We have used x-ray-absorption spectroscopy to study a series of compounds in which Cu assumes a formal valence between 0 and 3. We find that the shape, the threshold energy, and the intensity of the Cu L3 absorption edge is strongly inAuenced by the chemical state of the Cu atoms. We use the Cu 2p x-ray-absorption spectra of a large number of Cu compounds, including sulfides, oxides, LaSr-Cu-0 compounds, a phthalocyanine complex, and various minerals to show that the presence of a strong 2p-3d excitonic transition is a fingerprint of the Cu(d } contribution to the ground state. A simple ionic picture is generally inadequate to describe these compounds.
%'e consider the x-ray photoemission, inverse photoemission, and Cu L3VV Auger spectra of the high-T, superconductors.We conclude that the on-site correlation energies of the Cu 3d electrons are large and that the energy of the Cu d configurations is clearly too high for d configurations to play a significant role in the ground state.La2-, Sr, Cu04-y and RBa2Cu30qs (R Y and all rare-earth elements except Ce, Pr, and Tb) are superconducting at high temperatures. ' While at the present time there is little, if any, evidence here incompatible with the
We have been involved in a study of the physical properties of the crystalline substance of avian egg shells for the past two years. In this period both optical and x-ray measurements were carried out. On request of our colleagues in the section of Poultry Husbandry and Animal Physiology we had to investigate, among others, the orientation of the calcite crystals in the shell. In performing these measurements we proceeded in much the same way as investigators in the field of petrofabrics. Measurements of the orientation of calcite crystals carried out with a petrographic microscope equipped with a universal stage showed that the calcite crystals are more or less randomly arranged. In all the cases observed the c-axes
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