Helicity-protected ultrahigh mobility Weyl fermions in NbP

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 × 10 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 kz=1.16π/c plane (W1-WSM) and in the kz=0 plane near the high symmetry points Σ (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 ∼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.Topological Weyl semimetals (WSMs) are regarded as the next wonderland in condensed matter physics [1][2][3][4] for exploring fascinating quantum phenomena [5][6][7][8][9][10]. Unlike Dirac semimetals (DSMs) [11,12], band crossing points in WSMs, i.e. Weyl nodes, always appear in pair with opposite chirality, due to the lifting of spin degeneracy by breaking either time reversal symmetry [1] or inversion symmetry [3,4]. Fermi surfaces (FSs) enclosing the chiral Weyl nodes are characterized by helicity, i.e. the spin orientation is either parallel or antiparallel to the momentum. Such helical Weyl fermions are expected to be remarkably robust against non-magnetic disorders, and may lead to novel device concepts for spintronics and quantum computing.The recent proposed non-centrosymmetric TaAs, TaP, NbAs and NbP, have stimulated immense interests, due to the binary, non-magnetic crystal structure. The existence of Weyl nodes has soon been discovered in TaAs by angle-resolved photoemission spectroscopy (ARPES) [13,14], and by quantum transport measurements of NMR and a non-trivial Berry's phase (Φ B ) of π [15,16]. Transport studies of NbAs [17] and NbP [18] also show ultrahigh mobility and non-saturating MR, but no convincing evidence on the existence of Weyl fermions in these two compounds. However, ARPES resolves tadpoleshaped Fermi arcs on the (001) surface of both NbAs [19] and NbP [20]. It also shows pronounced changes in the * phyzhengyi@zju.edu.cn † zhuan@zju.edu.cn electronic structures of NbAs and NbP compared to TaAs [19], mainly due to weaker sp...

Fermi surface interconnectivity and topology in Weyl fermion semimetals TaAs, TaP, NbAs, and NbP

“…This discovery paves the way for realizing the many predicted exotic topological quantum phenomena of Weyl semimetals in the TaAs class of materials 9,[25][26][27][28][29][30][31][32][33][34] . For any ongoing [35][36][37][42][43][44][45][46][47][48][49][50][51] or future experiments on these materials, systematic knowledge of the band structure and Fermi surface is crucial and fundamental in understanding and interpreting their data and observations. However, surprisingly, a comprehensive study of the band structure details of the TaAs Weyl semimetal family has been lacking.…”

Possible origin of linear magnetoresistance: Observation of Dirac surface states in layered PtBi2

“…While the linear field dependent magnetoresistance as observed in graphene [1], topological insulators Bi 2 Te 3 and Bi 2 Se 3 [2,3], Ag 2+δ Te/Se [4,5], Dirac semimetals Cd 3 As 2 [6,7] and Na 3 Bi [8], and type-I Weyl semimetals NbAs(P) [9][10][11] and TaAs [12,13] is explained based on the linear dispersive Dirac states present near the Fermi level [14,15], the quadratic field dependent MR as observed in the type-II Weyl semimetals WTe 2 and MoTe 2 [16,17], LaSb [18], ZrSiS [19], and Bi [20] is explained based on the charge compensation [16]. However, the role of charge compensation for the XMR of these compounds is still not clear as some reports recently demonstrated quadratic field dependent MR without charge balance [21,22].…”

Prediction of Weyl semimetal in orthorhombicMoTe2

“…Recently the first WSM materials (Ta,Nb)(As,P) have been discovered by addressing the Fermi arcs in angleresolved photoemission spectra (ARPES) [14][15][16][17][18][19][20] , which was originally predicted by band structure calculations 21,22 . Meanwhile a great amount of efforts have also been devoted to their magneto-transport properties [23][24][25][26][27][28][29] , such as extremely large, positive transverse MR 23 and large, negative longitudinal MR 24,29 . These family of WSMs exhibit ideal Weyl cones in the bulk band structure, i.e.…”