1,425 research outputs found
Growing Cayley trees described by Fermi distribution
We introduce a model for growing Cayley trees with thermal noise. The
evolution of these hierarchical networks reduces to the Eden model and the
Invasion Percolation model in the limit , respectively.
We show that the distribution of the bond strengths (energies) is described by
the Fermi statistics. We discuss the relation of the present results with the
scale-free networks described by Bose statistics
Effects of azimuth-symmetric acceptance cutoffs on the measured asymmetry in unpolarized Drell-Yan fixed target experiments
Fixed-target unpolarized Drell-Yan experiments often feature an acceptance
depending on the polar angle of the lepton tracks in the laboratory frame.
Typically leptons are detected in a defined angular range, with a dead zone in
the forward region. If the cutoffs imposed by the angular acceptance are
independent of the azimuth, at first sight they do not appear dangerous for a
measurement of the cos(2\phi)-asymmetry, relevant because of its association
with the violation of the Lam-Tung rule and with the Boer-Mulders function. On
the contrary, direct simulations show that up to 10 percent asymmetries are
produced by these cutoffs. These artificial asymmetries present qualitative
features that allow them to mimic the physical ones. They introduce some
model-dependence in the measurements of the cos(2\phi)-asymmetry, since a
precise reconstruction of the acceptance in the Collins-Soper frame requires a
Monte Carlo simulation, that in turn requires some detailed physical input to
generate event distributions. Although experiments in the eighties seem to have
been aware of this problem, the possibility of using the Boer-Mulders function
as an input parameter in the extraction of Transversity has much increased the
requirements of precision on this measurement. Our simulations show that the
safest approach to these measurements is a strong cutoff on the Collins-Soper
polar angle. This reduces statistics, but does not necessarily decrease the
precision in a measurement of the Boer-Mulders function.Comment: 13 pages, 14 figure
A Superconductor Made by a Metal Heterostructure at the Atomic Limit Tuned at the "Shape Resonance": MgB2
We have studied the variation of Tc with charge density and lattice
parameters in Mg1-xAlxB2 superconducting samples at low Al doping x<8%. We show
that high Tc occurs where the chemical potential is tuned at a "superconducting
shape resonance" near the energy Ec of the quantum critical point (QCP) for the
dimensional transition from 2D to 3D electronic structure in a particular
subband of the natural superlattice of metallic atomic boron layers. At the
"shape resonance" the electrons pairs see a 2D Fermi surface at EF-w0 and a 3D
Fermi surface at EF+wo, where wo is the energy cut off of the pairing
interaction. The resonant amplification occurs in a narrow energy range where
EF-Ec is in the range of 2wo.Comment: 16 page
XANES Study of Structural Disorder in Amorphous Silicon
An investigation of the structure of several amorphous silicon (a-Si) films is presented. Samples were prepared by using the ion beam sputtering technique at different substrate deposition temperatures. X-ray absorption spectroscopy and multiple scattering formalism have been used to detect structural variations of the a-Si films. The analysis of the XANES (X-ray absorption near-edge structure) spectra shows that increasing the substrate deposition temperature leads to a structural change toward a higher-level short-range order.
The Feshbach resonance and nanoscale phase separation in a polaron liquid near the quantum critical point for a polaron Wigner crystal
The additional long range order parameter that competes with the high Tc
superconductivity long range order is identified as an electronic crystal of
pseudo Jahn-Teller polarons beyond the critical value of the electron lattice
interaction. We show that the region of quantum critical fluctuations in the
two variables phase diagram of cuprates: the doping (delta) and the chemical
pressure (i.e., the tolerance factor, or the average ionic radius of A-site
cations) can be measured via the microstrain (eta) of the Cu-O length in the
CuO2 lattice. The fluctuating order in the proximity of the microstrain quantum
critical point that competes with the superconducting long range order is the
polaron electronic crystalline phase called a Wigner polaron crystal and the
variation of the spin gap energy as a function of microstrain provides a strong
experimental support for this proposal.Comment: 13 pages, 3 figure
Quantum statistics in complex networks
In this work we discuss the symmetric construction of bosonic and fermionic
networks and we present a case of a network showing a mixed quantum statistics.
