Oxygen isotope effects on the critical and Curie temperatures and Raman modes in the ferromagnetic superconductorRuSr2GdCu2O8

Abstract

The effect of oxygen isotopic substitution on transition temperature ${T}_{c},$ the Curie temperature ${T}_{M},$ and Raman modes is investigated in the ferromagnetic superconductor ${\mathrm{RuSr}}_{2}{\mathrm{GdCu}}_{2}{\mathrm{O}}_{8}.$ A large positive oxygen isotope effect, ${\ensuremath{\alpha}}_{{T}_{c}}=1.6,$ is observed in the zero resistance ${T}_{c}$ value for as-prepared samples but a much smaller value ${\ensuremath{\alpha}}_{{T}_{c}}=0.12\ifmmode\pm\else\textpm\fi{}0.04$ is found in the true ${T}_{c},$ where the superconductivity order parameter is established as signified by the rapid rise in $d\ensuremath{\rho}/dT.$ Such a value is in keeping with other underdoped high-${T}_{c}$ superconductor cuprates. A rather small $({\ensuremath{\alpha}}_{\mathrm{FM}}\ensuremath{\approx}0.02)$ oxygen isotope effect is observed in the ferromagnetic transition. Raman modes for this material are identified by inspection of the spectra with progressive ${}^{18}\mathrm{O}$ exchange, cation substitution, and structural considerations.