Abstract
We present a direct evidence of the softening of the shear modulus constant c 66 by a measurement of the real part χ′( T) of ac susceptibility. A dip in χ′( T) for a single crystal of Bi 2Sr 2CaCu 2O 8 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 χ″ on magnetic field and current density shows the transition between vortex solid and vortex liquid. Thus, the dip in χ′( T) is illustrated as a precursor to vortex lattice melting in this narrow field region, indicative of lattice softening prior to melting. 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∝[1−( J/ J c) ν ] H μ with ν, μ>0.
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