Charge Ordering Geometries in Uniaxially-Strained NbSe$_2$

TL;DR

This paper models how tiny uniaxial strains in NbSe₂ can induce a transition from a threefold symmetric charge order to a uniaxial one, aligning with experimental observations and revealing changes in the phase transition nature.

Contribution

It introduces a theoretical model demonstrating that minimal uniaxial strain can switch charge order symmetry in NbSe₂, explaining experimental domain coexistence and transition characteristics.

Findings

0.1% uniaxial strain favors 1Q charge order

Transition shifts from weakly-first-order to second order

Local ordering wavevector shortens with temperature

Abstract

Recent STM 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 NbSe$_2$, 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 NbSe$_2$ 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 wavevector with increasing temperature, complementing recent X-ray diffraction observations within the low-temperature phase.