For applications of high-Tc superconductors as well as for the fundamental understanding of the electrical transport mechanism in these ceramics, the knowledge about pinning and motion of flux lines is essential. Therefore it is highly desirable to measure local flux and current distributions with high spatial resolution. This can be realized using scanning X-ray microscopy (STXM) in combination with the highly sensitive X-ray Magnetic Circular Dichroism (XMCD). A ferromagnet placed on top of the superconductor allows us to map the local magnetic field distribution.
Soft-magnetic amorphous CoFeB is deposited on YBCO (YBa₂Cu₃O_7-δ)¹ and used as field sensing layer. The local stray field of the superconductor leads to a reorientation of the magnetic moments of the ferromagnet², which are then mapped within the X-ray microscope. These measurements can be performed at the X-ray microscope MAXYMUS which has been upgraded with a nitrogen based Micro Miniature Joule-Thompson Cryostat. This cryostat offers measurements in a temperature range of 78K up to 580K³. We are able to map magnetic flux penetration into the superconductor below the superconducting transition with a spatial resolution in the nanometer range⁴.
1 C. Stahl, P. Walker, S. Treiber, G. Christiani, G. Schütz, and J. Albrecht, EPL (Europhysics Lett. 106, 27002 (2014).
2 C. Stahl, P. Audehm, J. Gräfe, S. Ruoß, M. Weigand, M. Schmidt, S. Treiber, M. Bechtel, E. Goering, G. Schütz, and J. Albrecht, Phys. Rev. B 90, 104515 (2014).
3 C. Stahl, S. Ruoß, M. Weigand, M. Bechtel, G. Schütz, and J. Albrecht, J. Appl. Phys. (2015).
4 S. Ruoß, C. Stahl, M. Weigand, G. Schütz, and J. Albrecht, Appl. Phys. Lett. 106, 022601 (2015).

The authors appreciate the support by Iuliia Bykova and Michael Bechtel as well as the whole team of BESSY II and the Helmholtz-Zentrum Berlin, Germany for the allocation of synchrotron radiation beamtime.

Thermal stability of Nb₃Sn quadrupole magnets in the High Luminosity LHC Upgrade (HL-LHC) accelerator at CERN during operation up to 4'000 fb-1 is an important issue. After introducing a W shield, the radiation load by multiple energy sources on the innermost windings of the HL-LHC quadrupoles has been calculated and a total displacement per atom value dpa of 2.4x10^-4 has been found [1], mostly due to the hadronic shower component.In order to predict the irradiation effects the same Ta and Ti added multifilamentary Nb₃Sn wires were irradiated with high energy protons (65 MeV and 24 GeV, up to 1.4 x 10²¹ p/m²) and neutrons (1 MeV, up to 1.8 x 10²² n/m²). Within these fluences, T_c exhibited a slight, linear decrease and B_c2 increased by approx. 5 and 3 %, respectively. Jc showed a strong increase by a factor of 2 and 1.5, respectively, reaching the remarkably high values of the order of 11x10³ A/mm² at 4.2 K/10 T. This enhancement is the result of a strong enhancement of the point pinning contribution, and was satisfactorily described by a quantitative two-force model [2].
The variation of T_c in Nb₃Sn wires as a function of the dpa value (determined using the FLUKA code) for both, proton and neutron irradiation in the dpa range between 1x10^-4 and 1.4x10^-3 was found to fall almost on the same curve. The variation of J_c vs. dpa shows some similarities between proton and neutron irradiation, too, but the problem is more complex: the correlation T_c vs. dpa is governed by the change in atomic ordering, while J_c vs. dpa depends on the enhanced point pinning correlated with the radiation induced defect clusters. With the new relationship between T_c and dpa for both, protons and neutrons, the decrease of T_c in the quadrupoles at the maximum luminosity can now be estimated to ~ 0.3 K.
1) A. Lechner et al., CERN, 2015, unpublished
2) T. Spina et al. Published in Applied Superconductivity, IEEE Transactions on (Volume:25 , Issue: 3 )

