Evidence that all high-temperature superconductors are p-type

Abstract

The ground charge state of Ce substituting for Nd+3 in Nd2 CuO4 is usually assumed to be Ce+4 , indicating that Ce is a donor which causes Nd2−z Cez CuO4 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 Nd2 CuO4 , implying that monatomic Ce dopants are isoelectronic centers, not n-type donors. The observed Ce doping of Nd2−z Cez CuO4 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 host p-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 R2−z Cez CuO4 to superconduct. A similar situation occurs for the infinite-layer material Sr1−v Ndv CuO2 , 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.

Source-reported materials — not catalogue approval

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