Emergent anomalies and generalized Luttinger theorems in metals and semimetals
Chong Wang, Alexander Hickey, Xuzhe Ying, A. A. Burkov
Abstract
Whether a given material is a metal or an insulator is determined by a single parameter, the number of electrons per unit cell per spin, that is, by the filling number. When the latter is integer, the material is an insulator or a compensated semimetal. When it is fractional, the material is a metal with a Fermi surface, enclosing a volume in momentum space, determined by the fractional part of the filling -- a statement known as Luttinger's theorem. Topological semimetals provide a realization of the metallic phase, distinct from ordinary metals, as they exist at integer fillings, which normally correspond to insulators. Here, the authors develop a framework that unifies Luttinger's theorem for ordinary metals and analogous exact statements for topological semimetals. They use the language of quantum anomalies of the emergent low-energy charge conservation symmetry at each point on the Fermi surface. This framework allows one to connect topological responses in all nodal semimetals to their low-energy properties, such as the distance between the point nodes in momentum space or the area, enclosed by the nodal line.