  Temperature dependence of the zero-field critical current density, Jc0(T), in
the CuO2 planes has been investigated for c-axis oriented YBa2Cu3O7-d (Y123)
thin films with different hole content, p. p-values were varied by changing the
oxygen deficiency, d. Jc0(T) were calculated from the magnetic hysteresis (M-H)
loops obtained at different temperatures. From the analysis of the Jc0(T) data
for different sample compositions, we have extracted valuable information
regarding the nature of the underlying magnetic flux pinning mechanisms in Y123
superconductors. It is found that the oxygen defects play only a secondary role
in pinning of the vortices, the superfluid density, on the other hand plays a
significant role. The temperature exponent, n, governing the Jc0(T) behavior
for a given sample composition showed a systematic behavior as hole contents
are changed.
