The oxygen isotope effect on the in-plane penetration depth in cuprate superconductors

Muon spin rotation (μSR) studies of the oxygen isotope (16O/18O) effect (OIE) on the in-plane magnetic field penetration depth λab in cuprate high-temperature superconductors (HTS) are presented. First, the doping dependence of the OIE on the transition temperature Tc in various HTS is briefly discussed. It is observed that different cuprate families show similar doping dependences of the OIE on Tc. Then, bulk μSR, low-energy μSR, and magnetization studies of the total and site-selective OIE on λab are described in some detail. A substantial OIE on λab was observed in various cuprate families at all doping levels, suggesting that cuprate HTS are non-adiabatic superconductors. The experiments clearly demonstrate that the total OIE on Tc and λab arise from the oxygen sites within the superconducting CuO2 planes, demonstrating that the phonon modes involving the movement of planar oxygen are dominantly coupled to the supercarriers. Finally, it is shown that the OIE on Tc and λab exhibit a relation that appears to be generic for different families of cuprate HTS. The observation of these unusual isotope effects implies that lattice effects play an essential role in cuprate HTS and have to be considered in any realistic model of high-temperature superconductivity.