Chern bands with higher-order Van Hove singularities on topological moiré surface states

In two-dimensional electronic lattices, changes in the topology of the Fermi surface (Lifshitz transitions) lead to Van Hove singularities characterized by a divergence in the electronic density of states. Van Hove singularities can enhance the effect of electronic interactions, providing a platform to explore novel correlated electronic states. In this work, we investigate the emergence of topological Chern bands on the surface of three-dimensional topological insulators, which host higher-order Van Hove singularities that are characterized by the power-law diverging density of states. These singularities can arise from the interplay between a time-reversal-breaking Zeeman field induced by proximity to a ferromagnetic insulator and a time-reversal-invariant moir\'e potential on the surface electrons, created by quintuple layer misalignment in a family of topological insulators such as ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, which host a single surface Dirac fermion. We establish the onset of Chern bands near charge neutrality with Chern numbers $C=\ifmmode\pm\else\textpm\fi{}1$ that also possess a manifold of higher-order Van Hove singularities on the moir\'e Brillouin zone valleys controlled by the Zeeman and moir\'e potential energy scales, unveiling a platform to realize exotic Lifshitz transitions in topological bands. Furthermore, we show that the strong peaks in the density of states in the vicinity of Lifshitz transitions give rise to characteristic features in the low-temperature intrinsic anomalous Hall conductivity, yielding a path to probe Van Hove singularities in Chern bands through anomalous transport measurements.