Higher critical currents yet faster vortex creep in EuBa$_2$Cu$_3$O$_y$ films containing coherent artificial pinning centers

Abstract

The electromagnetic properties of type-II superconductors depend on vortices -- magnetic flux lines whose motion introduces dissipation that can be mitigated by pinning from material defects. The material disorder landscape is tuned by the choice of materials growth technique and incorporation of impurities that serve as vortex pinning centers. For example, metal organic deposition (MOD) and pulsed laser deposition (PLD) produce high-quality superconducting films with uncorrelated versus correlated disorder, respectively. Here, we study vortex dynamics in PLD-grown EuBa$_2$Cu$_3$O$_y$ films containing varying concentrations of BaHfO$_3$ inclusions and compare our results with those of MOD-grown (Y,Gd)Ba$_2$Cu$_3$O$_y$ films. Despite both systems exhibiting behavior consistent with strong pinning theory, which predicts the critical current density $J_c$ based on vortex trapping by randomly distributed spherical inclusions, we find striking differences in the vortex dynamics owing to the correlated versus uncorrelated disorder. Specifically, we find that the EuBa$_2$Cu$_3$O$_y$ films grown without inclusions exhibit surprisingly slow vortex creep, comparable to the slowest creep rates achieved in (Y,Gd)Ba$_2$Cu$_3$O$_y$ films containing high concentrations of BaHfO$_3$. Whereas adding inclusions to (Y,Gd)Ba$_2$Cu$_3$O$_y$ is effective in slowing creep, BaHfO$_3$ increases creep in EuBa$_2$Cu$_3$O$_y$ even while concomitantly improving $J_c$. Lastly, we find evidence of variable range hopping and that $J_c$ is maximized at the BaHfO$_3$ concentration that hosts creep of large vortex bundles or a Bose glass state.