Copper and oxygen isotope effects in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$

The copper and oxygen isotope effects were investigated in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ for x=0.15 and x=0.125. It is found that the partial isotope exponents ${\mathrm{\alpha }}_{\mathrm{Cu}}$ and ${\mathrm{\alpha }}_{\mathrm{O}}$ are close to each other. For x=0.15 we find ${\mathrm{\alpha }}_{\mathrm{Cu}}$=0.15 and ${\mathrm{\alpha }}_{\mathrm{O}}$=0.11, while for x=0.125 anomalously large isotope exponents of ${\mathrm{\alpha }}_{\mathrm{Cu}}$=0.93 and ${\mathrm{\alpha }}_{\mathrm{O}}$=0.91 are found. The results show that large parts of the phonon spectrum influence the superconducting transition temperature.