OXYGEN-ISOTOPE EFFECTS IN LA2-XSRXCUO4 - EVIDENCE FOR POLARONIC CHARGE-CARRIERS AND THEIR CONDENSATION

Magnetization measurements have been performed on the oxygen-isotope e xchanged samples (O-16 and O-18) of the one-layer cuprate superconduct ors La2-xSrxCuO4 (0.06 less than or equal to x less than or equal to 0 .15). From magnetization measurements on the fine-grained and decouple d samples in the Meissner state, we find substantial oxygen-isotope ef fects on both the penetration depth lambda(0) and T-c. From normal-sta te susceptibility measurements, we are able to show that there is a ne gligible oxygen-isotope effect on the carrier density n. The combined results strongly suggest that there is an oxygen-isotope effect on the effective supercarrier mass m*, which is huge for x = 0.06, and redu ced to a smaller value for x = 0.15. We discuss the isotope effects, s upercarrier mass anisotropy, normal-state gap, in-plane penetration de pth, and mid-infrared spectra for x less than or equal to 0.09 on the basis of the small polaron theory of superconductivity. We find that t he agreement between the calculated and experimental results is excell ent without any adjustable parameters.