Studies of cracking behavior in melt-processed YBCO bulk superconductors

An important phenomenon in bulk superconductors fabricated by top-seeded-melt growth (TSMG) is the formation of cracks due to the inherent brittleness of the YBa2Cu3O7-δ (Y-123) phase matrix. These form during the fabrication of the superconducting monolith and play an important role in the limitation of current flow. However, cracks may also form during cooling cycles of the sample to liquid nitrogen temperatures. In this investigation, macrocracks along the c-direction, in particular were analyzed microscopically before and after cooling. In addition we attempt to resolve the c-axis macrocrack formation pattern using the magnetoscan technique

Epitaxial LaFeAsOF thin films grown by pulsed laser deposition

Superconducting and epitaxially grown LaFeAsOF thin films were successfully prepared on (001)-oriented LaAlO3 substrates using pulsed laser deposition. The prepared thin films show exclusively a single in-plane orientation with epitaxial relation (001)[100] parallel to (001)[100] and a FWHM value of 1deg. Furthermore, resistive measurement of the superconducting transition temperature revealed a Tc90 of 25K with a high residual resistive ratio of 6.8. The applied preparation technique, standard thin film pulsed laser deposition at room temperature in combination with a subsequent post annealing process, is suitable for fabrication of high quality LaFeAsO1-xFx thin films. A high upper critical field of 76.2 T was evaluated for magnetic fields applied perpendicular to the c-axis and the anisotropy was calculated to be 3.3 assuming single band superconductivity.Comment: 6 pages, 4 Figure

A Novel Magnetoscan Setup

Due to a modification in the original magnetoscan setup, a significant improvement in resolution was obtained. The paper focuses on experimental results which should support the idea of the new setup using two magnets with opposite direction of magnetization. This contribution to the characterization techniques of melt-grown bulk superconductors should promote the easy installation of this technique in industry. The improved magnetoscan technique may further help to investigate growth-induced inhomogeneities of the top-seeded-melt-growth process in the submillimeter range, and it offers new possibilities to the characterization of smaller structures such as superconducting films or coated conductors.Comment: 7 pages, 7 figure

BaFe_{1.8}Co_{0.2}As_2 thin film hybrid Josephson junctions

Josephson junctions with iron pnictides open the way for fundamental experiments on superconductivity in these materials and their application in superconducting devices. Here, we present hybrid Josephson junctions with a BaFe_{1.8}Co_{0.2}As_2 thin film electrode, an Au barrier and a PbIn counter electrode. The junctions show RSJ-like current-voltage characteristics up to the critical temperature of the counter electrode of about 7.2K. The temperature dependence of the critical current, IC, does not show an Ambegaokar-Baratoff behavior. Well-pronounced Shapiro steps are observed at microwave frequencies of 10-18GHz. Assuming an excess current, I_ex, of 200 {\mu}A at 4.2K we get an effective I_C R_N product of 6 {\mu}V.Comment: submitted to Appl. Phys. Let

Quantitative assessment of pinning forces and the superconducting gap in NbN thin films from complementary magnetic force microscopy and transport measurements

Epitaxial niobium-nitride thin films with a critical temperature of Tc=16K and a thickness of 100nm were fabricated on MgO(100) substrates by pulsed laser deposition. Low-temperature magnetic force microscopy (MFM) images of the supercurrent vortices were measured after field cooling in a magnetic field of 3mT at various temperatures. Temperature dependence of the penetration depth has been evaluated by a two-dimensional fitting of the vortex profiles in the monopole-monopole model. Its subsequent fit to a single s-wave gap function results in the superconducting gap amplitude Delta(0) = 2.9 meV = 2.1*kB*Tc, in perfect agreement with previous reports. The pinning force has been independently estimated from local depinning of individual vortices by lateral forces exerted by the MFM tip and from transport measurements. A good quantitative agreement between the two techniques shows that for low fields, B << Hc2, MFM is a powerful and reliable technique to probe the local variations of the pinning landscape. We also demonstrate that the monopole model can be successfully applied even for thin films with a thickness comparable to the penetration depth.Comment: 6 pages, 6 figures, 2 table

Studies of cracking behavior in melt-processed YBCO bulk superconductors

An important phenomenon in bulk superconductors fabricated by top-seeded-melt growth (TSMG) is the formation of cracks due to the inherent brittleness of the YBa2Cu3O7-δ (Y-123) phase matrix. These form during the fabrication of the superconducting monolith and play an important role in the limitation of current flow. However, cracks may also form during cooling cycles of the sample to liquid nitrogen temperatures. In this investigation, macrocracks along the c-direction, in particular were analyzed microscopically before and after cooling. In addition we attempt to resolve the c-axis macrocrack formation pattern using the magnetoscan technique