Evidence that all high-temperature superconductors are p-type

The ground charge state of Ce substituting for Nd+3 in Nd2CuO4 is usually assumed to be Ce+4, indicating that Ce is a donor which causes Nd2-zCezCuO4 to be an n-type superconductor. Using the self-consistent bond/charge method, we show that the Ce ground state is Ce+3, not Ce+4, in ideal Nd2CuO4, implying that monatomic Ce dopants are isoelectronic centers, not n-type donors. The observed Ce doping of Nd2-zCezCuO4 can be understood if an interstitial oxygen binds to the Ce, elevating its charge state to Ce+4. This is a paired defect of Ce and oxygen, which dopes the host p-type, not n-type, because of oxygen's 2p holes. The condition that the interstitial oxygen does dope the hostp-type translates into an approximate steric constraint that the ionic radius of the rare-earth ion R(+3) must exceed that of Tb+3 for Ra2-zCezCuO4 to superconduct. A similar situation occurs for the infinite-layer material Sr1-vNdvCuO2, where the Nd dopant is isoelectronic Nd+2, not donor Nd+3 as assumed, unless an extra oxygen is adjacent to the Nd. The data are reviewed in light of these results, and show no convincing evidence either of particle-hole symmetry in doping, or of n-type doping of any high-temperature superconductor. Instead they hint that the superconductivity is associated with oxygen, rather than with any particle-hole-symmetric cuprate-plane phenomenon.