Topological Van Hove singularities at phase transitions in Weyl metals.

We show that in 3D topological metals a subset of the Van Hove singularities of the density of states sits exactly at the transitions between topological and trivial gapless phases. We may refer to these as topological Van Hove singularities. By investigating two minimal models, we show that they originate from energy saddle points located between Weyl points with opposite chiralities and we portrait their topological nature through their magnetotransport properties in the ballistic regime. We exemplify the relation between Van Hove singularities and topological phase transitions in Weyl systems by analysing the 3D Hofstadter model, which offers a simple and interesting playground to consider different kinds of Weyl metals and understand the features of their density of states. In this model, as a function of the magnetic flux, the occurrence of topological Van Hove singularities can be explicitly checked.