Fig. 5.

(A) Phase diagram without coupling of electrons to phonons (λ = 0). In this case, there is a U(1) symmetry in the electronic sector and a Z₂ symmetry in the structural sector which can be broken independently, leading to four distinct phases. (B) Phase diagram with finite electron–phonon coupling (λ > 0). U(1) symmetry is explicitly broken by linear coupling to the phonons, leading to the three symmetry-broken phases to merge into only one Z₂ phase. The purple dots signify the two limiting cases where the transition is driven by the excitonic order parameter or the crystal structure.