This model takes into account the different nature of nodes, described by a
random parameter that we call energy, and includes rewiring of the links. The
system described by the mixed statistics is an inhomogemeous system formed by
two class of nodes. In fact there is a threshold energy such that
nodes with lower energy increase their connectivity
while nodes with higher energy decrease their
connectivity in time.Comment: 5 pages, 2 figure
Two-bands superconductivity with intra- and interband pairing for synthetic superlattices
We consider a model for superconductivity in a two-band superconductor,
having an anisotropic electronic structure made of two partially overlapping
bands with a first hole-like and a second electron-like fermi surface. In this
pairing scenario, driven by the interplay between interband and
intraband pairing terms, we have solved the two gap equations at the
critical temperature and calculate and the chemical potential
as a function of the number of carriers for various values of pairing
interactions, , , and . The results show the
complexity of the physics of condensates with multiple order parameters with
the chemical potential near band edges.Comment: 6 pages, 2 figure
Effect of Al doping on the optical phonon spectrum in Mg(1-x)Al(x)B(2)
Raman and infrared absorption spectra of Mg(1-x)Al(x)B(2) have been collected
for 0<x<0.5 in the spectral range of optical phonons. The x-dependence of the
peak frequency, the width and the intensity of the observed Raman lines has
been carefully analized. A peculiar x-dependence of the optical modes is
pointed out for two different Al doping ranges. In particular the onset of the
high-doping structural phase previously observed in diffraction measurements is
marked by the appearence of new spectral components at high frequencies. A
connection between the whole of our results and the observed suppression of
superconductivity in the high doping region is established
From Majorana theory of atomic autoionization to Feshbach resonances in high temperature superconductors
The Ettore Majorana paper - Theory of incomplete P triplets- published in
1931, focuses on the role of selection rules for the non-radiative decay of two
electron excitations in atomic spectra, involving the configuration interaction
between discrete and continuum channels. This work is a key step for
understanding the 1935 work of Ugo Fano on the asymmetric lineshape of two
electron excitations and the 1958 Herman Feshbach paper on the shape resonances
in nuclear scattering arising from configuration interaction between many
different scattering channels. The Feshbach resonances are today of high
scientific interest in many different fields and in particular for ultracold
gases and high Tc superconductivity.Comment: 13 pages, 7 figures. Journal of Superconductivity and Novel Magnetism
to be publishe
Visual image analysis to estimate morphological and weight measurements in rabbits
[EN] Visual Image Analysis (VIA) has been evaluated to estimate morphological traits and weights of live rabbits and carcasses to improve the body conformation of the new breed Leprino di Viterbo. The reliability of VIA was firstly tested on a sample of 30 does. Then, a total of 365 animal (130 additional does and 205 rabbits at slaughtering weight of kg 2.5) was utilised to calculate some surface traits and correlations between live and carcass weights that were later validated over a new sample of 112 rabbits (37 does and 75 fattened). VIA gave very good results to evaluate morphological and weight measurements. Maximum observed individual difference between data measured by metre and by VIA was only 3.6%. Since standard error (SE) of VIA was lower than SE of metre (0.06 and 0.33 respectively), VIA was considered more reliable than the instrumental measurement. A new parameter, Body Side Surface, was made available by the Image Analysis to estimate Live Weight and Carcass Weight efficiently. Correlations were high (P<0.01) both in does (R2=0.87 for live weight equation) and at slaughter time (R2=0.82 and 0.76 for live and carcass weight equations, respectively). It was concluded that VIA is a viable, quick and practical mean to measure and select for weight and morphological traits as head length, ear length, body length and body side surface.Negretti, P.; Bianconi, G.; Finzi, A. (2007). Visual image analysis to estimate morphological and weight measurements in rabbits. World Rabbit Science. 15(1):37-41. doi:10.4995/wrs.2007.606374115
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