Oxygen isotope effect on the effective mass of carriers from magnetic measurements on ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$

Oxygen isotope effects on ${\mathit{T}}_{\mathit{c}}$ and the Meissner fraction f have been investigated in fine-grained, decoupled ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ with x=0.105, 0.110, and 0.115. We find that these oxygen isotope effects are related to each other: d ln${\mathit{T}}_{\mathit{c}}$/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 ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ isotope are nearly equal The observed large oxygen isotope effects on ${\mathit{T}}_{\mathit{c}}$ and the Meissner fraction can be explained as due to the oxygen-mass dependence of ${\mathit{m}}^{\mathrm{*}}$ (effective mass of carriers). The results may suggest that the conducting carriers in the cuprate superconductors are of polaronic type.