Oxygen isotope effect on the effective mass of carriers from magnetic measurements on La2-xSrxCuO4

Oxygen isotope effects on T_c and the Meissner fraction f have been investigated in fine-grained, decoupled La_2-xSr_xCuO₄ with x=0.105, 0.110, and 0.115. We find that these oxygen isotope effects are related to each other: d lnT_c/d lnM~=d lnf/d lnM (where M is the mass of oxygen). We also show that the trapped flux due to intergrain weak-link and intragrain flux pinning is negligible in our samples, and that the Meissner fractions for the samples with the same ¹⁶O isotope are nearly equal The observed large oxygen isotope effects on T_c and the Meissner fraction can be explained as due to the oxygen-mass dependence of m^* (effective mass of carriers). The results may suggest that the conducting carriers in the cuprate superconductors are of polaronic type.