Evidence for an Instability Near Twice the Fermi Wave Vector in the Low Electronic Density Liquid MetalLi(NH3)4

We report high-resolution inelastic x-ray scattering measurements in the metallic liquid Li(NH3)4, which to a good approximation can be treated as a dilute alkali metal. We see a well-defined excitation out to large momentum transfers. This excitation shows a strong softening at wave vectors near the first peak in the structure factor, which occurs near twice the Fermi momentum.