Vortex depinning temperature in YBa2Cu3O7films with BaZrO3nanorods

Abstract

The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts the depinning of vortices due to the renormalization of the pinning barriers by thermal fluctuations. In the case of YB${}_{2}$Cu${}_{3}$O${}_{7}$ theoretical estimates give a depinning temperature ${T}_{dp}$ very close to the critical temperature ${T}_{c}$ (${T}_{dp}$ \ensuremath{\sim} 0.95${T}_{c}$), while the results of standard magnetization relaxation experiments are repeatedly interpreted in terms of a much lower ${T}_{dp}$ (\ensuremath{\sim}0.5${T}_{c}$). We determined the temperature $T$ variation of the normalized magnetization relaxation rate $S$ for YB${}_{2}$Cu${}_{3}$O${}_{7}$ films containing BaZrO${}_{3}$ nanorods preferentially oriented along the $c$ axis, with the external magnetic field applied along the nanorods. By extending the $T$ interval up close to ${T}_{c}$, below the matching field a rich nonmonotonous $S$($T$) variation was observed. It is shown here that the often analyzed $S$($T$) maximum occurring at relatively low $T$ (which was connected to a disappointing ${T}_{dp}$) has an extrinsic origin, related to thermomagnetic instabilities. The accommodation of vortices to the columnar pins in the presence of the $T$-dependent macroscopic currents induced in the sample during experiments is actually signaled by a pronounced $S$($T$) deep located at high $T$, indicating that ${T}_{dp}$ remains close to ${T}_{c}$, in agreement with the theoretical prediction.