Electron sampling depth and saturation effects in perovskite films investigated by soft x-ray absorption spectroscopy

Knowledge of the electron sampling depth and related saturation effects is important for quantitative analysis of X-ray absorption spectroscopy data, yet for oxides with the perovskite structure no quantitative values are so far available. Here we study absorption saturation in films of two of the most-studied perovskites, LCMO and YBCO, at the L2,3 edge of Mn and Cu, respectively. By measuring the electron-yield intensity as a function of photon incidence angle and film thickness, the sampling depth d, photon attenuation length, lambda, and the ratio lambda/d have been independently determined between 50 and 300 K. The extracted sampling depth dLCMO=3 nm for LCMO at high temperatures in its polaronic insulator state (150-300K) is not much larger than values reported for pure transition metals (d Co or Ni=2-2.5nm) at room temperature, but it is smaller than dYBCO=3.9 nm for metallic YBCO that is in turn smaller than the value reported for Fe3O4 (d=4.5 nm) The measured dLCMO increases to 4.5 nm when LCMO is in the metallic state at low temperatures. These results indicate that a universal rule of thumb for the sampling depth in oxides cannot be assumed, and that it can be measurably influenced by electronic phase transitions that derive from strong correlations.Comment: 19 pages, 7 figure

Imaging spontaneous currents in superconducting arrays of pi-junctions

Superconductors separated by a thin tunneling barrier exhibit the Josephson effect that allows charge transport at zero voltage, typically with no phase shift between the superconductors in the lowest energy state. Recently, Josephson junctions with ground state phase shifts of pi proposed by theory three decades ago have been demonstrated. In superconducting loops, pi-junctions cause spontaneous circulation of persistent currents in zero magnetic field, analogous to spin-1/2 systems. Here we image the spontaneous zero-field currents in superconducting networks of temperature-controlled pi-junctions with weakly ferromagnetic barriers using a scanning SQUID microscope. We find an onset of spontaneous supercurrents at the 0-pi transition temperature of the junctions Tpi = 3 K. We image the currents in non-uniformly frustrated arrays consisting of cells with even and odd numbers of pi-junctions. Such arrays are attractive model systems for studying the exotic phases of the 2D XY-model and achieving scalable adiabatic quantum computers.Comment: Pre-referee version. Accepted to Nature Physic

Nondestructive detection of damage in carbon fibre composites by SQUID magnetometry

Monitoring of structural integrity is an essential issue in enhancing the affordability as well the safety of modern aircraft and spacecraft structures. Increasingly, metallic parts of aircrafts are being replaced by carbon fibre composite components due to their high strength and stiffness combined with low density. This paper reviews of the use of superconducting quantum interference devices (SQUIDs) in the detection of different types of damage in carbon fibre panels. The results presented here on impact damage on carbon fibre reinforced polymer and cracks induced by tensile loads on carbon fibre reinforced carbon matrix show that this method is sensitive not only to the presence but also the severity of damage. Indeed, it enables one to distinguish between the different failure mechanisms as the damage process evolves. SQUIDs response to artificial delaminations, flaws and deep-lying defects are also presented. The application of a neural network system for the detection of impact damage in a noisy environment is discussed. Experimental results demonstrate that nondestructive evaluation using SQUID magnetometers is a suitable technique to investigate composites to improve their mechanical properties

Magnetic response of damaged carbon fibre reinforced plastics measured by a HTS-SQUID magnetometer

Controlled states of damage were imparted to carbon fibre reinforced plastic panels, simply supporting them on a circular ring and applying a quasi-statically increasing transverse load at the centre. The damage state was controlled by interrupting the loading stage at predetermined values of the deflection. After mechanical tests, the panels were non-destructively inspected by a highly sensitive superconductive magnetometer (HTS SQUID), from which the maps of the parallel component of the magnetic field above the sample surface were obtained. A destructive analysis of the specimens was also carried out by optical microscopy, to assess damage features. The existence of a defect in the material was clearly signalled by a distortion in the magnetic field, even when an elastic behaviour could be guessed from the load-displacement curve. Until the loading conditions only resulted in failures in the matrix, the slope of the SQUID response along a line-scan was linearly dependent on the maximum energy applied to the specimen during the mechanical tests. The dependence of the slope on the energy was strongly altered when fibre failures were induced in the laminate. The magnetometer response was also influenced by the coil excitation frequency. The results obtained at different frequencies indicate that the optimum frequency can vary, depending on the scope of the non-destructive inspection under concern

Experimental and numerical results of electromagnetic nondestructive testing with HTc SQUIDS

International audienceWe 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