Unraveling liquid polymorphism in silicon driven out-of-equilibrium

TL;DR

This study uses nonequilibrium molecular dynamics simulations to explore how shear stabilizes supercooled silicon liquids in different polymorphic forms, revealing conditions that favor LDL and HDL phases and their electronic properties.

Contribution

It demonstrates how shear can stabilize metastable liquid polymorphs of silicon, providing insights into their structural, energetic, and electronic characteristics.

Findings

Shear stabilizes LDL and HDL forms of supercooled silicon.

Different densities and shear rates determine the dominant polymorph.

Shear influences the electronic properties, with HDL being metallic and LDL semimetallic.

Abstract

Using nonequilibrium molecular dynamics (NEMD) simulations, we study the properties of supercooled liquids of Si under shear at T=1060K over a range of densities encompassing the low-density liquid (LDL) and high-density liquid (HDL) forms. This enables us to generate nonequilibrium steady-states of the LDL and HDL polymorphs, that remain stabilized in their liquid forms for as long as the shear is applied. This is unlike the LDL and HDL forms at rest, which are metastable under those conditions and, when at rest, rapidly undergo a transition towards the crystal, i.e. the thermodynamically stable equilibrium phase. In particular, through a detailed analysis of the structural and energetic features of the liquids under shear, we identify the range of densities, as well as the range of shear rates, that give rise to the two forms. We also show how the competition between shear and tetrahedral order impacts the two-body entropy in steady-states of Si under shear. These results open the door to new ways of utilizing shear to stabilize forms that are metastable at rest and can exhibit unique properties, since, for instance, experiments on Si have shown that HDL is metallic, with no band gap, while LDL is semimetallic, with a pseudogap.