# Pressure induced electronic structure transformation of topological semimetal

## TL;DR

This study investigates how applying hydrostatic pressure transforms the electronic structure of yttrium trihydride, transitioning from a topological semimetal with a nodal surface to a trivial insulator.

## Contribution

It provides a detailed first-principles analysis of pressure-induced topological phase transitions in YH₃, revealing the evolution from a pseudo nodal surface to a nodal ring and eventually to an insulator.

## Key findings

- At zero pressure, YH₃ exhibits a pseudo nodal surface semimetallic state.
- Increasing pressure breaks particle-hole symmetry, transforming the band structure.
- Above 31 GPa, YH₃ becomes a trivial insulator.

## Abstract

We study the electronic structure change of yttrium trihydride $\mathrm{YH_3}$ by applying a hydrostatic pressure. At zero pressure, $\mathrm{YH_3}$ has the structure with energy favored symmetry group $P\bar{3}c1$ (165). From first principle calculation, we argue that the band crossings are caused by overlapping of an electron- and hole-like bands. Besides the space inversion symmetry ($\mathcal{I}$) and the time reversal symmetry, the band structure also exhibits an approximate particle-hole symmetry. Thus, $\mathrm{YH_3}$ can be viewed as a pseudo nodal surface semimetal belongs to class BDI of the ten-fold AZ+ $\mathcal{I}$ classifications of gapless topological matter. As pressure increases, the approximate particle-hole symmetry is gradually broken and the pseudo nodal surface turns into a nodal ring belonging to the class AI with fewer non-spatial symmetries. Also, the nodal ring is shrinking in the process. At about $31$ GPa, which is higher than the reported structure phase transition pressure $21$ GPa, the nodal ring shrinks to a nodal point. When above $31$ GPa, all band crossings are gapped out and $\mathrm{YH_3}$ becomes a trivial insulator eventually.