Unusual temperature dependence of the oxygen-isotope effect on the exchange-energy of $La_{1-x}Ca_xMnO_3$ at high temperatures

We report magnetic susceptibility $chi(T)$ measurements on oxygen-isotope exchanged La$_{1-x}$Ca$_{x}$MnO$_{3+y}$ up to 700 K. The $1/chi(T)$ data show that the ferromagnetic exchange-energy $J$ depends strongly on the oxygen-isotope mass. The isotope effect on $J$ decreases with temperature up to 400 K and then increases again with temperature above 400 K. This unusual temperature dependence of the isotope effect cannot be explained by existing theories of the colossal magnetoresistance effect for doped manganites. The present results thus provide essential constraints on the physics of manganites.

Large oxygen-isotope shift above the quantum critical point of $Y_{1-x}Ca_xBa_2Cu_3O_{7-delta}$

We have studied the oxygen-isotope effect on the superconducting transition temperature $T_{c}$ in overdoped Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-delta}$ with $x$ = 0.10, 0.20, and 0.25. We find the oxygen-isotope exponent $alpha_{O}$ to be small ($sim$0.02) for $x$ = 0.10 but substantial ($sim$0.1) for $x$ = 0.20 and 0.25. The doping level above which $alpha_{O}$ increases sharply coincides with a quantum critical point where the normal-state pseudogap starts to diminish. The present isotope-effect experiments provide direct and quantitative constraints on the pairing mechanism of high-temperature superconductivity in cuprates.