512 research outputs found
General duality for abelian-group-valued statistical-mechanics models
We introduce a general class of statistical-mechanics models, taking values
in an abelian group, which includes examples of both spin and gauge models,
both ordered and disordered. The model is described by a set of ``variables''
and a set of ``interactions''. A Gibbs factor is associated to each variable
and to each interaction. We introduce a duality transformation for systems in
this class. The duality exchanges the abelian group with its dual, the Gibbs
factors with their Fourier transforms, and the interactions with the variables.
High (low) couplings in the interaction terms are mapped into low (high)
couplings in the one-body terms. The idea is that our class of systems extends
the one for which the classical procedure 'a la Kramers and Wannier holds, up
to include randomness into the pattern of interaction. We introduce and study
some physical examples: a random Gaussian Model, a random Potts-like model, and
a random variant of discrete scalar QED. We shortly describe the consequence of
duality for each example.Comment: 26 pages, 2 Postscript figure
Numerical Evidence for Spontaneously Broken Replica Symmetry in 3D Spin Glasses
By numerical simulations of the Ising spin glass we find evidence that
spontaneous replica symmetry breaking theory and not the droplet model
describes with good accuracy the equilibrium behavior of the system.Comment: PHYSREV format, 2 .ps figures added with figure command in uufiles
forma
A General Limitation on Monte Carlo Algorithms of Metropolis Type
We prove that for any Monte Carlo algorithm of Metropolis type, the
autocorrelation time of a suitable ``energy''-like observable is bounded below
by a multiple of the corresponding ``specific heat''. This bound does not
depend on whether the proposed moves are local or non-local; it depends only on
the distance between the desired probability distribution and the
probability distribution for which the proposal matrix satisfies
detailed balance. We show, with several examples, that this result is
particularly powerful when applied to non-local algorithms.Comment: 8 pages, LaTeX plus subeqnarray.sty (included at end),
NYU-TH-93/07/01, IFUP-TH33/9
The Critical Hopping Parameter in O(a) improved Lattice QCD
We calculate the critical value of the hopping parameter, , in O(a)
improved Lattice QCD, to two loops in perturbation theory. We employ the
Sheikholeslami-Wohlert (clover) improved action for Wilson fermions.
The quantity which we study is a typical case of a vacuum expectation value
resulting in an additive renormalization; as such, it is characterized by a
power (linear) divergence in the lattice spacing, and its calculation lies at
the limits of applicability of perturbation theory.
The dependence of our results on the number of colors , the number of
fermionic flavors , and the clover parameter , is shown
explicitly. We compare our results to non perturbative evaluations of
coming from Monte Carlo simulations.Comment: 11 pages, 2 EPS figures. The only change with respect to the original
version is inclusion of the standard formulae for the gauge fixing and ghost
parts of the action. Accepted for publication in Physical Review
O(N) and RP^{N-1} Models in Two Dimensions
I provide evidence that the 2D model for is equivalent
to the -invariant non-linear -model in the continuum limit. To
this end, I mainly study particular versions of the models, to be called
constraint models. I prove that the constraint and models are
equivalent for sufficiently weak coupling. Numerical results for their
step-scaling function of the running coupling are
presented. The data confirm that the constraint model is in the samei
universality class as the model with standard action. I show that the
differences in the finite size scaling curves of i and models
observed by Caracciolo et al. can be explained as a boundary effect. It is
concluded, in contrast to Caracciolo et al., that and models
share a unique universality class.Comment: 14 pages (latex) + 1 figure (Postscript) ,uuencode
il terremoto del 30 Ottobre 1901 e la sismicità del versante occidentale del Garda
On November 24, 2004 a strong earthquake (Ml 5.2), followed by a small seismic sequence, hit the western side of Lake Garda area, producing moderate but widespread damage in the main locality of the area (Salò) and more
serious damage in some small villages of the Val Sabbia (Clibbio, Pompegnino). The macroseismic study of the 2004 earthquake has been the opportunity to reappraise the seismicity of the area. The most significant historical earthquake of the area is certainly that occurred on October 30, 1901, well known by the local historical memory. We carried out a revision of the 1901 earthquake, which has significantly improved the informative background; at the same time we reviewed and reassessed the available information on minor earthquakes affecting the area over the past two centuries. The deep review of the 1901 earthquake together with the new data-set allow a better definition of the characteristics of local
seismicity and its seismic hazard
Nonequilibrium Reweighting on the Driven Diffusive Lattice Gas
The nonequilibrium reweighting technique, which was recently developed by the
present authors, is used for the study of the nonequilibrium steady states. The
renewed formulation of the nonequlibrium reweighting enables us to use the very
efficient multi-spin coding. We apply the nonequilibrium reweighting to the
driven diffusive lattice gas model. Combining with the dynamical finite-size
scaling theory, we estimate the critical temperature Tc and the dynamical
exponent z. We also argue that this technique has an interesting feature that
enables explicit calculation of derivatives of thermodynamic quantities without
resorting to numerical differences.Comment: Accepted for publication in J. Phys. A (Lett.
The nature of the continuum limit in the 2D gauge model
The RP(2) gauge model is studied in 2D. We use Monte-Carlo renormalization
techniques for blocking the mean spin-spin interaction, , and the mean gauge
field plaquette, . The presence of the O(3) renormalized trajectory is
verified and is consistent with the known three-loop beta-function. The
first-order `vorticity' transition observed by Solomon et al. is confirmed, and
the location of the terminating critical point is established. New scaling
flows in (,) are observed associated with a large exponent kappa in the
range 4~5. The scaling flows give rise to a strong cross-over effect between
regions of high and low vorticity and are likely to induce an apparent signal
for scaling in the cross-over region which we propose explains the scaling
observed for RP(2), RP(3) and SO(4)-matrix models. The signal for this `pseudo'
scaling will occur for the RP(2) spin model in the cross-over region which is
the region in which computer simulations are done. We find that the RP(2) spin
model is in the same universality class as the O(3) spin model but that it is
likely to require a very large correlation length before the true scaling of
this class sets in. We conjecture that the scaling flows are due either to the
influence of a nearby new renormalized trajectory or to the ghost of the
Kosterlitz-Thouless trajectory in the associated XY model.Comment: 29 pages, LATEX2e, 10 figures, uses styles[epsfig,latexsym
Structural stability and increase in size rationalize the efficiency of lipoplexes in serum.
We have investigated the effect of serum on nanometric structure, size, surface potential, DNA-hinding capacity, and transfection efficiency of DDAB-DOPE/DNA and DC-Chol-DOPE/DNA lipoplexes as a function of membrane charge density and cationic lipid/DNA charge ratio. In the absence of serum, the nanometric structure and DNA binding capacity of lipoplexes determined the transfection efficiency. When serum was added, the transfection efficiency of all lipoplex formulations was found to increase. We identified structural stability and an increase in size in serum as major parameters regulating the efficiency of lipofection. By extrapolation, we propose that serum, regulating the size of resistant lipid - DNA complexes, can control the mechanism of internalization of lipoplexes and, in turn, their efficiency
Random Walks with Long-Range Self-Repulsion on Proper Time
We introduce a model of self-repelling random walks where the short-range
interaction between two elements of the chain decreases as a power of the
difference in proper time. Analytic results on the exponent are obtained.
They are in good agreement with Monte Carlo simulations in two dimensions. A
numerical study of the scaling functions and of the efficiency of the algorithm
is also presented.Comment: 25 pages latex, 4 postscript figures, uses epsf.sty (all included)
IFUP-Th 13/92 and SNS 14/9
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