Enhanced superconductivity, Kondo behavior and negative-curvature resistivity of oxygen-irradiated thin films of aluminium

TL;DR

This study investigates how oxygen irradiation modifies aluminum thin films, leading to enhanced superconductivity, Kondo behavior, and negative-curvature resistivity, through defect creation and charge transfer processes.

Contribution

It reveals the dual role of oxygen in irradiation-induced modifications, introducing a multistep process involving defect stabilization and charge transfer that explains the observed phenomena.

Findings

Enhanced superconductivity observed after oxygen irradiation.

Identification of Kondo behavior due to paramagnetic defects.

Negative-curvature resistivity linked to charge liberation.

Abstract

We followed the evolution of the normal and superconducting properties of Al thin films after each session of various successive oxygen irradiations at ambient temperature. Such irradiated films, similar to the granular ones, exhibit enhanced superconductivity, Kondo behavior and negative-curvature resistivity. Two distinct roles of oxygen are identified: as a damage-causing projectile and as an implanted oxidizing agent. The former gives rise to the processes involved in the conventional recovery stages. The latter, considered within the context of the Cabrera-Mott model, gives rise to a multistep process which involves charges transfer and creation of stabilized vacancies and charged defects. Based on the outcome of this multistep process, we consider (i) the negative curvature resistivity as a manifestation of a thermally-assisted liberation of trapped electric charges, (ii) the Kondo contribution as a spin-flip scattering from paramagnetic, color-center-type defects, and (iii) the enhancement of T_{c} as being due to a lattice softening facilitated by the stabilized defects and vacancies. The similarity in the phase diagrams of granular and irradiated films as well as the aging effects are discussed along the same line of reasoning.