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Some quasi-atomic effects in the solid state

Abstract It has been shown that, in some solid-state systems, electrons with energies close to the Fermi level behave quasi-atomically. X-ray absorption and electron-energy-loss spectroscopy involving the excitation of core electrons to empty electronic states can then be classified by initial and final states described by L, S and J quantum numbers, and allowed transitions that follow strict atomic selection rules. These ideas are illustrated for 4d → 4f transitions of Ba in high-T c. superconductors, 2p → 3d transitions of Ca2 + in CaF2, 4f7–4f7 transitions in Gd metal, and X-ray dichroism of 3d–4f transitions in Dy. Such effects are compared with situations where the electrons are band like in behaviour and where electron correlations take more complex forms.