Temperature dependence of critical currents in REBCO thin films with artificial pinning centers

Conventionally, delta T-c type (order parameter modulation) and delta l type (mean free path modulation) pinning mechanisms have been proposed to explain the temperature dependence of the flux pinning of superconducting materials. According to previous studies, it is assumed that the temperature dependence of J(c) of REBa2Cu3O7 (REBCO, RE = Y, Gd, Sm, etc) films without artificial pinning centers (APCs) is delta l type, but it is unidentified when APCs are introduced into the films. In this paper, GdBCO thin films doped with BaHfO3 (BHO) deposited on LaAlO3 substrates by pulsed laser deposition were studied. A target exchange method was used to alternately ablate two targets of pure GdBCO and BHO for introducing nanorods as APCs into GdBCO films. Since the insulative BHO acts as a strong pinning center, the delta T-c pinning mechanism is expected for the temperature dependence of J(c) of these thin films. However, the experimental results showed that the J(c) of the films with BHO nanorods was determined by the delta l pinning mechanism over a wide temperature range. In order to explain these unexpected results, we examined the pinning mechanism by nanorods based on a resultant pinning force model.