85,939 research outputs found
A Terradynamics of Legged Locomotion on Granular Media
The theories of aero- and hydrodynamics predict animal movement and device
design in air and water through the computation of lift, drag, and thrust
forces. Although models of terrestrial legged locomotion have focused on
interactions with solid ground, many animals move on substrates that flow in
response to intrusion. However, locomotor-ground interaction models on such
flowable ground are often unavailable. We developed a force model for
arbitrarily-shaped legs and bodies moving freely in granular media, and used
this "terradynamics" to predict a small legged robot's locomotion on granular
media using various leg shapes and stride frequencies. Our study reveals a
complex but generic dependence of stresses in granular media on intruder depth,
orientation, and movement direction and gives insight into the effects of leg
morphology and kinematics on movement
Rate of Convergence in Nonlinear Hartree Dynamics with Factorized Initial Data
The mean field dynamics of an -particle weekly interacting Boson system
can be described by the nonlinear Hartree equation. In this paper, we present
estimates on the 1/N rate of convergence of many-body Schr\"{o}dinger dynamics
to the one-body nonlinear Hartree dynamics with factorized initial data with
two-body interaction potential in .Comment: AMS LaTex, 21 page
Early Time Dynamics of Gluon Fields in High Energy Nuclear Collisions
Nuclei colliding at very high energy create a strong, quasi-classical gluon
field during the initial phase of their interaction. We present an analytic
calculation of the initial space-time evolution of this field in the limit of
very high energies using a formal recursive solution of the Yang-Mills
equations. We provide analytic expressions for the initial chromo-electric and
chromo-magnetic fields and for their energy-momentum tensor. In particular, we
discuss event-averaged results for energy density and energy flow as well as
for longitudinal and transverse pressure of this system. For example, we find
that the ratio of longitudinal to transverse pressure very early in the system
behaves as where
is the longitudinal proper time, is related to the saturation scales
of the two nuclei, and with a scale to
be defined later. Our results are generally applicable if .
As already discussed in a previous paper, the transverse energy flow of
the gluon field exhibits hydrodynamic-like contributions that follow transverse
gradients of the energy density . In addition, a
rapidity-odd energy flow also emerges from the non-abelian analog of Gauss' Law
and generates non-vanishing angular momentum of the field. We will discuss the
space-time picture that emerges from our analysis and its implications for
observables in heavy ion collisions.Comment: 26 pages, 9 figure
A Comprehensive Four-Quark Interpretation of D_s(2317), D_s(2457) and D_s(2632)
The recently observed new member of the charm-strange family D_s(2632) which
has a surprisingly narrow width is challenging our theory. D_s(2317) and
D_s(2457) which were observed earlier have similar behaviors and receive
various theoretical explanations. Some authors use the heavy hadron chiral
effective theory to evaluate heavy-light quark systems and obtain a reasonable
evaluation on the masses of D_s(2317) and D_s(2457). An alternative picture is
to interpret them as four-quark or molecular states. In this work, we are
following the later and propose a unitive description for all the three new
members D_s(2632), D_s(2317) and D_s(2457) and at least, so far our picture is
consistent with the data.Comment: 6 page
Nuclear matter symmetry energy and the neutron skin thickness of heavy nuclei
Correlations between the thickness of the neutron skin in finite nuclei and
the nuclear matter symmetry energy are studied in the Skyrme Hartree-Fock
model. From the most recent analysis of the isospin diffusion data in heavy-ion
collisions based on an isospin- and momentum-dependent transport model with
in-medium nucleon-nucleon cross sections, a value of MeV for the
slope of the nuclear symmetry energy at saturation density is extracted, and
this imposes stringent constraints on both the parameters in the Skyrme
effective interactions and the neutron skin thickness of heavy nuclei.
Predicted thickness of the neutron skin is fm for Pb,
fm for Sn, and fm for Sn.Comment: 6 pages, 4 figures, 1 table, revised version, to appear in PR
Spectral and optical properties in the antiphase stripe phase of the cuprate superconductors
We investigate the superconducting order parameter, the spectral and optical
properties in a stripe model with spin (charge) domain-derived scattering
potential (). We show that the charge domain-derived scattering
is less effective than the spin scattering on the suppression of
superconductivity. For , the spectral weight concentrates on
the () antinodal region, and a finite energy peak appears in the optical
conductivity with the disappearance of the Drude peak. But for , the spectral weight concentrates on the () nodal region,
and a residual Drude peak exists in the optical conductivity without the finite
energy peak. These results consistently account for the divergent observations
in the ARPES and optical conductivity experiments in several high-
cuprates, and suggest that the "insulating" and "metallic" properties are
intrinsic to the stripe state, depending on the relative strength of the spin
and charge domain-derived scattering potentials.Comment: 7 pages, 4 figure
Extremely sharp carbon nanocone probes for atomic force microscopy imaging
A simple and reliable catalyst patterning technique combined with electric-field-guided growth is utilized to synthesize a sharp and high-aspect-ratio carbon nanocone probe on a tipless cantilever for atomic force microscopy. A single carbon nanodot produced by an electron-beam-induced deposition serves as a convenient chemical etch mask for catalyst patterning, thus eliminating the need for complicated, resist-based, electron-beam lithography for a nanoprobe fabrication. A gradual, sputtering-induced size reduction and eventual removal of the catalyst particle at the probe tip during electric-field-guided growth creates a sharp probe with a tip radius of only a few nanometers. These fabrication processes are amenable for the wafer-scale synthesis of multiple probes. High resolution imaging of three-dimensional features and deep trenches, and mechanical durability enabling continuous operation for many hours without noticeable image deterioration have been demonstrated
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