Chern numbers and chiral anomalies in Weyl butterflies

The Hofstadter butterfly of lattice electrons in a strong magnetic field is a cornerstone of condensed matter physics, exploring the competition between periodicities imposed by the lattice and the field. Here, we introduce and characterize the Weyl butterfly, which emerges when a large magnetic field is applied to a three-dimensional Weyl semimetal. Using an experimentally motivated lattice model for cold-atomic systems, we solve this problem numerically. We find that Weyl nodes reemerge at commensurate fluxes and propose using wave-packet dynamics to reveal their chirality and location. Moreover, we show that the chiral anomaly-a hallmark of the topological Weyl semimetal-does not remain proportional to the magnetic field at large fields, but rather inherits a fractal structure of linear regimes as a function of the external field. The slope of each linear regime is determined by the difference of two Chern numbers in the gaps of the Weyl butterfly and can be measured experimentally in time of flight