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Abstract: The properties of spatial distribution of luminous matter are investigated analysing all the available three dimensional catalogues of galaxies. In standard view, galaxies are believed to have a fractal distribution at small... more
Abstract: The properties of spatial distribution of luminous matter are investigated analysing all the available three dimensional catalogues of galaxies. In standard view, galaxies are believed to have a fractal distribution at small scale with a crossover to an homogeneous one at large scale. However up to now, the quantitative determination of this presumed homogeneity scale is still lacking. Contrary to such expectations, observational results show, in fact, a very inhomogeneous galaxy distribution.
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One of the most important issue in observational cosmology is the identification of the homogneity scale, ie the scale beyond which a constant density can be defined. In this respect, the authors study the Perseus Pisces redshift survey.... more
One of the most important issue in observational cosmology is the identification of the homogneity scale, ie the scale beyond which a constant density can be defined. In this respect, the authors study the Perseus Pisces redshift survey. They find that this sample shows no tendency towards homogenization, and that the galaxy distribution has a fractal long-range correlation up to the sample depth (≈ 130 h-1 Mpc), with fractal dimension D≈ 2. Moreover, the authors find clear evidence that the luminosity segregation phenomenon ...
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There is nowadays a general agreement on a key role of the σ bands in the superconducting properties of MgB 2. We show that peculiar characteristics of the σ bands give rise to nonadiabatic and anharmonic effects which break the... more
There is nowadays a general agreement on a key role of the σ bands in the superconducting properties of MgB 2. We show that peculiar characteristics of the σ bands give rise to nonadiabatic and anharmonic effects which break the conventional Migdal-Eliashberg framework. Both these features are governed by the small value of the Fermi energy due to the vicinity of the hole doping level to the top of the σ bands. In this context we discuss how the nonadiabatic theory leads to a coherent interpretation of the superconducting properties of MgB 2 without invoking very large couplings and it naturally explains the role of the disorder on T c . It also leads to various specific predictions for the properties of MgB 2 and for the material optimization of these type of compounds. Anharmonicity is also investigated by means of LDA calculations. We find that the anharmonic character of the E2g phonon is essentially driven by the small Fermi energy of the σ holes. We present a simple analytic m...
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ABSTRACT
High-Tc superconductors are characterized by very low carrier densities. This feature leads to two fundamental consequences: on one hand the Fermi energies are correspondingly small and they can be of the same order of phonon frequencies.... more
High-Tc superconductors are characterized by very low carrier densities. This feature leads to two fundamental consequences: on one hand the Fermi energies are correspondingly small and they can be of the same order of phonon frequencies. In such a situation nonadiabatic corrections arising from the breakdown of Migdal's theorem can not be longer neglected. In addition, small carrier densities imply poor screening and correlation effects have to be taken into account. We present a comprehensive overview of the theory of superconductivity generalized into the nonadiabatic regime which is qualitatively different from the conventional one. In this framework some of the observed properties of the cuprates and the fullerene compounds can be naturally accounted for, and a number of theoretical predictions are proposed that can be experimentally tested.
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High-T c superconductors have Fermi energies EF much smaller than conventional metals comparable to phonon frequencies. In such a situation nonadiabatic effects are important. A generalization of Eliashberg theory in the nonadiabatic... more
High-T c superconductors have Fermi energies EF much smaller than conventional metals comparable to phonon frequencies. In such a situation nonadiabatic effects are important. A generalization of Eliashberg theory in the nonadiabatic regime has previously been shown to reproduce some anomalous features of the high-T c superconductors as for instance the enhancement of T c or the isotopic effects on T c and m*. In this contribution we address the issue of the symmetry of the gap in the context of nonadiabatic superconductivity. We show that vertex corrections have a momentum structure which favours d-wave superconductivity when forward scattering is predominant. An additional increase of T c is also found.
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The electrical conductivity of a charged layer of graphite is calculated from first principles within a tight-binding framework. The Fermi surface consists of circles around the P points in the Brillouin zone. In the neighborhood of these... more
The electrical conductivity of a charged layer of graphite is calculated from first principles within a tight-binding framework. The Fermi surface consists of circles around the P points in the Brillouin zone. In the neighborhood of these points we obtain analytical expressions for the electron-phonon coupling gk-->k-->'. The longitudinal and transverse phonons are shown to give exactly the same average
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Page 1. Granular Shearing and Barkhausen Noise Andrea Baldassarri1, Fergal Dalton2, Alberto Petri2, Luciano Pietronero1,3, Giorgio Pontuale2, and Stefano Zapperi1,3 ... F. Heslot, T. Baumberger, B. Perrin, B. Caroli, C. Caroli: Phys. Rev.... more
Page 1. Granular Shearing and Barkhausen Noise Andrea Baldassarri1, Fergal Dalton2, Alberto Petri2, Luciano Pietronero1,3, Giorgio Pontuale2, and Stefano Zapperi1,3 ... F. Heslot, T. Baumberger, B. Perrin, B. Caroli, C. Caroli: Phys. Rev. E 49, 4973 (1994) 29. ...
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Summary The problem of existence and uniqueness of solutions of the Hartree-Fock equations is studied by means of the Lyusternik-Schrinlmann category theory. In the case of a box with all the states with negative energies the existence... more
Summary The problem of existence and uniqueness of solutions of the Hartree-Fock equations is studied by means of the Lyusternik-Schrinlmann category theory. In the case of a box with all the states with negative energies the existence and uniqueness of the solutions is discussed.
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Fullerene compounds have phonon frequencies up to ωmax=0.2 eV and a Fermi energy of the order E F=0.3 eV. It is, therefore, expected that the adiabatic parameter λωph/E F (where λ is the electron-phonon coupling constant and ωph is a... more
Fullerene compounds have phonon frequencies up to ωmax=0.2 eV and a Fermi energy of the order E F=0.3 eV. It is, therefore, expected that the adiabatic parameter λωph/E F (where λ is the electron-phonon coupling constant and ωph is a typical phonon frequency) is not a priori negligible and the conventional theory of phonon-mediated superconductivity is inapplicable in this case.
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ABSTRACT