Weyl Semimetal Phase in Noncentrosymmetric Transition-Metal Monophosphides

Abstract

Based on first-principle calculations, we show that a family of nonmagnetic materials including TaAs, TaP, NbAs, and NbP are Weyl semimetals (WSM) without inversion centers. We find twelve pairs of Weyl points in the whole Brillouin zone (BZ) for each of them. In the absence of spin-orbit coupling (SOC), band inversions in mirror-invariant planes lead to gapless nodal rings in the energy-momentum dispersion. The strong SOC in these materials then opens full gaps in the mirror planes, generating nonzero mirror Chern numbers and Weyl points off the mirror planes. The resulting surface-state Fermi arc structures on both (001) and (100) surfaces are also obtained, and they show interesting shapes, pointing to fascinating playgrounds for future experimental studies.

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Plain Language Summary

The Dirac equation was proposed in 1928, and only one year later, H. Weyl pointed out that one of the massless solutions of the equation represents a new particle. An intense experimental search for so-called Weyl fermions has been ongoing ever since. Recent developments in condensed-matter physics, especially in topological insulators and topological semimetals, have suggested an alternative way to realize Weyl fermions: a low-energy excitation in a solid, where two nondegenerate linear dispersion electron bands cross each other at an arbitrary momentum in three-dimensional momentum space. Many remarkable new phenomena are expected on the basis of these Weyl points, including surface Fermi arcs, chiral anomalies, negative magnetoresistance, nonlocal transport, and novel superconductivity. We find that four materials are so-called Weyl semimetals, which possess 12 pairs of Weyl nodes.

We study a family of naturally existing stoichiometric, noncentrosymmetric, and nonmagnetic transition-metal monophosphides. These crystals have similar band structures, and we focus on TaAs, which has a body-centered tetragonal shape. We search for Weyl semimetals, which have Weyl points only close to or at the Fermi energy. Most studies thus far have been devoted to magnetic materials, although experimental tests with these materials are more difficult than those with nonmagnetic materials. Taking into account spin-orbit coupling in our calculations, we computed the surface Fermi arcs, which are accessible through experimental techniques. Two experimental observations have already verified our predictions.

We expect that our results will pave the way for studies of magnetoresistance and superconductivity in these Weyl semimetals.