172,870 research outputs found
Epistola a Dinet
No full textLa curatela dell'Epistola a Dinet è di Belgioioso G. e Capone A., la traduzione è a c. di Capone A., le note sono a cura di Agostini I. e Capone A. Nello stesso volume è a c. di Capone A. la revisione della traduzione delle seguenti opere: Regole (pp. 673-815), La ricerca della verità (pp. 817-871) e Meditazioni (pp. 659-1417)
Enhancement of superconductivity close to a Mott transition
No full textAn s-wave superconducting phase is shown to appear in the proximity of the Mott transition in an extended three-orbital Hubbard model suitable for the description of alkali-doped fullerides. We discuss the key features and the possible generality of this phase diagram. (C) 2004 Elsevier B.V. All rights reserved. RI Capone, Massimo/A-7762-200
Electron-phonon interaction in proximity of a Mott transition
No full textThe effects of electron-phonon interaction on the electronic properties of a strongly correlated metal on the verge of a Mott metal-insulator transition are studied by means of dynamical mean-field theory. We show that electron-phonon interactions induce a phase separation instability close to the density driven Mott transition, and give rise to significant renormalization of electronic properties like charge susceptibility. Nonetheless, the strong correlation reduces the effectiveness of the electron-phonon coupling in driving a polaronic crossover. (c) 2005 Elsevier B.V. All rights reserved. RI Grilli, Marco/C-6309-2009; Capone, Massimo/A-7762-200
First-Order Pairing Transition and Phase Separation in the Attractive Hubbard model
No full textThe normal state properties of the Hubbard model are studied by means of the Dynamical Mean-Field Theory. Even in the gauge-symmetric subspace, a first-order transition occurs between a Fermi-liquid phase and a strong-coupling bound-pairs phase, which can be thought as a "disordered" superconductor. The transition is of first order for all densities different from n = 1, and it is accompanied by a region of phase separation between the two phases at different densities. RI Grilli, Marco/C-6309-2009; Capone, Massimo/A-7762-200
First-order pairing transition and single-particle spectral function in the attractive Hubbard model
No full textA dynamical mean-field theory analysis of the attractive Hubbard model in the normal phase is carried out upon restricting to solutions where superconducting order is not allowed. A clear first-order pairing transition as a function of the coupling takes place at all the electron densities out of half filling between a Fermi liquid, stable for U < U-c, and an insulating bound pairs phase for U > Uc, and it is accompanied by phase separation. The spectral function in the metallic phase is constituted by a low-energy structure around the Fermi level, which disappears discontinuously at U = U-c, and two high-energy features (Hubbard bands), which persist in the insulating phase. RI Grilli, Marco/C-6309-2009; Capone, Massimo/A-7762-200
Interplay of strong correlation and Jahn-Teller effect in orbitally degenerate systems
No full textWe study the unconventional insulating state in A(4)C(60) by means of the dynamical mean-held theory. The interplay between the electron-electron correlation and the Jahn-Teller interaction determines the properties of these compounds. The system is a Mott-Jahn-Teller insulator. In that state, conduction between molecules is blocked by on-site Coulomb repulsion, magnetism is suppressed by intra-molecular Jahn-Teller effect, and important excitations (such as optical and spin gap) are found to be essentially intra-molecular. Experimental values of the optical and spin gaps are recovered by our calculations. RI Capone, Massimo/A-7762-200
The small-polaron crossover: Comparison between exact results and vertex correction approximation
No full textWe study the crossover from quasi-free electron to small polaron in the Holstein model for a single electron by means of both exact and self-consistent calculations in one dimension and on an infinite coordination lattice. We show that the crossover occurs when both strong coupling (lambda > 1) and multiphonon (alpha(2) > 1) conditions are fulfilled leading to different relevant coupling constants (lambda) in the adiabatic and (alpha(2)) antiadiabatic region of the parameters space. We also show that the self-consistent calculations obtained by including the first electron-phonon vertex correction give accurate results in a sizeable region of the phase diagram well separated from the polaronic crossover. RI Grimaldi, Claudio/B-1641-2010; Capone, Massimo/A-7762-200
Detecting pairing and polarization crossovers in systems with retarded interactions
No full textIn a system where a boson (e.g., a phonon) of finite frequency omega(0) is coupled to electrons, two phenomena occur as the coupling is increased: local electron pairing via multiple boson exchanges and polarization of the boson field. Within a path integral formalism in a dynamical mean-field approach, we introduce two distribution functions which allow us to pinpoint the two effects. When coo is smaller than the bandwidth D, pairing and polarization occur for fairly similar couplings for all considered temperatures. When omega(0) > D, the two phenomena tend to coincide only for T >> omega(0), but are no longer tied for low temperatures so that a state of local pairs without finite polarization is stabilized. (c) 2007 Elsevier B.V. All rights reserved. RI Capone, Massimo/A-7762-200
Energetic balance of the superconducting transition across the BCS-Bose Einstein crossover in the attractive Hubbard model
No full textWe investigate by means of Dynamical mean-field theory the crossover from BCS superconductivity to Bose-Einstein (BE) condensation of preformed pairs which occurs in the attractive Hubbard model by increasing the attraction strength. Fixing the density to a generic value n=0.75, we follow the evolution of the two energy scales underlying the superconducting phenomenon, the gap Delta(0), and the superfluid stiffness D-S, which controls the phase coherence. The BCS-BE crossover is clearly mirrored in a change in the hierarchy of these two scales, the smallest of the two controlling the critical temperature. In the whole intermediate-to-strong-coupling region T-c scales with D-S, while T-C is proportional to Delta(0) only in the BCS regime. This evolution as a function of the interaction qualitatively resembles what happens in the cuprates when the doping is decreased towards the Mott insulator. This continuous change reflects also in the energetic balance at the superconducting transition. While, as it is well known, superconductivity is stabilized by a potential energy gain in the BCS regime, the strong-coupling superconductivity is made stable by a reduction of kinetic energy. Interestingly the intermediate-coupling region, where the maximum T-c is achieved, behaves similarly to the strong-coupling regime, and its gain in kinetic energy is the largest as a function of the coupling. Since the integral of the optical conductivity is proportional to the kinetic energy, the above finding implies that the attractive Hubbard model can account qualitatively for the anomalous behavior of optical spectra around T-c, where an increase of spectral weight is observed in under and optimally doped cuprates, while the overdoped samples have a more standard behavior. This qualitative agreement is lost in the normal phase, specifically at strong coupling, calling for the inclusion of strong correlation effects in the theoretical description. RI Capone, Massimo/A-7762-200
Paring and Superconductivity from weak to strong coupling in the Attractive Hubbard model
No full textThe finite-temperature phase diagram of the attractive Hubbard model is studied by means of the dynamical mean-field theory. We first consider the normal phase of the model by explicitly frustrating the superconducting ordering. In this case, we obtain a first-order pairing transition between a metallic phase and a paired phase formed by strongly coupled incoherent pairs. The transition line ends in a finite temperature critical point, but a crossover between two qualitatively different solutions still occurs at higher temperature. Comparing the superconducting- and the normal-phase solutions, we find that the superconducting instability always occurs before the pairing transition in the normal phase, i.e. T-c > T-pairing. Nevertheless, the high-temperature phase diagram at T > T-c is still characterized by a crossover from a metallic phase to a preformed pair phase. We characterize this crossover by computing different observables that can be used to identify the pseudogap region, like the spin susceptibility, the specific heat and the single-particle spectral function. RI Capone, Massimo/A-7762-2008; Barone, Paolo/C-8918-201
- …
