The charge carriers in all of the well-known layered copper oxide high-temperature superconductors1-4 are holes located in the CuO2 planes, 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 Nd2-xCexCuO4, 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 states8-10. As a result, the details of the electron-pairing mechanism could be very different in the two systems11.
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