New scaling laws for pinning force density in superconductors

Abstract

Since the report by Fietz and Webb (1968 Phys. Rev. 178 657), who considered the pinning force density, $|F_p| = |J_c \times B|$ (where $J_c$ is the critical current density and $B$ is applied magnetic field), in isotropic superconductors as a unique function of the reduced applied field, $B/B_{c2}$ (where $B_c2$ is the upper critical field), $|F_p|$ has been scaled based on B/B_c2 ratio, for which there is widely used scaling law of $|F_p(B)|=F_{p,max}((B/B_{c2})^p)((1-B/B_{c2})^q)$, where $F_{p,max}$, $B_{c2}$, $p$, and $q$ are free-fitting parameters, proposed by Kramer (1973 J. Appl. Phys. 44 1360) and Dew-Hughes (1974 Phil. Mag. 30 293). To describe $|F_p(B)|$ in high-temperature superconductors, Kramer-Dew-Hughes scaling law was modified by (a) an assumption of the angular dependence of all free-fitting parameters on the rotation angle and (b) by the replacement of the upper critical field, $B_{c2}$, by the irreversibility field, $B_{irr}$. Here we note that the pinning force density is also a function of critical current density and, thus, $|F_p(J_c)|$ scaling law should exist. In an attempt to reveal this law, we considered the full $|F_p(B,J_c)|$ function and reported that there are three distinctive characteristic ranges of $(B/B_{c2}, J_c/(J_c(sf)))$ (where $J_c(sf)$ is the self-field critical current density) on which $|F_p(B,J_c)|$ can be splatted. Several new scaling laws for $|F_p(J_c)|$ were proposed, discussed, and applied to scale $|F_p(J_c)|$ in MgB2, NdFeAs(O,F), REBCO, and near-room temperature superconducting super hydrides (La,Y)H10 and YH6. We pointed out that the general scaling law for the pinning force density is on the quest.