Friedel oscillations
= Friedel oscillations
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The sharp <Fermi surface> gives a nonlocal oscillatory electronic response around an impurity. In a three-dimensional isotropic metal its far-field oscillations have characteristic wavevector $2k_F$ and algebraically decaying amplitude. They are absent from the purely exponential <Thomas-Fermi screening> potential. The local screening formula is a useful long-wavelength model, not an exact asymptotic expression at every distance.