Vortex fluctuations in underdoped Bi2Sr2CaCu2O8+d crystals

Vortex thermal fluctuations in heavily underdoped Bi2Sr2CaCu2O8+d (Tc=69.4 K) are studied using Josephson plasma resonance (JPR). From the data in zero magnetic field, we obtain the penetration depth along the c-axis, lambda_{L,c}(0) = 229 micrometers and the anisotropy ratio gamma(0) = 600. The low plasma frequency allows us to study phase correlations over the whole vortex solid (Bragg-glass) state. The JPR results yield a wandering length r_{w} of vortex pancakes. The temperature dependence of r_{w} as well as its increase with applied dc magnetic field can only be explained by the renormalization of the tilt modulus by thermal fluctuations, and suggest the latter is responsible for the dissociation of the vortices at the first order transition.Comment: 4 pages, 5 figures. Submitted to Phys. Rev. Let

Nature of c-axis coupling in underdoped Bi2Sr2CaCu2O8 with varying degrees of disorder

The dependence of the Josephson Plasma Resonance (JPR) frequency in heavily underdoped Bi2Sr2CaCu2O8+\delta on temperature and controlled pointlike disorder, introduced by high-energy electron irradiation, is cross-correlated and compared to the behavior of the ab-plane penetration depth. It is found that the zero temperature plasma frequency, representative of the superfluid component of the c-axis spectral weight, decreases proportionally with T_c when the disorder is increased. The temperature dependence of the JPR frequency is the same for all disorder levels, including pristine crystals. The reduction of the c-axis superfluid density as function of disorder is accounted for by pair-breaking induced by impurity scattering in the CuO2 planes, rather than by quantum fluctuations of the superconducting phase. The reduction of the c-axis superfluid density as function of temperature follows a T^{2}--law and is accounted for by quasi-particle hopping through impurity induced interlayer states.Comment: 10 pages, 9 Figure