Votrex avalanche phenomena in MgB2 superconducting film studied by current noise measurements

Vortex avalanche phenomenon enhanced by thermomagnetic instabilities suppresses critical current in MgB2 films. This effect is investigated at 4.2 K by spectral noise power measurements up to 1 kHz with a dc superconducting quantum interference device apparatus. 1/f(gamma) behavior is shown at low frequencies in magnetic fields and feeding currents below j(c). gamma increases from 1 to 2 at increasing magnetic field, typical of fluxon avalanche processes. A peak in the power spectrum frequency behavior, observed only in magnetic field, is attributed to vortex-antivortex annihilations. Thermal instabilities disappear when an efficient thermal link to the He bath is attained and no vortex avalanche extra-noise is observed

Enhanced reactivity at metal-oxide inerface water interaction with MgO ultrathin films

The interest in thin and ultrathin oxide films is increasing rapidly due to the new properties and the many possible technological applications of such materials. In this frame the interaction with water, a major constituent of our atmosphere, is an essential issue for a better characterization of oxide film-based devices. We report here a detailed high-resolution electron energy loss spectroscopy and X-ray photoelectron spectroscopy study of the reactivity of ultrathin MgO films grown on Ag(100) toward H2O. We find that only OH groups are detected at 3 10 K, while at low temperature molecular adsorption prevails. In the former case we observe a strongly enhanced dissociation probability for monolayer and submonolayer MgO films, indicative of an active role of the Ag substrate in the dissociation process. The active sites are suggested to be low-coordinated ions at the border of monolayer MgO islands. Aging phenomena, previously observed on MgO ultrathin films,(1) have been confirmed. Although their origin could not be definitively determined, our data strongly suggest water adsorption not to be the major cause of this process