Large oxygen-isotope shift above the quantum critical point of $Y_{1-x}C{a}_{x}B{a}_{2}C{u}_3{O}_{7-delta}$

Abstract

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.

4 pages, 4 Figures