Experimental discovery of a topological Weyl semimetal state in TaP

TL;DR

This paper reports the experimental discovery of a Weyl semimetal state in TaP, demonstrating Weyl fermions, Fermi arc surface states, and the bulk-boundary correspondence using photoemission spectroscopy.

Contribution

It provides the first experimental realization of a Weyl semimetal in TaP and introduces a practical method to confirm bulk Weyl fermions via surface state measurements.

Findings

Observation of Weyl fermion cones and nodes in TaP

Detection of topological Fermi arcs and trivial surface states

Confirmation of bulk-boundary correspondence in TaP

Abstract

Weyl semimetals are expected to open up new horizons in physics and materials science because they provide the first realization of Weyl fermions and exhibit protected Fermi arc surface states. However, they had been found to be extremely rare in nature. Recently, a family of compounds, consisting of TaAs, TaP, NbAs and NbP was predicted as Weyl semimetal candidates. Here, we experimentally realize a Weyl semimetal state in TaP. Using photoemission spectroscopy, we directly observe the Weyl fermion cones and nodes in the bulk and the Fermi arcs on the surface. Moreover, we find that the surface states show an unexpectedly rich structure, including both topological Fermi arcs and several topologically-trivial closed contours in the vicinity of the Weyl points, which provides a promising platform to study the interplay between topological and trivial surface states on a Weyl semimetal's surface. We directly demonstrate the bulk-boundary correspondence and hence establish the topologically nontrivial nature of the Weyl semimetal state in TaP, by resolving the net number of chiral edge modes on a closed path that encloses the Weyl node. This also provides, for the first time, an experimentally practical approach to demonstrating a bulk Weyl fermion from a surface state dispersion measured in photoemission.