A Weyl semimetal is a new state of matter that hosts Weyl fermions as emergent quasiparticles and admits a topological classification that protects Fermi arc surface states on the boundary of a bulk ...sample. This unusual electronic structure has deep analogies with particle physics and leads to unique topological properties. We report the experimental discovery of a Weyl semimetal, tantalum arsenide (TaAs). Using photoemission spectroscopy, we directly observe Fermi arcs on the surface, as well as the Weyl fermion cones and Weyl nodes in the bulk of TaAs single crystals. We find that Fermi arcs terminate on the Weyl fermion nodes, consistent with their topological character. Our work opens the field for the experimental study of Weyl fermions in physics and materials science.
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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.
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Rectification is a process that converts electromagnetic fields into a direct current. Such a process underlies a wide range of technologies such as wireless communication, wireless charging, energy ...harvesting, and infrared detection. Existing rectifiers are mostly based on semiconductor diodes, with limited applicability to small-voltage or high-frequency inputs. Here, we present an alternative approach to current rectification that uses the intrinsic electronic properties of quantum crystals without using semiconductor junctions. We identify a previously unknown mechanism for rectification from skew scattering due to the inherent chirality of itinerant electrons in time-reversal invariant but inversion-breaking materials. Our calculations reveal large, tunable rectification effects in graphene multilayers and transition metal dichalcogenides. Our work demonstrates the possibility of realizing high-frequency rectifiers by rational material design and quantum wave function engineering.
Single-atom catalysts (SACs) have demonstrated superior catalytic performance in numerous heterogeneous reactions. However, producing thermally stable SACs, especially in a simple and scalable way, ...remains a formidable challenge. Here, we report the synthesis of Ru SACs from commercial RuO
powders by physical mixing of sub-micron RuO
aggregates with a MgAl
Fe
O
spinel. Atomically dispersed Ru is confirmed by aberration-corrected scanning transmission electron microscopy and X-ray absorption spectroscopy. Detailed studies reveal that the dispersion process does not arise from a gas atom trapping mechanism, but rather from anti-Ostwald ripening promoted by a strong covalent metal-support interaction. This synthetic strategy is simple and amenable to the large-scale manufacture of thermally stable SACs for industrial applications.
The Milky Way Imaging Scroll Painting project is an unbiased Galactic plane CO survey for mapping regions of to +250° and 5 2 with the 13.7 m telescope of the Purple Mountain Observatory. The legacy ...survey aims to observe the , , and C18O (J = 1-0) lines simultaneously with full-sampling using the nine-beam Superconducting SpectroScopic Array Receiver system with an instantaneous bandwidth of 1 GHz. In this paper, the completed 250 deg2 data from l = +25 8 to +49 7 are presented with a grid spacing of 30″ and a typical rms noise level of ∼0.5 K for at the channel width of 0.16 km and ∼0.3 K for and C18O at 0.17 km . The high-quality data, with moderate resolution (∼50″), uniform sensitivity, and high spatial dynamic range, allow us to investigate the details of molecular clouds (MCs) traced by the three CO isotope lines. Three interesting examples are briefly investigated, including distant Galactic spiral arms traced by CO emission with VLSR < 0 km s−1, the bubble-like dense gas structure near the H ii region W40, and the MCs distribution perpendicular to the Galactic plane.
Topological metals and semimetals (TMs) have recently drawn significant interest. These materials give rise to condensed matter realizations of many important concepts in high-energy physics, leading ...to wide-ranging protected properties in transport and spectroscopic experiments. It has been well-established that the known TMs can be classified by the dimensionality of the topologically protected band degeneracies. While Weyl and Dirac semimetals feature zero-dimensional points, the band crossing of nodal-line semimetals forms a one-dimensional closed loop. In this paper, we identify a TM that goes beyond the above paradigms. It shows an exotic configuration of degeneracies without a well-defined dimensionality. Specifically, it consists of 0D nexus with triple-degeneracy that interconnects 1D lines with double-degeneracy. We show that, because of the novel form of band crossing, the new TM cannot be described by the established results that characterize the topology of the Dirac and Weyl nodes. Moreover, triply-degenerate nodes realize emergent fermionic quasiparticles not present in relativistic quantum field theory. We present materials candidates. Our results open the door for realizing new topological phenomena and fermions including transport anomalies and spectroscopic responses in metallic crystals with nontrivial topology beyond the Weyl/Dirac paradigm.
