1,232 research outputs found
Reach-to-Grasp Actions Under Direct and Indirect Viewing Conditions
The purpose of this study was to determine whether indirectly viewing an object through a mirror causes reach-to-grasp kinematics to differ from those performed under normal conditions, when participants directly observe the object to be grasped. Participants, in a supine position, reached for objects placed on a presentation platform (workspace) resting above their thighs. In the indirect viewing condition, a two-mirror viewing system was placed above the head that allowed participants to view the workspace. In the direct viewing condition, participants\u27 heads were elevated and tilted forward such that they could directly view the workspace. Three infrared markers were attached to participants\u27 right index fingertip, the tip of the right thumb and the knuckle of the right index-finger. Hand movements were captured using an OPTOTRAK 3020 camera system and kinematics were calculated offline following the experiment. It was found that reaches made under the indirect viewing condition were performed at a slower speed, and overall took longer than reaches made under the direct viewing condition. Hand shaping was effected by viewing condition and object size interaction. The results from this study reveal that reach-to-grasp movements performed under different viewing conditions are quantifiably different and may suggest that different neural substrates and/or different computational loads are present during these two viewing conditions. These results suggest that there may be experimental confounds in several of the previously published reach-to-grasp neuroimaging studies, specifically those that have used optical manipulations to view the hand and workspace within the fMRI scanning environment
An electric-field representation of the harmonic XY model
The two-dimensional harmonic XY (HXY) model is a spin model in which the
classical spins interact via a piecewise parabolic potential. We argue that the
HXY model should be regarded as the canonical classical lattice spin model of
phase fluctuations in two-dimensional condensates, as it is the simplest model
that guarantees the modular symmetry of the experimental systems. Here we
formulate a lattice electric-field representation of the HXY model and contrast
this with an analogous representation of the Villain model and the
two-dimensional Coulomb gas with a purely rotational auxiliary field. We find
that the HXY model is a spin-model analogue of a lattice electric-field model
of the Coulomb gas with an auxiliary field, but with a temperature-dependent
vacuum (electric) permittivity that encodes the coupling of the spin vortices
to their background spin-wave medium. The spin vortices map to the Coulomb
charges, while the spin-wave fluctuations correspond to auxiliary-field
fluctuations. The coupling explains the striking differences in the
high-temperature asymptotes of the specific heats of the HXY model and the
Coulomb gas with an auxiliary field. Our results elucidate the propagation of
effective long-range interactions throughout the HXY model (whose interactions
are purely local) by the lattice electric fields. They also imply that global
spin-twist excitations (topological-sector fluctuations) generated by local
spin dynamics are ergodically excluded in the low-temperature phase. We discuss
the relevance of these results to condensate physics.Comment: 13 pages, 10 figure
Topological-sector fluctuations and ergodicity breaking at the Berezinskii-Kosterlitz-Thouless transition
The Berezinskii-Kosterlitz-Thouless (BKT) phase transition drives the
unbinding of topological defects in many two-dimensional systems. In the
two-dimensional Coulomb gas, it corresponds to an insulator-conductor
transition driven by charge deconfinement. We investigate the global
topological properties of this transition, both analytically and by numerical
simulation, using a lattice-field description of the two-dimensional Coulomb
gas on a torus. The BKT transition is shown to be an ergodicity breaking
between the topological sectors of the electric field, which implies a
definition of topological order in terms of broken ergodicity. The breakdown of
local topological order at the BKT transition leads to the excitation of global
topological defects in the electric field, corresponding to different
topological sectors. The quantized nature of these classical excitations, and
their strict suppression by ergodicity breaking in the low-temperature phase,
afford striking global signatures of topological-sector fluctuations at the BKT
transition. We discuss how these signatures could be detected in experiments
on, for example, magnetic films and cold-atom systems.Comment: 11 pages, 6 figure
Crystal Shape-Dependent Magnetic Susceptibility and Curie Law Crossover in the Spin Ices Dy2Ti2O7 and Ho2Ti2O7
We present an experimental determination of the isothermal magnetic
susceptibility of the spin ice materials Dy2Ti2O7 and Ho2Ti2O7 in the
temperature range 1.8-300 K. The use of spherical crystals has allowed the
accurate correction for demagnetizing fields and allowed the true bulk
isothermal susceptibility X_T(T) to be estimated. This has been compared to a
theoretical expression based on a Husimi tree approximation to the spin ice
model. Agreement between experiment and theory is excellent at T > 10 K, but
systematic deviations occur below that temperature. Our results largely resolve
an apparent disagreement between neutron scattering and bulk measurements that
has been previously noted. They also show that the use of non-spherical
crystals in magnetization studies of spin ice may introduce very significant
systematic errors, although we note some interesting - and possibly new -
systematics concerning the demagnetizing factor in cuboidal samples. Finally,
our results show how experimental susceptibility measurements on spin ices may
be used to extract the characteristic energy scale of the system and the
corresponding chemical potential for emergent magnetic monopoles.Comment: 11 pages, 3 figures 1 table. Manuscript submitte
Non-Gaussian Resistance Noise near Electrical Breakdown in Granular Materials
The distribution of resistance fluctuations of conducting thin films with
granular structure near electrical breakdown is studied by numerical
simulations. The film is modeled as a resistor network in a steady state
determined by the competition between two biased processes, breaking and
recovery. Systems of different sizes and with different levels of internal
disorder are considered. Sharp deviations from a Gaussian distribution are
found near breakdown and the effect increases with the degree of internal
disorder. However, we show that in general this non-Gaussianity is related to
the finite size of the system and vanishes in the large size limit.
