1,399 research outputs found
Fast-Light in a Photorefractive Crystal for Gravitational Wave Detection
We demonstrate superluminal light propagation using two frequency multiplexed
pump beams to produce a gain doublet in a photorefractive crystal of Ce:BaTiO3.
The two gain lines are obtained by two-wave mixing between a probe field and
two individual pump fields. The angular frequencies of the pumps are
symmetrically tuned from the frequency of the probe. The frequency difference
between the pumps corresponds to the separation of the two gain lines; as it
increases, the crystal gradually converts from normal dispersion without
detuning to an anomalously dispersive medium. The time advance is measured as
0.28 sec for a pulse propagating through a medium with a 2Hz gain separation,
compared to the same pulse propagating through empty space. We also demonstrate
directly anomalous dispersion profile using a modfied experimental
configuration. Finally, we discuss how anomalous dispersion produced this way
in a faster photorefractive crystal (such as SPS: Sn2P2S6) could be employed to
enhance the sensitivity-bandwidth product of a LIGO type gravitational wave
detector augmented by a White Light Cavity.Comment: 14 pages, 5 figure
Cascaded Parametric Amplification for Highly Efficient Terahertz Generation
A highly efficient, practical approach to high-energy terahertz (THz)
generation based on spectrally cascaded optical parametric amplification
(THz-COPA) is introduced. The THz wave initially generated by difference
frequency generation between a strong narrowband optical pump and optical seed
(0.1-10% of pump energy) kick-starts a repeated or cascaded energy
down-conversion of pump photons. This helps to greatly surpass the
quantum-defect efficiency and results in exponential growth of THz energy over
crystal length. In cryogenically cooled periodically poled lithium niobate,
energy conversion efficiencies >8% for 100 ps pulses are predicted. The
calculations account for cascading effects, absorption, dispersion and
laser-induced damage. Due to the coupled nonlinear interaction of multiple
triplets of waves, THz-COPA exhibits physics distinct from conventional
three-wave mixing parametric amplifiers. This in turn governs optimal
phase-matching conditions, evolution of optical spectra as well as limitations
of the nonlinear process.Comment: 5 pages, double colum
The N-steps Invasion Percolation Model
A new kind of invasion percolation is introduced in order to take into
account the inertia of the invader fluid. The inertia strength is controlled by
the number N of pores (or steps) invaded after the perimeter rupture. The new
model belongs to a different class of universality with the fractal dimensions
of the percolating clusters depending on N. A blocking phenomenon takes place
in two dimensions. It imposes an upper bound value on N. For pore sizes larger
than the critical threshold, the acceptance profile exhibits a permanent tail.Comment: LaTeX file, 12 pages, 5 ps figures, to appear in Physica
Phase transition in the modified fiber bundle model
We extend the standard fiber bundle model (FBM) with the local load sharing
in such a way that the conservation of the total load is relaxed when an
isolated fiber is broken. In this modified FBM in one dimension (1D), it is
revealed that the model exhibits a well-defined phase transition at a finite
nonzero value of the load, which is in contrast to the standard 1D FBM. The
modified FBM defined in the Watts-Strogatz network is also investigated, and
found is the existences of two distinct transitions: one discontinuous and the
other continuous. The effects of the long-range shortcuts are also discussed.Comment: 7 pages, to appear in Europhys. Let
A low incidence of perineal hernia when using a biological mesh after extralevator abdominoperineal excision with or without pelvic exenteration or distal sacral resection in locally advanced rectal cancer patients
