Effect of doping on the linear temperature dependence of the magnetic penetration depth in cuprate superconductors

Abstract

We report on measurements of the low temperature dependence of the magnetic penetration depth λ ab in several single crystals of La s− x Sr x CuO 4, Bi 2Sr 2CaCu 2O 8 and YBa 2Cu 3O 6+ x at various doping levels ranging from the under- to the overdoped regimes by using a novel single-coil technique achieving 10 pm resolution. We have found a linear dependence of λ ab in all samples with a rapidly increasing slope dλ ab / dT as doping decreases. Our analysis of the data indicates that a superconducting gap with d-wave symmetry is sufficient to quantitatively account for the above slope values in the optimally or over-doped samples. In the underdoped samples, the d-wave model predicts much smaller values than those measured by assuming realistic values for the zero-temperature λ ab and gap Δ. The experimental data are compatible with a model of thermodynamic phase fluctuations of the order parameter. Therefore, we put forward the hypothesis that the gapless properties observed in cuprates may have qualitatively different physical origins depending on the doping level.