Study of the magnetic field distribution in high-temperature superconductors using muon-spin-rotation

Detailed and systematic μ+SR experiments have been performed in order to (i) investigate the temperature dependence of the magnetic penetration depth in various cuprate high-Tc superconductors and (ii) study the vortex structures and dynamics in the highly anisotropic Bi2Sr2CaCu2O8. The μ+SR method and its application to superconductivity has been discussed. The positive muon is a microscopic probe of the local magnetic field in the bulk of a sample. The μ+SR technique can therefore measure the magnetic field distribution p(B) which is determined by the flux structure in the superconductor. The second moment (ΔB2) of p(B) is closely related to the magnetic penetration depth λ, a fundamental parameter of superconductivity. It has been shown that in high-quality sintered samples a good estimate of the in-plane penetration depth λab can be given in terms of the muon-depolarization rate σ. Since the penetration depth is related to the superconducting order parameter, the temperature dependence of the penetration depth is a potential probe of the pairing state. Systematic measurements of the temperature dependence of σ have been performed in sintered samples of high quality in various members of the Y123 family, Pb and Y doped Tl1212 family, and also in Y124 and Bi2212. It is found that the extracted temperature behaviour of λab-2 is characteristic of each compound. This can be interpreted as a varying coupling strength in these systems. In well oxygenated Y123, λab-2(T) is well described by the two-fluid model indicating strong coupling. The rest of the cuprates investigated show a λab-2 (T) which points to weaker coupling, with λab-2(T) of highly oxygen deficient Y123 being similar to the weak-coupling BCS prediction. In the Y123 family the decreasing coupling strength with decreasing oxygen content is related to the increasing anisotropy. Comparison with theoretical predictions of λab-2(T) revealed that the observed temperature dependence of λab is not consistent with d-wave pairing symmetry. (author) 30 figs., 2 tabs., 125 refs