Electron momentum density in nickel

The Compton profiles of single crystals of nickel have been measured by two independent research groups using 412 keV gamma -ray Compton spectrometers. The two sets of data have been obtained to high statistical and systematic accuracy ( sigma approximately=0.2%J(0)) and are in good mutual agreement. The measured anisotropy is found to be significantly reduced with respect to that predicted by an LCGO band-structure calculation performed within the local density approximation for electron correlation. Absolute directional profiles strongly suggest the cause of this discrepancy to be the breakdown of the independent-particle approximation in the calculation of momentum density in the first-series transition metals. Correlation effects in nickel are shown to be twice as large as those in the case of copper reported previously.