71,916 research outputs found
Dynamics of polar vortex crystallization
Vortex crystals are commonly observed in ultra-thin ferroelectrics. However,
a clear physical picture of origin of this topological state is currently
lacking. Here, we show that vortex crystallization in ultra-thin
Pb(Zr0.4,Ti0.6)O3 films stems from the softening of a phonon mode and can be
thus described as a SU(2) symmetry-breaking transition. This result sheds light
on the topology of the polar vortex patterns and bridges polar vortices with
smectic phases, spin spirals, and other modulated states. Finally, we predict
an ac-field driven resonant switching of the vortex tube orientation which
could enable new low-power electronic technologies.Comment: 3 figure
Relaxation dynamics in fluids of platelike colloidal particles
The relaxation dynamics of a model fluid of platelike colloidal particles is
investigated by means of a phenomenological dynamic density functional theory.
The model fluid approximates the particles within the Zwanzig model of
restricted orientations. The driving force for time-dependence is expressed
completely by gradients of the local chemical potential which in turn is
derived from a density functional -- hydrodynamic interactions are not taken
into account. These approximations are expected to lead to qualitatively
reliable results for low densities as those within the isotropic-nematic
two-phase region. The formalism is applied to model an initially spatially
homogeneous stable or metastable isotropic fluid which is perturbed by
switching a two-dimensional array of Gaussian laser beams. Switching on the
laser beams leads to an accumulation of colloidal particles in the beam
centers. If the initial chemical potential and the laser power are large enough
a preferred orientation of particles occurs breaking the symmetry of the laser
potential. After switching off the laser beams again the system can follow
different relaxation paths: It either relaxes back to the homogeneous isotropic
state or it forms an approximately elliptical high-density core which is
elongated perpendicular to the dominating orientation in order to minimize the
surface free energy. For large supersaturations of the initial isotropic fluid
the high-density cores of neighboring laser beams of the two-dimensional array
merge into complex superstructures.Comment: low-resolution figures due to file size restrictions, revised versio
Spontaneous symmetry breaking in spinor Bose-Einstein condensates
We present an analytical model for the theoretical analysis of spin dynamics
and spontaneous symmetry breaking in a spinor Bose-Einstein condensate (BEC).
This allows for an excellent intuitive understanding of the processes and
provides good quantitative agreement with experimental results in Phys. Rev.
Lett. 105, 135302 (2010). It is shown that the dynamics of a spinor BEC
initially prepared in an unstable Zeeman state mF=0 (|0>) can be understood by
approximating the effective trapping potential for the state |+-1> with a
cylindrical box potential. The resonances in the creation efficiency of these
atom pairs can be traced back to excitation modes of this confinement. The
understanding of these excitation modes allows for a detailed characterization
of the symmetry breaking mechanism, showing how a twofold spontaneous breaking
of spatial and spin symmetry can occur. In addition a detailed account of the
experimental methods for the preparation and analysis of spinor quantum gases
is given.Comment: 12 pages, 14 figure
Quantum Dynamics in the Thermodynamic Limit
The description of spontaneous symmetry breaking that underlies the
connection between classically ordered objects in the thermodynamic limit and
their individual quantum mechanical building blocks is one of the cornerstones
of modern condensed matter theory and has found applications in many different
areas of physics. The theory of spontaneous symmetry breaking however, is
inherently an equilibrium theory, which does not address the dynamics of
quantum systems in the thermodynamic limit. Here, we will use the example of a
particular antiferromagnetic model system to show that the presence of a
so-called thin spectrum of collective excitations with vanishing energy -one of
the well-known characteristic properties shared by all symmetry-breaking
objects- can allow these objects to also spontaneously break time-translation
symmetry in the thermodynamic limit. As a result, that limit is found to be
able, not only to reduce quantum mechanical equilibrium averages to their
classical counterparts, but also to turn individual-state quantum dynamics into
classical physics. In the process, we find that the dynamical description of
spontaneous symmetry breaking can also be used to shed some light on the
possible origins of Born's rule. We conclude by describing an experiment on a
condensate of exciton polaritons which could potentially be used to
experimentally test the proposed mechanism.Comment: 13 pages, 4 figures; typos corrected, references updated, minor
changes in tex
Coverage, Continuity and Visual Cortical Architecture
The primary visual cortex of many mammals contains a continuous
representation of visual space, with a roughly repetitive aperiodic map of
orientation preferences superimposed. It was recently found that orientation
preference maps (OPMs) obey statistical laws which are apparently invariant
among species widely separated in eutherian evolution. Here, we examine whether
one of the most prominent models for the optimization of cortical maps, the
elastic net (EN) model, can reproduce this common design. The EN model
generates representations which optimally trade of stimulus space coverage and
map continuity. While this model has been used in numerous studies, no
analytical results about the precise layout of the predicted OPMs have been
obtained so far. We present a mathematical approach to analytically calculate
the cortical representations predicted by the EN model for the joint mapping of
stimulus position and orientation. We find that in all previously studied
regimes, predicted OPM layouts are perfectly periodic. An unbiased search
through the EN parameter space identifies a novel regime of aperiodic OPMs with
pinwheel densities lower than found in experiments. In an extreme limit,
aperiodic OPMs quantitatively resembling experimental observations emerge.
Stabilization of these layouts results from strong nonlocal interactions rather
than from a coverage-continuity-compromise. Our results demonstrate that
optimization models for stimulus representations dominated by nonlocal
suppressive interactions are in principle capable of correctly predicting the
common OPM design. They question that visual cortical feature representations
can be explained by a coverage-continuity-compromise.Comment: 100 pages, including an Appendix, 21 + 7 figure
Chiral charge pumping in graphene deposited on a magnetic insulator
We demonstrate that a sizable chiral charge pumping can be achieved at room
temperature in graphene/Yttrium Iron Garnet (YIG) bilayer systems. The effect,
which cannot be attributed to the ordinary spin pumping, reveals itself in the
creation of a dc electric field/voltage in graphene as a response to the
dynamic magnetic excitations (spin waves) in an adjacent out-of-plane
magnetized YIG film. We show that the induced voltage changes its sign when the
orientation of the static magnetization is reversed, clearly indicating the
broken spatial inversion symmetry in the studied system. The strength of effect
shows a non-monotonous dependence on the spin-wave frequency, in agreement with
the proposed theoretical model.Comment: 8 pages, 5 figure
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