Analysis of pinning inNdBa2Cu3O7−δsuperconductors

Current densities $j_s$ and volume pinning forces $F_p$ are obtained in a wide temperature $(5\mathrm{}<~T<~92\mathrm{K})$ and field range $(0\mathrm{}<~{\mu }_0{H}_a<~9\mathrm{T})$ on different ${\mathrm{NdBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ samples. Above 60 K a good scaling of the volume pinning force $F_p$ versus the reduced field ${h=H}_a{/H}_{\mathrm{irr}}$ can be established. The scaled pinning curves are compared to several theoretical predictions. Experimental evidence for strong pinning at extended superconducting defects (interaction volume $V_{\mathrm{pin}}\sim {\xi }^{2}d$) is given. These defects are ascribed to spatial composition fluctuations found in light rare-earth superconductors, providing a scatter of the transition temperature $T_c.$ Such a pinning mechanism is especially important for applications of high-$T_c$ superconductors operating at $T=77\mathrm{}\mathrm{K}.$