Microstructurally clean, isovalently P-doped BaFe2As2 (Ba-122) single crystalline thin films have been prepared on MgO (001) substrates by molecular beam epitaxy. These films show a superconducting transition temperature (Tc) of over 30 K although the P content is around 0.22, which is lower than the optimal one for single crystals (i.e., 0.33). The enhanced Tc at this doping level is attributed to the in-plane tensile strain. The strained film shows high transport self-field critical current densities (Jc) of over 6 MA/cm2 at 4.2 K, which are among the highest for Fe based superconductors (FeSCs). In-field Jc exceeds 0.1 MA/cm2 at µ0H = 35 T for H||ab and µ0H = 18 T for H||c at
4.2 K, respectively, in spite of moderate upper critical fields compared to other FeSCs with similar Tc. Structural investigations reveal no defects or misoriented grains pointing to strong pinning centers. We relate this unexpected high Jc to a strong enhancement of the vortex core energy at optimal Tc, driven by in-plane strain and doping. These unusually high Jc make P-doped Ba-122 very favorable for high-field magnet applications.

EU - SFP (FP7/2007-2013) No. 283141 (IRON-SEA)
EU-Japan project (No. 283204 SUPER-IRON)
NHMFL, National Science Foundation Cooperative Agreement No. DMR-1157490, Florida, the U.S. Department of Energy
Graduate school GRK 1621
SICORP Japan Science and Technology Agency
Grant-in-Aid MEXT

The iron-based superconductors are promising for high-field magnet applications because of their large upper critical fields (H_c2 > 100 T) with small anisotropy (γ ~ 1-2). Indeed, superconducting wires have been fabricated by the powder-in-tube method using iron-based superconductors, especially 122-type materials, e.g. (Ba,K)Fe₂As₂. The knowledge and understanding of critical current properties, such as chemical composition dependence or nature of pinning, are necessary to improve the performance of wires. Here we investigated the doping dependence of critical current properties of K, Co, and P-doped BaFe₂As₂ using high quality single crystals. We estimated the critical current density (J_c) from the magnetic hysteresis loop measurements and obtained J_c curves as a function of doping for each compound. In all the three cases, the J_c curves show a dome-like shape with the maximum around the phase boundary between superconductivity and antiferromagnetic-orthorhombic phase. We found that the slightly underdoped (Ba,K)Fe₂As₂ possesses highest J_c among the three compounds. With overdoping, the magnitude of J_c rapidly decreases in spite of relatively high T_c, resulting in a contrasting doping evolution of J_c and T_c. We also observed the fish-tail effect, or the second peak at higher magnetic fields except for (Ba,K)Fe₂As₂ with doping above optimal level. Comparison of the magnetic-field and temperature dependence of J_c in those three compounds gives clues to unveil the pinning mechanism in iron-based superconductors.

This work was supported by Strategic International Collaborative Research Program (SICORP), Japan Science and Technology Agency (JST), and the Sasakawa Scientific Research Grant from The Japan Science Society.

We measured critical currents and irreversibility fields of Ag-sheath (Sr,K)Fe₂As₂ tapes at various temperatures, magnetic fields and magnetic field directions. The sample tapes were prepared by a powder-in-tube method. A few samples were made by a hot-press process in the final heat treatments. In case of the hot-pressed samples, the J_c values for B perpendicular to the tape, J_c^perp, are larger than those for B//tapes, J_c^para at 4.2 K. The other hot-pressed sample shows that the crossover from J_c^perp > J_c^para in low fields to J_c^perp < J_c^para in high fields. The crossover field decreases with increasing temperature. In addition, we found that a broad peak in angular dependence of J_c around B perpendicular to the tape appears in a low field region and it decreases with increasing field and temperature. On the other hands, the irreversibility field and the upper critical field of the same sample show the usual anisotropy. These unusual anisotropic J_c behaviors will be discussed on the basis of the flux pinning combined with texture and microstructure studies.

