\textbf{Understanding and eliminating the fast creep problem in Fe-based superconductors}

One surprising characteristic of Fe-based superconductors is that they exhibit flux creep rates ($S)$ as large as, or larger than, those found in oxide high temperature superconductors (HTS). This very fast vortex dynamics appears to be inconsistent with the estimate of the influence of the thermal fluctuations as quantified by the Ginzburg number (\textit{Gi}), which measures the ratio of the thermal energy to the condensation energy in an elemental superconducting volume. In particular, compounds of the $A$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ family (``122'') have \textit{Gi} $\sim $10$^{\mathrm{-5}}$ to 10$^{\mathrm{-4}}$, so $S$ could be expected to lie between that of low $T_{\mathrm{c}}$ materials (where typically \textit{Gi }$\sim $ 10$^{\mathrm{-8}})$ and HTS such as YBa$_{\mathrm{2}}$Cu$_{\mathrm{3}}$O$_{\mathrm{7}}$ (\textit{Gi }$\sim $ 10$^{\mathrm{-2}})$, as indeed occurs in other superconductors with intermediate fluctuations, such as MgB$_{\mathrm{2}}$ (\textit{Gi} $\sim $10$^{\mathrm{-6}}$ to 10$^{\mathrm{-4}})$. We have found the solution to this puzzle: the fast creep rates in 122 compounds are due to non-optimized pinning landscapes. Initial evidence comes from our previous studies showing that the introduction of additional disorder by irradiation decreases creep significantly in 122 single crystals, although still remaining well above the ideal limit. We now have new evidence from 122 thin films demonstrating that S can be reduced to the lower limit set by \textit{Gi} by appropriate engineering of the pinning landscape.