When current streamlines in superconducting materials are forced to depart from their rectilinear trajectory due to the presence of an obstacle, they conglomerate at the sharp bends of the path. This so-called current crowding effect has been recognized as an important factor limiting the performance of superconducting devices, such as single-photon detectors [1,2]. Besides that, flux avalanches may occur, in a window of temperatures and applied magnetic fields, undermining any practical application of superconducting films. This phenomenon is caused by instabilities of thermomagnetic origin which lead to the sudden propagation of flux of dendritic pattern. On the other hand, it is well-known that the inclusion of engineered pinning centers in a film promotes guidance of such avalanches. Both the geometry of the pinning centers and the lattice symmetry of the arrangement have impact on the morphology of the avalanches.
In the present work, we show how the avalanche morphology can be controlled by inserting antidots arranged periodically. Guided flux avalanches develop in a treelike structure, with the main trunk perpendicular to the borders of the sample, while secondary branches follow well-defined directions determined by the current crowding effects of the underlying periodic pinning landscape. Strikingly, we demonstrate that in a superconductor with relatively weak random pinning the morphology of such flux avalanches can be fully controlled by proper combinations of lattice symmetry and antidot geometry [3]. Moreover, the resulting flux patterns can be reproduced by simulations based on a phenomenological thermomagnetic model which, in turn, can be used to estimate physical variables of more difficult experimental access, such as the local values of temperature and electric field.
[1] J. R. Clem and K. K. Berggren, Phys. Rev. B 84, 174510 (2011).
[2] O.-A. Adami et al. Appl. Phys. Lett. 102, 052603 (2013).
[3] M. Motta et al. Phys. Rev. B 89, 134508 (2014).

This work is supported by the FNRS projects, crédit de démarrage U.Lg., FWO projects, Methusalem Funding of the Flemish Government, the Brazilian agencies FAPESP and CNPq, and by the program for scientific cooperation FNRS-CNPq. MM is grateful for the support from CNPq (Grant 151257/2014-0).

Nowadays YBa₂Cu₃O_7-x (YBCO) films are beengused in HTS coated conductors (CC). For high-field applications the introduction of pinning centers is essential to immobilize fluxoids penetrating the superconducting layer. In this work pinning centers have been created through the production of nanocomposites of YBCO with Ba₂Y(Nb,Ta)O₆ (BYNTO) or BaHfO₃ (BHO) nanocolumns. Variation of the substrate temperature and deposition rate alters the defect structure of the film resulting in changes of the superconducting transition temperature and of the critical current density in magnetic fields.
Through transmission electron microscopy (TEM) we investigated the micro- and nanostructure of the nanocomposites producedby pulsed laser deposition on a SrTiO₃ (STO) substrate at different conditions.
Under optimal deposition conditions the YBCO matrix contains homogenously distributed nanocolumns of the addend. The columns start at the YBCO/STO interface and run through the whole YBCO layer. In most cases, a thin layer of highly distorted YBCO is present at the substrate/superconductor interface. In these areas the nanocolumns are generated above the distorted YBCO layer. Columns are about 2 to 10 nm thick and are distributed with ~ 1 column per 20-30 nm. Non-optimal deposition conditions lead to an inhomogeneous second phase distribution, interruption of the nanorods or formation of planar precipitates. All of them will be discussed in this presentation.
The structural relationship between columns and YBCO is: (001)_BYNTO/BHO // (001)_YBCO, (100)_BYNTO/BHO // (100)_YBCO. Nanocolumns cause expansion of the YBCO structure in the [001]_YBCO and [100]_YBCO direction and induce defects, such as stacking faults, dislocations and extra planes.
Non-optimal deposition conditions lead to an inhomogeneous second phase distribution, interruption of the nanorods or formation of planar precipitates. All of them will be discussed in this presentation.

This work is supported in part via EC FP7 EUROTAPES.

Thermal Analysis (TA) denotes a set of techniques that monitors the evolution of a property when submitted to a controlled temperature program. Thus, TA is especially suited to study the processes that take place during precursor decomposition and oxide crystallization in the synthesis of oxide films through the chemical solution deposition (CSD) route. In contrast, the use of TA in CSD is quite limited and little practical information is obtained from these techniques. The reason is that TA is usually applied in samples in the form of powders while the aim of CSD is the synthesis of films.

Precursor decomposition involves solid-gas reactions that critically depend on gas transport mechanisms. Due to their very different aspect ratio, gas and heat transport is significantly enhanced in films with respect to powders. Therefore, the information obtained from TA on powders may be very inaccurate or misleading when applied to films [1,2]. For instance, TA analysis of YBCO precursors in the form of powders shows that BaF₂ decomposes at temperatures well above the decomposition temperature of YBCO; thus, the synthesis of YBCO powders from these precursors is impossible. On the contrary, TA on films of the same precursors shows that the enhanced water and HF diffusion lowers the BaF₂ decomposition temperature making possible the synthesis of YBCO films.

