The topology of the electronic structure of a crystal is manifested in its surface states. Recently, a distinct topological state has been proposed in metals or semimetals whose spin-orbit band structure features three-dimensional Dirac quasiparticles. We used angle-resolved photoemission spectroscopy to experimentally observe a pair of spin-polarized Fermi arc surface states on the surface of the Dirac semimetal Na3Bi at its native chemical potential. Our systematic results collectively identify a topological phase in a gapless material. The observed Fermi arc surface states open research frontiers in fundamental physics and possibly in spintronics.

Silicene is a one-atom-thick two-dimensional crystal of silicon with a hexagonal lattice structure that is related to that of graphene but with atomic bonds that are buckled rather than flat. This buckling confers advantages on silicene over graphene, because it should, in principle, generate both a band gap and polarized spin-states that can be controlled with a perpendicular electric field. Here we use first-principles calculations to show that field-gated silicene possesses two gapped Dirac cones exhibiting nearly 100% spin-polarization, situated at the corners of the Brillouin zone. Using this fact, we propose a design for a silicene-based spin-filter that should enable the spin-polarization of an output current to be switched electrically, without switching external magnetic fields. Our quantum transport calculations indicate that the proposed designs will be highly efficient (nearly 100% spin-polarization) and robust against weak disorder and edge imperfections. We also propose a Y-shaped spin/valley separator that produces spin-polarized current at two output terminals with opposite spins.

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Lithium-sulphur batteries have high theoretical energy density and potentially low cost, but significant challenges such as severe capacity degradation prevent its widespread adoption. Here we report a new design of lithium-sulphur battery using electrically connected graphite and lithium metal as a hybrid anode to control undesirable surface reactions on lithium. Lithiated graphite placed in front of the lithium metal functions as an artificial, self-regulated solid electrolyte interface layer to actively control the electrochemical reactions and minimize the deleterious side reactions, leading to significant performance improvements. Lithiumsulphur cells incorporating this hybrid anodes deliver capacities of 4800 mAh g À 1 for 400 cycles at a high rate of 1,737 mA g À 1 , with only 11% capacity fade and a Coulombic efficiency 499%. This simple hybrid concept may also provide scientific strategies for protecting metal anodes in other energy-storage devices.

Novel magnetic topological materials pave the way for studying the interplay between band topology and magnetism. However, an intrinsically ferromagnetic topological material with only topological bands at the charge neutrality energy has so far remained elusive. Using rational design, we synthesized MnBi8Te13, a natural heterostructure with [MnBi2Te4] and [Bi2Te3] layers. Thermodynamic, transport, and neutron diffraction measurements show that despite the adjacent [MnBi2Te4] being 44.1 Å apart, MnBi8Te13 manifests long-range ferromagnetism below 10.5 K with strong coupling between magnetism and charge carriers. First-principles calculations and angle-resolved photoemission spectroscopy measurements reveal it is an axion insulator with sizable surface hybridization gaps. Our calculations further demonstrate the hybridization gap persists in the two-dimensional limit with a nontrivial Chern number. Therefore, as an intrinsic ferromagnetic axion insulator with clean low-energy band structures, MnBi8Te13 serves as an ideal system to investigate rich emergent phenomena, including the quantized anomalous Hall effect and quantized magnetoelectric effect.

