Phonon-Assisted Magnetic Mott-Insulating State in the Charge Density Wave Phase of Single-Layer 1T-NbSe2

We study the structural, electronic, and vibrational properties of single-layer 1T-NbSe2 from first principles. Within the generalized gradient approximation, the 1T polytype is highly unstable with respect to the 2H. The density functional theory with the Hubbard U method improves the stability of the 1T phase, explaining its detection in experiments. A charge density wave occurs with a 13×13R30° periodicity, in agreement with STM data. At U=0, the David-star reconstruction displays a flatband below the Fermi level with a marked dz2-r2 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 dz2-r2 orbital and the neighboring Se p states, and lifts in energy the empty dz2-r2 flatband becoming nonbonding. This cooperative Jahn-Teller and correlation effect amplifies the Mott gap. Our results are relevant for the broad class of correlated insulator in the presence of a strong Jahn-Teller effect.