Vortex softening: origin of the second peak effect in Bi(2)Sr(2)CaCu(2)O(8+delta).

Magnetic hysteresis and transverse ac permeability measurements in Bi(2)Sr(2)CaCu(2)O(8+delta) allow a comparative analysis of the critical current with the elastic response of vortex structures, in equilibrium with their pinning potential, in the field and temperature region where the second peak is detected. This study provides strong evidence that the second peak has its origin in changes of the elastic equilibrium properties of the vortex structures.