The in-plane magnetic field penetration depth (λ ab ) in single-crystal La1.83Sr0.17CuO4 was investigated by means of the muon-spin rotation (µSR) technique. The temperature dependence of λ

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The temperature dependence of the London penetration depth lambda was measured for an untwinned single crystal of YBa_{2}Cu_{3}O_{7-delta} along the three principal crystallographic directions (a, b, and c). Both in-plane components (lambda_{a};{-2} and lambda_{b};{-2}) show an inflection point in their temperature dependence which is absent in the component along the c direction (lambda_{c};{-2}). The data provide convincing evidence that the in-plane superconducting order parameter is a mixture of (s+d)-wave symmetry whereas it is mainly s wave along the c direction. In conjunction with previous results it is concluded that coupled s+d-order parameters are universal and intrinsic to cuprate superconductors.

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Various unconventional isotope effects have been reported in high-temperature superconducting copper oxides which are beyond the scheme of BCS theory. Their origin is investigated within polaron theory which leads to a renormalization of the single particle energies and introduces a level shift here. It is found that the exponential squeezing of the second nearest neighbour hopping integral carries the correct isotope effect on the superconducting transition temperature T c , as well as the one on the penetration depth. The average superconducting gap is predicted to have an isotope effect comparable to the one on the penetration depth. The results imply that the coupling of the electronic degrees of freedom to the Jahn-Teller Q 2 -type mode is the origin of these isotope effects.High-temperature cuprate superconductors (HTSC) are one of most intensely studied systems due to the yet lacking understanding of the pairing mechanism. The antiferromagnetic properties of the undoped compounds are a consequence of the large Coulomb repulsion at the copper site. The energy scale given by it is the largest, and this has been taken as evidence that it must play a crucial role for the pairing mechanism. Consequently effects stemming from the lattice have mostly been ignored, especially in view of the fact that the isotope effect on the superconducting transition temperature T c almost vanishes at optimum doping 1,2 . The failure of BCS theory to account for many of the observed exotic properties has contributed to interpret the pairing mechanism in terms of a purely electronically driven one. However, various unexpected isotope effects have been reported 3,4,5 which are neither expected within the BCS mechanism nor within m odels based on strong correlations only. Since the Cu ion is one of the strongest Jahn-Teller systems 6 , polaron formation can take place here and be the origin of unconventional isotope effects. In spite of that the majority thought that the physics of high-temperature cuprate superconductors are dominated by a purely electronic mechanism, we address here the possibility of unconventional charge lattice coupling leading to Jahn-Teller polaron formation. We are motivated by various experimental findings as e.g. the observation of an isotope effect on the London penetration depth λ L 3,4 , the isotope dependence of the electronic energy bands 5 , the interpretation of EPR data in terms of three spin polarons 7 , the EXAFS data revealing the coexistence of different length scales 8 , the isotope effect on T c 1,2 , which exceeds the BCS value in the underdoped regime, the strain induced enhancement of T c in HTSC films 9,10 . In addition, we address the inherent inhomogeneity observed in HTSC as revealed by EXAFS 8 , STM 11 , NMR 12 , EPR 13 , which requires that a multi-component scenario has to be considered. Especially the observation of not only a d-wave order parameter but also an swave order parameter 14 is included in the modelling of HTSC.For the undoped parent compounds we start with the t -J sce...