High Vortex Depinning Temperatures in YBCO Films with BZO Nanorods

The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts the depinning of vortices due to the renormalization of the vortex pinning barriers by thermal fluctuations. For YB₂Cu₃O₇ (YBCO) in external magnetic fields H oriented along the columnar pins generated by various techniques theoretical estimates give a depinning temperature T _dp very close to the critical temperature T _c (T _dp～0.95T _c), whereas the results of standard magnetization relaxation experiments are repeatedly interpreted in terms of a much lower T _dp (～0.5T _c). We investigated the temperature T variation of the normalized magnetization relaxation rate S for YBCO thin films containing BaZrO₃ (BZO) nanorods preferentially oriented along the c axis, with H along the nanorods. The nonmonotonous S(T) variation below the matching field observed up to close to T _c does not support a low T _dp. The often considered S(T) maximum occurring at relatively low T (which was connected to a disappointing T _dp) is related to the occurrence of thermomagnetic instabilities. We show that the accommodation of vortices to the columnar pins in the presence of the T dependent macroscopic currents induced in the sample is signaled by a pronounced S(T) deep located at high T, in agreement with a T _dp close to T _c. By increasing the film thickness and using the substrate decoration the BZO nanorods splay out, leading to the inhibition of (detrimental) vortex excitations involving double vortex kink or superkink formation, characteristic for high-quality thin films and single crystals with columnar pins along the c axis.