12,577 research outputs found
The effect of Pressure in Higher Dimensional Quasi-Spherical Gravitational Collapse
We study gravitational collapse in higher dimensional quasi-spherical
Szekeres space-time for matter with anisotropic pressure. Both local and global
visibility of central curvature singularity has been studied and it is found
that with proper choice of initial data it is possible to show the validity of
CCC for six and higher dimensions. Also the role of pressure in the collapsing
process has been discussed.Comment: 11 pages, 6 figures, RevTeX styl
model with Hopf interaction: the quantum theory
The model with Hopf interaction is quantised following the
Batalin-Tyutin (BT) prescription. In this scheme, extra BT fields are
introduced which allow for the existence of only commuting first-class
constraints. Explicit expression for the quantum correction to the expectation
value of the energy density and angular momentum in the physical sector of this
model is derived. The result shows, in the particular operator ordering that we
have chosen to work with, that the quantum effect has a divergent contribution
of in the energy expectation value. But, interestingly
the Hopf term, though topological in nature, can have a finite contribution to energy density in the homotopically nontrivial
topological sector. The angular momentum operator, however, is found to have no
quantum correction, indicating the absence of any fractional spin even at this
quantum level. Finally, the extended Lagrangian incorporating the BT auxiliary
fields is computed in the conventional framework of BRST formalism exploiting
Faddeev-Popov technique of path integral method.Comment: LaTeX, 28 pages, no figures, typos corrected, journal ref. give
Determinant Quantum Monte Carlo Study of the Screening of the One Body Potential near a Metal-Insulator Transition
In this paper we present a determinant quantum monte carlo study of the two
dimensional Hubbard model with random site disorder. We show that, as in the
case of bond disorder, the system undergoes a transition from an Anderson
insulating phase to a metallic phase as the onsite repulsion U is increased
beyond a critical value U_c. However, there appears to be no sharp signal of
this metal-insulator transition in the screened site energies. We observe that,
while the system remains metallic for interaction values upto twice U_c, the
conductivity is maximal in the metallic phase just beyond U_c, and decreases
for larger correlation.Comment: 6 pages, 10 eps figures, Revtex
Why Do Granular Materials Stiffen with Shear Rate? : Test of Novel Stress-Based Statistics
Peer reviewedPublisher PD
Fluctuations in Shear-Jammed States: A Statistical Ensemble Approach
Granular matter exists out of thermal equilibrium, i.e. it is athermal. While
conventional equilibrium statistical mechanics is not useful for characterizing
granular materials, the idea of constructing a statistical ensemble analogous
to its equilibrium counterpart to describe static granular matter was proposed
by Edwards and Oakshott more than two decades ago. Recent years have seen
several implementations of this idea. One of these is the stress ensemble,
which is based on properties of the force moment tensor, and applies to
frictional and frictionless grains. We demonstrate the full utility of this
statistical framework in shear jammed (SJ) experimental states [1,2], a special
class of granular solids created by pure shear, which is a strictly
non-equilbrium protocol for creating solids. We demonstrate that the stress
ensemble provides an excellent quantitative description of fluctuations in
experimental SJ states. We show that the stress fluctuations are controlled by
a single tensorial quantity: the angoricity of the system, which is a direct
analog of the thermodynamic temperature. SJ states exhibit significant
correlations in local stresses and are thus inherently different from
density-driven, isotropically jammed (IJ) states.Comment: 6 pages, 4 figure
Deconvolving mutational patterns of poliovirus outbreaks reveals its intrinsic fitness landscape.
Vaccination has essentially eradicated poliovirus. Yet, its mutation rate is higher than that of viruses like HIV, for which no effective vaccine exists. To investigate this, we infer a fitness model for the poliovirus viral protein 1 (vp1), which successfully predicts in vitro fitness measurements. This is achieved by first developing a probabilistic model for the prevalence of vp1 sequences that enables us to isolate and remove data that are subject to strong vaccine-derived biases. The intrinsic fitness constraints derived for vp1, a capsid protein subject to antibody responses, are compared with those of analogous HIV proteins. We find that vp1 evolution is subject to tighter constraints, limiting its ability to evade vaccine-induced immune responses. Our analysis also indicates that circulating poliovirus strains in unimmunized populations serve as a reservoir that can seed outbreaks in spatio-temporally localized sub-optimally immunized populations
Spin Polarizations at and about the Lowest Filled Landau Level
The spin polarization versus temperature at or near a fully filled lowest
Landau level is explored for finite-size systems in a periodic rectangular
geometry. Our results at which also include the finite-thickness
correction are in good agreement with the experimental results. We also find
that the interacting electron system results are in complete agreement with the
results of the sigma model, i.e., skyrmions on a torus have a topological
charge of and the Q=1 solution is like a single spin-flip excitation.
Our results therefore provide direct evidence for the skyrmionic nature of the
excitations at this filling factor.Comment: 4 pages, REVTEX, and 4 .ps files, To be published in Europhysics
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