Emergent Z2 symmetry near a charge density wave multicritical point

We consider the critical behavior associated with incommensurate unidirectional charge-density-wave ordering in a weakly orthorhombic system subject to uniaxial strain as an experimentally significant example of U (1) x U (1) multicriticality. We show that, depending on microscopic details, the phase diagram can have qualitatively different structures which can involve a vestigial metanematic critical point, a pair of tricritical points, a decoupled tetracritical point, or (at least at mean-field level) a bicritical point. We analyze the emergent symmetries in the critical regime and find that these can-at least in some cases-involve an emergent Z2 order parameter symmetry.