Generic phase diagram of ``electron-doped'' T' cuprates

The electronic phase diagram of cuprate superconductors is a key ingredient to understand the still unresolved mechanism of high-temperature superconductivity. A particular interesting question is the differences and similarities between the hole- and electron-doped sides. The phase diagram of hole-doped high-$T_{c}$ cuprates has been well established, and shows a well-known ``dome'' shape with maximal superconductivity at a doping level of about 0.15. In contrast, the phase diagram of the electron-doped side is controversial. This is because the superconductivity in the T$'$ cuprates deteriorates seriously by the presence of impurity oxygen (O$_{ap})$ atoms, which have to be cleaned up in order to unveil the generic phase diagram of the $T'$-cuprates. We investigated the generic phase diagram of the electron doped superconductor, Nd$_{2-x}$Ce$_{x}$CuO$_{4}$, using films prepared by metal organic decomposition. After careful oxygen reduction treatment to remove interstitial O$_{ap}$ atoms, we found that the $T_{c}$ increases monotonically from 24 K to 29 K with decreasing $x$ from 0.15 to 0.00, demonstrating a quite different phase diagram from the previous bulk one. The implication of our results is discussed on the basis of tremendous influence of O$_{ap}$ ``impurities'' on superconductivity and also magnetism in T' cuprates.