Non-Hermitian higher-Order Weyl semimetal with surface diabolic points

Abstract: Higher-order topology in non-Hermitian (NH) systems has recently become one of the most promising and rapidly developing fields in condensed matter physics. Many distinct phases that were not present in the Hermitian equivalents are shown in these systems. In this work, we examine how higher-order Weyl semimetals are impacted by NH perturbation. We identify a new type of topological semimetal, i.e., non-Hermitian higher-order Weyl semimetal (NHHOWS) with surface diabolic points. We demonstrate that in such an NHHOWS, new exceptional points inside the bulk can be created and annihilated, therefore allowing us to manipulate their number. At the boundary, these exceptional points are connected through unique surface states with diabolic points and hinge states. For specific system parameters, the surface of NHHOWS behaves as a Dirac phase with linear dispersion or a Luttinger phase with a quadratic dispersion, thus paving a way for Dirac-Luttinger switching. Finally, we employ the biorthogonal technique to reinstate the standard bulk boundary correspondence for NH systems and compute the topological invariants. The obtained quantized biorthogonal Chern number and quadruple moment topologically protect the unique surface and hinge states, respectively.