Michael Rudolf Koblischka

Struts of a superconducting YBa2Cu3Oy (YBCO) foam prepared by the infiltration growth method on the base of commercial polyurethane foams were extracted from the bulk, and thoroughly characterized concerning the microstructure and the magnetoresistance, measured by the four-point technique. Optical microscopy, electron microscopy, electron backscatter diffraction and atomic force microscopy observations indicate a unique microstructure of the foam struts which shows a large amount of tiny Y2BaCuO5 (Y-211) particles (with diameters between 50 and 100 nm) being enclosed in channel-like grain boundaries between the YBCO grains and a one-of-a-kind surface of the struts covered with Ba3Cu5Oy-particles. The resistance data obtained at temperatures in the range 4.2 K ≤T≤ 150 K (applied magnetic fields ranging from 0 to 7 T) were analyzed in the framework of the fluctuation-induced conductivity (FIC) approach using the models of Aslamazov–Larkin (AL) and Lawrence–Doniach (LD). The resulting FIC curves reveal the presence of five distinct fluctuation regimes, namely, the short-wave (SWF), one-dimensional (1D), two-dimensional (2D), three-dimensional (3D), and critical (CR) fluctuation domains. The analysis of the FIC data enable the coherence length in the direction of the c-axis at zero-temperature (ξc(0)), the irreversibility field (Birr), the upper critical magnetic field (Bc2), the critical current density at T= 0 K (Jc(0)) and several other parameters describing the the material’s superconducting properties to be determined. The present data reveal that the minuscule Y-211 particles found along the YBCO grain boundaries alter the excess conductivity and the fluctuation behavior as compared to conventional YBCO samples, leading to a quite high value for Jc(0) for a sample with a non-optimized pinning landscape.

The experimental data in the literature concerning the Paramagnetic Meissner Effect (PME) or also called Wohlleben effect are reviewed with the emphasis on the PME exhibited by metallic, s-wave superconductors. The PME was observed in field-cool cooling (FC-C) and field-cool warming (FC-W) m(T)-measurements on Al, Nb, Pb, Ta, in compounds such as, e.g., NbSe2, In-Sn, ZrB12, and others, and also in MgB2, the metallic superconductor with the highest transition temperature. Furthermore, samples with different shapes such as crystals, polycrystals, thin films, bi- and multilayers, nanocomposites, nanowires, mesoscopic objects, and porous materials exhibited the PME. The characteristic features of the PME, found mainly in Nb disks, such as the characteristic temperatures T1 and Tp and the apparative details of the various magnetic measurement techniques applied to observe the PME, are discussed. We also show that PME can be observed with the magnetic field applied parallel and perpendicular to the sample surface, that PME can be removed by abrading the sample surface, and that PME can be introduced or enhanced by irradiation processes. The PME can be observed as well in magnetization loops (MHLs, m(H)) in a narrow temperature window Tp<Tc, which enables the construction of a phase diagram for a superconducting sample exhibiting the PME. We found that the Nb disks still exhibit the PME after more than 20 years, and we present the efforts of magnetic imaging techniques (scanning SQUID microscopy, magneto-optics, diamond nitrogen-vacancy (NV)-center magnetometry, and low-energy muon spin spectroscopy, (LE-μSR)). Various attempts to explain PME behavior are discussed in detail. In particular, magnetic measurements of mesoscopic Al disks brought out important details employing the models of a giant vortex state and flux compression. Thus, we consider these approaches and demagnetization effects as the base to understand the formation of the paramagnetic signals in most of the materials investigated. New developments and novel directions for further experimental and theoretical analysis are also outlined.