Stability and fine structure of symmetry-enriched quantum criticality in a spin ladder triangular model

In this letter, we propose and study a ladder triangular cluster model which possesses a $\mathbb{Z}_2$ symmetry and an anti-unitary $\mathbb{Z}^{\mathbb{T}}_2$ symmetry generated by the spin-flip and complex conjugation, respectively. The phase diagram of the model hosts a critical line between a spontaneous symmetry breaking phase and a symmetry protected topological phase. Along the critical line, one endpoint exhibits symmetry-enriched Ashkin-Teller universality (SEATU), while other critical points fall into the symmetry-enriched Ising universality (SEIU). Both universality classes accommodate symmetry protected degenerate edge modes under open boundary conditions. This degeneracy can be lifted with a gap opening when proper perturbation is applied to the boundary. With system size ($L$) increasing, at the point of SEATU, the gap closes following $L^{-1}$. In contrast, for the critical points of SEIU apart from a point with the known gap closing as $L^{-14}$, other points surprisingly show exponentially gap closing. The coexistence of different gap closing behaviors for critical points of the same symmetry-enriched universality goes beyond the the usual understanding of symmetry-enriched universality class, implying a fine and rich structure of phase transition and universality class.