C. Castellani

Singular quasiparticle scattering in the proximity of charge instabilities

We analyze the behavior of the dynamic scattering amplitude between Fermi liquid quasiparticles at the Fermi surface in the proximity of a charge instability, which may occur in the high temperature superconducting cuprates. Within the infinite-U Hubbard-Holstein model in the slave-boson large-N technique we find that, in the absence of long-range Coulomb forces the scattering amplitude is strongly singular at zero momentum transfer close to the phase separation instability and it has the same form provided by gauge-field theories. In the presence of long-range Coulomb forces the charge instability occurs at finite wavevectors and concomitantly the scattering is still singular but anisotropic. Nevertheless it remains strong over extended regions of the momentum space. In both cases we show how normal state properties are largely affected by this scattering.

Strongly Correlated Superconductivity

High-temperature superconductivity in doped Mott insulators such as the cuprates contradicts the conventional wisdom that electron repulsion is detrimental to superconductivity. Because doped fullerene conductors are also strongly correlated, the recent discovery of high-critical-temperature, presumably s-wave, superconductivity in C60 field effect devices is even more puzzling. We examine a dynamical mean-field solution of a model for electron-doped fullerenes that shows how strong correlations can indeed enhance superconductivity close to the Mott transition. We argue that the mechanism responsible for this enhancement could be common to a wider class of strongly correlated models, including those for cuprate superconductors.

d-wave superconductivity near charge instabilities.

We investigate the symmetry of the superconducting order parameter in the proximity of a phase separation or of an incommensurate charge-density-wave instability. The attractive effective interaction at small or intermediate transferred momenta is singular near the instability. This strongly {ital q}-dependent interaction, together with a residual local repulsion between the quasiparticles and an enhanced density of states for band structures appropriate for the high-temperature superconducting oxides, strongly favors the formation of {ital d}-wave superconductivity. The relative stability with respect to superconductivity in the {ital s}-wave channel is discussed in detail, finding this latter hardly realized in the above conditions. The superconducting temperature is mostly determined by the closeness to the quantum critical point associated with the charge instability and displays a stronger dependence on doping with respect to a simple proximity to a van Hove singularity. The relevance of this scenario and the generic agreement of the resulting phase diagram with the properties displayed by high-temperature superconducting oxides is discussed. {copyright} {ital 1996 The American Physical Society.}

Non-Fermi-liquid behavior and d-wave superconductivity near the charge-density-wave quantum critical point

AbstractA scenario is presented, in which the presence of a quantum critical point due to formation of incommensurate charge density waves accounts for the basic features of the high temperature superconducting cuprates, both in the normal and in the superconducting states. Specifically, the singular interaction arising close to this charge-driven quantum critical point gives rise to the non-Fermi liquid behavior universally found at optimal doping. This interaction is also responsible for d-wave Cooper pair formation with a superconducting critical temperature strongly dependent on doping in the overdoped region and with a plateau in the optimally doped region. In the underdoped region a temperature dependent pairing potential favors local pair formation without superconducting coherence, with a peculiar temperature dependence of the pseudogap and a non-trivial relation between the pairing temperature and the gap itself. This last property is in good qualitative agreement with so far unexplained features of the experiments.

Metallic phase and metal-insulator transition in two-dimensional electronic systems

The recent experimental observation of a metal-insulator transition in two dimensions prompts a re-examination of the theory of disordered interacting systems. We argue that the existing theory permits the existence of a metallic phase and propose a number of experiments such as magnetoconductance and tunnelling in the presence of a parallel field, which should provide diagnostic tests as to whether a given experimental system is in fact in this regime. We also comment on a generic flow diagram which predicts a maximum metallic resistivity.

Electron-phonon interactions in the presence of strong correlations.

We investigate the effect of strong electron-electron repulsion on the electron-phonon interaction from a Fermi-liquid point of view. In particular, we show that the strong interaction is responsible for vertex corrections, which are strongly dependent on the

Modelling the unconventional superconducting properties of expanded A3C60 Fullerides

{\mathit{v}}_{\mathit{F}}

Two-gap model for underdoped cuprate superconductors

q/\ensuremath{\omega} ratio, where

First-order pairing transition and single-particle spectral function in the attractive hubbard model.

{\mathit{v}}_{\mathit{F}}

Pairing and superconductivity from weak to strong coupling in the attractive Hubbard model

is the Fermi velocity and q and \ensuremath{\omega} are the transferred momentum and frequency, respectively. These corrections generically lead to a strong suppression of the effective coupling between quasiparticles mediated by a single phonon exchange in the