185,881 research outputs found
Density-correlator signatures of the vulcanization transition
Certain density correlators, measurable via various experimental techniques,
are studied in the context of the vulcanization transition. It is shown that
these correlators contain essential information about both the vulcanization
transition and the emergent amorphous solid state. Contact is made with various
physical ingredients that have featured in experimental studies of amorphous
colloidal and gel systems and in theoretical studies of the glassy state.Comment: 7 pages, 1 figur
Measuring effective temperatures in out-of-equilibrium driven systems
We introduce and solve a model of a thermometric measurement on a driven
glassy system in a stationary state. We show that a thermometer with a
sufficiently slow response measures a temperature higher than that of the
environment, but that the measured temperature does not usually coincide with
the effective temperature related to the violation of the
Fluctuation-Dissipation Theorem.Comment: 8 pages, 8 figures, LaTeX, Springer Journal class (included
Density-correlator signatures of the vulcanization transition
Certain density correlators, measurable via various experimental techniques,
are studied in the context of the vulcanization transition. It is shown that
these correlators contain essential information about both the vulcanization
transition and the emergent amorphous solid state. Contact is made with various
physical ingredients that have featured in experimental studies of amorphous
colloidal and gel systems and in theoretical studies of the glassy state.Comment: 7 pages, 1 figur
Modelling Collective Opinion Formation by Means of Active Brownian Particles
The concept of active Brownian particles is used to model a collective
opinion formation process. It is assumed that individuals in community create a
two-component communication field that influences the change of opinions of
other persons and/or can induce their migration. The communication field is
described by a reaction-diffusion equation, the opinion change of the
individuals is given by a master equation, while the migration is described by
a set of Langevin equations, coupled by the communication field. In the
mean-field limit holding for fast communication we derive a critical population
size, above which the community separates into a majority and a minority with
opposite opinions. The existence of external support (e.g. from mass media)
changes the ratio between minority and majority, until above a critical
external support the supported subpopulation exists always as a majority.
Spatial effects lead to two critical ``social'' temperatures, between which the
community exists in a metastable state, thus fluctuations below a certain
critical wave number may result in a spatial opinion separation. The range of
metastability is particularly determined by a parameter characterizing the
individual response to the communication field. In our discussion, we draw
analogies to phase transitions in physical systems.Comment: Revised text version. Accepted for publication in European Physics
Journal B. For related work see
http://summa.physik.hu-berlin.de/~frank/active.html and
http://www.if.pw.edu.pl/~jholys
A generalized spin model of financial markets
We reformulate the Cont-Bouchaud model of financial markets in terms of
classical "super-spins" where the spin value is a measure of the number of
individual traders represented by a portfolio manager of an investment agency.
We then extend this simplified model by switching on interactions among the
super-spins to model the tendency of agencies getting influenced by the opinion
of other managers. We also introduce a fictitious temperature (to model other
random influences), and time-dependent local fields to model slowly changing
optimistic or pessimistic bias of traders. We point out close similarities
between the price variations in our model with super-spins and total
displacements in an -step Levy flight. We demonstrate the phenomena of
natural and artificially created bubbles and subsequent crashes as well as the
occurrence of "fat tails" in the distributions of stock price variations.Comment: 11 pages LATEX, 7 postscript figures; longer text with theoretical
analysis, more accurate numerical data, better terminology, additional
references. Accepted for publication in European Physical Journal
Ground states versus low-temperature equilibria in random field Ising chains
We discuss with the aid of random walk arguments and exact numerical
computations the magnetization properties of one-dimensional random field
chains. The ground state structure is explained in terms of absorbing and
non-absorbing random walk excursions. At low temperatures, the magnetization
profiles follow those of the ground states except at regions where a local
random field fluctuation makes thermal excitations feasible. This follows also
from the non-absorbing random walks, and implies that the magnetization length
scale is a product of these two scales. It is not simply given by the
Imry-Ma-like ground state domain size nor by the scale of the thermal
excitations.Comment: 7 pages LaTeX, 8 eps-figures include
Il Presente e il futuro della legge sulle società in Italia
Nell’ordinamento italiano non si può parlare – come a tutti noto – di una legge sulle
società di capitali, in quanto la disciplina di tali società è contenuta all’interno del sistema
codicistico, che contempla sia le società personali sia le società di capitali. La distinzione,
articolata per tipi di società (tre tipi di società personali a fronte di tre tipi di società di capitali)
ed integrata dalla disciplina delle società mutualistiche e consortili, è stata confermata anche
nella recente riforma del diritto societario italiano, la quale ha riguardato esclusivamente le
società di capitali (società per azioni, società in accomandita per azioni e società a
responsabilità limitata) e le società cooperative.
Pertanto, in questa sede, si farà riferimento alle disposizioni della riforma (d.legvo 17
gennaio 2003, n. 6) per tracciare un quadro della disciplina vigente in Italia e delle sue
prospettive, con espressa esclusione dell’esame relativo alla disciplina delle società con azioni
quotate (d. leg.vo 24 febbraio 1998, n. 58) ed a quella delle cooperative
Crowd-Anticrowd Theory of Multi-Agent Market Games
We present a dynamical theory of a multi-agent market game, the so-called
Minority Game (MG), based on crowds and anticrowds. The time-averaged version
of the dynamical equations provides a quantitatively accurate, yet intuitively
simple, explanation for the variation of the standard deviation (`volatility')
in MG-like games. We demonstrate this for the basic MG, and the MG with
stochastic strategies. The time-dependent equations themselves reproduce the
essential dynamics of the MG.Comment: Presented at APFA2 (Liege) July 2000. Proceedings: Eur.Phys.J. B
[email protected]
Random quantum magnets with broad disorder distribution
We study the critical behavior of Ising quantum magnets with broadly
distributed random couplings (J), such that , , for large (L\'evy flight statistics).
For sufficiently broad distributions, , the critical behavior
is controlled by a line of fixed points, where the critical exponents vary with
the L\'evy index, . In one dimension, with , we obtaind
several exact results through a mapping to surviving Riemann walks. In two
dimensions the varying critical exponents have been calculated by a numerical
implementation of the Ma-Dasgupta-Hu renormalization group method leading to
. Thus in the region , where the
central limit theorem holds for the broadness of the distribution is
relevant for the 2d quantum Ising model.Comment: 10pages, 13figures, final for
Room temperature stable single-photon source
We report on the realization of a stable solid state room temperature source
for single photons. It is based on the fluorescence of a single
nitrogen-vacancy (NV) color center in a diamond nanocrystal. Antibunching has
been observed in the fluorescence light under both continuous and pulsed
excitation. Our source delivers 2*10^4 single-photon pulses per second at an
excitation repetition rate of 10 MHz. The number of two-photon pulses is
reduced by a factor of five compared to strongly attenuated coherent sources.Comment: 7 pages, 10 figures, accepted to the special issue of the European
Physical Journal D on "Quantum interference and cryptographic keys: novel
physics and advancing technologies", proceedings of the conference QUICK 200
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