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In recent years Bi₂Sr₂CaCu₂O_x(Bi-2212) received increasing attention due to its round wire multifilamentary architecture, a unique feature in high-T_csuperconductor. In fact, round wires are preferable for magnet designs, including solenoids for nuclear magnetic resonance (NMR) or research purpose and accelerator magnets. However, due to the narrow over-pressure heat treatment conditions necessary to obtain highJ_cand to the peculiar microstructure of Bi-2212 wires, a full understanding of the correlations between the different properties has not yet been developed. In this paper we investigate the effect of a vital part of Bi-2212 optimization, the maximum heat-treatment temperatureT_maxin the range of 885 °C–896 °C, on the variations ofJ_c, effective filament diameterd_eff, anisotropyγ, INTER- and intra-grain irreversibility fields and pinning energiesU₀, all critical parameters in unravelling the complex mix of vortex pinning and connectivity that ultimately determines the critical current density. We found thatd_effof the higherJ_cwires heat-treated at lower temperature is much smaller than for the lowerJ_cwires. Moreover, a systematic increase of the irreversibility field and a decrease of the intrinsic Bi-2212 anisotropy underpins the higherJ_c. The analysis of the pinning energies reveals that there is little sample-to-sample variation in the INTER-grain pinning, whereas in all samples the intra-grain pinning has an enhancement below ∼40–45 K becoming more and more evident with increasingJ_c. These results suggest that the overallJ_cperformance are not only related to the wire microstructure and connectivity, which obviously affect the INTER-grain properties, but they are also intimately related to the intrinsic and intra-grain properties such asγandU₀.

Although nanoscale deformation, such as nanostrain in iron-chalcogenide (FeSe_xTe_1−x, FST) thin films, has attracted attention owing to its enhancement of general superconducting properties, including critical current density (J_c) and critical transition temperature, the development of this technique has proven to be an extremely challenging and complex process thus far. Herein, we successfully fabricated an epitaxial FST thin film with uniformly distributed nanostrain by injection of a trace amount of CeO₂inside an FST matrix using sequential pulsed laser deposition. By means of transmission electron microscopy and geometric phase analysis, we verified that the injection of a trace amount of CeO₂forms nanoscale defects, with a nanostrained region of tensile strain (ε_zz ≅ 0.02) along thec-axis of the FST matrix. This nanostrained FST thin film achieves a remarkableJ_cof 3.5 MA/cm²under a self-field at 6 K and a highly enhancedJ_cunder the entire magnetic field with respect to those of a pristine FST thin film.