  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.

Superconductivity