Abstract
A density functional theory study of NbSe2 single-layers in the normal non-modulated and the 3 × 3 CDW states is reported. We show 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–Nb 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. We perform 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 single-layers. The relevance of these results in understanding recent physical measurements for NbSe2 single-layers is discussed.
Highlights
Transition metal dichalcogenides are layered materials, exfoliable due to the van der Waals forces linking their layers
We report first-principles density functional theory (DFT) results which provide a comprehensive description of the electronic structure of this system, bring some light into the long-debated mechanism of the charge density wave (CDW), report in depth calculations of the DOS near the Fermi level to discuss the surprising quasi-band gap feature reported by Ugeda et al [16] and compare our scanning tunneling microscope (STM) simulations with the experimental images, relating them with the structural features of the CDW
The electronic structure of NbSe2 single-layers in the normal non-modulated and the 3 × 3 CDW states has been studied in detail
Summary
José Ángel Silva-Guillén, Pablo Ordejón, Francisco Guinea and Enric Canadell. Keywords: charge density wave, transition metal dichalcogenides, single-layer, density functional theory. In the single layer, the CDW barely affects the Fermi surface of this work must maintain attribution to the the system, ruling out a nesting mechanism as the driving force for the modulation. 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 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 single-layers. The relevance of these results in understanding recent physical measurements for NbSe2 single-layers is discussed
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