# Emergence of topological electronic phases in elemental lithium under pressure

## TL;DR

This study uses first-principles calculations to reveal that elemental lithium under high pressure can host various topological electronic phases, including Dirac nodal line semimetals and Dirac crossings, with potential measurable consequences.

## Contribution

It demonstrates the emergence of topological electronic structures in elemental lithium under pressure, a novel finding for a simple metal.

## Key findings

- Li becomes a Dirac nodal line semimetal at 80 GPa
- Li exhibits a Lifshitz transition at 220 GPa
- High-pressure phases host topological nodes near the Fermi level

## Abstract

Lithium, a prototypical simple metal under ambient conditions, has a surprisingly rich phase diagram under pressure, taking up several structures with reduced symmetry, low coordination numbers, and even semiconducting character with increasing density. Using first-principles calculations, we demonstrate that some predicted high-pressure phases of elemental Li also host topological electronic structures. Beginning at 80 GPa and coincident with a transition to the Pbca phase, we find Li to be a Dirac nodal line semimetal. We further calculate that Li retains linearly-dispersive energy bands in subsequent predicted higher pressure phases, and that it exhibits a Lifshitz transition between two Cmca phases at 220 GPa. The Fd-3m phase at 500 GPa forms buckled honeycomb layers that give rise to a Dirac crossing 1 eV below the Fermi energy. The well-isolated topological nodes near the Fermi level in these phases result from increasing p-orbital character with density at the Fermi level, itself a consequence of rising 1s core wavefunction overlap, and a preference for nonsymmorphic symmetries in the crystal structures favored at these pressures. Our results provide evidence that under pressure, bulk 3D materials with light elements, or even pure elemental systems, can undergo topological phase transitions hosting nontrivial topological properties near the Fermi level with measurable consequences; and that, through pressure, we can access these novel phases in elemental lithium.