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The improvement of catalysts for the oxygen-reduction reaction is an important challenge for fuel cells and other electrochemical-energy technologies. A composite nanoporous Ni–Pt alloy with a tailored geometric architecture is now shown to exhibit high mass activity for oxygen reduction. The improvement of catalysts for the four-electron oxygen-reduction reaction (ORR; O 2 +4H + +4e − →2H 2 O) remains a critical challenge for fuel cells and other electrochemical-energy technologies. Recent attention in this area has centred on the development of metal alloys with nanostructured compositional gradients (for example, core–shell structure) that exhibit higher activity than supported Pt nanoparticles (Pt–C; refs  1 , 2 , 3 , 4 , 5 , 6 , 7 ). For instance, with a Pt outer surface and Ni-rich second atomic layer, Pt 3 Ni(111) is one of the most active surfaces for the ORR (ref.  8 ), owing to a shift in the d -band centre of the surface Pt atoms that results in a weakened interaction between Pt and intermediate oxide species, freeing more active sites for O 2 adsorption 2 , 9 . However, enhancements due solely to alloy structure and composition may not be sufficient to reduce the mass activity enough to satisfy the requirements for fuel-cell commercialization 10 , especially as the high activity of particular crystal surface facets may not easily translate to polyfaceted particles. Here we show that a tailored geometric and chemical materials architecture can further improve ORR catalysis by demonstrating that a composite nanoporous Ni–Pt alloy impregnated with a hydrophobic, high-oxygen-solubility and protic ionic liquid has extremely high mass activity. The results are consistent with an engineered chemical bias within a catalytically active nanoporous framework that pushes the ORR towards completion.

Icosahedral order has been suggested as the prevalent atomic motif of supercooled liquids and metallic glasses for more than half a century, because the icosahedron is highly close-packed but is difficult to grow, owing to structure frustration and the lack of translational periodicity. By means of angstrom-beam electron diffraction of single icosahedra, we report experimental observation of local icosahedral order in metallic glasses. All the detected icosahedra were found to be distorted with partially face-centered cubic symmetry, presenting compelling evidence on geometric frustration of local icosahedral order in metallic glasses.

Amorphous shear bands are the main deformation and failure mode of super-hard boron carbide subjected to shock loading and high pressures at room temperature. Nevertheless, the formation mechanisms of the amorphous shear bands remain a long-standing scientific curiosity mainly because of the lack of experimental structure information of the disordered shear bands, comprising light elements of carbon and boron only. Here we report the atomic structure of the amorphous shear bands in boron carbide characterized by state-of-the-art aberration-corrected transmission electron microscopy. Distorted icosahedra, displaced from the crystalline matrix, were observed in nano-sized amorphous bands that produce dislocation-like local shear strains. These experimental results provide direct experimental evidence that the formation of amorphous shear bands in boron carbide results from the disassembly of the icosahedra, driven by shear stresses. Boron carbide is known to deform and fail via the formation of amorphous shear bands, but the atomic-scale events by which this occurs are unknown. Reddy et al. study the atomic structure of these shear bands and find that they form via the disassembly of icosahedra.

A quantum simulation platform based on quantum dots is reported that can operate at relatively low temperatures, and its utility is shown by simulating a Fermi–Hubbard model. Quantum simulations on quantum dots Quantum simulations have been performed on various different platforms, for example using vacancies in diamond or ultracold quantum gases. Quantum dots have been regarded as a promising constituent of quantum simulation platforms for some time, but owing to difficulties in calibrating them it has so far been impossible to run a successful simulation. Here, the authors overcome these difficulties and demonstrate a quantum simulation of a Fermi–Hubbard model, which is a famous model in condensed matter physics. Quantum simulation platforms based on quantum dots are predicted to be able to reach lower temperatures than atomic-physics-based platforms. This could help to clarify puzzles in condensed matter physics, such as high-temperature superconductivity. Interacting fermions on a lattice can develop strong quantum correlations, which are the cause of the classical intractability of many exotic phases of matter 1 , 2 , 3 . Current efforts are directed towards the control of artificial quantum systems that can be made to emulate the underlying Fermi–Hubbard models 4 , 5 , 6 . Electrostatically confined conduction-band electrons define interacting quantum coherent spin and charge degrees of freedom that allow all-electrical initialization of low-entropy states and readily adhere to the Fermi–Hubbard Hamiltonian 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 . Until now, however, the substantial electrostatic disorder of the solid state has meant that only a few attempts at emulating Fermi–Hubbard physics on solid-state platforms have been made 18 , 19 . Here we show that for gate-defined quantum dots this disorder can be suppressed in a controlled manner. Using a semi-automated and scalable set of experimental tools, we homogeneously and independently set up the electron filling and nearest-neighbour tunnel coupling in a semiconductor quantum dot array so as to simulate a Fermi–Hubbard system. With this set-up, we realize a detailed characterization of the collective Coulomb blockade transition 20 , which is the finite-size analogue of the interaction-driven Mott metal-to-insulator transition 1 . As automation and device fabrication of semiconductor quantum dots continue to improve, the ideas presented here will enable the investigation of the physics of ever more complex many-body states using quantum dots.

