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Non-centrosymmetric transition metal monopnictides, including TaAs, TaP,NbAs, and NbP, are emergent topological Weyl semimetals (WSMs) hosting exoticrelativistic Weyl fermions. In this letter, we elucidate the physical origin ofthe unprecedented charge carrier mobility of NbP, which can reach$1\times10^{7}$ cm $^{2}$V$^{-1}$s$^{-1}$ at 1.5 K. Angle- andtemperature-dependent quantum oscillations, supported by density functiontheory calculations, reveal that NbP has the coexistence of p- and n-type WSMpockets in the $k_{z}$=1.16$\pi$/c plane (W1-WSM) and in the $k_{z}$=0 planenear the high symmetry points $\Sigma$ (W2-WSM), respectively. Uniquely, eachW2-WSM pocket forms a large dumbbell-shaped Fermi surface (FS) enclosing twoneighboring Weyl nodes with the opposite chirality. The magneto-transport inNbP is dominated by these highly anisotropic W2-WSM pockets, in which Weylfermions are well protected from defect backscattering by real spinconservation associated to the chiral nodes. However, with a minimal doping of$\sim$1\% Cr, the mobility of NbP is degraded by more than two order ofmagnitude, due to the invalid of helicity protection to magnetic impurities.Helicity protected Weyl fermion transport is also manifested in chiral anomalyinduced negative magnetoresistance, controlled by the W1-WSM states. In thequantum regime below 10 K, the intervalley scattering time by impuritiesbecomes a large constant, producing the sharp and nearly identical conductivityenhancement at low magnetic field.

Helicity protected ultrahigh mobility Weyl fermions in NbP