Tunable Dirac nodal line in orthorhombic RuO$_2$

TL;DR

This study investigates how pressure affects the crystal structure and electronic properties of RuO2, revealing a pressure-induced phase transition and the presence of a tunable Dirac nodal line crossing the Fermi level.

Contribution

It reports the discovery of a pressure-induced orthorhombic phase in RuO2 and demonstrates the tunability of its Dirac nodal line via compression.

Findings

Orthorhombic RuO2 forms above 13 GPa under quasi-hydrostatic pressure.

The Dirac nodal line shifts across the Fermi level with pressure.

Pressure increases the crystal-field splitting in RuO2.

Abstract

Pressure evolution of RuO2 is studied using single-crystal x-ray diffraction in a diamond anvil cell, combined with \textit{ab initio} band-structure calculations. The tetragonal rutile structure transforms into the orthorhombic CaCl$_2$-type structure above 13 GPa under quasi-hydrostatic pressure conditions. This second-order transition is ferroelastic in nature and accompanied by tilts of the RuO$_6$ octahedra. Orthorhombic RuO$_2$ is expected to be paramagnetic metal, similar to ambient-pressure RuO$_2$. It shows the increased $t_{2g}-e_g$ crystal-field splitting that is responsible for the pressure-induced color change. It further features the Dirac nodal line that shifts across the Fermi level upon compression.