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 γ for the generic (T, x) phase diagram of these materials. A possible microscopic model that can explain the doping dependence of γ is based on polaronic charge carriers. That lattice effects may play a role for high-temperature superconductivity is further supported by measurements of the oxygen-isotope effect on the in-plane penetration depth λab(0) in underdoped La2−xSrxCuO4 single crystals that are reported in this paper. The oxygen-isotope effect on λ−2ab(0) is found to be around −9% for the samples investigated. It arises mainly from the oxygen–mass dependence of the in-plane effective mass m*ab. The experimental facts presented in this paper suggest that lattice vibrations are important for the occurrence of high-temperature superconductivity.