Generalized Chiral Symmetry and Stability of Zero Modes for Tilted Dirac Cones

While it has been well-known that the chirality is an important symmetry for Dirac-fermion systems that gives rise to the zero-mode Landau level in graphene, here we explore whether this notion can be extended to tilted Dirac cones as encountered in organic metals. We have found that there exists a "generalized chiral symmetry" that encompasses the tilted Dirac cones, where a generalized chiral operator $\gamma$, satisfying $\gamma^{\dagger} H + H\gamma =0$ for the Hamiltonian $H$, protects the zero mode. We can use this to show that the $n=0$ Landau level is delta-function-like (with no broadening) by extending the Aharonov-Casher argument. We have numerically confirmed that a lattice model that possesses the generalized chirality has an anomalously sharp Landau level for spatially correlated randomness.