# Low energy phases of bilayer Bi predicted by structure search in two dimensions

## TL;DR

This study uses ab-initio structure search to identify new low-energy bilayer Bi phases, revealing diverse electronic properties and potential for phase transition studies, with three novel structures predicted and characterized.

## Contribution

The paper introduces three new bilayer Bi structures predicted via structure search, expanding the known phase diagram and exploring their electronic and vibrational properties.

## Key findings

- Recovered known low-energy structures of bilayer Bi.
- Predicted three new bilayer Bi phases ($\alpha$, $\beta$, $\gamma$).
- Identified metallic and semiconducting phases and topological features.

## Abstract

We employ an ab-initio structure search algorithm to explore the configurational space of Bi in quasi two dimensions. A confinement potential restricts the movement of atoms within a pre-defined thickness during structure search calculations within the minima hopping method to find the stable and metastable forms of bilayer Bi. In addition to recovering the two known low-energy structures (puckered monoclinic and buckled hexagonal), our calculations predict three new structures of bilayer Bi. We call these structures the $\alpha$, $\beta$, and $\gamma$ phases of bilayer Bi, which are, respectively, 63, 72, and 83 meV/atom higher in energy than that of the monoclinic ground state, and thus potentially synthesizable using appropriate substrates. We also compare the structural, electronic, and vibrational properties of the different phases. The puckered monoclinic, buckled hexagonal, and $\beta$ phases exhibit a semiconducting energy gap, whereas $\alpha$ and $\gamma$ phases are metallic. We notice an unusual Mexican-hat type band dispersion leading to a van Hove singularity in the buckled hexagonal bilayer Bi. Notably, we find symmetry-protected topological Dirac points in the electronic spectrum of the $\gamma$ phase. The new structures suggest that bilayer Bi provides a novel playground to study distortion-mediated metal-insulator phase transitions.