Weyl nodes close to the Fermi energy in NbAs

TL;DR

This study investigates the Fermi surface and Weyl pockets in NbAs, revealing that some pockets are likely Weyl pockets with nodes close to the Fermi energy, which is crucial for understanding its topological properties.

Contribution

It provides the first comprehensive quantum oscillation analysis of NbAs, identifying potential Weyl pockets near the Fermi level and comparing experimental data with theoretical predictions.

Findings

Six distinct quantum oscillation frequencies observed.

Two of the pockets are likely topologically trivial.

Weyl nodes are within a few meV of the Fermi energy.

Abstract

The noncentrosymmetric transition metal monopnictides NbP, TaP, NbAs and TaAs are a family of Weyl semimetals in which pairs of protected linear crossings of spin-resolved bands occur. These so-called Weyl nodes are characterized by integer topological charges of opposite sign associated with singular points of Berry curvature in momentum space. In such a system anomalous magnetoelectric responses are predicted, which should only occur if the crossing points are close to the Fermi level and enclosed by Fermi surface pockets penetrated by an integer flux of Berry curvature, dubbed Weyl pockets. TaAs was shown to possess Weyl pockets whereas TaP and NbP have trivial pockets enclosing zero net flux of Berry curvature. Here, via measurements of the magnetic torque, resistivity and magnetisation, we present a comprehensive quantum oscillation study of NbAs, the last member of this family where the precise shape and nature of the Fermi surface pockets is still unknown. We detect six distinct frequency branches, two of which have not been observed before. A comparison to density functional theory calculations suggests that the two largest pockets are topologically trivial, whereas the low frequencies might stem from tiny Weyl pockets. The enclosed Weyl nodes are within a few meV of the Fermi energy.