Increase of critical currents and peak effect in Mo substituted YBa$_{2}$Cu$_{3}$O$_{7}$

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 Kramers 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.