Symmetry-Protected Topological Phases in the Triangular Majorana-Hubbard Ladder

We uncover a family of symmetry-protected topological phases in a model of interacting Majorana fermions on triangular-lattice ladders. Included among these phases is one in which symmetry-protected topological order coexists with spontaneous symmetry breaking associated with an adiabatically evolving set of symmetries. Our results indicate a mechanism through which interactions among Majorana modes generate nontrivial symmetry-fractionalized orders. These phases may be realized in arrays of vortex-bound Majorana zero modes on the surface of topological superconductors.