Journal
JOURNAL OF PHYSICS-CONDENSED MATTER
Volume 30, Issue 32, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/1361-648X/aad215
Keywords
TMDC; Mott insulator; charge-transfer insulator; charge-density wave; Star of David
Categories
Funding
- Deutsche Forschungsgemeinschaft [RTG 2247]
- HLRN [hbp00045]
- Alexander von Humboldt Foundation
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Using ab initio calculations we reveal the nature of the insulating phase recently found experimentally in monolayer 1T-NbSe2. We find soft phonon modes in a large part of the Brillouin zone indicating the strong-coupling nature of a charge-density-wave instability. Structural relaxation of a root 13 x root 13 supercell reveals a Star-of-David reconstruction with an energy gain of 60 meV per primitive unit cell. The band structure of the distorted phase exhibits a half-filled flat band which is associated with orbitals that are delocalized over several atoms in each Star of David. By including many-body corrections through a combined GW, hybrid-functional, and DMFT treatment, we find the flat band to split into narrow Hubbard bands. The lowest energy excitation across the gap turns out to be between itinerant Se-p states and the upper Hubbard band, determining the system to be a charge-transfer insulator. Combined hybrid-functional and GW calculations show that long-range interactions shift the Se-p states to lower energies. Thus, a delicate interplay of local and long-range correlations determines the gap nature and its size in this distorted phase of the monolayer 1T-NbSe2.
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