Type-III Weyl semimetals: ( TaSe 4 ) 2 I

The Fermi surface plays a crucial rule in understanding the fundamental physics of crystals, such as superconductivity, the charge/spin density wave, and anomalous transport. Here, the authors discover the type-III Weyl semimetal, whose novel Fermi surface consists of two electron or hole pockets touching at the double Weyl points in Ta${}_{2}$Se${}_{8}$I, supporting the axionic charge density wave phase at low temperature. A unified and transparent picture to classify all of type-I, type-II, and type-III Weyl fermions based on the general tilting mechanism is also found.