Massimiliano Capezzali

One-electron spectral functions of the attractive Hubbard model at intermediate coupling

We calculate the one-electron spectral function of the attractive-U Hubbard model in two dimensions. We work in the intermediate coupling and low density regime and evaluate analytically the self-energy. The results are obtained in a framework based on the self-consistent T-matrix approximation. We also calculate the chemical potential of the bound pairs as a function of temperature. On the basis of this calculation we analyze the low-temperature resistivity and specific heat in the normal state of this system. We compare our results with recent beautiful tunneling experiments in the underdoped regime of HTSC-materials.We calculate the one-electron spectral function of the attractive-U Hubbard model in two dimensions. We work in the intermediate coupling and low-density regime and evaluate analytically the self energy. The results are obtained in a framework based on the self-consistent T-matrix approximation. We also calculate the chemical potential of the bound pairs as a function of temperature. On the basis of this calculation we analyze the low-temperature resistivity and specific heat in the normal state of this system. We compare our results with recent beautiful tunneling experiments in the underdoped regime of HTSC-materials.

PSEUDOGAP, QUANTUM PHASE FLUCTUATIONS AND SPECTROSCOPY OF HTCS CUPRATES

Superconducting critical temperatures Tc are calculated as a function of the doping parameter x for generic layered Cu oxide superconductors, within the framework of a 2D-XY model with quantum phase fluctuations. The model accounts for the lower doping threshold observed at x=0.05, and allows for a simple interpretation of the ‘‘pseudogap’’ in terms of condensation of pre-formed pairs. On the other hand, a novel pairing mechanism, based on the 2D confinement of carriers and on the in-plane overscreening of long-range Coulomb repulsion, is proposed. A 2–3D dimensional crossover which suppresses 2D pairing is considered in the overdoped regime

Screening effects in superconductors

The partition function of the Hubbard model with local attraction and long-range Coulomb repulsion between electrons is written as a functional integral with an action A involving a pairing field Δ and a local potential V. After integration over V and over fluctuations in ∥δ∥2, the final form of A involves a Josephson coupling between the local phases of δ and a “kinetic energy” term, representing the screened Coulomb interaction between charge fluctuations. The competition between Josephson coupling and charging energy allows to understand the relation between Tc and composition in high-TC materials, in particular superlattices, alloys and bulk systems of low doping.

Pseudogap in the one-electron spectral functions of the attractive Hubbard model

We calculate the one-electron Greens function of the two-dimensional (2D) attractive Hubbard model by coupling the electrons to pair fluctuations. The latter are approximated by homogeneous amplitude fluctuations and phase correlations corresponding to the XY-model. The electronic density of states shows a pseudogap at temperatures well above the transition temperature Tc. For a quasi-3D system, a superconducting gap emerges out of the pseudogap below Tc.

Excitation spectrum of the two-dimensional attractive Hubbard model

We calculate the one-particle spectral functions above the superconducting transition temperature TC, in the framework of a functional integral approach. The coupling of the electronic self-energy to pair fluctuations, which are treated by means of a time-dependent Ginzburg–Landau equation, yields a double-peak structure, around the Fermi wavenumber. The peak separation is essentially temperature-independent, but the structure sharpens when TC is approached.

Dynamics of vortices in disordered Josephson junctions arrays

We have studied a two-dimensional disordered Josephson Junction Array (JJA), where sites (or bonds) have been randomly removed with a given probability 1 − p. These defect regions of the array are modelled by a fixed distribution of holes, which are considered as being circular. We calculate the vortex mobility as a function of frequency, ω. When the holes are considered as having the same size, the former increases almost monotonically when the frequency gets higher, tending then to the value of the regular lattice as ω → ∞. For holes of different sizes, a similar behavior results from the superposition of the contribution from each family of holes.