F.J. Himpsel, G.V. Chandrashekhar, A.B. McLean, M.W. Shafer, "Orientation of the O 2p holes in Bi2Sr2Ca1Cu2O8," Physical Review B, 38, 11,946-8,1 December 1988. Franz J. Himpsel (IBM T.J. Watson Research Center, Yorktown Heights, N.Y.): "In the quest for understanding the mechanism of high-temperature superconductivity it is important to know the electronic states that cause superconductivity. Previous work had identified the copper oxide planes as active structural units in these materials, and the copper 3d-orbitals together with the oxygen 2p-orbitals as the significant electronic states. However, the orientation of these oxygen hole orbitals was not known. In our article, we used the polarization dependence of optical transitions from the oxygen 1s core level to the oxygen 2p orbitals to show that they are oriented parallel to the superconducting copper oxide layer in the superconductor Bi2Sr2Ca1Cu2O8. This finding significantly narrows down the possible choices for a theory of high-temperature superconductivity."...

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