Hidden Bethe states in a partially integrable model

We present a one-dimensional multicomponent model, known to be partially integrable when restricted to the subspaces made of only two components. By constructing fully antisymmetrized bases, we find integrable excited eigenstates corresponding to the totally antisymmetric irreducible representation of the permutation operator in the otherwise nonintegrable subspaces. We establish rigorously the breakdown of integrability in those subspaces by showing explicitly the violation of the Yang-Baxter equation. We further solve the constraints from the Yang-Baxter equation to find exceptional momenta that allows Bethe ansatz solutions of solitonic bound states. These integrable eigenstates have distinct dynamical consequence from the embedded integrable subspaces previously known, as they do not span their separate Krylov subspaces, and a generic initial state can partly overlap with them and therefore have slow thermalization. However, this novel form of weak ergodicity breaking contrasts with that of quantum many-body scars in that the integrable eigenstates involved do not have necessarily low entanglement. Our approach provides a complementary route to arrive at exact excited states in nonintegrable models: instead of solving towers of single-mode excited states based on a solvable ground state in a nonintegrable model, we identify the integrable eigenstates that survive in a deformation of the Hamiltonian away from its integrable point.