Charge ordering geometries in uniaxially strained NbSe₂

Recent scanning tunneling microscopy experiments reveal niobium diselenide to support domains of striped (1Q) charge order side by side with its better-known triangular (3Q) phase, suggesting that small variations in local strain may induce a quantum phase transition between the two. We use a theoretical model of the charge order in NbSe₂, based on a strong momentum- and orbital-dependent electron-phonon coupling, to study the effect of uniaxial strain. We find that as little as 0.1% anisotropic shift in phonon energies breaks the threefold symmetry in favor of a 1Q state, in agreement with the experimental results. The altered symmetries change the transition into the ordered state from weakly first order in the 3Q case to second order in the 1Q regime. Modeling the pseudogap phase of NbSe₂ as the range of temperatures above the onset of long-range order in which phase coherence is destroyed by local phonon fluctuations, we find a shortening of the local ordering wave vector with increasing temperature, complementing recent x-ray diffraction observations within the low-temperature phase.