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 ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ crystals near $T_c.$ At low magnetic fields we relate linear in field corrections to the plasma frequency to the average distance between the pancake vortices in the neighboring layers (wandering length). We calculate the wandering length in the case of thermal wiggling of vortex lines, taking into account both Josephson and magnetic interlayer coupling of pancakes. Analyzing experimental data, we found that (i) the wandering length becomes comparable with the London penetration depth near $T_c$ and (ii) at small melting fields $(<\mathrm{}20\mathrm{}\mathrm{G})$the wandering length 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 columnar defects affects weakly the field dependence of the plasma resonance frequency near $T_c.$