Unconventional isotope effects in cuprate high-temperature superconductors

A brief review on unconventional oxygen-isotope (O-16/O-18) effects (OIE) in cuprate high-temperature superconductors (HTSC) is presented. First the doping dependence of the OIE on the superconducting transition T-c in various HTSC and investigations of the site-selective OIE in Y1-xPrxBa2Cu3O7-delta are discussed. These experiments clearly demonstrate that the planar oxygen ions mainly (greater than or equal to 80%) contribute to the total OIE on T-c at all doping levels. Magnetization and magnetic torque OIE studies of the in-plane penetration depth lambda(ab)(0) in La2-xSrxCuO4 indicate a substantial oxygen-mass dependence of lambda(ab)(-2) (0) which increases with reduced doping. These results are in agreement with recent muSR OIE studies of lambda(ab)(-2)(0) in polycrystalline samples of Y1-xPrxBa2Cu3O7-delta and La2-xSrxCuO4. The OIE exponents of T-c and lambda(ab)(-2)(0) exhibit a relation that appears to be generic for various families of HTSC. The unusual isotope effects observed in HTSC strongly suggest that lattice effects play an essential role. (C) 2002 Elsevier Science B.V. All rights reserved.