Hydrostatic pressure: a very effective approach to significantly enhance critical current density in granular iron pnictide superconductors

Pressure is well known to significantly raise the superconducting transition temperature, T_c, in both iron pnictides and cuprate based superconductors. Little work has been done, however, on how pressure can affect the flux pinning and critical current density in the Fe-based superconductors. Here, we propose to use hydrostatic pressure to significantly enhance flux pinning and T_c in polycrystalline pnictide bulks. We have chosen Sr₄V₂O₆Fe₂As₂ polycrystalline samples as a case study. We demonstrate that the hydrostatic pressure up to 1.2 GPa can not only significantly increase T_c from 15 K (underdoped) to 22 K, but also significantly enhance the irreversibility field, H_irr, by a factor of 4 at 7 K, as well as the critical current density, J_c, by up to 30 times at both low and high fields. It was found that pressure can induce more point defects, which are mainly responsible for the J_c enhancement. Our findings provide an effective method to significantly enhance T_c, J_c, H_irr, and the upper critical field, H_c2, for other families of Fe-based superconductors in the forms of wires/tapes, films, and single crystal and polycrystalline bulks.