Negative longitudinal magnetoresistance in Dirac and Weyl metals

TL;DR

This paper develops a theoretical understanding of the negative longitudinal magnetoresistance observed in Dirac and Weyl metals, attributing it to chiral anomaly effects and Berry curvature-induced charge coupling, especially near Dirac or Weyl nodes.

Contribution

It introduces a theory explaining the negative quadratic magnetoresistance in Dirac and Weyl metals based on chiral anomaly and Berry curvature effects, highlighting the importance of long chiral charge relaxation times.

Findings

Negative quadratic magnetoresistance arises from chiral anomaly.

Berry curvature induces coupling between chiral and total charge densities.

Long chiral charge relaxation times are crucial for the effect.

Abstract

It has recently been found that Dirac and Weyl metals are characterized by an unusual weak-field longitudinal magnetoresistance: large, negative, and quadratic in the magnetic field. This has been shown to arise from chiral anomaly, i.e. nonconservation of the chiral charge in the presence of external electric and magnetic fields, oriented collinearly. In this paper we report on a theory of this effect in both Dirac and Weyl metals. We demonstrate that this phenomenon contains two important ingredients. One is the magnetic-field-induced coupling between the chiral and the total (or vector, in relativistic field theory terminology) charge densities. This arises from the Berry curvature and is present in principle whenever the Berry curvature is nonzero, i.e. is nonspecific to Dirac and Weyl metals. This coupling, however, leads to a large negative quadratic magnetoresistance only when the second ingredient is present, namely when the chiral charge density is a nearly conserved quantity with a long relaxation time. This property is specific to Dirac and Weyl metals and is realized only when the Fermi energy is close to Dirac or Weyl nodes, expressing an important low-energy property of these materials, emergent chiral symmetry.