Dynamic electronic correlation effects in NbO$_2$ as compared to VO$_2$

TL;DR

This study compares the electronic structures of NbO$_2$ and VO$_2$, revealing the significant role of dynamic electronic correlations in their metallic and insulating phases, and challenging the purely Peierls insulator description of NbO$_2$.

Contribution

It provides a detailed analysis of dynamic correlations in NbO$_2$ and VO$_2$, highlighting their impact on gap formation and electronic structure, with novel insights into NbO$_2$'s insulating mechanism.

Findings

Electronic correlations strongly renormalize $t_{2g}$ bands in both oxides.

Nonlocal dynamic correlations influence gap formation in NbO$_2$.

NbO$_2$ is not a purely Peierls insulator, due to nonlocal correlations.

Abstract

In this work we present a comparative investigation of the electronic structures of NbO$_2$ and VO$_2$ obtained within the combination of density functional theory and cluster-dynamical mean field theory calculations. We investigate the role of dynamic electronic correlations on the electronic structure of the metallic and insulating phases of NbO$_2$ and VO$_2$, with focus on the mechanism responsible for the gap opening in the insulating phases. For the rutile metallic phases of both oxides, we obtain that electronic correlations lead to strong renormalization of the $t_{2g}$ subbands, as well as the emergence of incoherent Hubbard subbands, signaling that electronic correlations are also important in the metallic phase of NbO$_2$. Interestingly, we find that nonlocal dynamic correlations do play a role in the gap formation of the (bct) insulating phase of NbO$_2$, by a similar physical mechanism as that recently proposed by us in the case of the (M$_1$) dimerized phase of VO$_2$ (\textit{Phys. Rev. Lett. 117, 056402 (2016)}). Although the effect of nonlocal dynamic correlations in the gap opening of bct phase is less important than in the (M$_1$ and M$_2$) monoclinic phases of VO$_2$, their presence indicates that the former is not a purely Peierls-type insulator, as it was recently proposed.