  The temperature dependences of AC magnetic susceptibility at different
magnetic field amplitudes and frequencies are investigated for underdoped and
overdoped R(1-x)Ca(x)Ba(2)Cu(3)O(7-delta)(R=Y; Gd and x=0; 0.2) polycrystalline
samples. The activation energy, Ea, for thermally assisted flux flow (TAFF) in
intergranular region is determined. It was established that a correlation
exists between the intergranular critical current and flux pinning activation
energy. In underdoped samples the intergranular current shows S-I-S behaviour
and the activation energy is small, while in overdoped samples the
intergranular current is changed to S-N-S type and the activation energy
increases. 2D pancake vortices are characteristic of underdoped samples, while
3D vortex system exists in overdoped samples. In fact we demonstrate that Ca
substitution not only increases carrier concentration, but improves
intergranular activation energy for TAFF.
