ANISOTROPY OF THE CORE-HOLE RELAXATION IN X-RAY-ABSORPTION SPECTROSCOPY AS PROBED IN SQUARE-PLANAR CUPRATES

It has been shown that the one-electron approach is not sufficient to understand core-level photoexcitation in narrow-band systems, such as the 4f of the rare-earth elements or the 3d of the late transition-metal elements. Refinements that take into account the strong Coulomb interaction of the core hole with all localized or quasilocalized electrons in the final state have been introduced; this approach still leaves the photoelectron as a spectator in the full relaxation process. In this paper we show the need to take into account the role of the slow photoelectron in the many-body problem. Thanks to two sets of polarization-dependent K-edge experiments on single crystals of La2CuO4 and Pr2NiO4, the role of the photoelectron is shown to be anisotropic for cuprates but not anisotropic for nickelates. This different behavior is expected because the relaxation leads to the electron transfer O 2p only if Cu 3d in cuprates, where only one channel with x2-y2 symmetry is available, whereas in nickelates two quasiequivalent channels x2-y2 and 3z2-r2 are opened with almost equal probability.