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

Abstract

Adensity functional theory study of NbSe2 single-layers in the normal non-modulated and the 3×3 CDWstates is reported.Weshow that, in the single layer, theCDWbarely affects the Fermi surface of the system, thus ruling out a nesting mechanism as the driving force for the modulation. TheCDW 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.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 theCDW. 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. Introduction Transition metal dichalcogenides are layered materials, easily exfoliable due to the van der Waals forces linking their layers. They have been the focus of large attention in the past few years because they are ideal systems where to study the influence of the reduced electronic screening brought about by lowering the dimensionality from bulk to layers of different thickness. Among them, 2H-NbSe2 (from now on we will refer to it just as NbSe2) is metallic at room temperature, becomes superconducting (SC) at around 7 K [1, 2] and there are strong indications that it is a twogap superconductor [3–7]. Before reaching the SC state it undergoes a charge density wave (CDW) distortion at around 30 K [8, 9]. The bulk structure of NbSe2 is built from hexagonal layers containing Nb atoms in a trigonal prismatic coordination (see figure 1(a)) [10], but there are also relatively short interlayer Se–Se contacts providing a substantial interlayer coupling. Although