Stacking-induced symmetry-protected topological phase transitions

We study symmetry-protected topological (SPT) phase transitions induced by stacking two gapped one-dimensional subsystems in the BDI symmetry class. The topological invariant of the entire system is a sum of three topological invariants: two from each subsystem and an emerging topological invariant from the stacking. We find that any symmetry-preserving stacking of topologically trivial subsystems can drive the entire system into a topologically nontrivial phase for a certain coupling strength. We explain this intriguing SPT phase transition by conditions set by orbital degrees of freedom and time-reversal symmetry. To exemplify the SPT transition, we provide a concrete model which consists of an atomic chain and a spinful nanowire with spin-orbit interaction and s-wave superconducting order. The stacking-induced SPT transition drives this heterostructure into a zero-field topological superconducting phase.