We present a thorough study of the nucleation and growth processes of the solution-based YBa 2 Cu 3 O 7 -Ba 2 YTaO 6 (YBCO-BYTO) system, carried out with a view to controlling the characteristics of the BYTO phase to meet the requirements for specific power applications. Scanning transmission electron microscopy and x-ray diffraction have been used to characterize the BYTO nucleation and phase evolution during the YBCO-BYTO conversion. At high BYTO loads (>10 mol%), the nanoparticles tend to aggregate, resulting in much less efficiency for generating nanostrained areas in the YBCO matrix, and enhancement of the vortex pinning. Our experiments show that by modifying the nucleation kinetics and thermodynamics of the BYTO, the nucleation mode (homogeneous versus heterogeneous), the particle size and the particle orientation can be controlled. We demonstrate that YBCO-BYTO nanocomposites with high concentration of nanoparticles can be prepared in such a way as to obtain small and randomly oriented nanoparticles (i.e. high incoherent interface), generating highly strained nanoareas in the YBCO, with enhancement in the vortex pinning. We have also observed that the incoherent interface is not the only parameter controlling the nanostrain. The Cu-O intergrowth characteristics must also be a key factor for controlling the nanostrain in future tuning of YBCO vortex pinning.

No abstract

According to a generic temperature vs. carrier-doping (T − p) phase diagram of high-temperature superconductors it has been proposed that as doping increases to the overdoped region they approach gradually a conventional (canonical) Fermi Liquid. However, Hall effect measurements in several systems reported by different authors show a still strong T -dependence in overdoped samples. We report here electrical transport measurements of Y1−xCaxBa2Cu3O 7−δ thin films presenting a temperature dependence of the Hall constant, RH , which does not present a gradual transition towards the T -independent behavior of a canonical Fermi Liquid. Instead, the T -dependence passes by a minimum near optimal doping and then increases again in the overdoped region. We discuss the theoretical predictions from two representative Fermi Liquid models and show that they can not give a satisfactory explanation to our data. We conclude that this region of the phase diagram in YBCO, as in most HTSC, is not a canonical Fermi Liquid, therefore we call it Anomalous Fermi Liquid.

No abstract

No abstract