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Register a new userInterplay between magnetism and charge instabilities in layered NbSe$_{2}$
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Using ab initio methods based on density functional theory, the electronic and magnetic structure of layered hexagonal NbSe$_{2}$ is studied. In the case of single-layer NbSe$_{2}$ it is found that, for all the functionals considered, the magnetic solution is lower in energy than the non-magnetic solution. The magnetic ground-state is ferrimagnetic with a magnetic moment of 1.09 $mu_{B}$ at the Nb atoms and a magnetic moment of 0.05 $mu_{B}$, in the opposite direction, at the Se atoms. Our calculations show that single-layer NbSe$_{2}$ does not display a charge density wave instability unless a graphene layer is considered as a substrate. Then, two kinds of 3$times$3 charge density waves are found, which are observed in our STM experiments. This suggest that the driving force of charge instabilities in NbSe$_{2}$ differ in bulk and in the single-layer limit. Our work sets magnetism into play in this highly-correlated 2D material, which is crucial to understand the formation mechanisms of 2D superconductivity and charge density wave order.
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Charge density waves are ubiquitous phenomena in metallic transition metal dichalcogenides. In NbSe$_2$, a triangular $3times3$ structural modulation is coupled to a charge modulation. Recent experiments reported evidence for a triangular-stripe transition at the surface, due to strain or accidental doping and associated to a $4times4$ modulation. We employ textit{ab-initio} calculations to investigate the strain-induced structural instabilities in a pristine single layer and analyse the energy hierarchy of the structural and charge modulations. Our results support the observation of phase separation between triangular and stripe phases in 1H-NbSe$_2$, relating the stripe phase to compressive isotropic strain, favouring the $4times4$ modulation. The observed wavelength of the charge modulation is also reproduced with good accuracy.
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We use scanning tunneling microscopy (STM) to study charge density wave (CDW) states in the rare-earth di-telluride, CeTe$_{2}$. In contrast to previous experimental and first-principles studies of the rare-earth di-tellurides, our STM measurements surprisingly detect a unidirectional CDW with $textit{q}$ ~ 0.28 $textit{a}$*, which is very close to what is found in experimental measurements of the related rare-earth tri-tellurides. Furthermore, in the vicinity of an extended sub-surface defect, we find spatially-separated as well as spatially-coexisting unidirectional CDWs at the surface of CeTe$_{2}$. We quantify the nanoscale strain and its variations induced by this defect, and establish a correlation between local lattice strain and the locally-established CDW states. Our measurements probe the fundamental properties of a weakly-bound two-dimensional Te-sheet, which experimental and theoretical work has previously established as the fundamental component driving much of the essential physics in both the rare-earth di- and tri-telluride compounds.
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