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...

SnSe is a promising thermoelectric material with record-breaking figure of merit. However, to date a comprehensive understanding of the electronic structure and most critically, the self-hole-doping mechanism in SnSe is still absent. Here we report the highly anisotropic electronic structure of SnSe investigated by angle-resolved photoemission spectroscopy, in which a unique pudding-mould-shaped valence band with quasi-linear energy dispersion is revealed. We prove that p-type doping in SnSe is extrinsically controlled by local phase segregation of SnSe2 microdomains via interfacial charge transferring. The multivalley nature of the pudding-mould band is manifested in quantum transport by crystallographic axis-dependent weak localisation and exotic non-saturating negative magnetoresistance. Strikingly, quantum oscillations also reveal 3D Fermi surface with unusual interlayer coupling strength in p-SnSe, in which individual monolayers are interwoven by peculiar point dislocation defects. Our results suggest that defect engineering may provide versatile routes in improving the thermoelectric performance of the SnSe family.

Chiral anomaly induced negative magnetoresistance (NMR) has been widely used as a critical transport evidence on the existence of Weyl fermions in topological semimetals. In this mini review, we discuss the general observation of the NMR phenomena in non-centrosymmetric NbP and NbAs. We show that NMR can be contributed by intrinsic chiral anomaly of Weyl fermions and/or extrinsic effects, such as superimposition of Hall signals, field-dependent inhomogeneous current flow in the bulk, i.e. current jetting, and weak localization (WL) of coexistent trivial carriers. Such WL controlled NMR is heavily dependent on sample quality, and is characterized by pronounced crossover from positive to negative MR growth at elevated temperatures, as a result of the competition between the phase coherence time and the spin-orbital scattering constant of the bulk trivial pockets. Thus, the correlation of NMR and chiral anomaly needs to be scrutinized, without the support of other complimentary techniques. Due to the lifting of spin degeneracy, the spin orientations of Weyl fermions are either parallel or antiparallel to the momentum, a unique physical property known as helicity. The conservation of helicity provides strong protection for the transport of Weyl fermions, which can only be effectively scattered by magnetic impurities. Chemical doping of magnetic and non-magnetic impurities are thus more convincing in probing the existence of Weyl fermions than the NMR method.

Negative magnetoresistance (NMR) induced by the Adler-Bell-Jackiw anomaly is regarded as the most prominent quantum signature of Weyl semimetals when electrical field E is collinear with the external magnetic field B. In this article, we report universal NMR in nonmagnetic, centrosymmetric transition metal dipnictides MPn2 (M=Nb and Ta; Pn=As and Sb), in which the existence of Weyl fermions can be explicitly excluded. Using temperature-dependent magnetoresistance, Hall and thermoelectric coefficients of Nernst and Seebeck effects, we determine that the emergence of the NMR phenomena in MPn2 is coincident with a Lifshitz transition, corresponding to the formation of unique electron-hole-electron (e-h-e) pockets along the I − L − I direction. First-principles calculations reveal that, along the I − L − I line, the dxy and d x 2 −y 2 orbitals of the transition metal form tilted nodal rings of band crossing well below the Fermi level. Strong spin-orbital coupling gaps all the crossing points and creates the characteristic e-h-e structure, making MPn2 a topological semimetal with Z2 indices of [0;(111)]. By excluding the weak localization contribution of the bulk states, we conclude that the universal NMR in MPn2 may have an exotic origin in topological surface states, which appears in pairs with opposite spin-momentum locking on nontrivial surfaces.

The startup process is the crucial transition phase of the supercritical carbon-dioxide Brayton cycle, so it is essential to focus on and investigate the transient performance for the system’s safety and stability. The pressure in the buffer tank approaches the safety upper limit with different startup schemes during the joint commissioning of the compressor and heater in a MWe-scale experiment system, while the maximum temperature is 309 °C. Hence, dynamic simulations are carried out to explore the dynamic startup characteristics from a cold state or a warm state to the turbine pre-start condition, in which 60% of the rated mass flow rate and 67% of the rated compressor speed are reached in the end. The results show that, when starting from a cold state, the startup scheme of simultaneously heating and speeding up has a limited effective application scope. Two venting operations during the above process help the system establish heat regeneration and promote temperature uniformity in the system. Furthermore, when starting from a warm state with an existing temperature gradient in the system, the startup scheme of simultaneously heating and speeding up is more effective and has a more extensive range of control.