Grain size dependence of the current-voltage characteristics and critical current density in the self-field approximation
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In the self-field approximation, the influence of the grain radius on the current-voltage characteristics and associated critical current density J(c) are investigated by considering the conventional flux flow motion of vortices. It is shown that the direct summation theory of Campbell et al. could be adopted to the granular systems [A. M. Campbell, J. E. Evetts, and D. Dew-Hughes, Philos. Mag. 10, 333 (1964)]. We found that J, is enhanced as the size of the grains is decreased. The case of the critical current density being greater for smaller grains, which is consistent with both transport and low-field magnetization measurements is discussed by the weak links between the superconducting grains and also with the percolation model. (C) 1998 American Institute of Physics.