Radio frequency vortex dynamics in oriented platelets of (Bi-Pb)₂Sr₂Ca₂Cu₃O₁₀ superconductor

Abstract

We report measurements of temperature and field dependence of the radio frequency (rf) penetration depth λ (H,T) in oriented platelets of (Bi-Pb)₂Sr₂Ca₂Cu₃O₁₀ superconductor. For the DC field aligned perpendicular to the plane of the platelet (predominantly H||c configuration), three distinct regimes of field variation of λ(H,T) are identified which correspond, respectively to grain boundary/weak link pinning, intragrain pinning and flux flow with the increasing field. In the low flux density regime investigated here, λ(H,T) follows the dynamics of a single vortex subjected to a periodic pinning potential, a viscous damping and an oscillating Lorentz force of rf currents. The pinning force constants for H perpendicular (⊥ ) and parallel (||) to the plane of the platelet are k_p^⊥2.2× 10⁴ N/m² and k_p^||~1× 10⁵ N/m² at 65 K, respectively for bulk pinning and are considerably lower for intergrain pinning. The force constant follows an angular dependence of the type, k_p^⊥k_p(φ ) sinφ for φ > 8° , where φ is the angle between the applied field and the plane of the platelet. The critical fields H₁^∗ and H₂^∗ at which the dynamics changes from grain boundary dominated to bulk pinning and from bulk pinning to flux flow are determined, and have been identified with H_c1 of the grains and irreversibility field, respectively. The trapping of flux between grain boundaries, which has eluded detection in the studies of DC magnetization is distinctly established through these high-frequency measurements.