LOCAL-FIELD EFFECTS IN NIO AND NI

We have calculated the electron energy-loss function for NiO and Ni using the random phase approximation. Experimental results are presented for NiO, and it is shown that theory is in rather good agreement with experiment. The local-field effects are shown to be small for the loss function, although the system is very inhomogeneous, while they are large for the static dielectric function. We demonstrate that the size of the local-field effects are governed by a certain Coulomb integral zu which is determined by the important transitions. These are 3d --> 4f transitions for the loss function, where w(df) is small, and 3d --> 3d and 4s --> 4s for the static dielectric function, where w(dd) is large. We discuss how the local-field effects reduce the static screening. We show that this reduction is so efficient for the 3d --> 3d contribution that the 4s --> 4s contribution plays a substantial role for the static dielectric function, although the 4s density of states is much smaller than the 3d density of states.