Unconventional Isotope Effects in Cuprate Superconductors

Abstract

A brief review on unconventional oxygen-isotope (¹⁶O/¹⁸O) 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 HTS is discussed. For all cuprate HTS families the OIE exponent of T_c (α_O) shows a generic trend: in the underdoped regime α_O is large (α_O > 0.5) and becomes small in the optimally doped and overdoped regime. Magnetization, magnetic torque, and muon-spin rotation (μSR) studies of the OIE on the in-plane penetration depth λ_ab (0) in La_2–x SrxCuO₄ and Y_1–xPr_xBa₂Cu₃O_7–δ indicate a substantial oxygen-mass dependence of λ_ab (0) which increases with reduced doping. It is remarkable that even in optimally doped YBa₂Cu₃O_7–δ and La_2–xSr_xCuO₄ a substantial OIE on λab(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 λ_ab (0) exhibit a correlation that appears to be generic for various families of HTS. Furthermore, siteselective OIE investigations of Y_1–x Pr _x Ba₂Cu₃O_7–δ clearly reveal that the planar oxygen atoms mainly contribute to the total OIE on T_c as well as on λ_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.