188 research outputs found
Simulated Dark-Matter Halos as a Test of Nonextensive Statistical Mechanics
In the framework of nonextensive statistical mechanics, the equilibrium
structures of astrophysical self-gravitating systems are stellar polytropes,
parameterized by the polytropic index n. By careful comparison to the
structures of simulated dark-matter halos we find that the density profiles, as
well as other fundamental properties, of stellar polytropes are inconsistent
with simulations for any value of n. This result suggests the need to
reconsider the applicability of nonextensive statistical mechanics (in its
simplest form) to equilibrium self-gravitating systems.Comment: Accepted for publication in Physical Review
Simple proof of gauge invariance for the S-matrix element of strong-field photoionization
The relationship between the length gauge (LG) and the velocity gauge (VG)
exact forms of the photoionization probability amplitude is considered. Our
motivation for this paper comes from applications of the Keldysh-Faisal-Reiss
(KFR) theory, which describes atoms (or ions) in a strong laser field (in the
nonrelativistic approach, in the dipole approximation). On the faith of a
certain widely-accepted assumption, we present a simple proof that the
well-known LG form of the exact photoionization (or photodetachment)
probability amplitude is indeed the gauge-invariant result. In contrast, to
obtain the VG form of this probability amplitude, one has to either (i) neglect
the well-known Goeppert-Mayer exponential factor (which assures gauge
invariance) during all the time evolution of the ionized electron or (ii) put
some conditions on the vector potential of the laser field.Comment: The paper was initially submitted (in a previous version) on 16
October 2006 to J. Phys. A and rejected. This is the extended version (with 2
figures), which is identical to the paper published online on 12 December
2007 in Physica Script
Interplay of chiral and helical states in a Quantum Spin Hall Insulator lateral junction
We study the electronic transport across an electrostatically-gated lateral
junction in a HgTe quantum well, a canonical 2D topological insulator, with and
without applied magnetic field. We control carrier density inside and outside a
junction region independently and hence tune the number and nature of 1D edge
modes propagating in each of those regions. Outside the 2D gap, magnetic field
drives the system to the quantum Hall regime, and chiral states propagate at
the edge. In this regime, we observe fractional plateaus which reflect the
equilibration between 1D chiral modes across the junction. As carrier density
approaches zero in the central region and at moderate fields, we observe
oscillations in resistance that we attribute to Fabry-Perot interference in the
helical states, enabled by the broken time reversal symmetry. At higher fields,
those oscillations disappear, in agreement with the expected absence of helical
states when band inversion is lifted.Comment: 5 pages, 4 figures, supp. ma
Velocity quantization approach of the one-dimensional dissipative harmonic oscillator
Given a constant of motion for the one-dimensional harmonic oscillator with
linear dissipation in the velocity, the problem to get the Hamiltonian for this
system is pointed out, and the quantization up to second order in the
perturbation approach is used to determine the modification on the eigenvalues
when dissipation is taken into consideration. This quantization is realized
using the constant of motion instead of the Hamiltonian.Comment: 10 pages, 2 figure
One-dimensional relativistic dissipative system with constant force and its quantization
For a relativistic particle under a constant force and a linear velocity
dissipation force, a constant of motion is found. Problems are shown for
getting the Hamiltoninan of this system. Thus, the quantization of this system
is carried out through the constant of motion and using the quantization of the
velocity variable. The dissipative relativistic quantum bouncer is outlined
within this quantization approach.Comment: 11 pages, no figure
A nonextensive entropy approach to solar wind intermittency
The probability distributions (PDFs) of the differences of any physical
variable in the intermittent, turbulent interplanetary medium are scale
dependent. Strong non-Gaussianity of solar wind fluctuations applies for short
time-lag spacecraft observations, corresponding to small-scale spatial
separations, whereas for large scales the differences turn into a Gaussian
normal distribution. These characteristics were hitherto described in the
context of the log-normal, the Castaing distribution or the shell model. On the
other hand, a possible explanation for nonlocality in turbulence is offered
within the context of nonextensive entropy generalization by a recently
introduced bi-kappa distribution, generating through a convolution of a
negative-kappa core and positive-kappa halo pronounced non-Gaussian structures.