Detailed investigation Critical current density behaviour in FeSe_0.5Te_0.5 films synthesized by pulsed laser ablation

S. Kawale¹, E. Sarnelli², M. Adamo², E. Enrico³, V. Braccini¹, L. Pellegrino¹, M. Putti^1,4, E. Bellingeri¹, C. Ferdeghini¹

1 CNR – SPIN Genova, Corso Perrone 24, 16152 Genova, Italy.
2 CNR SPIN Napoli, Via Campi Flegrei 34, I-80078 Pozzuoli, Napoli, Italy
3 l'Istituto Nazionale di Ricerca Metrologica (INRIM), Torino, Italy
4 DIFI, University of Genova, Via Dodecaneso 33, 16145 Genova, Italy.

One of the most interesting family of Fe-based Superconductors (FBS) is 11 system i.e. Fe-Chalcogenides (Fe-CH). Thin films of FeSe_1-xTe_x are of huge interest considering their recently enhanced superconducting properties such as critical temperature (T_C) and critical current densities (J_C) and intrinsically high critical field (H_C2). In our efforts, we have achieved very high J_C values, of the order of 1 MA/cm² with very weak and isotropic dependence on the magnetic field, on epitaxial single crystal FeSe_0.5Te_0.5 thin films fabricated by pulsed laser ablation on CaF₂ substrate. Here we discuss and compare the J_C behaviour of FeSe_0.5Te_0.5 pristine single-crystalline thin films with artificially and naturally defected films, nano bridges fabricated by focused ion beam (FIB) lithography and along the artificial grain boundaries on SrTiO₃ bicrystal. Defects can be introduced into the films by means of various methods, for example intinsically during the growth or artificially, in a post deposition process. Nano bridges were fabricated by FIB without damage to superconducting properties i.e. T_C and J_C values remain almost unchanged even after FIB patterning. The current-voltage (I-V) curves were studied for these nano-bridges and fast switch from 0 voltages to resistive branch was observed which is typical for S/C nanowire. The effect of an arificial grain boundary (twist misorientation) in micro (nano) bridges on critical current will also be discussed. In all these structures very high and promising J_C values, larger than 1 MA/cm², were observed, which underlines the potential of this material from application perspectives.

*Corresponding author: Shrikant Kawale, CNR-SPIN Institute, Corso Perrone 24, Genova-16152, Italy.
email-shrikant.kawale@spin.cnr.it

YBa2Cu3O7-d (YBCO) thin films with high supercurrent carrying capability and low-anisotropy over wide range of temperature and applied magnetic field are strongly desired for various technological applications [1-2]. The enhancement of critical current density (JC) and the reduction of the anisotropy in JC has been the subject of intense activity in recent years. The nanoinclusions of several non-perovskite, perovskite and double-perovskite materials such as Y2BaCuO5, Y2O3, BaZrO3, BaSnO3, YBa2NbO6 etc. have been demonstrated to enhance the critical current properties of YBCO thin films deposited by pulsed laser deposition (PLD) technique. The critical current density of YBCO films exhibit intrinsic anisotropy where at a particular magnetic field, JC along the ab-plane is higher than along the c-axis which is attributed to the anisotropic crystal structure of YBCO. The self-assembled nanocolumns of perovskites such as BZO, BSO, YBNO etc. have been very effective in improving the JC particularly when the magnetic field is oriented along the c-axis of the films. Recently, the combination of differently shaped pinning structures, which have been termed as hybrid APCs, have been also attempted to improve the anisotropy in the JC of YBCO films.
In this paper, the effect of hybrid nanostructures (BSO nanocolumns together with Y211 nanoparticles) on the critical current properties of YBCO thin films is presented. For incorporating BSO nanocolumns, premixed YBCO+BSO targets are used whereas for incorporating Y211 nanoparticles, thin sectored Y211 pieces attached to the target surface are used. The YBCO thin films comprising of hybrid APCs showed superior JC-B characteristics along with sifnificantly improved angular dependence both at 77 K as well as 65 K.

