Pressure-induced dimerization and molecular orbitals formation in Na2RuO3 with strong correlation-enhanced spin-orbit coupling effect

TL;DR

This study uses first-principles calculations to reveal pressure-induced structural and electronic transitions in Na2RuO3, highlighting the roles of Coulomb correlation and spin-orbit coupling in its nonmagnetic insulating states.

Contribution

It demonstrates that Coulomb correlation enhanced SOC interactions induce a unique nonmagnetic insulating state and predicts a pressure-driven dimerization transition with molecular orbital formation in Na2RuO3.

Findings

Pressure induces structural dimerization around 15-17.5 GPa.

Coulomb correlation and SOC together produce a nonmagnetic insulator in the dimerized phase.

Formation of molecular orbitals accompanies the structural transition.

Abstract

First-principles calculations and simulations are conducted to clarify the nonmagnetic insulating ground state of the honeycomb lattice compound Na2RuO3 with 4d^4 electronic configuration and explore the evolutions of crystal structure and electronic property under pressure. We reveal that individual Coulomb correlation or spin-orbit coupling (SOC) effect cannot reproduce the experimentally observed nonmagnetic insulating behavior of Na2RuO3, whereas the Coulomb correlation enhanced SOC interactions give rise to an unusual spin-orbital-entangled J = 0 nonmagnetic insulating state, which contrasts with the SOC assisted Mott insulating state in d^5 ruthenates and iridates. Furthermore, a pressure-induced structural dimerization transition has been predicted around 15-17.5 GPa. The honeycomb lattice of the high-pressure dimerized phase features with parallel pattern of the short Ru-Ru dimers aligning along the crystallographic b direction. Accompanied with the structural dimerization, the electronic structure shows striking reconstruction by formation of molecular orbitals. Interestingly, the cooperation of Coulomb correlation together with SOC can realize a nonmagnetic insulating state in the high-pressure dimerized phase. The d^4 ruthenate Na2RuO3 with honeycomb lattice will provide a new platform to explore unusual physics and rich phase diagram due to the delicate interplay of lattice degree of freedom, electron correlations, and SOC interactions.