The PDFs of solar wind scalar field differences are computed from WIND and ACE
data for different time lags and compared with the characteristics of the
theoretical bi-kappa functional, well representing the overall scale dependence
of the spatial solar wind intermittency. The observed PDF characteristics for
increased spatial scales are manifest in the theoretical distribution
functional by enhancing the only tuning parameter , measuring the
degree of nonextensivity where the large-scale Gaussian is approached for
. The nonextensive approach assures for experimental studies
of solar wind intermittency independence from influence of a priori model
assumptions. It is argued that the intermittency of the turbulent fluctuations
should be related physically to the nonextensive character of the
interplanetary medium counting for nonlocal interactions via the entropy
generalization.Comment: 17 pages, 7 figures, accepted for publication in Astrophys.
Progress in Classical and Quantum Variational Principles
We review the development and practical uses of a generalized Maupertuis
least action principle in classical mechanics, in which the action is varied
under the constraint of fixed mean energy for the trial trajectory. The
original Maupertuis (Euler-Lagrange) principle constrains the energy at every
point along the trajectory. The generalized Maupertuis principle is equivalent
to Hamilton's principle. Reciprocal principles are also derived for both the
generalized Maupertuis and the Hamilton principles. The Reciprocal Maupertuis
Principle is the classical limit of Schr\"{o}dinger's variational principle of
wave mechanics, and is also very useful to solve practical problems in both
classical and semiclassical mechanics, in complete analogy with the quantum
Rayleigh-Ritz method. Classical, semiclassical and quantum variational
calculations are carried out for a number of systems, and the results are
compared. Pedagogical as well as research problems are used as examples, which
include nonconservative as well as relativistic systems
Tyrosine 23 Phosphorylation-Dependent Cell-Surface Localization of Annexin A2 Is Required for Invasion and Metastases of Pancreatic Cancer
The aggressiveness of pancreatic ductal adenocarcinoma (PDA) is characterized by
its high metastatic potential and lack of effective therapies, which is the
result of a lack of understanding of the mechanisms involved in promoting PDA
metastases. We identified Annexin A2 (ANXA2), a member of the Annexin family of
calcium-dependent phospholipid binding proteins, as a new molecule that promotes
PDA invasion and metastases. We found ANXA2 to be a PDA-associated antigen
recognized by post-treatment sera of patients who demonstrated prolonged
survival following treatment with a PDA-specific vaccine. Cell surface ANXA2
increases with PDA development and progression. Knockdown of ANXA2 expression by
RNA interference or blocking with anti-ANXA2 antibodies inhibits in
vitro invasion of PDA cells. In addition, post-vaccination patient
sera inhibits in vitro invasion of PDA cells, suggesting that
therapeutic anti-ANXA2 antibodies are induced by the vaccine. Furthermore,
cell-surface localization of ANXA2 is tyrosine 23 phosphorylation-dependent; and
tyrosine 23 phosphorylation is required for PDA invasion. We demonstrated that
tyrosine 23 phosphorylation resulting in surface expression of ANXA2 is required
for TGFβ-induced, Rho-mediated epithelial-mesenchymal transition (EMT),
linking the cellular function of ANXA2 which was previously shown to be
associated with small GTPase-regulated cytoskeletal rearrangements, to the EMT
process in PDA. Finally, using mouse PDA models, we showed that shRNA knock-down
of ANXA2, a mutation at tyrosine 23, or anti-ANXA2 antibodies,
inhibit PDA metastases and prolong mouse survival. Thus, ANXA2 is part of a
novel molecular pathway underlying PDA metastases and a new target for
development of PDA therapeutics
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