Plasma resonance at low magnetic fields as a probe of vortex line meandering in layered superconductors

We consider the magnetic-field dependence of the plasma resonance frequency in pristine and in irradiated Bi2Sr2CaCu2O8 crystals near T-c. At low magn etic fields we relate linear in field corrections to the plasma frequency t o the average distance between the pancake vortices in the neighboring laye rs (wandering length). We calculate the wandering length in the case of the rmal wiggling of vortex lines, taking into account both Josephson and magne tic interlayer coupling of pancakes. Analyzing experimental data, we found that (i) the wandering length becomes comparable with the London penetratio n depth near T-c and (ii) at small melting fields (<20 G) the wandering len gth does not change much at the melting transition. This shows existence of the line liquid phase in this field range. We also found that pinning by c olumnar defects affects weakly the field dependence of the plasma resonance frequency near T-c.