Superconductivity and Critical Current of Iron-Based Superconductors in High Field

Although high-temperature superconducting cuprates have been discovered for more than 26 years, high-field applications are still based on low-temperature superconductors (LTS), such as Nb$_{\mathrm{3}}$Sn. The high anisotropies, brittle textures and high manufacturing costs limit the applicability of the cuprates. Recently, we demonstrated that the iron superconductors, without most of the drawbacks of the cuprates, have a superior high-field performance over LTS at 4.2 K [Nat. Commun. \textbf{4}:1347 (2013); Rep. Prog. Phys. \textbf{74} 124510 (2011)]. In this presentation, I will discuss recent progress aimed at understanding the relationships between superconductivity, critical current, and nano-scaled structure defects in iron-based superconductors, with emphasis on the properties of superconducting iron chalcogenide films. Critical current densities $J_{\mathrm{c}}$ $\sim$ 10$^{\mathrm{7}}$ A/cm$^{\mathrm{2}}$ were observed in FeSe$_{\mathrm{0.5}}$Te$_{\mathrm{0.5}}$ films grown on CeO$_{\mathrm{2}}$ buffered single-crystalline and flexible metal substrates. These films are capable of carrying $J_{\mathrm{c}}$ exceeding 10$^{\mathrm{5}}$ A/cm$^{\mathrm{2}}$ under 30 T magnetic fields. Furthermore, we found that these films have significantly higher $T_{\mathrm{c}}$ (\textgreater 20K) as compared to bulk samples (bulk $T_{\mathrm{c}}$ $\sim$ 15 K) for the entire doping regime of FeSe$_{\mathrm{1-x}}$Te$_{\mathrm{x}}$. Structural analysis revealed that these films generally have significantly smaller c-axis and a-axis lattice constant than the bulk value, suggesting that the crystal structure changes have a dominating impact on the superconducting transition in iron-based superconductors. Large $J_{\mathrm{c}}$ enhancement can also be realized in iron based superconductors by irradiation with proton and heavy ions that opens a new avenue for a tailored landscape of effective vortex pinning defects.