LONGITUDINAL ELASTIC CORRELATION LENGTH OF FLUX LINES ALONG THE C-AXIS IN SUPERCONDUCTING BI-2212 SINGLE-CRYSTAL IN THE FIELD RANGE NEAR THE PEAK EFFECT

The longitudinal elastic correlation length, l(44), of flux lines in a superconducting Bi-2212 single crystal in the low field region near t he peak effect is directly measured using the Campbell method under th e magnetic field parallel to the c-axis. It is found that l(44) is lon ger than several mu m even in this field region similarly to the previ ous measurement at much high fields. The obtained result agrees well w ith the pinning correlation length estimated using the measured critic al current density, J(c). The apparent pinning potential estimated fro m the relaxation rate of magnetization has a peak near the peak field and can be explained well by the collective flux creep theory in which the obtained J(c) and l(44) are used. It is concluded from these resu lts that the flux lines are strongly coupled over a fairly long distan ce along the c-axis even in the vicinity of peak field. Discussion is given on the correlation with the observed l(44) and the local distort ions of flux lines which are speculated to be the reason for the peak effect.