Electronic structure of 2H-NbSe₂ single-layers in the CDW state

Adensity functional theory study of NbSe₂"Qsingle-layers in the normal non-modulated and the 3"Q¡Ñ"Q3 CDWstates is reported.Weshow that, in the single layer, the CDW barely affects the Fermi surface of the system, thus ruling out a nesting mechanism as the driving force for the modulation. The CDW stabilizes levels lying around 1.35 eV below the Fermi level within the Se-based valence band but having a substantial Nb-VNb bonding character. The absence of interlayer interactions leads to the suppression of the pancake-like portion of the bulk Fermi surface in the single-layer.Weperform scanning tunneling microscopy simulations and find that the images noticeably change with the sign and magnitude of the voltage bias. The atomic corrugation of the Se sublayer induced by the modulation plays a primary role in leading to these images, but the electronic reorganization also has an important contribution. The analysis of the variation of these images with the bias voltage does not support a Fermi surface nesting mechanism for the CDW. It is also shown that underlying graphene layers (present in some of the recent experimental work) do not modify the conduction band, but do affect the shape of the valence band of NbSe₂"Qsingle-layers. The relevance of these results in understanding recent physical measurements for NbSe₂"Qsingle-layers is discussed.