The critical temperature T_c and the critical current density J_c determine the limits to large-scale superconductor applications. Superconductivity emerges at T_c. The practical current-carrying capability, measured by J_c, is the ability of defects in superconductors to pin the magnetic vortices, and that may reduce T_c. Simultaneous increase of T_c and J_c in superconductors is desirable but very difficult to realize. Here we demonstrate a route to raise both T_c and J_c together in iron-based superconductors. By using low-energy proton irradiation, we create cascade defects in FeSe_0.5Te_0.5 films. T_c is enhanced due to the nanoscale compressive strain and proximity effect, whereas J_c is doubled under zero field at 4.2 K through strong vortex pinning by the cascade defects and surrounding nanoscale strain. At 12 K and above 15 T, one order of magnitude of J_c enhancement is achieved in both parallel and perpendicular magnetic fields to the film surface.

The loss-less electrical current-carrying capability of type II superconductors, measured by the critical current density J _c , can be increased by engineering desirable defects in superconductors to pin the magnetic vortices. Here, we demonstrate that such desirable defects can be created in superconducting FeSe _0.5 Te _0.5 films by 6 MeV Au-ions irradiations that produce cluster-like defects with sizes of 10–15 nm over the entire film. The pristine FeSe _0.5 Te _0.5 film exhibits a low anisotropy in the angular dependence of J _c . A clear improvement in the J _c is observed upon Au-ion irradiation for all field orientations at 4.2 K. Furthermore, a nearly 70% increase in J _c is observed at a magnetic field of 9 T applied parallel to the crystallographic c -axis at 10 K with little reduction of the superconducting transition temperature T _c . Our studies show that a dose of 1 × 10 ¹² Au cm ^–2 irradiation at a few MeV is sufficient in order to provide a strong isotropic pinning defect landscape in iron-based superconducting films.

A variety of morphologies of irradiation defects ranging from columnar defects (CDs) to spherical ones were introduced into NdBa ₂ Cu ₃ O _y thin films by using three kinds of heavy-ion beams. The highest critical current density J _c was obtained for the 50-MeV Kr irradiation at the matching field of B _φ = 10 T, where spherical defects were installed. In contrast, J _c of the 200 MeV Xe irradiation which produces elongated CDs continues to increase only up to B _φ = 5 T, resulting in lower value. Interestingly, the volume fraction of the spherical defects at B _φ = 10 T was about 10 vol.%, which is almost the same as that of elongated CDs at B _φ = 5 T. These results suggest that spherical defects provide weak elementary pinning force, but can realize the introduction of much higher density without suppressing the superconducting percolation path at the same volume fraction condition. The systematic segmentation of CDs also enabled us to reveal the influence of the morphology of pinning centers on thermal relaxation rate of J _c .

Iron-chalcogenide FeSe _x Te _{1− x} (Fe(Se,Te)) superconductors have attracted a great deal of interest in both fundamental physics and potential applications. While Majorana zero-modes hosted by the Fe(Se,Te) superconductors hold great promise for topological quantum computing, low-anisotropy and very high upper critical fields make Fe(Se,Te) an excellent candidate for high-magnetic-field and energy applications. Recently, we demonstrated a route for creating structural defects by ion irradiation in Fe(Se,Te) films, leading to strong flux pinning. Ion irradiation into Fe(Se,Te) films significantly improved the critical current density J _c over a wide field range and for all orientations with respect to magnetic field. In particular, a robust enhancement of critical temperature T _c and J _c was realized simultaneously in the Fe(Se,Te) film irradiated with protons. Here, we provide a review of the ion-irradiated Fe(Se,Te) films, including the film growth and characteristics before and after irradiation, design of the irradiation process for different ion energies and species with irradiation simulations and experimental setups that are scalable for long-length Fe(Se,Te)-coated conductors.

The morphologies of ion tracks tilted at different angles were systematically investigated by TEM observations on GdBCO coated conductors irradiated with 80 MeV Xe ions. The 80 MeV Xe ion beam along the c -axis produced shortly segmented (discontinuous) columnar defects (CDs) along the ion path throughout the sample. When the ion beam was tilted at slight angle of 20° from the c -axis, by contrast, not discontinuous CDs but thick and elongated (continuous) ones were formed in the irradiation direction. The formation of continuous CDs was observed to a deeper depth from the surface for the irradiation tilted at larger angle from the c -axis. The variation of the microstructures against the irradiation angles was caused by the anisotropic temperature spread during the formation of CDs, which was attributed to the anisotropic thermal diffusivity in high- T _c superconductors. We also demonstrated that discontinuous CDs could be formed at the irradiation angle of ±45° relative to the c -axis into YBCO thin films by using 50 MeV Kr ions, which was the irradiation condition estimated backwards from the directional dependent morphology of CDs.

Magnesium diboride (MgB₂) thin films on r-cut sapphire (r-Al₂O₃) single crystals were fabricated by a precursor, which was obtained at room temperature via a pulsed laser deposition (PLD) method using a Nd:YAG laser, and an in situ postannealing process. The onset superconducting transition, T_c^onset, and zero-resistivity transition, T_c^zero, were observed at 33.6 and 31.7 K, respectively, in the MgB₂ thin films prepared by a Mg-rich target with a ratio of Mg:B = 3:2. The critical current density, J_c, calculated from magnetization measurements reached up to 0.9 × 10⁶ A cm⁻² at 20 K and 0 T. The broad angular J_c peak was found at 28 K when the magnetic fields were applied in a direction parallel to the film surface (θ = 90°). This could be indicative of the granular structure with randomly oriented grains. Our results demonstrate that this process is a promising candidate for the fabrication of MgB₂ superconducting devices.