Each discovery of a new high temperature superconductor drives the expectation that advanced engineering of materials defect structures will enable effective vortex pinning and high values of the electrical current density. Here, we demonstrate that single crystals of the iron-based superconductor Ba0.6K0.4Fe2As2 with Tc = 37.5 K can accommodate an unprecedented large concentration of strong-pinning defects in the form of discontinuous nm-sized nanorods with no degradation of the superconducting transition temperature. At a temperature of 5 K, we find a critical current density of 5 MA/cm2 that is magnetic field independent in fields up to 7 T.

A new critical-current-by-design paradigm is presented. It aims at predicting the optimal defect landscape in superconductors for targeted applications by elucidating the vortex dynamics responsible for the bulk critical current. To this end, critical current measurements on commercial high-temperature superconductors are combined with large-scale time-dependent Ginzburg-Landau simulations of vortex dynamics.

We report on the specific heat determination of the anisotropic phase diagram of single crystals of optimally doped SmFeAsO 1-x F x . In zero-field, we find a clear cusp-like anomaly in C/T with ΔC/T c = 24 mJ/molK 2 at T c = 49.5 K. In magnetic fields along the c-axis, pronounced superconducting fluctuations induce broadening and suppression of the specific heat anomaly which can be described using three-dimensional lowestLandau-level scaling with an upper critical field slope of -3.5 T/K and an anisotropy of Γ = 8. The small value of ΔC/T c yields a Sommerfeld coefficient γ ~ 8 mJ/molK 2 indicating that SmFeAsO 1-x F x is characterized by a modest density of states and strong coupling.

Iron-based superconductors could be useful for electricity distribution and superconducting magnet applications because of their relatively high critical current densities and upper critical fields. SmFeAsO 0.8 F 0.15 is of particular interest as it has the highest transition temperature among these materials. Here we show that by introducing a low density of correlated nano-scale defects into this material by heavy-ion irradiation, we can increase its critical current density to up to 2 Â 10 7 A cm À 2 at 5 K-the highest ever reported for an ironbased superconductor-without reducing its critical temperature of 50 K. We also observe a notable reduction in the thermodynamic superconducting anisotropy, from 8 to 4 upon irradiation. We develop a model based on anisotropic electron scattering that predicts that the superconducting anisotropy can be tailored via correlated defects in semimetallic, fully gapped type II superconductors.

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