Long-range polarization interaction in simple liquid metals

In earlier work [F. Perrot and N. H. March, Phys. Rev. A 41, 4521 (1990); 42, 4884 (1990)], the electron theory of pair potentials in simple s-p liquid metals, such as Na and K near freezing, was set out. The potential thus derived for Na is in good agreement in its major features with that obtained by inverting diffraction measurements of the liquid structure factor S(k). The present study addresses polarization effects in the interatomic interaction in such s-p metals, a simple physical model being introduced. By a lower-bound argument, this is shown to lead to an attractive contribution to the effective pair potential which decays as ${\mathit{R}}^{\mathrm{-}4}$ at large interionic separation R. Since it is recognized that some difficulties still remain in justifying the simple model employed in a dense system, it is important to test experimentally whether such a long-range interaction is actually present in liquid Na or K near freezing; it is proposed that highly accurate small-angle neutron scattering could resolve this question precisely. Though less decisive for detection of the long-range polarization effect, the possible relevance to (i) long-wavelength phonon properties in hot (and cold) crystalline Na metal and (ii) the interaction between charged defects in metals (e.g., a divacancy) is also noted.