J.M. Tranquada, S.M. Heald, A.R. Moodenbaugh, G. Liang, M. Croft, "Nature of the charge carriers in electron-doped copper oxide superconductors," Nature, 337, 720-1, 23 February 1989.
John M. Tranquada (Brookhaven National Laboratories, Upton, N.Y.): "The discovery of an electron-doped copper oxide superconductor by Takagi, Uchida, and Tokura (Physical Review Letters, 62:1197-1200, 1989) created a great deal of excitement, because all previously known cuprate superconductors were hole-doped (electron deficient). Our paper was one of the first in which a spectroscopic technique (X-ray absorption spectroscopy) was used to study the nature of the added electrons. Our conclusion that the extra electrons tend to fill the empty d state on the copper atoms has proved to be somewhat controversial, but it has received support from several other groups. The similarities and differences between the electron- and hole-doped systems continue to stimulate a good deal of research."