Torsional Landau levels and geometric anomalies in condensed matter Weyl systems

Abstract

We consider the role of coordinate dependent tetrads ("Fermi velocities"), momentum space geometry, and torsional Landau levels (LLs) in condensed matter systems with low-energy Weyl quasiparticles. In contrast to their relativistic counterparts, they arise at finite momenta and an explicit cutoff to the linear spectrum. Via the universal coupling of tetrads to momentum, they experience geometric chiral and axial anomalies with gravitational character. We show how torsion and the Nieh-Yan (NY) anomaly appear, with a ultraviolet (UV) parameter with dimensions of momentum, in condensed matter Weyl systems with background fields corresponding to emergent anisotropic Riemann-Cartan and Newton-Cartan spacetimes, depending on the precise form of the low-energy dispersion. The torsional NY anomaly arises in simplest terms from the spectral flow of torsional LLs coupled to the nodes at finite momenta and the linear approximation with a cutoff. We carefully review this for relativistic fermions at zero momentum. We contrast this with the spectral flow and non-zero torsional anomaly and the appearance the dimensionful UV-cutoff parameter in condensed matter systems at finite momentum. We apply this to chiral transport anomalies sensitive to the emergent tetrads in non-homogenous chiral superconductors, superfluids and Weyl semimetals under elastic strain. We also briefly discuss the role torsion in anomalous thermal transport for non-relativistic Weyl fermions, which arises via Luttinger's fictitious gravitational field corresponding to thermal gradients.