Normal metal-superconductor decoupling as a source of thermal fluctuation noise in transition-edge sensors J. Appl. Phys. 112, 034515 (2012) Transport critical-current density of superconducting films with hysteretic ferromagnetic dots AIP Advances 2, 022166 (2012) Pressure effects on strained FeSe0.5Te0.5 thin films J. Appl. Phys. 111, 112610 (2012) Magnetoresistance and transistor-like behavior of a double quantum-dot via crossed Andreev reflections J. Appl. Phys. 111, 113905 (2012) What are the internal field and the vortex density along the edges of a coated conductor or a superconducting bridge carrying current?

No abstract

The biaxially textured growth of superconducting Co-doped BaFe2As2 (Ba-122) thin films has been realized on ion beam assisted deposition (IBAD) MgO coated conductor templates by employing an iron buffer architecture. The iron pnictide coated conductor showed a superconducting transition temperature of 21.5 K, which is slightly lower than that of Co-doped Ba-122 films on single crystalline MgO substrates. A self-field critical current density of over 10 5 A·cm −2 has already been achieved even at 8 K. The current experiment highlights the potential of possible coated conductor applications of the iron-based superconductors.

The temperature-dependent optical reflectivity and complex transmissivity of an epitaxially grown Ba͑Fe 0.9 Co 0.1 ͒ 2 As 2 thin film were measured over a wide frequency range ͑4 -35 000 cm −1 ͒. The opening of the superconducting gap 2⌬ 0 = ͑3.7Ϯ 0.3͒ meV is directly observed by vanishing optical conductivity at 30 cm −1 for T Ͻ T c = 20 K. While in this range the measured temperature-and frequency-dependent electrodynamic properties agree well with the BCS predictions of a nodeless order parameter, unexpectedly a strong quasiparticle absorption shows up below 1.5 meV. The spectral weight of the condensate 1.94ϫ 10 7 cm −2 corresponds to a penetration depth = 3600 Å.

We report on the transport properties of clean, epitaxial Fe(Se,Te) thin films prepared on Febuffered MgO (001) single crystalline substrates by pulsed laser deposition. Near Tc a steep slope of the upper critical field for H||ab was observed (74.1 T/K), leading to a very short out-of-plane coherence length, ξc(0), of 0.2 nm, yielding 2ξc(0) ≈ 0.4 nm. This value is shorter than the interlayer distance (0.605 nm) between Fe-Se(Te) planes, indicative of modulation of the superconducting order parameter along the c-axis. An inverse correlation between the power law exponent N of the electric field-current density(E-J) curve and the critical current density, Jc, has been observed at 4 K, when the orientation of H was close to the ab-plane. These results prove the presence of intrinsic pinning in Fe(Se,Te). A successful scaling of the angular dependent Jc and the corresponding exponent N can be realized by the anisotropic Ginzburg Landau approach with appropriate Γ values 2∼3.5. The temperature dependence of Γ behaves almost identically to that of the penetration depth anisotropy.

We have measured the complex dynamical conductivity, σ = σ1 + iσ2, of superconducting Ba(Fe0.9Co0.1)2As2 (Tc = 22 K) at terahertz frequencies and temperatures 2 -30 K. In the frequency dependence of σ1 below Tc, we observe clear signatures of the superconducting energy gap opening. The temperature dependence of σ1 demonstrates a pronounced coherence peak at frequencies below 15 cm −1 (1.8 meV). The temperature dependence of the penetration depth, calculated from σ2, shows power-law behavior at the lowest temperatures. Analysis of the conductivity data with a two-gap model, gives the smaller isotropic s-wave gap of ∆A = 3 meV, while the larger gap is highly anisotropic with possible nodes and its rms amplitude is ∆0 = 8 meV. Overall, our results are consistent with a two-band superconductor with an s± gap symmetry.

In general, the critical current density, Jc, of type II superconductors and its anisotropy with respect to magnetic field orientation is determined by intrinsic and extrinsic properties. The Fe-based superconductors of the ‘122’ family with their moderate electronic anisotropies and high yet accessible critical fields (Hc2 and Hirr) are a good model system to study this interplay. In this paper, we explore the vortex matter of optimally Co-doped BaFe2As2 thin films with extended planar and c-axis correlated defects. The temperature and angular dependence of the upper critical field is well explained by a two-band model in the clean limit. The dirty band scenario, however, cannot be ruled out completely. Above the irreversibility field, the flux motion is thermally activated, where the activation energy U0 is going to zero at the extrapolated zero-kelvin Hirr value. The anisotropy of the critical current density Jc is both influenced by the Hc2 anisotropy (and therefore by multi-band effects) as well as the extended planar and columnar defects present in the sample.

Fe-based superconductors (FBS) present a large variety of compounds whose properties are affected to different extents by their crystal structures. Amongst them, the REFeAs(O,F) (RE1111, RE being a rare-earth element) is the family with the highest critical temperature Tc but also with a large anisotropy and Josephson vortices as demonstrated in the flux-flow regime in Sm1111 (Tc ∼ 55 K). Here we focus on the pinning properties of the lower-Tc Nd1111 in the flux-creep regime. We demonstrate that for H//c critical current density Jc at high temperatures is dominated by point-defect pinning centres, whereas at low temperatures surface pinning by planar defects parallel to the c-axis and vortex shearing prevail. When the field approaches the ab-planes, two different regimes are observed at low temperatures as a consequence of the transition between 3D Abrikosov and 2D Josephson vortices: one is determined by the formation of a vortex-staircase structure and one by lock-in of vortices parallel to the layers. This is the first study on FBS showing this behaviour in the full temperature, field, and angular range and demonstrating that, despite the lower Tc and anisotropy of Nd1111 with respect to Sm1111, this compound is substantially affected by intrinsic pinning generating a strong ab-peak in Jc.