Isotope effects and possible pairing mechanism in optimally doped cuprate superconductors

We have studied the oxygen-isotope effects on T_{c} and in-plane penetration depth \lambda_{ab}(0) in an optimally doped 3-layer cuprate Bi_{1.6}Pb_{0.4}Sr_{2}Ca_{2}Cu_{3}O_{10+y} (T_{c} \sim 107 K). We find a small oxygen-isotope effect on T_{c} (\alpha_{O} = 0.019), and a substantial effect on \lambda_{ab} (0) (\Delta \lambda_{ab} (0)/\lambda_{ab} (0) = 2.5\pm0.5%). The present results along with the previously observed isotope effects in single-layer and double-layer cuprates indicate that the isotope exponent \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 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.Comment: 5 pages, 4 figure

Large copper isotope effect in oxygen depleted YBa<SUB>2</SUB>Cu<SUB>3</SUB>Oy: importance of Cu-dominated phonon modes in the pairing mechanism

We have measured the copper isotope effect in YBa2Cu3Oy. We find a large positive copper-isotope exponent (α Cu≈0.37) in the underdoped plateau region (y~6.6). The exponent decreases to zero (or slightly negative) in the optimally doped region (y~6.94). The α Cu in YBa2Cu3O6.6 is similar to the α O (oxygen-isotope exponent) in YBa2-xLaxCu3Oy with the same Tc. These comparable values of α Cu and α O show that phonons which involve Cu motion as well as the (high-frequency) modes of the oxygen in the CuO2 planes are important in the pairing mechanism