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We report phase coherent electron transport in PbTe nanowire networks with a loop geometry. Magneto-conductance shows Aharonov-Bohm (AB) oscillations with periods of $h/e$ and $h/2e$ in flux. The amplitude of $h/2e$ oscillations is enhanced near zero magnetic field, possibly due to interference between time-reversal paths. Temperature dependence of the AB amplitudes suggests a phase coherence length $\sim$ 8 - 12 $\mu$m at 50 mK. This length scale is larger than the typical geometry of PbTe-based hybrid semiconductor-superconductor nanowire devices., Comment: Raw data and processing codes within this paper are available at https://zenodo.org/record/5798424

Fermi arcs on Weyl semimetals exhibit many exotic quantum phenomena. Usually found on atomically flat surfaces with approximate translation symmetry, Fermi arcs are rooted in the peculiar topology of bulk Bloch bands of 3D crystals. The fundamental question of whether a 1D Fermi arc can be probed remains unanswered. Such an answer could significantly broaden potential applications of Weyl semimetals. Here, we report a direct observation of robust edge states on atomic-scale ledges in TaAs using low-temperature scanning tunneling microscopy/spectroscopy. Spectroscopic signatures and theoretical calculations reveal that the 1D Fermi arcs arise from the chiral Weyl points of bulk crystals. The crossover from 2D Fermi arcs to eventual complete localization on 1D edges was arrested experimentally on a sequence of surfaces. Our results demonstrate extreme robustness of the bulk-boundary correspondence, which offers topological protection for Fermi arcs, even in cases in which the boundaries are at the atomic-scale. The persistent 1D Fermi arcs can be profitably exploited in miniaturized quantum devices.

Topological quantum computing is based on braiding of Majorana zero modes encoding topological qubits. A promising candidate platform for Majorana zero modes is semiconductor-superconductor hybrid nanowires. The realization of topological qubits and braiding operations requires scalable and disorder-free nanowire networks. Selective area growth of in-plane InAs and InSb nanowires, together with shadow-wall growth of superconductor structures, have demonstrated this scalability by achieving various network structures. However, the noticeable lattice mismatch at the nanowire-substrate interface, acting as a disorder source, imposes a serious obstacle along with this roadmap. Here, combining selective area and shadow-wall growth, we demonstrate the fabrication of PbTe-Pb hybrid nanowires - another potentially promising Majorana system - on a nearly perfectly lattice-matched substrate CdTe, all done in one molecular beam epitaxy chamber. Transmission electron microscopy shows the single-crystal nature of the PbTe nanowire and its atomically sharp and clean interfaces to the CdTe substrate and the Pb overlayer, without noticeable inter-diffusion or strain. The nearly ideal interface condition, together with the strong screening of charge impurities due to the large dielectric constant of PbTe, hold promise towards a clean nanowire system to study Majorana zero modes and topological quantum computing., Comment: 19 pages, 4+7 figures

This paper proposes a novel directional pilot protection method based on the transient energy for bipolar HVDC line. Supposing the positive direction of current is from DC bus to the DC line, in the case of an internal line fault, the transient energies detected on both sides of the line are all negative within a short time, which denotes the positive direction fault; while for an external fault, the transient energy on one end is positive, which means a negative direction fault, but the transient energy on the opposite end is negative, which indicates that the fault direction is positive. According to these characteristics, an integration criterion identifying fault direction is constructed. In addition, through setting a fixed energy threshold, the faulted line and lightning disturbance can also be discriminated. Simulation results from UHVDC transmission system show the validity of the proposed protection method.

