations at various
temperatures to obtain the temperature dependence of the lattice constant and
of the atomic mean-square-displacement, as well as the phonon density-of-states
and phonon-dispersion curves at finite temperature. We further tested the
transferability of the model to different atomic environments by simulating
liquid gold. Whenever possible we compared these results to experimental
values.Comment: 7 pages, 9 encapsulated Postscript figures, submitted to Physical
Review
Unveiling Order behind Complexity: Coexistence of Ferromagnetism and Bose-Einstein Condensation
We present an algebraic framework for identifying the order parameter and the
possible phases of quantum systems that is based on identifying the local
dimension of the quantum operators and using the SU(N) group representing
the generators of generalized spin-particle mappings. We illustrate this for
=3 by presenting for any spatial dimension the exact solution of the
bilinear-biquadratic =1 quantum Heisenberg model at a high symmetry point.
Through this solution we rigorously show that itinerant ferromagnetism and
Bose-Einstein condensation may coexist.Comment: 5 pages, 1 psfigur
Absence of string order in the anisotropic S=2 Heisenberg antiferromagnet
We study an AFM Heisenberg S=2 quantum spin chain at T=0 with both
interaction and on-site anisotropy, H = \sum_{i}
{1/2}(S^{+}_{i}S^{-}_{i+1}+S^{-}_{i}S^{+}_{i+1})
+J^{z}S^{z}_{i}S^{z}_{i+1}+D(S^{z}_{i})^{2}. Contradictory scenarios exist for
the S=2 anisotropic phase diagram, implying different mechanisms of the
emergence of the classical limit. One main AKLT-based scenario predicts the
emergence of a cascade of phase transitions not seen in the S=1 case. Another
scenario is in favor of an almost classical phase diagram for S=2; the S=1 case
then is very special with its dominant quantum effects. Numerical studies have
not been conclusive. Using the DMRG, the existence of hidden topological order
in the anisotropic S=2 chain is examined, as it distinguishes between the
proposed scenarios. We show that the topological order is zero in the
thermodynamical limit in all disordered phases, in particular in the new phase
interposed between the Haldane and large- phases. This excludes the
AKLT-model based scenario in favor of an almost classical phase diagram for the
spin chains.Comment: 9 pages, 9 eps figures, uses RevTeX, submitted to PR
Intermediate phase in the spiral antiferromagnet Ba_2CuGe_2O_7
The magnetic compound Ba_2CuGe_2O_7 has recently been shown to be an
essentially two-dimensional spiral antiferromagnet that exhibits an
incommensurate-to-commensurate phase transition when a magnetic field applied
along the c-axis exceeds a certain critical value H_c. The T=0 dynamics is
described here in terms of a continuum field theory in the form of a nonlinear
sigma model. We are thus in a position to carry out a complete calculation of
the low-energy magnon spectrum for any strength of the applied field throughout
the phase transition. In particular, our spin-wave analysis reveals
field-induced instabilities at two distinct critical fields H_1 and H_2 such
that H_1 < H_c < H_2. Hence we predict the existence of an intermediate phase
whose detailed nature is also studied to some extent in the present paper.Comment: 15 pages, 11 figures, 2 table
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