Weyl semimetals are crystalline solids that host emergent relativistic Weyl fermions and have characteristic surface Fermi-arcs in their electronic structure. Weyl semimetals with broken time reversal symmetry are difficult to identify unambiguously. In this work, using angle-resolved photoemission spectroscopy, we visualized the electronic structure of the ferromagnetic crystal Co3Sn2S2 and discovered its characteristic surface Fermi-arcs and linear bulk band dispersions across the Weyl points. These results establish Co3Sn2S2 as a magnetic Weyl semimetal that may serve as a platform for realizing phenomena such as chiral magnetic effects, unusually large anomalous Hall effect and quantum anomalous Hall effect.

Topological semimetals (TSs) in structurally chiral crystals (which possess a handedness due to a lack of mirror and inversion symmetries) are expected to display numerous exotic physical phenomena, such as new fermionic excitations with large topological charge 1 , long Fermi-arc surface states 2,3 , unusual magnetotransport 4 and lattice dynamics 5 , as well as a quantized response to circularly polarized light 6 . To date, however, all experimentally confirmed TSs crystallize in space groups that contain mirror operations, which forces the aforementioned properties to vanish. Here, by employing angle-resolved photoelectron spectroscopy and ab-initio calculations, we show that AlPt is a structurally chiral TS that hosts new fourfold and sixfold fermions, which can be viewed as a higher spin generalization of Weyl fermions without equivalence in elementary particle physics. Remarkably, these multifold fermions are located at high symmetry points with Chern numbers larger than those in Weyl-semimetals, thus resulting in multiple Fermi-arcs that thread through the full diagonal of the surface Brillouin zone (BZ), spanning the largest portion of the BZ of any material. By imaging these long Fermi-arcs, we can experimentally determine the magnitude and sign of their Chern number, which allows us to relate their dispersion to the handedness of their host crystal.Revised manuscript 2 An object that cannot be superimposed with its mirror image is said to be chiral, a concept first proposed by Lord Kelvin 7 that has found widespread applications across the modern sciences, from high energy physics to biology 8,9 . In condensed matter physics (where the properties of crystalline materials are tightly constrained by spatial lattice symmetries), chiral crystals can only be found in the 65 Sohncke space groups, which contain only orientation-preserving operations, and can therefore be assigned a handedness. The structural chirality of these systems can endow them with fascinating properties, such as natural optical activity 10 , negative refraction in metamaterials 11 , non-reciprocal effects such as magnetochiral birefringence of light or electronic magnetochiral anisotropy 12,13 , chiral magnetic textures such as helices and skyrmions 14 , or unusual superconductivity 15 . In the recently discovered Weyl-semimetals, not just the crystal structure itself, but also the electronic wavefunctions can exhibit a chirality at point-like twoband crossings of the quasiparticle dispersion, which are known as Weyl fermions [16][17][18][19] . These crossings are topologically protected because they carry a topological charge, a quantized Berry flux through any surface enclosing them in momentum space. This charge has integer magnitude C= ±1, which is known as the Chern number and gives the electronic wavefunction a handedness. This topological property results in a plethora of exotic phenomena, such as Fermiarc surface states 20 , unconventional magnetoresistance 16,21,22 , nonlocal transport 23 , and many

A new layered oxide semiconductor (Bi2O2Se) is found with excellent electronic properties for promising applications.

Topological semimetals feature protected nodal band degeneracies characterized by a topological invariant known as the Chern number (C). Nodal band crossings with linear dispersion are expected to have at most |C|=4, which sets an upper limit to the magnitude of many topological phenomena in these materials. Here, we show that the chiral crystal palladium gallium (PdGa) displays multifold band crossings, which are connected by exactly four surface Fermi arcs, thus proving that they carry the maximal Chern number magnitude of 4. By comparing two enantiomers, we observe a reversal of their Fermi-arc velocities, which demonstrates that the handedness of chiral crystals can be used to control the sign of their Chern numbers.

Twisted bilayer graphene (tBLG) has recently attracted growing interest due to its unique twist-angle-dependent electronic properties. The preparation of high-quality large-area bilayer graphene with rich rotation angles would be important for the investigation of angle-dependent physics and applications, which, however, is still challenging. Here, we demonstrate a chemical vapor deposition (CVD) approach for growing high-quality tBLG using a hetero-site nucleation strategy, which enables the nucleation of the second layer at a different site from that of the first layer. The fraction of tBLGs in bilayer graphene domains with twist angles ranging from 0° to 30° was found to be improved to 88%, which is significantly higher than those reported previously. The hetero-site nucleation behavior was carefully investigated using an isotope-labeling technique. Furthermore, the clear Moiré patterns and ultrahigh room-temperature carrier mobility of 68,000 cm2 V−1 s−1 confirmed the high crystalline quality of our tBLG. Our study opens an avenue for the controllable growth of tBLGs for both fundamental research and practical applications.

Twisted bilayer graphene provides a new two-dimensional platform for studying electron interaction phenomena and flat band properties such as correlated insulator transition, superconductivity and ferromagnetism at certain magic angles. Here,...

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Magnetic Weyl semimetals are a newly discovered class of topological materials that may serve as a platform for exotic phenomena, such as axion insulators or the quantum anomalous Hall effect. Here, we use angle-resolved photoelectron spectroscopy and ab initio calculations to discover Weyl cones in CoS2, a ferromagnet with pyrite structure that has been long studied as a candidate for half-metallicity, which makes it an attractive material for spintronic devices. We directly observe the topological Fermi arc surface states that link the Weyl nodes, which will influence the performance of CoS2 as a spin injector by modifying its spin polarization at interfaces. In addition, we directly observe a minority-spin bulk electron pocket in the corner of the Brillouin zone, which proves that CoS2 cannot be a true half-metal.