Stanene has been theoretically predicted to be a 2D topological insulator with a large band gap, potentially hosting quantum spin Hall effect at room temperature. Here, few-layer stanene films have been epitaxially grown on Sb-terminated InSb (1 1 1) surface and their structural and electrical properties are characterized. Scanning tunneling spectrum results reveal a large bulk bandgap in single-layer stanene (over 0.2 eV). Moreover, spectroscopy evidence for a filled edge state near the steps was observed. The gap decreases dramatically with increasing number of layers, and multilayer stanene should become a Dirac semimetal in the bulk limit. The changeover may involve nontrivial topological phase transitions. Clear and reproducible Shubnikov–de Haas oscillations were observed on the single-layer stanene films that were exposed to atmospheric conditions for an extended period of time, showing the possibility for device experiments using nanofabrication and magneto-transport. Our results demonstrate that the single-layer stanene is a promising topological material for exploring fundamental physics and quantum applications.

We report comprehensive studies of the single crystal growth and electrical transport properties for various samples of TaAs, the first experimentally confirmed inversion symmetry-breaking Weyl semimetal. The transport parameters for different samples are obtained through the fitting of the two band model and the analysis of Shubnikov de Haas oscillations. We find that the ratio factor of transport lifetime to quantum lifetime is intensively enhanced when the Fermi level approaches the Weyl node. This result is consistent with the side-jump interpretation derived from a chirality-protected shift in the scattering process for a Weyl semimetal., This is a modified version of arXiv:1502.00251

The family of materials defined as ZrSiX (X = S, Se, Te) has been established as Dirac node-line semimetals, and subsequent study is urgent to exploit the promising applications of unusual magnetoresistance (MR) properties. Herein, we systematically investigated the anisotropic MR in the newly-discovered Dirac node-line material ZrSiSe. By applying a magnetic field of 3 T by a vector field, three-dimensional (3D) MR shows the strong anisotropy. The MR ratio of maximum and minimum directions reaches 7 at 3 T and keeps increasing at the higher magnetic field. The anisotropic MR forms a butterfly-shaped curve, indicating the quasi-2D electronic structures. This is further confirmed by the angular dependent Shubnikov-de Haas oscillations. The first-principles calculations establish the quasi-2D tubular-shaped Fermi surface near the X point in the Brillouin zone. Our finding sheds light on the 3D mapping of MR and the potential applications in magnetic sensors based on ZrSiSe.

Using the characteristic difference of transient energy, a transient energy based directional unit protection is proposed. Assuming the positive direction of current is from DC busbar to DC line, for an internal line fault, the detected transient energies on both ends of the line are all negative, which denote the positive direction fault. When an external fault occurs, the transient energy on one end is positive, which denotes a negative direction fault, but the transient energy on opposite end is negative, which means that the fault direction is positive. Through comparing fault directions on both ends, the internal or external DC line fault can be identified. Besides, according to the magnitude difference of transient energy, the faulty line can also be discriminated for bipolar line. Finally, this paper constructs a ±800kV DC transmission system model, and simulation results verify the validity and feasibility of the proposed method.

The traveling wave protection performance of traditional DC transmission line can be influenced by fault conditions, so this paper proposes a traveling wave based directional pilot protection method. Analysis shows, for an internal DC line fault, that the forward traveling wave is less than the backward traveling wave at two ends of the proposed line within a short time of post-fault; but in the case of an external fault, the forward traveling wave is greater than the backward one at one end of the line while the forward traveling wave is less than the backward one at the opposite end. According to this characteristic difference, an integral criterion identifying fault direction is constructed here. By comparing the identification results of fault direction at both ends of the line, the internal fault can be distinguished from the external fault. For bipolar HVDC system, the faulted line can be determined according to the polarity difference between the aerial mode component and the zero mode component of the backward traveling wave. This paper builds a UHVDC line model, and the simulation results verify the validity of the proposed method.

With the integration of DG, the power flow direction of distribution grid may be changed from single to bidirectional. In this situation, the conventional current protection scheme faces the extreme challenge, even does not meet the requirement of distribution grid with DG. Therefore, this paper proposes a directional pilot protection method to protect the upstream line of DG. Using the fault components of voltage and current, an energy function is constructed. Through analyzing the characteristics of energy function, the fault direction can be easily identified, further the faulted section of line is also determined. Simulation results show the proposed method is valid and feasible.