J. Gutiérrez, A. Llordés, J. Gazquez, M. Gibert, N. Romá, S. Ricart, A. Pomar, F. Sandiumenge, N. Mestres, T. Puig and X. Obradors Nat. Mater. 6, 367-373 (2007) DOI:10.1038/nmat1893 Power applications of superconductors will be tremendously boosted if an effective method for magnetic flux immobilization is discovered. Here, we report the most efficient vortex-pinning mechanism reported so far which, in addition, is based on a low-cost chemical solution deposition technique. A dense array of defects in the superconducting matrix is induced in YBa₂Cu₃O_7-x–BaZrO₃ nanocomposites where BaZrO₃ nanodots are randomly oriented. Non-coherent interfaces are the driving force for generating a new type of nanostructured superconductor. Angle-dependent critical-current measurements demonstrate that a strong and isotropic flux-pinning mechanism is extremely effective at high temperatures and high magnetic fields leading to high-temperature superconductors with record values of pinning force. The maximum vortex-pinning force achieved at 65 K, 78 GN m^-3, is 500% higher than that of the best low-temperature NbTi superconductors at 4.2 K and so a great wealth of high-field applications will be possible at high temperatures.

J. Gazquez, F. Sandiumenge, M. Coll, A. Pomar, N. Mestres, T. Puig, X. Obradors, Y. Kihn, M. J. Casanove and C. Ballesteros Chem. Mat. 18, 6211-6219 (2006) DOI: 10.1021/cm0617891 We describe the conversion of yttrium, barium, and copper trifluoracetate-derived solid precursors to epitaxial YBa₂Cu₃O_x superconducting ceramics on (001)-oriented LaAlO₃substrates. Transmission electron microscopy, electron energy loss spectroscopy, energy-dispersive X-ray analysis, and X-ray diffraction are used to characterize the reaction path and nucleation mechanism yielding high critical current YBa₂Cu₃O₇. Our results show that the pyrolysis of the trifluoracetate solutions yields a nanostructured, partially amorphous Ba_1-xY_xF_2+x matrix having a Ba/Y ratio close to 2, with homogeneously dispersed CuO nanoparticles. Upon heating, the chemical trajectory of the fluoride matrix and the overall microstructural evolution of the ceramic precursor prior to YBa₂Cu₃O₇ nucleation is driven by the decomposition and oxidation of this solid solution. The Y solid solubility decreases with temperature yielding Y₂O₃ which reacts with the CuO particles forming Y₂Cu₂O₅ at about 700 °C. In addition, electron energy loss spectroscopy reveals a high oxygen concentration and almost no Y in the matrix quenched from 795 °C, at a stage where the YBa₂Cu₃O_x phase still forms disconnected 50−100 nm thick islands spaced by 1−2 μm. The observed evolution from Ba_1-xY_xF_2+x to a barium oxyfluoride mostly occurs prior to the heteroepitaxial nucleation of YBa₂Cu₃O_x at about 700 °C. Hence, a microstructural scenario is defined which favors competitive nucleation growth between heteroepitaxial YBa₂Cu₃O_x and bulk Y₂Cu₂O₅. X-ray diffraction pole-figure analysis reveals that the oxyfluoride phase is heavily textured, exhibiting two epitaxial relationships with the (001)-LaAlO₃ substrate:  (001)OF//(001)LaAlO₃, [110]OF//[100]LaAlO₃ and (111)OF//(001)LaAlO₃, [110]OF//[100]LaAlO₃ (OF stands for oxyfluoride). High-resolution observations of the growth front support that (111)-oriented oxyfluoride regions provide low-barrier nucleation sites for c-axis-oriented YBa₂Cu₃O_x on the buried (001)-LaAlO₃ substrate. However, owing to its high mismatch, this orientation only represents roughly 15% percent of the total OF volume. Considering that the nucleation of YBa₂Cu₃O_x is confined to those regions, this would lead to an anomalously large internuclei spacing, as is indeed observed, favoring the formation of large YBa₂Cu₃O_x grains and films with a low mosaic spread.

J. Figueras, T. Puig, X. Obradors, W. K. Kwok, L. Paulius, G. W. Crabtree and G. Deutscher Nat. Phys. 2, 402-407 (2006) DOI:10.1038/nphys311 In high-temperature superconductors, magnetic field lines penetrate the samples through vortices arranged in an Abrikosov vortex lattice. In a magnetic field H_m(T ) below the upper critical field H_c2(T ) that destroys bulk superconductivity, the vortex lattice melts to a liquid vortex state, in which each vortex line must be ‘pinned’ individually to prevent dissipation. Linear and planar defects have been found to be effective for pinning the vortex liquid because they trap an entire vortex within a single extended defect. However, up to now it is not known how far into the liquid state this pinning process can be effective. Here, we show that there is a universal magnetic field line H_l(T ) betweenH_m(T ) and H_c2(T ), where thermodynamic fluctuations of the order parameter can cause vortex unpinning from extended defects. This magnetic field H_l(T ) sets an upper limit to the irreversibility line H_irr(T ) marking the onset of dissipation. For that reason it determines a new magnetic-field–temperature region in which a superconductor can remain useful.