Isotope effects and possible pairing mechanism in optimally doped cuprate superconductors

Abstract

We have studied the oxygen-isotope effects on ${T}_{c}$ and in-plane penetration depth ${\ensuremath{\lambda}}_{\mathrm{ab}}(0)$ in an optimally doped three-layer cuprate ${\mathrm{Bi}}_{1.6}{\mathrm{Pb}}_{0.4}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{10+y}$ ${(T}_{c}\ensuremath{\sim}107 \mathrm{K}).$ We find a small oxygen-isotope effect on ${T}_{c}$ $({\ensuremath{\alpha}}_{O}=0.019),$ and a substantial effect on ${\ensuremath{\lambda}}_{\mathrm{ab}}(0)$ $[\ensuremath{\Delta}{\ensuremath{\lambda}}_{\mathrm{ab}}(0)/{\ensuremath{\lambda}}_{\mathrm{ab}}(0)=2.5\ifmmode\pm\else\textpm\fi{}0.5%].$ The present results along with the previously observed isotope effects in single-layer and double-layer cuprates indicate that the isotope exponent ${\ensuremath{\alpha}}_{O}$ in optimally doped cuprates is small while the isotope effect on the in-plane effective supercarrier mass is substantial and nearly independent of the number of the ${\mathrm{CuO}}_{2}$ layers. A plausible pairing mechanism is proposed to explain the isotope effects, high-${T}_{c}$ superconductivity, and tunneling spectra in a consistent way.