Background Extralevator abdominoperineal excision (ELAPE), abdominoperineal excision (APE) or pelvic exenteration (PE) with or without sacral resection (SR) for locally advanced rectal cancer leaves a significant defect in the pelvic floor. At first, this defect was closed primarily. To prevent perineal hernias, the use of a biological mesh to restore the pelvic floor has been increasing. The aim of this study, was to evaluate the outcome of the use of a biological mesh after ELAPE, APE or PE with/without SR. Methods A retrospective study was conducted on patients who had ELAPE, APE or PE with/without SR with a biological mesh (Permacol™) for pelvic reconstruction in rectal cancer in our center between January 2012 and April 2015. The endpoints were the incidence of perineal herniation and wound healing complications. Results Data of 35 consecutive patients [22 men, 13 women; mean age 62 years (range 31–77 years)] were reviewed. Median follow-up was 24 months (range 0.4–64 months). Perineal hernia was reported in 3 patients (8.6%), and was asymptomatic in 2 of them. The perineal wound healed within 3 months in 37.1% (n = 13), within 6 months in 51.4% (n = 18) and within 1 year in 62.9% (n = 22). In 17.1% (n = 6), the wound healed after 1 year. It was not possible to confirm perineal wound healing in the remaining 7 patients (20.0%) due to death or loss to follow-up. Wound dehiscence was reported in 18 patients (51.4%), 9 of whom needed vacuum-assisted closure therapy, surgical closure or a flap reconstruction. Conclusions Closure of the perineal wound after (EL)APE with a biological mesh is associated with a low incidence of perineal hernia. Wound healing complications in this high-risk group of patients are comparable to those reported in the literature
New results for virial coefficients of hard spheres in D dimensions
We present new results for the virial coefficients B_k with k <= 10 for hard
spheres in dimensions D=2,...,8.Comment: 10 pages, 5 figures, to appear in conference proceedings of STATPHYS
2004 in Pramana - Journal of Physic
Discrete Symmetry and Stability in Hamiltonian Dynamics
In this tutorial we address the existence and stability of periodic and
quasiperiodic orbits in N degree of freedom Hamiltonian systems and their
connection with discrete symmetries. Of primary importance in our study are the
nonlinear normal modes (NNMs), i.e periodic solutions which represent
continuations of the system's linear normal modes in the nonlinear regime. We
examine the existence of such solutions and discuss different methods for
constructing them and studying their stability under fixed and periodic
boundary conditions. In the periodic case, we employ group theoretical concepts
to identify a special type of NNMs called one-dimensional "bushes". We describe
how to use linear combinations such NNMs to construct s(>1)-dimensional bushes
of quasiperiodic orbits, for a wide variety of Hamiltonian systems and exploit
the symmetries of the linearized equations to simplify the study of their
destabilization. Applying this theory to the Fermi Pasta Ulam (FPU) chain, we
review a number of interesting results, which have appeared in the recent
literature. We then turn to an analytical and numerical construction of
quasiperiodic orbits, which does not depend on the symmetries or boundary
conditions. We demonstrate that the well-known "paradox" of FPU recurrences may
be explained in terms of the exponential localization of the energies Eq of
NNM's excited at the low part of the frequency spectrum, i.e. q=1,2,3,....
Thus, we show that the stability of these low-dimensional manifolds called
q-tori is related to the persistence or FPU recurrences at low energies.
Finally, we discuss a novel approach to the stability of orbits of conservative
systems, the GALIk, k=2,...,2N, by means of which one can determine accurately
and efficiently the destabilization of q-tori, leading to the breakdown of
recurrences and the equipartition of energy, at high values of the total energy
E.Comment: 50 pages, 13 figure
Monte Carlo study of the magnetic critical properties of the two-dimensional Ising fluid
A two-dimensional fluid of hard spheres each having a spin and
interacting via short-range Ising-like interaction is studied near the second
order phase transition from the paramagnetic gas to the ferromagnetic gas
phase. Monte Carlo simulation technique and the multiple histogram data
analysis were used. By measuring the finite-size behaviour of several different
thermodynamic quantities,we were able to locate the transition and estimate
values of various static critical exponents. The values of exponents
and are close to the ones for the two-dimensional
lattice Ising model. However, our result for the exponent is very
different from the one for the Ising universality class.Comment: 6 pages, 8 figures. To appear in Phys. Rev.
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