8,688 research outputs found
A Comment on General Formulae for Polarization Observables in Deuteron Electrodisintegration and Linear Relations
We establish a simple, explicit relation between the formalisms employed in
the treatments of polarization observables in deuteron two-body
electrodisintegration published by Arenh\"ovel, Leidemann, and Tomusiak in
Few-Body Systems {\bf 15}, 109 (1993) and the results of the present authors
published in Phys.~Rev.~C {\bf 40}, 2479 (1989). We comment on the overlap
between the two sets of results.Comment: 9 pages, no figure
Hexatic phase and water-like anomalies in a two-dimensional fluid of particles with a weakly softened core
We study a two-dimensional fluid of particles interacting through a
spherically-symmetric and marginally soft two-body repulsion. This model can
exist in three different crystal phases, one of them with square symmetry and
the other two triangular. We show that, while the triangular solids first melt
into a hexatic fluid, the square solid is directly transformed on heating into
an isotropic fluid through a first-order transition, with no intermediate
tetratic phase. In the low-pressure triangular and square crystals melting is
reentrant provided the temperature is not too low, but without the necessity of
two competing nearest-neighbor distances over a range of pressures. A whole
spectrum of water-like fluid anomalies completes the picture for this model
potential.Comment: 26 pages, 14 figures; printed article available at
http://link.aip.org/link/?jcp/137/10450
Residual Multiparticle Entropy for a Fractal Fluid of Hard Spheres
The residual multiparticle entropy (RMPE) of a fluid is defined as the
difference, , between the excess entropy per particle (relative to an
ideal gas with the same temperature and density), , and the
pair-correlation contribution, . Thus, the RMPE represents the net
contribution to due to spatial correlations involving three,
four, or more particles. A heuristic `ordering' criterion identifies the
vanishing of the RMPE as an underlying signature of an impending structural or
thermodynamic transition of the system from a less ordered to a more spatially
organized condition (freezing is a typical example). Regardless of this, the
knowledge of the RMPE is important to assess the impact of non-pair
multiparticle correlations on the entropy of the fluid. Recently, an accurate
and simple proposal for the thermodynamic and structural properties of a
hard-sphere fluid in fractional dimension has been proposed [Santos,
A.; L\'opez de Haro, M. \emph{Phys. Rev. E} \textbf{2016}, \emph{93}, 062126].
The aim of this work is to use this approach to evaluate the RMPE as a function
of both and the packing fraction . It is observed that, for any given
dimensionality , the RMPE takes negative values for small densities, reaches
a negative minimum at a packing fraction
, and then rapidly increases, becoming positive beyond a
certain packing fraction . Interestingly, while both
and monotonically decrease as dimensionality
increases, the value of exhibits a nonmonotonic
behavior, reaching an absolute minimum at a fractional dimensionality . A plot of the scaled RMPE shows a
quasiuniversal behavior in the region .Comment: 10 pages, 3 figures; v2: minor change
Stability of solutions of the Sherrington-Kirkpatrick model with respect to replications of the phase space
We use real replicas within the Thouless, Anderson and Palmer construction to
investigate stability of solutions with respect to uniform scalings in the
phase space of the Sherrington-Kirkpatrick model. We show that the demand of
homogeneity of thermodynamic potentials leads in a natural way to a
thermodynamically dependent ultrametric hierarchy of order parameters. The
derived hierarchical mean-field equations appear equivalent to the discrete
Parisi RSB scheme. The number of hierarchical levels in the construction is
fixed by the global thermodynamic homogeneity expressed as generalized de
Almeida Thouless conditions. A physical interpretation of a hierarchical
structure of the order parameters is gained.Comment: REVTeX4, 22 pages, second extended version to be published in Phys.
Rev.
Representational momentum in the motor system?
PURPOSE: If presented with a moving object which suddenly disappears observers usually misjudge the object's last seen position as being further forward along the path of motion. This effect, called representational momentum, can also be seen in objects that change size or shape. It has been argued that the effect is due to perceptual anticipation. We tested whether a similar effect is present in the motor system. METHODS: Using stereo computer graphics we presented cubes of different sizes on a CRT monitor. In each trial three cubes were successively presented for 200 msec with increasing or decreasing size (steps of 1 cm width difference). Ten participants either compared the last cube to a comparison cube (perceptual task) or grasped the cube using a virtual haptic setup (motor task). The setup consisted of two robot arms (Phantom TM) attached to index finger and thumb. The robot arms were controlled to create forces equivalent to the forces created by real objects. The CRT monitor was viewed via a mirror such that the visual position of the cubes matched the position of the virtual haptic objects. RESULTS: In the motor task participants opened their fingers by 1.1+/-0.4 mm wider if they grasped a cube that was preceded by smaller cubes than if they grasped a cube that was preceded by larger cubes. This is the well-known representational momentum effect. In the perceptual task the effect was reversed (-2.2+/-0.4 mm). The effects correlated between observers (r=.71, p=.02). CONCLUSIONS: It seems that a representational momentum occurs also in grasping tasks. The correlation between observers suggests that the motor effect is related to the perceptual effect. However, our perceptual task showed a reversed effect. Reasons for this discrepancy will be discussed
On chaos in mean field spin glasses
We study the correlations between two equilibrium states of SK spin glasses
at different temperatures or magnetic fields. The question, presiously
investigated by Kondor and Kondor and V\'egs\"o, is approached here
constraining two copies of the same system at different external parameters to
have a fixed overlap. We find that imposing an overlap different from the
minimal one implies an extensive cost in free energy. This confirms by a
different method the Kondor's finding that equilibrium states corresponding to
different values of the external parameters are completely uncorrelated. We
also consider the Generalized Random Energy Model of Derrida as an example of
system with strong correlations among states at different temperatures.Comment: 19 pages, Late
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