Phonon-assisted magnetic Mott-insulating state in the charge density wave phase of single-layer 1TNbSe2

TL;DR

This study reveals that single-layer 1TNbSe2 exhibits a phonon-assisted magnetic Mott-insulating state within its charge density wave phase, driven by electron correlations and lattice distortions, as shown by first-principles calculations.

Contribution

It demonstrates the stabilization of a magnetic Mott insulator in 1TNbSe2 due to Hubbard interactions and lattice effects, providing new insights into correlated phases in 2D materials.

Findings

Charge density wave with specific periodicity matches experimental data.

Hubbard U induces a magnetic Mott insulating state.

Lattice distortion enhances the Mott gap and magnetic properties.

Abstract

We study the structural, electronic and vibrational properties of single-layer 1TNbSe$_2$ from first principles. Within the generalized gradient approximation, the 1T polytype is highly unstable with respect to the 2H. The DFT+U method improves the stability of the 1T phase, explaining its detection in experiments. A charge density wave occurs with a $\sqrt{13}\times\sqrt{13}~R30^{\circ}$ periodicity, in agreement with STM data. At $U=0$, the David-star reconstruction displays a flat band below the Fermi level with a marked d$_{z^2-r^2}$ orbital character of the central Nb. The Hubbard interaction induces a magnetic Mott insulating state. Magnetism distorts the lattice around the central Nb atom in the star, reduces the hybridization between the central Nb d$_{z^2-r^2}$ orbital and the neighbouring Se p-states and lifts in energy the flat band becoming non-bonding. This cooperative lattice and magnetic effect amplifies the Mott gap. Single-layer 1TNbSe$_2$ is then a phonon-assisted spin-$1/2$ Magnetic Mott insulator.