288 research outputs found
New algorithm and results for the three-dimensional random field Ising Model
The random field Ising model with Gaussian disorder is studied using a new
Monte Carlo algorithm. The algorithm combines the advantanges of the replica
exchange method and the two-replica cluster method and is much more efficient
than the Metropolis algorithm for some disorder realizations. Three-dimensional
sytems of size are studied. Each realization of disorder is simulated at
a value of temperature and uniform field that is adjusted to the phase
transition region for that disorder realization. Energy and magnetization
distributions show large variations from one realization of disorder to
another. For some realizations of disorder there are three well separated peaks
in the magnetization distribution and two well separated peaks in the energy
distribution suggesting a first-order transition.Comment: 24 pages, 23 figure
INNOVATION IN THE ECONOMY: KEY DRIVERS AND CHALLENGES
Innovation is at the heart of the modern economy. It is considered to be the main driver of economic growth, increased productivity and improved living standards
PATENT AND INNOVATION ACTIVITY IN RUSSIA: NUMBERS AND PROBLEMS
The article is devoted to the problems of patent and innovation activity in Russia. There is an observed discrepancy between the rather high patent activity in Russia, the stability of Russia's positions in the world innovation ratings and the low level of real innovation activity
Lower Neutrino Mass Bound from SN1987A Data and Quantum Geometry
A lower bound on the light neutrino mass is derived in the framework
of a geometrical interpretation of quantum mechanics. Using this model and the
time of flight delay data for neutrinos coming from SN1987A, we find that the
neutrino masses are bounded from below by eV, in
agreement with the upper bound
eV currently available. When the model is applied to photons with effective
mass, we obtain a lower limit on the electron density in intergalactic space
that is compatible with recent baryon density measurements.Comment: 22 pages, 3 figure
Critical behavior of a fluid in a disordered porous matrix: An Ornstein-Zernike approach
Using a liquid-state approach based on Ornstein-Zernike equations, we study
the behavior of a fluid inside a porous disordered matrix near the liquid-gas
critical point.The results obtained within various standard approximation
schemes such as lowest-order -ordering and the mean-spherical
approximation suggest that the critical behavior is closely related to that of
the random-field Ising model (RFIM).Comment: 10 pages, revtex, to appear in Physical Review Letter
Monte Carlo study of the random-field Ising model
Using a cluster-flipping Monte Carlo algorithm combined with a generalization
of the histogram reweighting scheme of Ferrenberg and Swendsen, we have studied
the equilibrium properties of the thermal random-field Ising model on a cubic
lattice in three dimensions. We have equilibrated systems of LxLxL spins, with
values of L up to 32, and for these systems the cluster-flipping method appears
to a large extent to overcome the slow equilibration seen in single-spin-flip
methods. From the results of our simulations we have extracted values for the
critical exponents and the critical temperature and randomness of the model by
finite size scaling. For the exponents we find nu = 1.02 +/- 0.06, beta = 0.06
+/- 0.07, gamma = 1.9 +/- 0.2, and gammabar = 2.9 +/- 0.2.Comment: 12 pages, 6 figures, self-expanding uuencoded compressed PostScript
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Equilibrium random-field Ising critical scattering in the antiferromagnet Fe(0.93)Zn(0.07)F2
It has long been believed that equilibrium random-field Ising model (RFIM)
critical scattering studies are not feasible in dilute antiferromagnets close
to and below Tc(H) because of severe non-equilibrium effects. The high magnetic
concentration Ising antiferromagnet Fe(0.93)Zn(0.07)F2, however, does provide
equilibrium behavior. We have employed scaling techniques to extract the
universal equilibrium scattering line shape, critical exponents nu = 0.87 +-
0.07 and eta = 0.20 +- 0.05, and amplitude ratios of this RFIM system.Comment: 4 pages, 1 figure, minor revision
OSTEOCONDUCTIVE COMPOSITE MATERIALS BASED ON POLY-L-LACTIDE AND NANOCRYSTALLINE CELLULOSE MODIFIED WITH POLY(GLUTAMIC ACID)
The aim of this work was to obtain the series of composite polymeric materials based on poly-L-lactide (PLLA) with different contents of hydrophilic nanocrystalline cellulose (NCC) and modified with poly(glutamic acid) (PGlu) nanocrystalline cellulose (NCC-PGlu) (5, 10 and 15 wt%) as fillers. For this purpose several methods for modifying NCC with poly(glutamic acid) were tested. The best result was demonstrated by the partial oxidation of the NCC and the subsequent interaction of the obtained aldehyde NCC groups with the terminal amino groups of PGlu.The research was carried out with the use of some equipment of the Research Park of St. Petersburg State University: “Interdisciplinary Center for Nanotechnology” and “Center for Chemical Analysis and Materials Research”
Power-law correlations and orientational glass in random-field Heisenberg models
Monte Carlo simulations have been used to study a discretized Heisenberg
ferromagnet (FM) in a random field on simple cubic lattices. The spin variable
on each site is chosen from the twelve [110] directions. The random field has
infinite strength and a random direction on a fraction x of the sites of the
lattice, and is zero on the remaining sites. For x = 0 there are two phase
transitions. At low temperatures there is a [110] FM phase, and at intermediate
temperature there is a [111] FM phase. For x > 0 there is an intermediate phase
between the paramagnet and the ferromagnet, which is characterized by a
|k|^(-3) decay of two-spin correlations, but no true FM order. The [111] FM
phase becomes unstable at a small value of x. At x = 1/8 the [110] FM phase has
disappeared, but the power-law correlated phase survives.Comment: 8 pages, 12 Postscript figure
Ground state numerical study of the three-dimensional random field Ising model
The random field Ising model in three dimensions with Gaussian random fields
is studied at zero temperature for system sizes up to 60^3. For each
realization of the normalized random fields, the strength of the random field,
Delta and a uniform external, H is adjusted to find the finite-size critical
point. The finite-size critical point is identified as the point in the H-Delta
plane where three degenerate ground states have the largest discontinuities in
the magnetization. The discontinuities in the magnetization and bond energy
between these ground states are used to calculate the magnetization and
specific heat critical exponents and both exponents are found to be near zero.Comment: 10 pages, 6 figures; new references and small changes to tex
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