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Laviano F.,Polytechnic University of Turin | Gerbaldo R.,Polytechnic University of Turin | Ghigo G.,Polytechnic University of Turin | Gozzelino L.,Polytechnic University of Turin | And 5 more authors.
Journal of Superconductivity and Novel Magnetism | Year: 2013

Spatial distributions of magnetic field and of current density in superconductors are obtained, in real-time, by the magneto-optical imaging technique with an indicator film. Quantitative values of the local magnetic field and then of the local current density were achieved by a careful optical calibration and custom algorithms for the inversion of Biot-Savart law. An iterative procedure was developed for the correction of artifacts due to the coupling of the indicator film magnetization with the in-plane magnetic field generated by the superconducting sample. This technique is highly valuable when the sample under study has nonuniform properties. Several cases of such systems are shown. Particular attention is devoted to the local variation of structural and chemical properties by means of heavy-ion irradiation. A comparative study of swift-ion irradiation effects between cuprate and pnictide superconductors is addressed. For the case of cuprate materials, some successful applications of the heavy ion irradiation engineering are presented and discussed. © 2013 Springer Science+Business Media New York.


Ding Q.-P.,University of Tokyo | Ding Q.-P.,JST Transformative Research Project on Iron Pnictides TRIP | Tsuchiya Y.,University of Tokyo | Sun Y.,University of Tokyo | And 5 more authors.
Journal of the Physical Society of Japan | Year: 2012

We report a study on the anisotropy of superconducting properties in single crystalline Ca 10(Pt 4-δAs 8)(Fe 1.79-Pt 0.21As 2) 5 with T c ∼ 13:6 K. Under a field of 5 Oe, the sample reaches fully-diamagnetic state at ∼10 K for H || c, and ∼8K for H || ab, indicating the presence of slight inhomogeneities. The magnetization measurements reveal fish-tail effect in the hysteresis loop for both H || c and H || ab. Averaged critical current densities at low magnetic fields along the c-axis and ab-plane J c H||c and J c H||ab at 5K are estimated to be 0:9 × 10 5 and 0:7 × 10 5 A/cm 2, respectively. Resistive transitions under magnetic field show broadening, which is consistent with a relatively large anisotropy of upper critical field. Magneto-optical images reveal homogenous current flow within the ab-plane. © 2012 The Physical Society of Japan.


Nishizaki T.,Tohoku University | Nakajima Y.,Tohoku University | Nakajima Y.,University of Tokyo | Nakajima Y.,JST Transformative Research Project on Iron Pnictides TRIP | And 4 more authors.
Journal of the Physical Society of Japan | Year: 2011

We present scanning tunneling microscopy/spectroscopy (STM/STS) on iron-based superconductor Ba(Fe0:93-Co0:07) 2As2 single crystals. The STM topography shows that the low-temperature cleaved surface consists of two types of ordered atomic structures: (1) the 2×1 surface, formed by half of the Ba atoms remaining on the As layer, and (2) the √2 × √2 surface of the As layer from which Ba atoms are completely removed. The superconducting spectra with the energy gap (2Δ ≳ 6:9kBTc) are observed only on the 2×1 surface. This value is approximately twice that for weak coupling s-wave BCS superconductors, indicating a strong pairing interaction similar to that observed in cuprate superconductors. We also find that vortices form a disordered structure and that the vortex core spectroscopy does not show a bound state, suggesting a strong disorder effect on the vortex pinning and quasiparticle scattering. © 2011 The Physical Society of Japan.


Taen T.,University of Tokyo | Yagyuda H.,University of Tokyo | Nakajima Y.,University of Tokyo | Nakajima Y.,JST Transformative Research Project on Iron Pnictides TRIP | And 9 more authors.
Physica C: Superconductivity and its Applications | Year: 2013

We report effects of particle energy on physical properties of Co-doped BaFe2As2 in the case of Xe and Au irradiations. The irradiation-induced suppression of Tc is found to be related with several factors, such as defect structures and the thickness of the crystal. Heavy-ion irradiation induces enhancement of critical current density J c, and a weak dip in the hysteresis loop. The fish-tail effect is completely suppressed by a modest density of defects. The enhancement of J c is closely related to the defect structure. Namely, splayed structure in columnar defects, inevitably introduced at lower energies, results in a strong enhancement of Jc by forced entanglement of vortices.

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