= Electrostatic screening
Mobile <electric charges> redistribute in response to an imposed <electric potential>, partly compensating its field. The induced density response and the Poisson equation must be solved self-consistently. Classical dilute mobile ions lead to the <Debye–Hückel approximation>; a degenerate electron system instead has a response set by its electronic compressibility and interactions. A high charge response can pin a partly occupied <Landau level> near the <Fermi energy>, whereas an incompressible gap suppresses that response. <electrostatic screening> is not a claim that every material perfectly excludes every static field.
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