3,408 research outputs found
Retrieving information from a noisy "knowledge network"
We address the problem of retrieving information from a noisy version of the
``knowledge networks'' introduced by Maslov and Zhang. We map this problem onto
a disordered statistical mechanics model, which opens the door to many
analytical and numerical approaches. We give the replica symmetric solution,
compare with numerical simulations, and finally discuss an application to real
datas from the United States Senate.Comment: 10 pages, 4 figures. Writing of the last section improved; version
accepted in JSTA
Phase separation frustrated by the long range Coulomb interaction II: Applications
The theory of first order density-driven phase transitions with frustration
due to the long range Coulomb (LRC) interaction develop on paper I of this
series is applied to the following physical systems: i) the low density
electron gas ii) electronic phase separation in the low density three
dimensional model iii) in the manganites near the charge ordered phase.
We work in the approximation that the density within each phase is uniform and
we assume that the system separates in spherical drops of one phase hosted by
the other phase with the distance between drops and the drop radius much larger
than the interparticle distance. For i) we study a well known apparent
instability related to a negative compressibility at low densities. We show
that this does not lead to macroscopic drop formation as one could expect
naively and the system is stable from this point of view. For ii) we find that
the LRC interaction significantly modifies the phase diagram favoring uniform
phases and mixed states of antiferromagnetic (AF) regions surrounded by
metallic regions over AF regions surrounded by empty space. For iii) we show
that the dependence of local densities of the phases on the overall density
found in paper I gives a non-monotonous behavior of the Curie temperature on
doping in agreement with experiments.Comment: Second part of cond-mat/0010092 12 pages, 12 figure
Phase separation frustrated by the long range Coulomb interaction I: Theory
We analyze the combined effect of the long range Coulomb (LRC) interaction
and of surface energy on first order density-driven phase transitions in the
presence of a compensating rigid background. We study mixed states formed by
regions of one phase surrounded by the other in the case in which the scale of
the inhomogeneities is much larger than the interparticle distance. Two
geometries are studied in detail: spherical drops of one phase into the other
and a layered structure of one phase alternating with the other. We find the
optimum density profile in an approximation in which the free energy is a
functional of the local density (LDA). It is shown that an approximation in
which the density is assumed to be uniform (UDA) within each phase region gives
results very similar to those of the more involved LDA approach. Within the UDA
we derive the general equations for the chemical potential and the pressures of
each phase which generalize the Maxwell construction to this situation. The
equations are valid for a rather arbitrary geometry. We find that the
transition to the mixed state is quite abrupt i.e. inhomogeneities of the first
phase appear with a finite value of the radius and of the phase volume
fraction. The maximum size of the inhomogeneities is found to be on the scale
of a few electric field screening lengths. Contrary to the ordinary Maxwell
construction, the inverse specific volume of each phase depends here on the
global density in the coexistence region and can decrease as the global density
increases. The range of densities in which coexistence is observed shrinks as
the LRC interaction increases until it reduces to a singular point. We argue
that close to this singular point the system undergoes a lattice instability as
long as the inverse lattice compressibility is finite.Comment: 17 pages, 14 figures. We added a section were the density profile of
inhomogeneities is arbitrary and included other geometries. The applications
of the original version are in a separate pape
Marginal Fermi liquid behavior from 2d Coulomb interaction
A full, nonperturbative renormalization group analysis of interacting
electrons in a graphite layer is performed, in order to investigate the
deviations from Fermi liquid theory that have been observed in the experimental
measures of a linear quasiparticle decay rate in graphite. The electrons are
coupled through Coulomb interactions, which remain unscreened due to the
semimetallic character of the layer. We show that the model flows towards the
noninteracting fixed-point for the whole range of couplings, with logarithmic
corrections which signal the marginal character of the interaction separating
Fermi liquid and non-Fermi liquid regimes.Comment: 7 pages, 2 Postscript figure
Striped phases in the two-dimensional Hubbard model with long-range Coulomb interaction
We investigate the formation of partially filled domain walls in the
two-dimensional Hubbard model in the presence of long-range interaction. Using
an unrestricted Gutzwiller variational approach we show that: i) the strong
local interaction favors charge segregation in stripe domain walls; ii) The
long-range interaction favors the formation of half-filled vertical stripes
with a period doubling due to the charge and a period quadrupling due to the
spins along the wall. Our results show that, besides the underlying lattice
structure, also the electronic interactions can contribute to determine the
different domain wall textures in Nd doped copper oxides and nickel oxides
Screening of Nuclear Reactions in the Sun and Solar Neutrinos
We quantitatively determine the effect and the uncertainty on solar neutrino
production arising from the screening process. We present predictions for the
solar neutrino fluxes and signals obtained with different screening models
available in the literature and by using our stellar evolution code. We explain
these numerical results in terms of simple laws relating the screening factors
with the neutrino fluxes. Futhermore we explore a wider range of models for
screening, obtained from the Mitler model by introducing and varying two
phenomenological parameters, taking into account effects not included in the
Mitler prescription. Screening implies, with respect to a no-screening case, a
central temperat reduction of 0.5%, a 2% (8%) increase of Beryllium
(Boron)-neutrino flux and a 2% (12%) increase of the Gallium (Chlorine) signal.
We also find that uncertainties due to the screening effect ar at the level of
1% for the predicted Beryllium-neutrino flux and Gallium signal, not exceeding
3% for the Boron-neutrino flux and the Chlorine signal.Comment: postscript file 11 pages + 4 figures compressed and uuencoded we have
replaced the previous paper with a uuencoded file (the text is the same) for
any problem please write to [email protected]
Optical excitation of phase modes in strongly disordered superconductors
According to the Goldstone theorem the breaking of a continuous U(1) symmetry
comes along with the existence of low-energy collective modes. In the context
of superconductivity these excitations are related to the phase of the
superconducting (SC) order parameter and for clean systems are optically
inactive. Here we show that for strongly disordered superconductors phase modes
acquire a dipole moment and appear as a subgap spectral feature in the optical
conductivity. This finding is obtained with both a gauge-invariant random-phase
approximation scheme based on a fermionic Bogoliubov-de Gennes state as well as
with a prototypical bosonic model for disordered superconductors. In the
strongly disordered regime, where the system displays an effective granularity
of the SC properties, the optically active dipoles are linked to the isolated
SC islands, offering a new perspective for realizing microwave optical devices
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