Direct observation of nonequivalent Fermi-arc states of opposite surfaces in the noncentrosymmetric Weyl semimetal NbP

Weyl semimetals (WSMs) provide a platform to realize long-sought massless Weyl fermions, which can host a variety of exotic quantum phenomena such as chiral anomalies and anomalous Hall conductivity. The most intriguing nature of WSMs is the emergence of an unconventional surface state called a Fermi arc, which is distinct from the well-known Fermi surface of ordinary metals. The authors have used angle-resolved photoemission spectroscopy on NbP and succeeded in observing the electronic states for two differently terminated surfaces of this noncentrosymmetric crystal. Corroborated by first-principles calculations, it is unambiguously demonstrated that NbP is a Weyl semimetal with reduced spin-orbit coupling, exhibiting a drastic difference in the Fermi-surface topology between the P- and Nb-terminated surfaces. Through a direct comparison of Fermi-arc surface states between the two surfaces, the momentum location of Weyl nodes has been elucidated. The present results provides a pathway for exploring new quantum phenomena utilizing Fermi-arc properties of WSMs.