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Defects in perovskite are key factors in limiting the photovoltaic performance and stability of perovskite solar cells (PSCs). Generally, choline halide (ChX) can effectively passivate defects by ...binding with charged point defects of perovskite. However, we verified that ChI can react with CsPbI3 to form a novel crystal phase of one‐dimensional (1D) ChPbI3, which constructs 1D/3D heterostructure with 3D CsPbI3, passivating the defects of CsPbI3 more effectively and then resulting in significantly improved photoluminescence lifetime from 20.2 ns to 49.4 ns. Moreover, the outstanding chemical inertness of 1D ChPbI3 and the repair of undesired δ‐CsPbI3 deficiency during its formation process can significantly enhance the stability of CsPbI3 film. Benefiting from 1D/3D heterostructure, CsPbI3 carbon‐based PSCs (C‐PSCs) delivered a champion efficiency of 18.05 % and a new certified record of 17.8 % in hole transport material (HTM)‐free inorganic C‐PSCs.
ChPbI3 was obtained by ChI treatment of the CsPbI3 surface to construct a 1D/3D heterostructure, which significantly improves the defect‐assisted recombination and stability of CsPbI3. Benefiting from 1D/3D heterostructure, the assembled carbon‐based perovskite solar cells (C‐PSCs) delivered a champion/certified efficiency of 18.05 %/17.8 %.
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Quantum materials hosting Weyl fermions have opened a new era of research in condensed matter physics. First proposed in 1929 in the context of particle physics, Weyl fermions have yet to be observed ...as elementary particles. In 2015, Weyl fermions were detected as collective electronic excitations in the strong spin–orbit coupled material tantalum arsenide, TaAs. This discovery was followed by a flurry of experimental and theoretical explorations of Weyl phenomena in materials. Weyl materials naturally lend themselves to the exploration of the topological index associated with Weyl fermions and their divergent Berry curvature field, as well as the topological bulk–boundary correspondence, giving rise to protected conducting surface states. Here, we review the broader class of Weyl topological phenomena in materials, starting with the observation of emergent Weyl fermions in the bulk and Fermi arc states on the surface of the TaAs family of crystals by photoemission spectroscopy. We then discuss several exotic optical and magnetic responses observed in these materials, as well as progress in developing related chiral materials. We discuss the conceptual development of high-fold chiral fermions, which generalize Weyl fermions, and we review the observation of high-fold chiral fermion phases by taking the rhodium silicide, RhSi, family of crystals as a prime example. Lastly, we discuss recent advances in Weyl line phases in magnetic topological materials. With this Review, we aim to provide an introduction to the basic concepts underlying Weyl physics in condensed matter, and to representative materials and their electronic structures and topology as revealed by spectroscopic studies. We hope this work serves as a guide for future theoretical and experimental explorations of chiral fermions and related topological quantum systems with potentially enhanced functionalities.Weyl fermions have yet to be observed as elementary particles but can be realized in topological quantum materials. This Review discusses the theoretical and experimental discovery of emergent Weyl fermions, high-fold chiral fermions, topological Weyl lines and related Dirac phases.
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Dry reforming of methane (DRM) is an attractive route to utilize CO
as a chemical feedstock with which to convert CH
into valuable syngas and simultaneously mitigate both greenhouse gases. Ni-based ...DRM catalysts are promising due to their high activity and low cost, but suffer from poor stability due to coke formation which has hindered their commercialization. Herein, we report that atomically dispersed Ni single atoms, stabilized by interaction with Ce-doped hydroxyapatite, are highly active and coke-resistant catalytic sites for DRM. Experimental and computational studies reveal that isolated Ni atoms are intrinsically coke-resistant due to their unique ability to only activate the first C-H bond in CH
, thus avoiding methane deep decomposition into carbon. This discovery offers new opportunities to develop large-scale DRM processes using earth abundant catalysts.
Abstract Designing active and stable electrocatalysts with economic efficiency for acidic oxygen evolution reaction is essential for developing proton exchange membrane water electrolyzers. Herein, ...we report on a cobalt oxide incorporated with iridium single atoms (Ir-Co 3 O 4 ), prepared by a mechanochemical approach. Operando X-ray absorption spectroscopy reveals that Ir atoms are partially oxidized to active Ir >4+ during the reaction, meanwhile Ir and Co atoms with their bridged electrophilic O ligands acting as active sites, are jointly responsible for the enhanced performance. Theoretical calculations further disclose the isolated Ir atoms can effectively boost the electronic conductivity and optimize the energy barrier. As a result, Ir-Co 3 O 4 exhibits significantly higher mass activity and turnover frequency than those of benchmark IrO 2 in acidic conditions. Moreover, the catalyst preparation can be easily scaled up to gram-level per batch. The present approach highlights the concept of constructing single noble metal atoms incorporated cost-effective metal oxides catalysts for practical applications.
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