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...

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We report the surface electronic structure of niobium phosphide NbP single crystal on (001) surface by vacuum ultraviolet angle-resolved photoemission spectroscopy. Combining with our first principle calculations, we identify the existence of the Fermi arcs originated from topological surface states. Furthermore, the surface states exhibit circular dichroism pattern, which may correlate with its non-trivial spin texture. Our results provide critical evidence for the existence of the Weyl Fermions in NbP, which lays the foundation for further investigations.PACS numbers: 71.55. Ak, 74.20.Pq, 74.25.Jb, In the past few years, great progress has been witnessed in the study of quantum materials with non-trivial topological electronic structures. By introducing topological orders, insulators can be further classified into trivial one and non-trivial one, that is, topological insulators (TIs) [1,2]. Due to their unique physical properties and great potential in applications, TIs have been one of the central research subject of condensed matter physics in the last decade.The recent proposal that Weyl fermions can be realized in the so-called topological Weyl semi-metals (TWSMs) broadens the classification of topological phases of matter beyond TIs [3][4][5][6][7]. The low energy excitations in this new type of topological quantum matter are described by the Weyl equation [8]. Thus, in the bulk, the conduction and valence bands disperse linearly cross pairs of discrete points (the Weyl points) along all three momentum directions. The Weyl points are associated with a chiral charge that protects gapless surface states (SSs) on the boundary of a bulk sample. These topological SSs take the form of unclosed curves connecting the Weyl points of opposite chirality [4], leading to the existence of the unique Fermi arcs on the surface. Due to the novel physical phenomena related to the Weyl fermions such as negative magneto-resistivity, quantum anomalous Hall effect and non-local quantum oscillations, the search of TWSMs have attracted worldwide attentions [9][10][11][12][13][14].In principle, by breaking either time reversal or spacial inversion symmetry of a Dirac semi-metal (DSM) to remove the spin degeneracy of the bands [15][16][17][18][19][20][21][22][23], the otherwise degenerate bulk Dirac point will be split into a pair of Weyl points of opposite chirality, leading to the realization of the TWSM. Early predictions of the TWSMs focus on magnetic materials, e.g. R 2 Ir 2 O 7 , HgCr 2 Se 4 , which naturally break the time reversal symmetry [4,5]. However, due to the intricate Y

This Letter presents a direct example of the shrinkage of flare loops from the radio observation of a limb X3.1 flare occurring at 00:49 UT on 2002 August 24. The relation between the positions of X-ray and radio images reveals that this flare is a typical Masuda flare. The GOES X-ray flux shows a flat time variation, and the time variation of the radio flux at 17 and 35 GHz shows that the flare had only one intense energy injection at about 01:00 UT. In the rising phase of 9 minutes, the loop in the radio image at 34 GHz shrank by about 30%, suggesting a downward speed of approximately 13 km s Ϫ1 . About 2 minutes after the peak of the intense energy injection, the shrinkage of the radio loop evolved into an expansion.