Grain Boundary Diffusion in Yukawa Crystals

TL;DR

This paper calculates diffusion coefficients in grain boundaries of Yukawa crystals relevant to astrophysics, revealing weak orientation dependence and similarities to supercooled liquids, with implications for neutron star crust viscosity.

Contribution

It introduces a method to compute grain boundary diffusion in Yukawa crystals, highlighting the impact of local disorder and screening effects on diffusion behavior.

Findings

Diffusion coefficients are weakly dependent on crystal orientation.

Grain boundary diffusion resembles that of supercooled liquids scaled down.

Boundary diffusion may significantly influence neutron star crust viscosity.

Abstract

We present calculations of diffusion coefficients in grain boundaries in Yukawa crystals for astrophysics. Our methods follow from our recent work calculating diffusion coefficients in perfect body-centered cubic crystals. These diffusion coefficients show only a weak dependence on the crystal orientations at the grain boundary and are consistent with those expected for a supercooled liquid scaled down by one to two orders of magnitude. We argue that the local disorder at the grain boundary produces a landscape of potential barriers similar to that of an amorphous liquid thin film, significantly reducing activation barriers to diffusive hops relative to the bulk solid. This also introduces a screening dependence, such that boundary diffusion does not exhibit the same universality as the bulk crystal. These diffusion coefficients suggest that grain boundaries may be a dominant source of viscous dissipation in neutron star crusts.