Origin of the dimpled critical-current-versus-magnetic-field-angle relation in YBa2Cu3O7 films studied using sub-MeV ion irradiation

Abstract

In high-temperature superconductor (HTSC) films, the magnetic-field-angle (θ) dependence of critical current density (Jc) is often observed to have a characteristic local maximum at intermediate θ between H||ab and H||c. This local maximum appears as a ‘shoulder’ when Jc(θ) has a predominant peak at H||c, and when such a peak is absent, it appears as a ‘dimple edge’ in a very broad dome-like Jc(θ) that is centered and dimpled at H||c. Despite such common observation, there is still no consensus on the physical origin of this anomalous Jc(θ). In this work, to determine this physical origin. we measured the temperature and microstructure dependence of Jc(B, θ) in YBa2Cu3O7 films irradiated with 3 MeV Au, 500 keV Si, and 200 keV B ions (where B is magnetic induction). The film microstructure was controlled by varying the mass of irradiation ions (Mi) known to alter the size and spacing of collision cascades. Results revealed two observations: (i) the dimple structure diminishes with decreasing temperature, and (ii) the maximum pinning force density in a θ profile, which is recorded at the dimple edge, systematically becomes more B-dependent with decreasing Mi. Both experimental findings suggest that the sharp-shaped anomaly in Jc(θ) in HTSC films originates from flux-line-lattice shear and electron mass anisotropy.