Nuclear Pasta Formation

TL;DR

This study uses large-scale molecular dynamics simulations to investigate the formation and transition mechanisms of nuclear pasta phases in nuclear matter, revealing shape evolution, nucleation processes, and the impact of non-equilibrium effects.

Contribution

It provides detailed insights into the formation pathways and topological characterization of nuclear pasta phases using large-scale simulations and explores the effects of expansion rates on shape outcomes.

Findings

Different pasta shapes depend on expansion rate.

Nucleation mechanisms for phase transitions are explicitly observed.

Non-equilibrium effects influence the final pasta configurations.

Abstract

The formation of complex nonuniform phases of nuclear matter, known as nuclear pasta, is studied with molecular dynamics simulations containing 51200 nucleons. A phenomenological nuclear interaction is used that reproduces the saturation binding energy and density of nuclear matter. Systems are prepared at an initial density of 0.10fm$^{-3}$ and then the density is decreased by expanding the simulation volume at different rates to densities of 0.01 fm$^{-3}$ or less. An originally uniform system of nuclear matter is observed to form spherical bubbles ("swiss cheese"), hollow tubes, flat plates ("lasagna"), thin rods ("spaghetti") and, finally, nearly spherical nuclei with decreasing density. We explicitly observe nucleation mechanisms, with decreasing density, for these different pasta phase transitions. Topological quantities known as Minkowski functionals are obtained to characterize the pasta shapes. Different pasta shapes are observed depending on the expansion rate. This indicates non equilibrium effects. We use this to determine the best ways to obtain lower energy states of the pasta system from MD simulations and to place constrains on the equilibration time of the system.