2,207 research outputs found
Three-Dimensional Quantum Gravity Coupled to Gauge Fields
We show how to simulate U(1) gauge fields coupled to three-dimensional
quantum gravity and then examine the phase diagram of this system. Quenched
mean field theory suggests that a transition separates confined and deconfined
phases (for the gauge matter) in both the negative curvature phase and the
positive curvature phase of the quantum gravity, but numerical simulations find
no evidence for such transitions.Comment: 16 page
High volumetric capacitance near insulator-metal percolation transition
A new type of a capacitor with a very high volumetric capacitance is
proposed. It is based upon the known phenomenon of a sharp increase of the
dielectric constant of the metal-insulator composite in the vicinity of the
percolation threshold, but still on the insulator side. The optimization
suggests that the metallic particles should be of nanoscale and that the
distance between planar electrodes should be somewhat larger than the
correlation length of the percolation theory and 10 to 20 times larger than the
size of the particles while the area of the electrodes might be unlimited. The
random electric field in the capacitors is found to be larger than the average
field corresponding to the potential difference of electrodes. This random
field is potentially responsible for dielectric breakdown. The estimated
breakdown voltage of the new capacitor shows that the stored energy density
might be significantly larger than that of electrolytic capacitors while the
volumetric capacitances might be comparable. The charging and discharging times
should be significantly smaller than corresponding times of batteries and even
electrolytic capacitors.Comment: 10 pages 1 EPS figur
Probability distribution of persistent spins in a Ising chain
We study the probability distribution of , the fraction of
spins unflipped till time , in a Ising chain with ferromagnetic
interactions. The distribution shows a peak at and in general is
non-Gaussian and asymmetric in nature. However for it shows a
Gaussian decay. A data collapse can be obtained when versus
is plotted with and .
Interestingly, shows a different behaviour compared to , the persistence probability which follows the well-known behaviour
. A quantitative estimate of the asymmetry and
non-Gaussian nature of is made by calculating its skewness and
kurtosis.Comment: 4 pages, submitted to J. Phys
Invaded cluster algorithm for a tricritical point in a diluted Potts model
The invaded cluster approach is extended to 2D Potts model with annealed
vacancies by using the random-cluster representation. Geometrical arguments are
used to propose the algorithm which converges to the tricritical point in the
two-dimensional parameter space spanned by temperature and the chemical
potential of vacancies. The tricritical point is identified as a simultaneous
onset of the percolation of a Fortuin-Kasteleyn cluster and of a percolation of
"geometrical disorder cluster". The location of the tricritical point and the
concentration of vacancies for q = 1, 2, 3 are found to be in good agreement
with the best known results. Scaling properties of the percolating scaling
cluster and related critical exponents are also presented.Comment: 8 pages, 5 figure
Surface Partition of Large Fragments
The surface partition of large fragments is derived analytically within a
simple statistical model by the Laplace-Fourier transformation method. In the
limit of small amplitude deformations, a suggested Hills and Dales Model
reproduces the leading term of the famous Fisher result for the surface entropy
with an accuracy of a few percent. The surface partition of finite fragments is
discussed as well.Comment: 4 pages, 1 figur
Roughness of Interfacial Crack Front: Correlated Percolation in the Damage Zone
We show that the roughness exponent zeta of an in-plane crack front slowly
propagating along a heterogeneous interface embeded in a elastic body, is in
full agreement with a correlated percolation problem in a linear gradient. We
obtain zeta=nu/(1+nu) where nu is the correlation length critical exponent. We
develop an elastic brittle model based on both the 3D Green function in an
elastic half-space and a discrete interface of brittle fibers and find
numerically that nu=1.5, We conjecture it to be 3/2. This yields zeta=3/5. We
also obtain by direct numerical simulations zeta=0.6 in excellent agreement
with our prediction. This modelling is for the first time in close agreement
with experimental observations.Comment: 4 pages RevTeX
2D Metal-Insulator transition as a percolation transition
By carefully analyzing the low temperature density dependence of 2D
conductivity in undoped high mobility n-GaAs heterostructures, we conclude that
the 2D metal-insulator transition in this system is a density inhomogeneity
driven percolation transition due to the breakdown of screening in the random
charged impurity disorder background. In particular, our measured conductivity
exponent of approaches the 2D percolation exponent value of 4/3 at
low temperatures and our experimental data are inconsistent with there being a
zero-temperature quantum critical point in our system.Comment: 5 pages, 3 figure
Tumor bed brachytherapy for locally advanced laryngeal cancer: a feasibility assessment of combination with ferromagnetic hyperthermia
Purpose. To assess the feasibility of adding hyperthermia to an original method of organ-preserving brachytherapy treatment for locally advanced head and neck tumors. Methods and materials. The method involves organ-preserving tumor resection and adjunctive high-dose-rate (HDR) brachytherapy delivered via afterloading catheters. These catheters are embedded in a polymeric implant prepared intraoperatively to fill the resection cavity, allowing precise computer planning of dose distribution in the surrounding at-risk tumor bed tissue. Theoretical and experimental analyzes address the feasibility of heating the tumor bed implant by coupling energy from a 100 kHz magnetic field applied externally into ferromagnetic particles, which are uniformly distributed within the implant. The goal is to combine adjuvant hyperthermia (40 °C–45 °C) to at-risk tissue within 5 mm of the resection cavity for thermal enhancement of radiation and chemotherapy response. Results. A five-year relapse free survival rate of 95.8% was obtained for a select group of 48 male patients with T3N0M0 larynx tumors, when combining organ-preserving surgery with HDR brachytherapy from a tumor bed implant. Anticipating the need for additional treatment in patients with more advanced disease, a theoretical analysis demonstrates the ability to heat at-risk tissue up to 10 mm from the surface of an implant filled with magnetically coupled ferromagnetic balls. Using a laboratory induction heating system, it takes just over 2 min to increase the target tissue temperature by 10 °C using a 19% volume fraction of ferromagnetic spheres in a 2 cm diameter silicone implant. Conclusion. The promising clinical results of a 48 patient pilot study demonstrate the feasibility of a new organ sparing treatment for laryngeal cancer. Anticipating the need for additional therapy, theoretical estimations of potential implant heating are confirmed with laboratory experiments, preparing the way for future implementation of a thermobrachytherapy implant approach for organ-sparing treatment of locally advanced laryngeal cancer
Fractal Dimensions of Confined Clusters in Two-Dimensional Directed Percolation
The fractal structure of directed percolation clusters, grown at the
percolation threshold inside parabolic-like systems, is studied in two
dimensions via Monte Carlo simulations. With a free surface at y=\pm Cx^k and a
dynamical exponent z, the surface shape is a relevant perturbation when k<1/z
and the fractal dimensions of the anisotropic clusters vary continuously with
k. Analytic expressions for these variations are obtained using a blob picture
approach.Comment: 6 pages, Plain TeX file, epsf, 3 postscript-figure
Surviving opinions in Sznajd models on complex networks
The Sznajd model has been largely applied to simulate many sociophysical
phenomena. In this paper we applied the Sznajd model with more than two
opinions on three different network topologies and observed the evolution of
surviving opinions after many interactions among the nodes. As result, we
obtained a scaling law which depends of the network size and the number of
possible opinions. We also observed that this scaling law is not the same for
all network topologies, being quite similar between scale-free networks and
Sznajd networks but different for random networks.Comment: 9 pages, 3 figure
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