Zero modes and index theorems for non-Hermitian Dirac fermions

Abstract: Dirac fermions, subject to external magnetic fields and in the presence of mass orders that assume topologically nontrivial spatial textures such as domain-wall and vortices, for example, bind robust mid-gap states at zero-energy, the number of which is governed by the Aharonov-Casher and Jackiw-Rebbi or Jackiw-Rossi index theorems, respectively. Here I extend the jurisdiction of these prominent index theorems to Lorentz invariant non-Hermitian (NH) Dirac operators, constructed by augmenting the celebrated Dirac Hamiltonian by a masslike anti-Hermitian operator that also scales linearly with momentum. The resulting NH Dirac operator manifests real eigenvalues over an extended NH parameter regime, characterized by a real effective Fermi velocity for NH Dirac fermions. From the explicit solutions of the zero-energy bound states, I show that in the presence of external magnetic fields of arbitrary shape such modes always exist when the system encloses a finite number of magnetic flux quanta, while in the presence of spatially non-trivial textures of the mass orders localized zero-energy modes can only be found in the spectrum when the effective Fermi velocity for NH Dirac fermions is real. These findings pave a concrete route to realize nucleation of competing orders from the topologically robust zero-energy manifold in NH or open Dirac systems. Possible experimental setups to test these predictions are discussed.