Diffusion in Coulomb Crystals

TL;DR

This study uses molecular dynamics to analyze diffusion in Coulomb crystals, revealing that diffusion can be faster than in other solids and influences the crystalline or amorphous nature of stellar crusts.

Contribution

It provides new insights into diffusion mechanisms in Coulomb crystals and their implications for neutron star and white dwarf crust structures.

Findings

Diffusion constants in Coulomb crystals can exceed those in Lennard-Jones solids.

Ion exchange occurs via ring-like hops in perfect lattices.

Rapid quenching leads to crystallization during simulations.

Abstract

Diffusion in coulomb crystals can be important for the structure of neutron star crusts. We determine diffusion constants $D$ from molecular dynamics simulations. We find that $D$ for coulomb crystals with relatively soft-core $1/r$ interactions may be larger than $D$ for Lennard-Jones or other solids with harder-core interactions. Diffusion, for simulations of nearly perfect body-centered-cubic lattices, involves the exchange of ions in ring-like configurations. Here ions "hop" in unison without the formation of long lived vacancies. Diffusion, for imperfect crystals, involves the motion of defects. Finally, we find that diffusion, for an amorphous system rapidly quenched from coulomb parameter $\Gamma=175$ to coulomb parameters up to $\Gamma=1750$, is fast enough so that the system starts to crystallize during long simulation runs. These results strongly suggest that coulomb solids in cold white dwarf stars, and the crust of neutron stars, will be crystalline and not amorphous.