7,092 research outputs found
Structural signatures of the unjamming transition at zero temperature
We study the pair correlation function for zero-temperature,
disordered, soft-sphere packings just above the onset of jamming. We find
distinct signatures of the transition in both the first and split second peaks
of this function. As the transition is approached from the jammed side (at
higher packing fraction) the first peak diverges and narrows on the small-
side to a delta-function. On the high- side of this peak, decays as a
power-law. In the split second peak, the two subpeaks are both singular at the
transition, with power-law behavior on their low- sides and step-function
drop-offs on their high- sides. These singularities at the transition are
reminiscent of empirical criteria that have previously been used to distinguish
glassy structures from liquid ones.Comment: 8 pages, 13 figure
Pulling adsorbed polymers from surfaces with the AFM: stick versus slip, peeling versus gliding
We consider the response of an adsorbed polymer that is pulled by an AFM
within a simple geometric framework. We separately consider the cases of i)
fixed polymer-surface contact point, ii) sticky case where the polymer is
peeled off from the substrate, and iii) slippery case where the polymer glides
over the surface. The resultant behavior depends on the value of the surface
friction coefficient and the adsorption strength. Our resultant force profiles
in principle allow to extract both from non-equilibrium force-spectroscopic
data.Comment: 6 pages, 3 figures; accepted for publication in Europhys. Lett.,
http://www.edpsciences.org/journal/index.cfm?edpsname=ep
Fast quantum algorithm for numerical gradient estimation
Given a blackbox for f, a smooth real scalar function of d real variables,
one wants to estimate the gradient of f at a given point with n bits of
precision. On a classical computer this requires a minimum of d+1 blackbox
queries, whereas on a quantum computer it requires only one query regardless of
d. The number of bits of precision to which f must be evaluated matches the
classical requirement in the limit of large n.Comment: additional references and minor clarifications and corrections to
version
Effects of P-wave Annihilation on the Angular Power Spectrum of Extragalactic Gamma-rays from Dark Matter Annihilation
We present a formalism for estimating the angular power spectrum of
extragalactic gamma-rays produced by dark matter annihilating with any general
velocity-dependent cross section. The relevant density and velocity
distribution of dark matter is modeled as an ensemble of smooth, universal,
rigid, disjoint, spherical halos with distribution and universal properties
constrained by simulation data. We apply this formalism to theories of dark
matter with p-wave annihilation, for which the relative-velocity-weighted
annihilation cross section is \sigma v=a+bv^2. We determine that this
significantly increases the gamma-ray power if b/a >> 10^6. The effect of
p-wave annihilation on the angular power spectrum is very similar for the
sample of particle physics models we explored, suggesting that the important
effect for a given b/a is largely determined by the cosmic dark matter
distribution. If the dark matter relic from strong p-wave theories is thermally
produced, the intensities of annihilation gamma-rays are strongly p-wave
suppressed, making them difficult to observe. If an angular power spectrum
consistent with a strong p-wave were to be observed, it would likely indicate
non-thermal production of dark matter in the early Universe.Comment: 20 pages, 3 figure
Constraints on the large-x d/u ratio from electron-nucleus scattering at x>1
Recently the ratio of neutron to proton structure functions F_2n/F_2p was
extracted from a phenomenological correlation between the strength of the
nuclear EMC effect and inclusive electron-nucleus cross section ratios at x>1.
Within conventional models of nuclear smearing, this "in-medium correction"
(IMC) extraction constrains the size of nuclear effects in the deuteron
structure functions, from which the neutron structure function F_2n is usually
extracted. The IMC data determine the resulting proton d/u quark distribution
ratio, extrapolated to x=1, to be 0.23 +- 0.09 with a 90% confidence level.
