1,580 research outputs found
Local structure of directed networks
Previous work on undirected small-world networks established the paradigm
that locally structured networks tend to have high density of short loops. On
the other hand, many realistic networks are directed. Here we investigate the
local organization of directed networks and find, surprisingly, that real
networks often have very few short loops as compared to random models. We
develop a theory and derive conditions for determining if a given network has
more or less loops than its randomized counterpart. These findings carry broad
implications for structural and dynamical processes sustained by directed
networks
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
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
Study of temperature dependent atomic correlations in MgB
We have studied the evolution with temperature of the local as well as the
average crystal structure of MgB using the real-space atomic pair
distribution function (PDF) measured by high resolution neutron powder
diffraction. We have investigated the correlations of the B-B and B-Mg nearest
neighbor pair motion by comparing, in the wide temperature range from T=10 K up
to T=600 K, the mean-square displacements (MSD) of single atoms with the
mean-square relative displacements (MSRD) obtained from the PDF peak
linewidths. The results show that the single atom B and Mg vibrations are
mostly decoupled from each other, with a small predominance of positive (in
phase) correlation factor for both the B-B and B-Mg pairs. The small positive
correlation is almost temperature independent, in contrast with our theoretical
calculations; this can be a direct consequence of the strong decay processes of
the anharmonic phonons
Ising spin glass models versus Ising models: an effective mapping at high temperature II. Applications to graphs and networks
By applying a recently proposed mapping, we derive exactly the upper phase
boundary of several Ising spin glass models defined over static graphs and
random graphs, generalizing some known results and providing new ones.Comment: 11 pages, 1 Postscript figur
An algorithm for counting circuits: application to real-world and random graphs
We introduce an algorithm which estimates the number of circuits in a graph
as a function of their length. This approach provides analytical results for
the typical entropy of circuits in sparse random graphs. When applied to
real-world networks, it allows to estimate exponentially large numbers of
circuits in polynomial time. We illustrate the method by studying a graph of
the Internet structure.Comment: 7 pages, 3 figures, minor corrections, accepted versio
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