Vortex fluctuations in underdoped Bi2Sr2CaCu2O8+δ crystals -: art. no. 137002

Vortex thermal fluctuations in heavily underdoped Bi2Sr2CaCu2O8+delta (T-c = 69.4 K) are studied using Josephson plasma resonance. From the zero-field data, we obtain the c-axis penetration depth lambda(L,c)(0) = 230 +/- 10 mum and the anisotropy ratio gamma(T). The low plasma frequency allows us to study phase correlations over the whole vortex solid state and to extract a wandering length r(w) of vortex pancakes. The temperature dependence of r(w) as well as its increase with dc magnetic field is explained by the renormalization of the vortex line tension by the fluctuations, suggesting that this softening is responsible for the dissociation of the vortices at the first order transition.