Exchange and Correlation Instabilities of Simple Metals

The influence of electron-electron correlation on exchange instabilities of a metal is examined. The employment of screened interactions does not constitute a proper treatment. Correlation effects suppress ferromagnetic instabilities, as is well known, but they need not supress instabilities of the spin-density-wave type. On the contrary, it is shown that correlation enhances exchange instability of the charge-density-wave type. For either type, the wave vector of such a state adjusts so that the Fermi surface makes critical contact with the energy gaps introduced by the instability. This circumstance optimizes the correlation energy. The observed conjunction of the long-period-superlattice periodicity with the Fermi surface in order-disorder alloys is probably an example of this phenomenon. It is suggested that charge-density-wave ground states are likely in simple metals having weak Born-Mayer ion-ion interactions, such as the alkali metals. The intensity of Bragg reflection satellites caused by a concomitant positive-ion modulation is computed.