arXiv subjects

Massimiliano Capezzali

Publications and source records attributed to Massimiliano Capezzali.

8 recordsLinked to original sources

arXiv2004

Vortex dynamics in dilute two-dimensional Josephson junction arrays

The dynamics of thermally excited vortices in a dilute two-dimensional Josephson junction array where a fraction of the superconducting islands is missing has been investigated using a multiple trapping model. An expression for the frequency dependent mobility of vortices has been calculated which allows to obtain the frequency dependent complex electrodynamic response of the array for different fractions of missing islands.

cond-mat.stat-mech↗

arXiv2002

Vortex dynamics in two-dimensional Josephson junction arrays

The dynamic response of unfrustrated two-dimensional Josephson junction arrays close to, but above the Kosterlitz-Thouless($KT$) transition temperature is described in terms of the vortex dielectric function $ε(ω)$. The latter is calculated by considering separately the contribution of $\ll$free$\gg$ vortices interacting by a screened Coulomb potential, and the $\ll$pair motion$\gg$ of vortices that are closer to each other than the $KT$ correlation length. This procedure allows to understand various anomalous features in $ε(ω)$ and in the flux noise spectra that have been observed experimentally and in dynamic simulations.

cond-mat.supr-con↗

arXiv1999

Dynamics of the 2D two-component plasma near the Kosterlitz-Thouless transition

We study the dynamics of a classical, two-component plasma in two dimensions, in the vicinity of the Kosterlitz-Thouless (KT) transition where the system passes from a dielectric low-temperature phase (consisting of bound pairs) to a conducting phase. We use two ``complementary'' analytical approaches and compare to simulations. The conventional, ``intuitive'' approach is built on the KT picture of independently relaxing, bound pairs. A more formal approach, working with Mori projected dynamic correlation functions, avoids to assume the pair picture from the start. We discuss successes and failures of both approaches, and suggest a way to combine the advantages of both.

cond-mat.stat-mech↗

arXiv1998

Excitation spectrum of the two-dimensional attractive Hubbard model

We calculate the one-particle spectral functions above the superconducting transition temperature $T_{C}$, 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 $T_{C}$ is approached.

cond-mat↗

arXiv1998

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

We calculate the one-electron Green's function of the 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 $T_{C}$. For a quasi-3D system, a superconducting gap emerges out of the pseudogap below $T_{C}$.

cond-mat↗

arXiv1997

One-Electron Spectral Functions of the Attractive Hubbard Model for Intermediate Coupling

We calculate the one-electron spectral function of the attractive (negative-$U$) Hubbard model. We work in the intermediate coupling and low density regime and obtain the self-energy in an approximate analytical form. The excitation spectrum is found to consist of three branches. The results are obtained in a framework, based on the self-consistent T-matrix approximation, which is compatible with the Mermin-Wagner theorem.

cond-mat↗

arXiv1996

Anomalous vortex dynamics in 2D superconductors

Low -frequency dynamic impedance ($σ^{-1}(ω,T)\equiv(σ_{1}+iσ_{2})^{-1}$) measurements on Josephson junction arrays with finite vortex screening length $ξ$, found that $σ_{1}\sim |\logω|$, $σ_{2}\sim$ constant. This implies anomalously sluggish vortex mobilities $μ_{V}(ω)\simσ_{1}^{-1}$, and is in conflict with general dynamical scaling expressions that yield, for low-$ω$, $σ_{1}\rightarrowξ^{2}$ and $σ_{2}\rightarrow 0$. We calculate : a) $σ(ω,T)$ by real-space vortex scaling; b) $μ_{V}(ω)$ using Mori's formalism for a screened Coulomb gas. We find, in addition to the usual critical (large-$ω$) and hydrodynamic (low-$ω$) regimes, a new intermediate-frequency scaling regime into which the experimental data fall. This resolves the above mentioned conflict and makes explicit predictions for the scaling form of $σ(ω,T)$, testable in SNS and SIS arrays.

cond-mat.supr-con↗

arXiv1996

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 $T_{C}$ and composition in high-$T_{C}$ materials, in particular superlattices, alloys and bulk systems of low doping.

cond-mat↗