Isotope effect in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ superconductors

A ‘‘nonzero’’ isotope effect for x=0.15, and an ‘‘anomalous’’ isotope effect, with the oxygen isotope-effect exponent ${\mathrm{\alpha }}_{\mathrm{O}}$ larger than the theoretical value of 0.5 for x=0.12, were reported in the past for the ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ system. In view of the importance of understanding the mechanisms of high-temperature superconductivity, a reinvestigation of the isotope effect with more precise materials synthesis and isotope characterization is called for. We synthesized in flowing ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}_2^{}{}_{}{}^{}$, or in flowing ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}_2^{}{}_{}{}^{}$, copper-rich and stoichiometric versions of the ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ materials at x=0.125 and x=0.15. We characterized the materials with x-ray diffraction, high-sensitivity, high-temperature mass spectrometry, and magnetic measurements. We found that in the stoichiometric materials, both at x=0.125 and x=0.15, the oxygen isotope-effect exponent, ${\mathrm{\alpha }}_{\mathrm{O}}$≃0.4. The copper-rich samples at x=0.125 have a larger isotope shift than the stoichiometric ones, due to a larger amount of the ${\mathrm{La}}_{1.67}$ ${\mathrm{Sr}}_{0.33}$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_5$ nonsuperconducting second phase in the ${}_{}{}^{18}{}_{}{}^{}\mathrm{containing}$ material, than in the ${}_{}{}^{16}{}_{}{}^{}\mathrm{containing}$ material, which leaves the superconducting phase with a lower strontium concentration, which further lowers ${\mathit{T}}_{\mathit{c}}$. For the copper-rich x=0.15 material we found an oxygen isotope-effect exponent ${\mathrm{\alpha }}_{\mathrm{O}}$=0.08.