Chern numbers for the two-body Hofstadter-Hubbard butterfly

We analyze the two -body spectrum within the Hofstadter-Hubbard model on a square lattice through an exact variational ansatz and study the topological properties of its low-lying two -body bound -state branches. In particular, we discuss how the Hofstadter-Hubbard butterfly of the two -body branches evolves as a function of onsite interactions and how to efficiently calculate their Chern numbers using the Fukui-Hatsugai-Suzuki approach. Our numerical results are fully consistent with the simple picture that appears in the strong -coupling limit, where the attraction between fermions forms a composite boson characterized by an effective hopping parameter and an effective magnetic -flux ratio.