Thermal fluctuations of vortices were studied by microwave measurements of the Josephson Plasma Resonance (JPR) in small single crystals of underdoped Bi₂Sr₂CaCu₂O_8+δ with a critical temperature T_c = 69.4 K and a zero-field JPR-frequency of approx. 61 GHz. The penetration depth along the c-axis and the anisotropy ratio γ were calculated from zero-field data. The dc magnetic field and temperature dependences of the wandering length of pancake vortices were examined over the entire vortex solid state and explained by the renormalization of the vortex line tension by the fluctuations, finally leading to the dissociation of the vortices at the first order transition.

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