= Kohn singularity of static Lindhard screening
{c}
{title2=$F(1+u)=\tfrac12+\tfrac{u}{2}\ln|u|+O(u)$}
The three-dimensional static screening function is finite at $q=2k_F$, but its <derivative> diverges logarithmically. Expanding the explicit <static Lindhard dielectric function> gives the title's nonanalytic term. The singularity comes from particle–hole excitations connecting opposite parts of the <Fermi surface>. It produces a <Friedel density tail from a derivative singularity>; analogous electronic response can cause a <Kohn anomaly> in a coupled lattice vibration.
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