Observation of a possible oxygen isotope effect on the effective mass of carriers in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.94}$

We have investigated oxygen-isotope effects on the effective mass of carriers ${\mathit{m}}^{\mathrm{*}}$ and carrier concentration ${\mathit{n}}_{\mathit{s}}$ in fine-grained ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ (y≊6.94) using measurements of onset magnetizations in Meissner and mixed states. We find a large negative oxygen-isotope effect on ${\mathit{m}}^{\mathrm{*}}$ with an exponent ${\mathrm{\alpha }}_{\mathrm{O}{\mathit{m}}^{\mathrm{*}}}$=-d ln(m*)/d ln${\mathit{M}}_{\mathrm{O}}$=-0.61±0.09, and a small oxygen-isotope effect on ${\mathit{n}}_{\mathit{s}}$ with an exponent ${\mathrm{\alpha }}_{\mathrm{O}\mathit{n}}$=-d ln(${\mathit{n}}_{\mathit{s}}$)/d ln${\mathit{M}}_{\mathrm{O}}$=0±0.13. We also observe a small positive oxygen-isotope effect on ${\mathit{T}}_{\mathit{c}}$ with an exponent ${\mathrm{\alpha }}_{\mathrm{O}}$=-d ln${\mathit{T}}_{\mathit{c}}$/d ln${\mathit{M}}_{\mathrm{O}}$=0.028±0.002. The large oxygen-isotope effect on ${\mathit{m}}^{\mathrm{*}}$ can be explained as due to the breakdown of the Migdal approximation and/or strong anharmonicity of phonon modes.