Hidden type-II Weyl points in the Weyl semimetal NbP

TL;DR

This paper reveals that NbP, a Weyl semimetal, hosts both type-I and type-II Weyl points, with the latter being tilted electron-hole touching points, expanding understanding of its topological electronic structure.

Contribution

The study identifies and characterizes the presence of type-II Weyl points in NbP, which were previously assumed to be only type-I, demonstrating a coexistence of both types in a single material.

Findings

NbP has both type-I and type-II Weyl points.

Type-II Weyl points are tilted touching points between electron and hole pockets.

Fermi arcs are present despite Lorentz symmetry violation.

Abstract

As one of Weyl semimetals discovered recently, NbP exhibits two groups of Weyl points with one group lying inside the $k_z=0$ plane and the other group staying away from this plane. All Weyl points have been assumed to be type-I, for which the Fermi surface shrinks into a point as the Fermi energy crosses the Weyl point. In this work, we have revealed that the second group of Weyl points are actually type-II, which are found to be touching points between the electron and hole pockets in the Fermi surface. Corresponding Weyl cones are strongly tilted along a line approximately $17^\circ$ off the $k_z$ axis in the $k_x - k_z$ (or $k_y - k_z$) plane, violating the Lorentz symmetry but still giving rise to Fermi arcs on the surface. Therefore, NbP exhibits both type-I ($k_z=0$ plane) and type-II ($k_z \neq 0$ plane) Weyl points.