G. Pepe

Analysis of low-velocity impact damage in reinforced carbon fiber composites by HTS-SQUID magnetometers

Composite materials are susceptible to damage which can be induced by service loads and accidental impacts. The detection of any signature produced by damage is critical to maintaining the integrity of aircraft parts during routine maintenance. A high critical temperature SQUID magnetometer has been successfully employed in the evaluation of the behavior of multi-ply carbon fibers reinforced composite panels for aeronautical applications under low-velocity impacts. Measurements of the induced magnetic field have been carried out above specimen damaged with energy impact from 1 to 40 J. A quasi-linear behavior in two different regimes between the SQUIDs response and the energy of the impact has been found. This suggests a correspondence to the detection of intrinsically different damage that occurs in the laminates at different energies.

Current-induced domain wall depinning and magnetoresistance in La0.7Sr0.3MnO3 planar spin valves

The authors have performed experiments on current-induced domain wall (DW) displacement in La0.7Sr0.3MnO3 nanostructures patterned by gallium (Ga) focused-ion-beam milling. A dc current is found to assist or hinder, according to polarity, an external magnetic field in the depinning of a DW trapped in a nanoconstriction. For large enough currents, the DW depinning occurs in the absence of external magnetic field. The depinning current depends on the transverse anisotropy constant of the region toward which the DW is displaced.

Strong critical current density enhancement in NiCu/NbN superconducting nanostripes for optical detection

We present measurements of ferromagnet/superconductor (NiCu/NbN) and plain superconducting (NbN) nanostripes with the linewidth ranging from 150 to 300 nm. The NiCu (3 nm)/NbN (8 nm) bilayers, as compared to NbN (8 nm), showed a up to six times increase in their critical current density, reaching at 4.2 K the values of 5.5 MA/cm2 for a 150 nm wide nanostripe meander and 12.1 MA/cm2 for a 300 nm one. We also observed six-time sensitivity enhancement when the 150 nm wide NiCu/NbN nanostripe was used as an optical detector. The strong critical current enhancement is explained by the vortex pinning strength and density increase in NiCu/NbN bilayers and confirmed by approximately tenfold increase in the vortex polarizability factor.

Superconducting device with transistor-like properties including large current amplification

We have fabricated and studied a stacked superconducting double tunnel junction device with transistor-like properties. The intermediate electrode is a bilayer consisting of a Nb film together with an Al film that acts as a quasiparticle trap. Large current gains of more than 50 are observed at 4.2 K when the Al layer is normal. The operation is highly directional. Results are explained on the basis of trapping of quasiparticles from a superconductor into a normal metal, together with a conversion of relaxation energy into electronic excitations. Similar devices should have wide applications in low-temperature measurement and detection systems.

The characteristic electron–phonon coupling time of unconventional superconductors and implications for optical detectors

Superconductors are highly suitable materials for radiation detection. Several detector types have been proposed, with properties of fast detection or high wavelength resolution over a wide range of optical frequencies. Their performances depend on the relaxation processes involving phonons, quasiparticles and Cooper pairs occurring during the energy cascade following the absorption of the radiation in the superconductor. The energy down-conversion processes are related to the electron–phonon scattering strength λe−ph which is usually expressed in terms of the electron–phonon coupling time, τ0, which is characteristic for each material. In this paper we estimate the value of τ0 for several classes of superconducting materials not yet investigated. It is calculated in the framework of the McMillan model for the superconducting critical temperature Tc within the Debye approximation. The values obtained for τ0 are discussed as regards new possibilities for unexplored materials in the field of superconducting detectors. In particular, we focus our attention on materials with τ0 values that could play a significant role in both hot electron photodetectors and superconducting tunnel junction devices.

High Tc SQUIDs and eddy-current NDE: a comprehensive investigation from real data to modelling

The interest in magnetometry for eddy-current non-destructive testing, e.g. of planar conductive structures encountered in the aircraft industry, using high-temperature superconducting quantum interference devices (SQUIDs) is primarily due to their high sensitivity to magnetic flux even at very low frequencies. Here it is shown how theoretical, numerical and measurement machineries are combined to get reasonable synthetic and experimental data and to reach a good understanding of the interaction of diffusive wavefields with a damaged non-magnetic metal plate (as a first step towards the retrieval of pertinent features of the defects). The measurement modalities are considered first. It is illustrated in some detail how laboratory-controlled experiments are performed by a SQUID-based probe displaced above artificially damaged plates. Experimental data are then confronted with simulation results in order to evaluate the accuracy and reliability of this measurement system. Simulations are carried out by a computationally fast vector volume integral method dedicated to a planar layering affected by a volumetric defect, which involves the construction of the dyadic Green system of the layering.

Optical emission studies in Au/Ag nanoparticles

Photoluminescence spectra of Au/Ag nanoparticles have been investigated experimentally. The clusters were obtained inside an amorphous polymeric matrix by thermal simultaneous decomposition of both gold and silver mercaptides. The optical characterization in terms of photoluminescence showed a red shift of position of the emission peak as a function of the Ag mercaptide concentration. The red shift is qualitatively explained in terms of the Jellium model along with the quantum size effects.

Effect of intense proton irradiation on properties of Josephson devices

We have experimentally investigated the effects of intense proton beam irradiation (up to 10/sup 15/ p/cm/sup 2/) on Josephson junctions and junction arrays. The devices we have studied were realized using state of the art full-Nb technology, employing same materials and thicknesses of common Josephson digital circuit designs. We have analysed in detail the magnetic field dependence of the junction critical current, and the quasiparticle tunneling current, in order to observe possible occurrence of permanent changes produced by the ionizing particles. No evidence of radiation induced damage on the properties of the junctions has been found.

Magnetic and magnetotransport properties of La0.7Sr0.3MnO3/Permalloy heterostructures

The magnetic and magnetotransport properties of La0.7Sr0.3MnO3/Permalloy heterostructures are investigated. La0.7Sr0.3MnO3 films were grown on (110) SrTiO3 in order to induce an in-plane anisotropy with the easy axis along the [001] direction. An easy axis of magnetization was induced in the Permalloy in the same direction through the application of a magnetic field during the growth. The magnetic characterization of the bilayer is presented and discussed. The transport properties of devices operated in the current perpendicular to plane mode are shown. The possibility of fabricating magnetoresistive devices by simply depositing a ferromagnetic counterelectrode on a manganite film is demonstrated.

Experimental and numerical results of electromagnetic nondestructive testing with HTc SQUIDS

We present here recent results on detection of surface and subsurface artificial features in Al-Ti planar structures, to show current performance of our eddy-current nondestructive evaluation system based on HTc SQUIDs. The anomalous magnetic fields generated by flaws with known electromagnetic characteristics have been modeled by three-dimensional codes based on finite element method and volume integral formulation and developed for the investigated problem. Both numerical solutions have correctly predicted the shape of the complicated magnetic field response which is mainly the result of the shape of the defect, the geometry of the inducing coil and the characteristics of the SQUID gradiometer.