Nevertheless, near the critical point of the conductor-insulator transition,
deviations from Gaussianity persist when the size is increased and the
distribution of resistance fluctuations is well fitted by the universal
Bramwell-Holdsworth-Pinton distribution.Comment: 8 pages, 6 figures; accepted for publication on Physica
Phase order in superfluid helium films
Classic experimental data on helium films are transformed to estimate a
finite-size phase order parameter that measures the thermal degradation of the
condensate fraction in the two-dimensional superfluid. The order parameter is
found to evolve thermally with the exponent , a
characteristic, in analogous magnetic systems, of the
Berezinskii-Kosterlitz-Thouless (BKT) phase transition. Universal scaling near
the BKT fixed point generates a collapse of experimental data on helium and
ferromagnetic films, and implies new experiments and theoretical protocols to
explore the phase order. These results give a striking example of experimental
finite-size scaling in a critical system that is broadly relevant to
two-dimensional Bose fluids.Comment: 6 pages, 2 figure
Universal Magnetic Fluctuations with a Field Induced Length Scale
We calculate the probability density function for the order parameter
fluctuations in the low temperature phase of the 2D-XY model of magnetism near
the line of critical points. A finite correlation length, \xi, is introduced
with a small magnetic field, h, and an accurate expression for \xi(h) is
developed by treating non-linear contributions to the field energy using a
Hartree approximation. We find analytically a series of universal non-Gaussian
distributions with a finite size scaling form and present a Gumbel-like
function that gives the PDF to an excellent approximation. We propose the
Gumbel exponent, a(h), as an indirect measure of the length scale of
correlations in a wide range of complex systems.Comment: 7 pages, 4 figures, 1 table. To appear in Phys. Rev.
Characterising anomalous transport in accretion disks from X-ray observations
Whilst direct observations of internal transport in accretion disks are not yet possible, measurement of the energy emitted from accreting astrophysical systems can provide useful information on the physical mechanisms at work. Here we examine the unbroken multi-year time variation of the total X-ray flux from three sources: Cygnus X-1 , the microquasar GRS 1915+105 , and for comparison the nonaccreting Crab nebula. To complement previous analyses, we demonstrate that the application of advanced statistical methods to these observational time-series reveals important contrasts in the nature and scaling properties of the transport processes operating within these sources. We find the Crab signal resembles Gaussian noise; the Cygnus X-1 signal is a leptokurtic random walk whose self-similar properties persist on timescales up to three years; and the GRS 1915+105 signal is similar to that from Cygnus X-1, but with self-similarity extending possibly to only a few days. This evidence of self-similarity provides a robust quantitative characterisation of anomalous transport occuring within the systems
Onsager's Wien Effect on a Lattice
The Second Wien Effect describes the non-linear, non-equilibrium response of
a weak electrolyte in moderate to high electric fields. Onsager's 1934
electrodiffusion theory along with various extensions has been invoked for
systems and phenomena as diverse as solar cells, surfactant solutions, water
splitting reactions, dielectric liquids, electrohydrodynamic flow, water and
ice physics, electrical double layers, non-Ohmic conduction in semiconductors
and oxide glasses, biochemical nerve response and magnetic monopoles in spin
ice. In view of this technological importance and the experimental ubiquity of
such phenomena, it is surprising that Onsager's Wien effect has never been
studied by numerical simulation. Here we present simulations of a lattice
Coulomb gas, treating the widely applicable case of a double equilibrium for
free charge generation. We obtain detailed characterisation of the Wien effect
and confirm the accuracy of the analytical theories as regards the field
evolution of the free charge density and correlations. We also demonstrate that
simulations can uncover further corrections, such as how the field-dependent
conductivity may be influenced by details of microscopic dynamics. We conclude
that lattice simulation offers a powerful means by which to investigate
system-specific corrections to the Onsager theory, and thus constitutes a
valuable tool for detailed theoretical studies of the numerous practical
applications of the Second Wien Effect.Comment: Main: 12 pages, 4 figures. Supplementary Information: 7 page
- …