This is well below the SU(6) symmetry limit of 1/2 and significantly above the
scalar diquark dominance limit of 0.Comment: 4 pages, 3 figure
Numerical simulations of two dimensional magnetic domain patterns
I show that a model for the interaction of magnetic domains that includes a
short range ferromagnetic and a long range dipolar anti-ferromagnetic
interaction reproduces very well many characteristic features of
two-dimensional magnetic domain patterns. In particular bubble and stripe
phases are obtained, along with polygonal and labyrinthine morphologies. In
addition, two puzzling phenomena, namely the so called `memory effect' and the
`topological melting' observed experimentally are also qualitatively described.
Very similar phenomenology is found in the case in which the model is changed
to be represented by the Swift-Hohenberg equation driven by an external
orienting field.Comment: 8 pages, 8 figures. Version to appear in Phys. Rev.
Black Holes with Zero Mass
We consider the spacetimes corresponding to static Global Monopoles with
interior boundaries corresponding to a Black Hole Horizon and analyze the
behavior of the appropriate ADM mass as a function of the horizon radius r_H.
We find that for small enough r_H, this mass is negative as in the case of the
regular global monopoles, but that for large enough r_H the mass becomes
positive encountering an intermediate value for which we have a Black Hole with
zero ADM mass.Comment: 10 pages, 2 ps figures, REVTeX, some minor change
One-loop fermionic corrections to the instanton transition in two dimensional chiral Higgs model
The one-loop fermionic contribution to the probability of an instanton
transition with fermion number violation is calculated in the chiral Abelian
Higgs model in 1+1 dimensions, where the fermions have a Yukawa coupling to the
scalar field. The dependence of the determinant on fermionic, scalar and vector
mass is determined. We show in detail how to renormalize the fermionic
determinant in partial wave analysis, which is convenient for computations.Comment: 36 pages, 5 figure
Exact Dynamics of Multicomponent Bose-Einstein Condensates in Optical Lattices in One, Two and Three Dimensions
Numerous exact solutions to the nonlinear mean-field equations of motion are
constructed for multicomponent Bose-Einstein condensates on one, two, and three
dimensional optical lattices. We find both stationary and nonstationary
solutions, which are given in closed form. Among these solutions are a
vortex-anti-vortex array on the square optical lattice and modes in which two
or more components slosh back and forth between neighboring potential wells. We
obtain a variety of solutions for multicomponent condensates on the simple
cubic lattice, including a solution in which one condensate is at rest and the
other flows in a complex three-dimensional array of intersecting vortex lines.
A number of physically important solutions are stable for a range of parameter
values, as we show by direct numerical integration of the equations of motion.Comment: 22 pages, 9 figure
Thermalization of gluons in ultrarelativistic heavy ion collisions by including three-body interactions in a parton cascade
We develop a new 3+1 dimensional Monte Carlo cascade solving the kinetic
on-shell Boltzmann equations for partons including the inelastic gg ggg
pQCD processes. The back reaction channel is treated -- for the first time --
fully consistently within this scheme. An extended stochastic method is used to
solve the collision integral. The frame dependence and convergency are studied
for a fixed tube with thermal initial conditions. The detailed numerical
analysis shows that the stochastic method is fully covariant and that
convergency is achieved more efficiently than within a standard geometrical
formulation of the collision term, especially for high gluon interaction rates.
The cascade is then applied to simulate parton evolution and to investigate
thermalization of gluons for a central Au+Au collision at RHIC energy. For this
study the initial conditions are assumed to be generated by independent
minijets with p_T > p_0=2 GeV. With that choice it is demonstrated that overall
kinetic equilibration is driven mainly by the inelastic processes and is
achieved on a scale of 1 fm/c. The further evolution of the expanding gluonic
matter in the central region then shows almost an ideal hydrodynamical
behavior. In addition, full chemical equilibration of the gluons follows on a
longer timescale of about 3 fm/c.Comment: 121 pages with 55 figures, revised version. Two eps-figures and
comments are added. Formula (54) which has typo in journal version is given
correctl
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