Isotope Effects and Possible Pairing Mechanism in Optimally Doped Cuprate Superconductors

Abstract

We have studied the oxygen-isotope effects on Tc and in-plane penetration depth l ab(0) in an optimally doped three-layer cuprate Bi1.6Pb0.4Sr2Ca2Cu3O101y (Tc;107 K). We find a small oxygen-isotope effect on Tc (a O50.019), and a substantial effect on l ab(0) @Dlab(0)/l ab(0)52.560.5%#. The present results along with the previously observed isotope effects in single-layer and double-layer cuprates indicate that the isotope exponent a 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 CuO2 layers. A plausible pairing mechanism is proposed to explain the isotope effects, high- Tc superconductivity, and tunneling spectra in a consistent way. The pairing mechanism responsible for high-T c superconductivity is still controversial. In conventional superconductors, a strong effect of changing ion mass M on the transition temperature Tc implies that lattice vibrations ~phonons! play an important role in the microscopic mechanism of superconductivity. An isotope exponent a(52d ln Tc /d ln M )o f about 0.5 is consistent with the phonon-mediated BCS theory. A nearly zero oxygen-isotope effect ( a O.0.03) was earlier observed in a double-layer cuprate superconductor