Doping dependence of the effective mass anisotropy and oxygen-isotope effect on the magnetic penetration depth: The role of lattice vibrations in high-temperature superconductivity

Studies of the critical behavior of high-temperature superconductors (HTSC) show the importance of the strong doping dependence of the effective mass anisotropy gamma for the generic (T, x) phase diagram of these materials. A possible microscopic model that can explain the doping dependence of gamma is based on polaronic charge carriers. That lattice effects may play a rol e for high-temperature superconductivity is further supported by measuremen ts of the oxygen-isotope effect on the in-plane penetration depth lambda (a b)(0) in underdoped La2-xSrx CuO4 single crystals that are reported in this paper. The oxygen-isotope effect on lambda (-2)(ab)(0) is found to be arou nd -9% for the samples investigated. It arises mainly from the oxygen-mass dependence of the in-plane effective mass m(ab)*. The experimental facts pr esented in this paper suggest that lattice vibrations are important for the occurrence of high-temperature superconductivity.