References:
[1] S.R. Foltyn, L. Civale, J.L.M. Driscoll, Q.X. Jia, B. Maiorov, H. Wang, M. Maley, Nat. Mater. 6, 631 (2007)
[3] K. Matsumoto and P. Mele, Supercond. Sci. Technol. 23, 014001 (2010)

This work was supported by KAKENHI, Grant-in-Aid for Science Research (S), Grant Number 23226014.

It is commonly accepted that grain boundaries limit the macroscopic currents in state-of-the-art coated conductors at least at low fields. If the superconducting layer is sufficiently textured, the currents across the grains limit the performance at high fields, where vortex pinning becomes comparatively weak. The cross-over from grain boundary to pinning limitation was investigated in a quantitative way. We derive the cross-over field as a function of temperature and orientation of the tape with respect to the magnetic field by analyzing the influence of new defects, introduced by neutron irradiation, on the critical current. The additional defects enhance flux pinning within the grains but have little or an adverse effect on the grain boundary currents. Thus, whenever the macroscopic currents increase after neutron irradiation, pinning and not granularity is the main limiting factor. We used spin-coated ^Low-FYBa₂Cu₃O_7−x/^MODCZO/^ABADYSZ/SS coated conductors in these irradiation experiments. The analysis is complemented by high-resolution scanning Hall probe measurements performed at various temperatures and applied fields. These results are compared with field mapping results on RABiTs tapes, where the superconducting layer was made by pulsed laser deposition.

The research leading to these results have received funding from the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement n0. NMP-LA-2012-280432.

Weak link (WL) superconductivity, although studied in depth before the 1980’s [1], became a major player after discovery of superconductivity in ceramic cuprates [2]. Its importance can be readily appreciated when one takes into account that, ultimately, it is their capability to transport supercurrents that defines the critical current of a granular sample. After almost three decades of dedicated studies aiming at producing high current-carrying granular materials of practical use, WL superconductivity in the micrometric scale is still an issue of undebatable importance. Our approach to the study of WL superconductivity employs a toy-model system, comprising two rectangular superconducting islands (the grains) interconnected through a weak superconducting link. In practice, samples are prepared starting from an excellent-quality superconducting film, with borders sharply defined by use of optical lithography. A WL is then created by excavation of a submicrometric groove between the grains, made in a Focused Ion Beam (FIB) apparatus. Such a WL can be either a “through groove”, i.e., crossing the entire extension of the film all the way from one border to the opposite one; or a “contained groove”, in which case a frame of pristine material is left during the FIB excavation, to protect the WL from the exterior. Samples are then investigated using global techniques, e.g., magnetometry and magneto-transport measurements, as well as local probing methods, such as micro-Hall sensing and magneto-optical imaging (MOI). While decreasing the field in a hysteresis loop, an anomaly on the magnetization curve is seen, also manifested in MOI experiments as a sudden fade-out of the so-called d-lines. The complete understanding of the phenomenon was made possible by simultaneous measurements of the local magnetic fields at the superconducting islands and at the WL, demonstrating that the field-dependent critical currents at both regions become equal momentarily, being this the reason for the corresponding transitory suppression of the d-lines. This work is part of a study involving microstructured superconducting films, including the occurrence, suppression, guidance and morphology of flux avalanches, as well as the enhancement of critical currents by insertion of arrays of artificial pinning centers [3-7].

[1] Likharev, Rev. Mod. Phys. 51, No. 1, January 1979; [2] Tinkham and Lobb, Physical Properties of the New Superconductors, Solid State Physics 42, 91-134 (1989); [3] Motta et al., PRB 89, 134508, 2014; Colauto et al., APL 103, 032604, 2013; Adami et al., APL 102, 052603, 2013; Motta, APL 102, 212601, 2013; Colauto et al., APL 96, 092512, 2010.