Nature of the charge carriers in electron-doped copper oxide superconductors

Abstract

The charge carriers in all of the well-known layered copper oxide high-temperature superconductors^1– are holes located in the CuO₂planes, the common structural element of these systems. Recently, Tokura et al.⁵ have reported that doping of Ln₂CuO₄, where Ln represents Pr, Nd or Sm, with formally tetravalent Ce can produce a high-temperature superconductor with electron carriers. To determine the character of the carrier states in Nd_2–xCe_xCuO₄, we have performed X-ray absorption measurements at the Cu K and Ce L₃ edges for several different Ce concentrations. We find clear evidence that the electrons introduced by doping fill 3d holes on the Cu atoms, thus converting some Cu²⁺ ions to Cu⁺. Hence the nature of the states nearest to the Fermi energy (that is, those responsible for conduction) is qualitatively different from that in the hole-doped superconductors, in which the holes move in a band formed predominantly from O 2p states^8–10. As a result, the details of the electron-pairing mechanism could be very different in the two systems¹¹.