Delocalization of vortex in SmBa₂Cu₃O_{7- δ} superconducting films with BaHfO₃ nano-rods

Transport measurements revealed flux pinning properties and vortex phases in SmBa₂Cu₃O_7-δ (Sm123) superconducting films with BaHfO₃ nano-rods on LaAlO₃ substrates. The films have large matching fields B_Φ up to 9.9 T, nano-rod diameters of ∼6 nm, and a slight T_c degradation with T_c ∼ 91.8 K by using the low temperature growth technique. According to the transport results, a small critical exponent ∼4 indicates the presence of a Bose-glass phase in the films. Double peaks of the flux pinning force density are unexpectedly observed at high temperatures over 80 K, which is accompanied by steep drops of the crossover magnetic fields between the single vortex pinning and the collective pinning states. The drops are explained by the delocalization of the vortex where the vortex is pinned by many nano-rods in the single vortex pinning state. From the viewpoint of the vortex delocalization, we conclude that B_Φ should be less than 11 T for applications at liquid nitrogen temperature.