We will show that TA can be used to monitor the structural transformations that take place in YBCO films grown by CSD (solvent evaporation, precursor pyrolysis, decomposition of intermediate compounds – BaF₂ and BaCO₃), and that this can be done at the practical conditions of film synthesis (thickness, temperature program, gas composition and pressure). We will give some examples of fluorinated and non-fluorinated precursors to illustrate the applicability of TA for the optimization of any step of the synthesis route.
[1] D. Sanchez-Rodriguez, J. Farjas, P. Roura, S. Ricart, N. Mestres, X. Obradors, et al., Thermal Analysis for Low Temperature Synthesis of Oxide Thin Films from Chemical Solutions, J. Phys. Chem. C. 117 (2013) 20133–20138.
[2] P. Roura, J. Farjas, H. Eloussifi, L. Carreras, S. Ricart, T. Puig, et al., Thermal analysis of metal organic precursors for functional oxide preparation: Thin films versus powders, Thermochim. Acta. 601 (2015) 1–8.

This work was partially funded by the Spanish Programa Nacional de Materiales through projects MAT2011-28874-C02-02 and by the Generalitat de Catalunya contract No. 2009SGR-185. The authors wish to thank the University of Girona for the PhD fellowship granted to Daniel Sánchez-Rodríguez.

The interplay between quantum interference effects (QIEs) and superconductivity has been investigated in infinite layer Sr1-xLaxCuO2±y (SLCO) thin films, by using a combination of standard dc electric transport measurements and noise characterization. The films were grown using a shuttered layer-by-layer process in a dual chamber Molecular Beam Epitaxy system, and both La/Sr chemical ratio and Oxygen content were varied.
The occurrence of a marked resistivity upturn at T < Tmin ~ 100K has been observed in under-doped samples, which also showed a superconductive transition at about 20K. The resistivity behavior has been attributed to Weak Localization (WL) and has been further investigated by detailed low-frequency voltage spectral density measurements. In particular, a peculiar linear dependence of the 1/f noise on the bias current is found in the Tc < T < Tmin region. Such behavior has been found also in other materials (cuprates and metals) and has been attributed to the existence of a WL regime. Conversely, in the superconducting (Tc) and metallic regions (T > Tmin), a usual quadratic behavior of the 1/f component is found. Even though a metal-to-insulator transition does not occur in optimally- and over-doped samples, a QIEs-driven correction to conductivity can still be identified. Such a scenario has been thoroughly investigated by resistivity analysis, which provides useful indications to reveal the true nature of the metallic phase of electron-doped cuprates. In this respect, non-Fermi liquid behavior seems to dominate at all temperatures, while the electronic correlations determine the arising of QIEs at low temperatures.
The interplay between quantum interference effects (QIEs) and superconductivity has been investigated in infinite layer Sr_1-xLa_xCuO_2±y (SLCO) thin films, by using a combination of standard dc electric transport measurements and noise characterization. The films were grown using a shuttered layer-by-layer process in a dual chamber Molecular Beam Epitaxy system, and both La/Sr chemical ratio and Oxygen content were varied.
The occurrence of a marked resistivity upturn at T < T_min ~ 100K has been observed in under-doped samples, which also showed a superconductive transition at about 20K. The resistivity behavior has been attributed to Weak Localization (WL) and has been further investigated by detailed low-frequency voltage spectral density measurements. In particular, a peculiar linear dependence of the 1/f noise on the bias current is found in the T_c < T < T_min region. Such behavior has been found also in other materials (cuprates and metals) and has been attributed to the existence of a WL regime. Conversely, in the superconducting (Tc) and metallic regions (T > T_min), a usual quadratic behavior of the 1/f component is found. Even though a metal-to-insulator transition does not occur in optimally- and over-doped samples, a QIEs-driven correction to conductivity can still be identified. Such a scenario has been thoroughly investigated by resistivity analysis, which provides useful indications to reveal the true nature of the metallic phase of electron-doped cuprates. In this respect, non-Fermi liquid behavior seems to dominate at all temperatures, while the electronic correlations determine the arising of QIEs at low temperatures.