Superconducting critical currents $j_{ c} > 10^{ 5}$ A/cm$^{ 2}$ at temperatures $T \sim 50$ K and magnetic fields $B \sim 6$ T are reported for the YBa$_{ 2}$Cu$_{ 3-x}$Mo$_{ x}$O$_{ 7+d}$ compound with $x = 0.02$. Clear evidence for the increased pinning force $F_p$ was found from a peak effect present for $j_{ c}(B)$. The pinning force was analyzed by a scaling procedure using Kramer's approach. For a wide range of fields and temperatures, we were able to express all $F_p$ data as a single function of a reduced field $b = B/B_k$, where the scaling field $B_{ k} << H_{ c2}$ was related to the irreversibility filed $B_{ irr}$. Analyses of the field dependence of $j_{ c}(B,T)$ and $F_{ p}(b)$ show that the effective pinning centers act as weakly interacting extended point-like defects. We propose that the pinning centers are randomly distributed small defects, most likely the dimers of MoO$_6$ octahedra in the CuO chains.
Categories: cond-mat.supr-con