  Superconductors are excellent testbeds for studying vortices, topological
excitations that also appear in superfluids, liquid crystals, and Bose-Einstein
condensates. Vortex motion can be disruptive; it can cause phase transitions,
glitches in pulsars, and losses in superconducting microwave circuits, and it
limits the current carrying capacity of superconductors. Understanding vortex
dynamics is therefore of fundamental and technological importance, and the
competition between the effects of thermal energy and energy barriers defined
by material disorder is not completely understood. In particular, early
measurements of thermally-activated vortex motion (creep) in iron-based
superconductors unveiled fast rates (S) comparable to measurements of YBa2Cu3O7
(YBCO). This was puzzling because S is thought to somehow positively correlate
with the Ginzburg number (Gi), and Gi is orders of magnitude lower in most
iron-based superconductors than in YBCO. Here, we report very slow creep in
BaFe2(As0.67P0.33)2 films, and propose that there is a universal minimum
realizable (where Tc is the superconducting transition temperature) that has
been achieved in our films, few other materials, and violated by none. This
limitation provides new clues on how to design materials with slow creep and
helps elucidate open questions regarding the interplay between system-specific
length scales and vortex dynamics.