The newly discovered topological crystalline insulators feature a complex band structure involving multiple Dirac cones [1][2][3][4][5][6] , and are potentially highly tunable by external electric field, temperature or strain. Theoretically, it has been predicted that the various Dirac cones, which are o set in energy and momentum, might harbour vastly di erent orbital character 7 . However, their orbital texture, which is of immense importance in determining a variety of a material's properties [8][9][10] remains elusive. Here, we unveil the orbital texture of Pb 1−x Sn x Se, a prototypical topological crystalline insulator. By using Fourier-transform scanning tunnelling spectroscopy we measure the interference patterns produced by the scattering of surface-state electrons. We discover that the intensity and energy dependences of the Fourier transforms show distinct characteristics, which can be directly attributed to orbital e ects. Our experiments reveal a complex band topology involving two Lifshitz transitions 11 and establish the orbital nature of the Dirac bands, which could provide an alternative pathway towards future quantum applications.A counterpart to charge and spin, electron orbitals are of great importance in the underlying physical processes of a variety of systems. The orbital degrees of freedom, for example, play a crucial role in the colossal magnetoresistance effect in manganese oxides, and contribute to the anisotropic electronic and magnetic properties in many other transition-metal oxide systems 8 . More recently, orbital ordering within the superconducting FeAs layer has been thought to govern structural phase transitions and 'stripe'-like antiferromagnetism in Fe-based high-temperature superconductors 9,10 . Similarly, topological materials host complex orbital arrangements often strongly coupled to other electronic degrees of freedom [12][13][14][15][16][17][18] . Topological crystalline insulators (TCIs) in particular are predicted to exhibit intricate band, spin and orbital textures, potentially relevant for interactions in the quantum Hall regime. Although previous experiments provided a glimpse into the complex band topology present in TCIs (refs 3-5,19,20), these experimental efforts have not been able to shed light onto its orbital texture. Here we use Fourier-transform (FT) scanning tunnelling spectroscopy (STS) to reveal the distinct orbital nature of the Dirac bands in the TCI, Pb 1−x Sn x Se.In its stoichiometric state, Pb 1−x Sn x Se with x = 0 is a trivial insulator under the Z 2 topological classification of materials owing to the absence of band inversion. The process of adding Sn, which substitutes for Pb, leads to band inversion at an even number of time-reversal points, and the solutions remain Z 2 trivial. However, for a particular region of the composition-temperature parameter space, such as x > ∼0.23 and room temperature, or x > ∼0.18 and 4 K, topologically protected surface states emerge owing to the non-trivial band topology classified by crystalline symmetries 3...

Honeycomb-like nitrogen and sulfur dual-doped hierarchical porous biomass-derived carbon/sulfur composites (NSHPC/S) are successfully fabricated for high energy density lithium-sulfur batteries. The effects of nitrogen, sulfur dual-doping on the structures and properties of the NSHPC/S composites are investigated in detail by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and charge/discharge tests. The results show that N, S dual-doping not only introduces strong chemical adsorption and provides more active sites but also significantly enhances the electronic conductivity and hydrophilic properties of hierarchical porous biomass-derived carbon, thereby significantly enhancing the utilization of sulfur and immobilizing the notorious polysulfide shuttle effect. Especially, the as-synthesized NSHPC-7/S exhibits high initial discharge capacity of 1204 mA h g at 1.0 C and large reversible capacity of 952 mA h g after 300 cycles at 0.5 C with an ultralow capacity fading rate of 0.08 % per cycle even at high sulfur content (85 wt %) and high active material areal mass loading (2.8 mg cm ) for the application of high energy density Li-S batteries.

The polysulfide shuttle effect is one of the most important problems hindering the commercial application of lithium−sulfur batteries (LSBs). In order to solve the above problem and promote LSBs commercialization, herein a holistic design strategy on the molybdenum disulfide-coated nitrogen-doped mesoporous carbon sphere/sulfur (NMCS@MoS 2 /S) composite cathode and carbon nanotube/chitosan modified separator (CNT/CH) is proposed. In the holistic design, the NMCS@MoS 2 plays a role in the host of sulfur and the lithium polysulfides (LiPSs) adsorbent; the CNT/CH modified separator also has an inestimable role in promoting lithium ion transport and chemical adsorption of LiPSs. The results show that the LSBs with the NMCS@MoS 2 /S-CNT/CH release a high reversible capacity of 827 mAh g −1 with a high capacity retention of 92.4% at 0.5 C after 200 cycles. The delicate design exhibits apparently excellent electrochemical performance and provides an exciting strategy for solving the shuttle effect of LiPSs and boosting industrialization of LSBs.