Helicity protected ultrahigh mobility Weyl fermions in NbP

TL;DR

This paper investigates the high charge carrier mobility in NbP, a Weyl semimetal, revealing that helicity protection of Weyl fermions in specific Fermi surface pockets underpins its exceptional transport properties.

Contribution

It uncovers the physical origin of NbP's ultrahigh mobility, emphasizing the role of helicity-protected Weyl fermions in W2-WSM pockets and their protection from backscattering.

Findings

NbP exhibits charge mobility up to 10^7 cm^2V^{-1}s^{-1} at 1.5 K.

W2-WSM pockets form large dumbbell-shaped Fermi surfaces enclosing Weyl nodes.

Magneto-transport is dominated by W2-WSM pockets, with helicity protection suppressed by magnetic impurities.

Abstract

Non-centrosymmetric transition metal monopnictides, including TaAs, TaP, NbAs, and NbP, are emergent topological Weyl semimetals (WSMs) hosting exotic relativistic Weyl fermions. In this letter, we elucidate the physical origin of the unprecedented charge carrier mobility of NbP, which can reach $1\times10^{7}$ cm $^{2}$V$^{-1}$s$^{-1}$ at 1.5 K. Angle- and temperature-dependent quantum oscillations, supported by density function theory calculations, reveal that NbP has the coexistence of p- and n-type WSM pockets in the $k_{z}$=1.16$\pi$/c plane (W1-WSM) and in the $k_{z}$=0 plane near the high symmetry points $\Sigma$ (W2-WSM), respectively. Uniquely, each W2-WSM pocket forms a large dumbbell-shaped Fermi surface (FS) enclosing two neighboring Weyl nodes with the opposite chirality. The magneto-transport in NbP is dominated by these highly anisotropic W2-WSM pockets, in which Weyl fermions are well protected from defect backscattering by real spin conservation 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 of magnitude, due to the invalid of helicity protection to magnetic impurities. Helicity protected Weyl fermion transport is also manifested in chiral anomaly induced negative magnetoresistance, controlled by the W1-WSM states. In the quantum regime below 10 K, the intervalley scattering time by impurities becomes a large constant, producing the sharp and nearly identical conductivity enhancement at low magnetic field.