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 3x3 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 4x4 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 4x4 modulation. The observed wavelength of the charge modulation is also reproduced with a good accuracy.