Unconventional isotope effects in cuprate superconductors

A brief review on unconventional oxygen-isotope (O-16/O-18) effects (OIE) in cuprate high-temperature superconductors (HTS) is presented. First the doping dependence of the OIE on the superconducting transition temperature T-c in various FITS is discussed. For all cuprate HTS families the OIE exponent of T-c (alpha(0)) shows a generic trend: in the underdoped regime alpha(0) is large (alpha(0) > 0.5) and becomes small in the optimally doped and overdoped regime. Magnetization, magnetic torque, and muon-spin rotation (mu SR) studies of the OIE on the in-plane penetration depth lambda(ab)(0) in La2-xSrxCuO4 and Y1-xPrxBa2Cu3O7-delta indicate a substantial oxygen-mass dependence of lambda(ab)(0) which increases with reduced doping. It is remarkable that even in optimally doped YBa2Cu3O7-delta and La2-xSrxCuO4 a substantial OIE on lambda(ab)(0) is observed, although the OIE on T-c is rather small. The observation of an OIE on the penetration depth indicates that cuprate HTS are non-adiabatic superconductors. The oxygen-isotope shifts of T-c and lambda(ab)(0) exhibit a correlation that appears to be generic for various families of HTS. Furthermore, site-selective OIE investigations of Y1-xPrxBa2Cu3O7-delta clearly reveal that the planar oxygen atoms mainly contribute to the total OIE on T-c as well as on lambda(ab)(0) at all doping levels. These unusual isotope effects strongly suggest that lattice effects play an essential role in the basic physics of cuprate HTS and have to be considered in any realistic theoretical model.