We report the effects of C 4+ -irradiation on the superconducting properties of BaFe 1.9 Ni 0.1 As 2 single crystal. The BaFe 1.9 Ni 0.1 As 2 single crystal before and after C 4+ -irradiation was characterized by magnetic, magneto-transport and magneto-optical techniques over a wide range of magnetic fields (0-13 T) and temperatures (2-200 K). We demonstrate that the C 4+ -irradiation significantly enhances the in-field critical current density (by a factor of up to 1.5 at 5 K) and induces enhanced flux jumping at 2 K, with only a small degradation (by 0.5 K) of the critical temperature, T c . The vortex phase diagram describing the evolution of the vortex-glass transition temperature with magnetic field and the upper critical field has been resolved for the C 4+ -irradiated sample. For temperatures below T c , the resistivity curves and the pinning potential were found to show good scaling, using a modified model for vortex-liquid resistivity. The vortex state is three dimensional at temperatures lower than a characteristic temperature. Good agreement between the thermally activated flux flow model, which is usually employed to account for the resistivity in the vortex-liquid region, and the modified vortex-liquid model, has been observed.

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We present a novel method for analysis of superconducting thin films using dynamic magneto-optical imaging, revealing hallmarks of flux penetration with temporal resolution around 1 ms (in the present work) or better. This method involves investigation of transient field and dynamic current distributions, which are calculated by an inversion procedure on the Biot-Savart Law, which we show to be valid under dynamic conditions. We compare and discuss the flux front penetration speed and evolution of current distribution in high quality YBa2Cu3O${}_{7-\delta }$ thin films with that of samples deliberately damaged in such a way as to reduce critical current density without causing macroscopic damage. We present a novel method for analysis of superconducting thin films using dynamic magnetooptical imaging, revealing hallmarks of flux penetration with temporal resolution around 1 ms (in the present work) or better. This method involves investigation of transient field and dynamic current distributions, which are calculated by an inversion procedure on the Biot-Savart Law, which we show to be valid under dynamic conditions. We compare and discuss the flux front penetration speed and evolution of current distribution in high quality YBa2Cu3O7−δ thin films with that of samples deliberately damaged in such a way as to reduce critical current density without causing macroscopic damage.

The glass-like vortex distribution in pulsed laser deposited YBa2Cu3O7 − x thin films is observed by scanning superconducting quantum interference device microscopy and analysed for ordering after cooling in magnetic fields significantly smaller than the Earth's field. Autocorrelation calculations on this distribution show a weak short-range positional order, while Delaunay triangulation shows a near-complete lack of orientational order. The distribution of these vortices is finally characterised as an isotropic vortex glass. Abnormally closely spaced groups of vortices, which are statistically unlikely to occur, are observed above a threshold magnetic field. The origin of these groups is discussed, but will require further investigation.

. (2016). Vibration effect on magnetization and critical current density of superconductors. Superconductor Science and Technology, 29 075002-1-075002-12. Vibration effect on magnetization and critical current density of superconductors AbstractIn this work the effect of vibrations on critical current density ( Jc) of superconductors has been studied. The vibrations are shown to affect Jc of all types of superconductors during their measurements, employing a vibrating sample magnetometer (VSM). Increasing vibration frequency ( f ) and/or amplitude (A) leads to progressive reduction of Jc as a function of magnetic field (Ba). The effect of vibrations is substantially stronger in thin films. It leads to development of unexpected kinks on Jc (Ba) curves. Analysis of magnetization loops and relaxation of magnetization in YBCO films revealed that the vibration effect can be treated as the effective reduction of pinning potential. The asymmetry of the vibration effect in ascending and descending Ba is observed, indicating differences in free energy of the corresponding vortex structures. Thermal effects induced by vibrations with large f and A are shown to have rather insignificant influence, while the vibrational vortex dynamics exhibit a strong impact. The irreversibility field (Birr) is shown to be instrumentally defined, and its value depends on VSM settings. In addition, the practical importance of Birr for Jc modeling is demonstrated. Abstract.In this work the effect of vibrations on critical current density (Jc) of superconductors has been studied. The vibrations are shown to affect Jc of all types of superconductors during their measurements, employing Vibrating Sample Magnetometer (VSM). Increasing vibration frequency (f ) and/or amplitude (A) lead to progressive reduction of Jc as a function of magnetic field (Ba). The effect of vibrations is substantially stronger in thin films. It leads to development of unexpected kinks on Jc(Ba) curves. Analysis of magnetization loops and relaxation of magnetization in YBCO films revealed that the vibration effect can be treated as the effective reduction of pinning potential. The asymmetry of the vibration effect in ascending and descending Ba is observed, indicating differences in free energy of the corresponding vortex structures. Thermal effects induced by vibrations with large f and A is shown to have rather insignificant influence, while the vibrational vortex dynamics exhibits a strong impact. The irreversibility field (B irr ) is shown to be instrumentally defined, and its value depends on VSM settings. In addition, the practical importance of B irr for Jc modeling is demonstrated.Vibration effect on magnetization and critical current density of superconductors 2

The dynamics of transient current distributions in superconducting YBa2Cu3O7−δ thin films were investigated during and immediately following an external field ramp, using high-speed (real-time) Magneto-Optical Imaging and calculation of dynamic current profiles. A number of qualitatively unique and previously unobserved features are seen in this novel analysis of the evolution of supercurrent during penetration. As magnetic field ramps up from zero, the dynamic current profile is characterized by strong peaks, the magnitude of which exceed the conventional critical current density (as determined from static current profiles). These peaks develop close to the sample edges, initially resembling screening currents but quickly growing in intensity as the external field increases. A discontinuity in field and current behaviour is newly observed, indicating a novel transition from increasing peak current toward relaxation behaviour. After this transition, the current peaks move toward the centre of the sample while reducing in intensity as magnetic vortices penetrate inward. This motion slows exponentially with time, with the current distribution in the long-time limit reducing to the expected Kim-model profile.

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