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The recent experimental discovery of a Weyl semimetal in TaAs provides thefirst observation of a Weyl fermion in nature and demonstrates a novel type ofanomalous surface state band structure, consisting of Fermi arcs. So far, workhas focused on Weyl semimetals with strong spin-orbit coupling (SOC). However,Weyl semimetals with weak SOC may allow tunable spin-splitting for deviceapplications and may exhibit a crossover to a spinless topological phase, suchas a Dirac line semimetal in the case of spinless TaAs. NbP, isostructural toTaAs, may realize the first Weyl semimetal in the limit of weak SOC. Here westudy the surface states of NbP by angle-resolved photoemission spectroscopy(ARPES) and we find that we $\textit{cannot}$ show Fermi arcs based on ourexperimental data alone. We present an $\textit{ab initio}$ calculation of thesurface states of NbP and we find that the Weyl points are too close and theFermi level is too low to show Fermi arcs either by (1) directly measuring anarc or (2) counting chiralities of edge modes on a closed path. Nonetheless,the excellent agreement between our experimental data and numericalcalculations suggests that NbP is a Weyl semimetal, consistent with TaAs, andthat we observe trivial surface states which evolve continuously from thetopological Fermi arcs above the Fermi level. Based on these results, wepropose a slightly different criterion for a Fermi arc which, unlike (1) and(2) above, does not require us to resolve Weyl points or the spin splitting ofsurface states. We propose that raising the Fermi level by $> 20$ meV wouldmake it possible to observe a Fermi arc using this criterion in NbP. Our workoffers insight into Weyl semimetals with weak spin-orbit coupling, as well asthe crossover from the spinful topological Weyl semimetal to the spinlesstopological Dirac line semimetal.

Observation of surface states derived from topological Fermi arcs in the Weyl semimetal NbP