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

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. 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. Collective phenomena, such as charge/spin density waves and superconductivity, may change abruptly at the surface or in single layers of layered materials, whose observation may be affected by epitaxial strain. In Niobium diselenide, a stripe charge density wave, in place of a usual triangular one, has been observed at the surface, but not in single-layer. Phonon calculations and total energy calculations shed light on the intrinsic nature of the periodic lattice distortions, finding good agreement with experimental observations of phase separation between triangular and stripe phases, attributing to isotropic compressive strain the emergence of the latter.