Decoupling band topology from criticality in bosonic systems
A new understanding of criticality in systems described by quadratic bosonic Hamiltonians (QBHs) ties the emergence of long-range correlations to boundaries of dynamical, not thermodynamical, stability in the parameter space. This separation occurs because the solution of the Heisenberg equations of motion is determined by an auxiliary pseudo- Hermitian dynamical system. The boundary points of a region of dynamical stability can be either exceptional points, generically associated with long-range correlations, or Krein collisions, where correlations can be either long- or short-range. We investigate the interplay of this landscape of possibilities with band topology and boundary physics, by relying on both specific examples and general arguments. The examples stem from a two-parameter, thermodynamically unstable family of QBHs obtained from the bosonic Su-Schrieffer-Heeger model by breaking particle conservation while preserving a chiral pseudo-symmetry. The dynamically stable regime breaks up into different regions labeled by an integer-valued symplectic analogue of the Berry phase. The topological phase transition is a line of Krein collisions, which coincides with the closing of a band gap at zero and causes the localization length of the topologically mandated boundary zero modes to diverge before disappearing. In the unstable regime, we show that the chiral pseudo-symmetry of our model induces, despite the broken particle-number symmetry, enough structure on its associated dynamical matrices to support a topological classification and a bulk-boundary correspondence, independently of dynamical stability. This strongly suggests that bosonic topological physics extracted from basic index theory is insensitive to dynamical stability and, a posteriori, to non-interacting criticality.