Strain-induced stripe phase in charge-ordered single layer NbSe2

Abstract

Charge density waves are ubiquitous phenomena in metallic transition metal dichalcogenides. In NbSe₂, a triangular 3 × 3 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 4 × 4 modulation. We employ 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-NbSe₂, relating the stripe phase to compressive isotropic strain, favouring the 4 × 4 modulation. The observed wavelength of the charge modulation is also reproduced with a good accuracy.

Condensed-matter physics: Strain causes a change in charge patterns

Researchers in South Korea, Hungary and Sweden have shown how strain influences the electronic patterns formed in two-dimensional materials. Some of the stranger properties of crystalline solids arise when electric charges organise themselves into ordered patterns. Charge density waves are one example of these so-called quantum phases in which the charges create a standing-wave pattern. These waves have previously been observed in the bulk and in single atomic layers of niobium diselenide. This material hosts superconductivity in coexistence with charge density waves in a manner which is not fully understood. To obtain a fuller understanding of niobium diselenide, Fabrizio Cossu and Alireza Akbari from the Asia Pacific Center for Theoretical Physics, Pohang, South Korea, and co-workers modelled the effect of strain on its properties. They showed how strain controls the charge density wave patterns.