Screening in Ionic Systems: Simulations for the Lebowitz Length

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Simulations of the Lebowitz length, ξₗ(T,ρ), are reported for t he restricted primitive model hard-core (diameter a) 1:1 electrolyte for densi ties ρ 4ρc and Tc T 40Tc. Finite-size eff ects are elucidated for the charge fluctuations in various subdomains that serve to evaluate ξₗ. On extrapolation to the bulk limit for T 10Tc the low-density expansions (Bekiranov and Fisher, 1998) are seen to fail badly when ρ> 1/10ρc (with ρc a³ ≃ 0.08). At highe r densities ξₗ rises above the Debye length, ξD to √T/ρ, by 10-30% (upto ρ≃ 1.3ρc); the variation is portrayed fairly well by generalized Debye-Hückel theory (Lee and Fisher, 19 96). On approaching criticality at fixed ρ or fixed T, ξₗ(T, ρ) remains finite with ξₗᶜ ≃ 0.30 a ≃ 1.3 ξDᶜ but displays a weak entropy-like singularity.