Pinning forces and scaling in high-T_c superconductors

The volume pinning forces, F_p, of several high-T_c superconductors are compared to each other using literature data and to those obtained in NdBa₂Cu₃O_7-δ samples. It is shown that the pinning at small normal inclusions which is dominant in most high-T_c systems, leads to a peak at around 0.3 when plotting the normalized volume pinning forces, F_p/F_p,max, versus the reduced field defined by h = H/H_irr. Furthermore, relaxation effects play a significant role in the F_p diagrams, especially in the high h regions thus causing the typical rounded-off 'tail'. In contrast to this, NdBCO shows the peak at ≈0.5, thus indicating a strong pinning at small superconducting regions (ΔK/ΔT_c-pinning).