Dip effect in χ′ of ac susceptibility in Bi2Sr2CaCu2O8 single crystal

We present a direct evidence of the softening of the shear modulus constant c(66) by a measurement of the real part chi'(T) of ac susceptibility. A dip in chi'(T) for a single crystal of Bi2Sr2CaCu2O8 has been found at an intermediate field region between 50 and 3000 G. We showed that the dip effect was due to the softening of the shear modulus constant c(66), equivalent to the peak effect in resistivity measurement. The dependence of the peak temperature T-p in chi " on magnetic field and current density shows the transition between vortex solid and vortex liquid. Thus, the dip in chi'(T) is illustrated as a precursor to vortex lattice melting in this narrow field region, indicative of lattice softening prior to malting. In the region between T-d and the melting temperature T-m, the flux lines move by a type of plastic deformation, and the pinning energy barrier U has been obtained as: U proportional to [1 - (J/J(c))(nu)]H-mu with nu, mu > 0. (C) 1999 Elsevier Science B.V. All rights reserved.