Correlation between oxygen isotope effects on transition temperature and magnetic penetration depth in high-temperature superconductors close to optimal doping

The oxygen-isotope (O-16-O-18) effect (OIE) on the in-plane magnetic penetration depth lambda(ab)(0) in optimally doped YBa2Cu3O7-delta and La1.85Sr0.15CuO4, and in slightly underdoped YBa2Cu4O8 and Y0.8Pr0.2Ba2Cu3O7-delta was studied by means of muon-spin rotation. A substantial OIE on lambda(ab)(0) with an OIE exponent beta(O)=-d ln lambda(ab)(0)/d ln M-O approximate to-0.2 (M-O is the mass of the oxygen isotope), and a small OIE on the transition temperature T-c with an OIE exponent alpha(O)=-d ln T-c/d ln M-O similar or equal to 0.02-0.1 were observed. The observation of a substantial isotope effect on lambda(ab)(0), even in cuprates where the OIE on T-c is small, indicates that lattice effects play an important role in cuprate high-temperature superconductors.