Mode-Shell correspondence, a unifying phase space theory in topological physics -- Part I: Chiral number of zero-modes

We propose a theory, that we call the $\mathit{\text{mode-shell}}\mathit{\text{correspondence}}$, which relates the topological zero-modes localised in phase space to a $\mathit{\text{shell}}$ invariant defined on the surface forming a shell enclosing these zero-modes. We show that the mode-shell formalism provides a general framework unifying important results of topological physics, such as the bulk-edge correspondence, higher-order topological insulators, but also the Atiyah-Singer and the Callias index theories. In this paper, we discuss the already rich phenomenology of chiral symmetric Hamiltonians where the topological quantity is the chiral number of zero-dimensionial zero-energy modes. We explain how, in a lot of cases, the shell-invariant has a semi-classical limit expressed as a generalised winding number on the shell, which makes it accessible to analytical computations.