Oxygen-isotope effect on the in-plane penetration depth in underdoped La2-xSrxCuO4 single crystals

We report measurements of the oxygen-isotope effect (OIE) on the in-plane penetration depth lambda(ub)(0) in underdoped La2-xSrxCuO4 single crystals. A highly sensitive magnetic torque sensor with a resolution of Delta tau similar or equal to 10(-12) N m was used for the magnetic measurements on microcrystals with a mass of approximate to 10 mu g. The OIE on lambda(ab)(-2)(0) is found to be - 10(2)% for x = 0.080 and -8(1)% for x = 0.086. it arises mainly from the oxygen-mass dependence of the in-plane effective mass m(ab)*. The present results suggest that lattice vibrations are important for the occurrence of high temperature superconductivity.