Nonzero isotope effect in ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$

The effect on the superconducting transition temperature of exchanging ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ in ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_4$ has been studied. The concentration of the two isotopes has been determined by Rutherford backscattering spectroscopy and Raman spectroscopy confirmed the mass-related phonon frequency shift. The ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ substitution is found to be the same on the two O sites. Magnetic measurements of the transition temperature reveal a nonzero isotope effect (α=0.16±0.02). The exponent α has also been calculated within a conventional electron-phonon interaction model and was found to be significantly larger.