Johari-Goldstein or β relaxation, persisting down to glassy state from a supercooled liquid, is a universal phenomenon of glassy dynamics. Nevertheless, the underlying micromechanisms leading to the relaxation are still in debate despite great efforts devoted to this problem for decades. Here we report experimental evidence on the structural origins of Johari-Goldstein relaxation in an ultra-quenched metallic glass. The measured activation energy of the relaxation (~26 times of the product of gas constant and glass transition temperature) is consistent with the dynamic characteristics of Johari-Goldstein relaxation. Synchrotron X-ray investigations demonstrate that the relaxation originates from short-range collective rearrangements of large solvent atoms, which can be realized by local cooperative bonding switch. Our observations provide experimental insights into the atomic mechanisms of Johari-Goldstein relaxation and will be helpful in understanding the low-temperature dynamics and properties of metallic glasses. The mechanisms by which Johari-Goldstein relaxation is accommodated in metallic glasses are difficult to clarify. Here, the authors elucidate the mechanism in an ultra-quenched metallic glass with a cooling rate of ~1010 K s −1 , by extended X-ray absorption fine structure spectroscopy.

Oxide nanoprecipitates with typical sizes of smaller than five nanometres have been known to considerably enhance the mechanical properties of steel. An atomic-scale characterization is now able to directly verify the crystal structure of these stable oxide nanoclusters. Oxide-dispersion-strengthened steels are the most promising structural materials for next-generation nuclear energy systems because of their excellent resistance to both irradiation damage and high-temperature creep 1 , 2 , 3 , 4 . Although it has been known for a decade that the extraordinary mechanical properties of oxide-dispersion-strengthened steels originate from highly stabilized oxide nanoclusters with a size smaller than 5 nm, the structure of these nanoclusters has not been clarified and remains as one of the most important scientific issues in nuclear materials research 2 , 3 , 4 , 5 , 6 , 7 . Here we report the atomic-scale characterization of the oxide nanoclusters using state-of-the-art Cs-corrected transmission electron microscopy. This study provides compelling evidence that the nanoclusters have a defective NaCl structure with a high lattice coherency with the bcc steel matrix. Plenty of point defects as well as strong structural affinity of nanoclusters with the steel matrix seem to be the most important reasons for the unusual stability of the clusters at high temperatures and in intensive neutron irradiation fields.

The electron spins in a linear array of three quantum dots can be manipulated, shuttled and read out individually with high fidelity, resembling the functioning principle of a CCD (charge-coupled device). Spin-based electronics or spintronics relies on the ability to store, transport and manipulate electron spin polarization with great precision 1 , 2 , 3 , 4 . In its ultimate limit, information is stored in the spin state of a single electron, at which point quantum information processing also becomes a possibility 5 , 6 . Here, we demonstrate the manipulation, transport and readout of individual electron spins in a linear array of three semiconductor quantum dots. First, we demonstrate single-shot readout of three spins with fidelities of 97% on average, using an approach analogous to the operation of a charge-coupled device (CCD) 7 . Next, we perform site-selective control of the three spins, thereby writing the content of each pixel of this ‘single-spin charge-coupled device’. Finally, we show that shuttling an electron back and forth in the array hundreds of times, covering a cumulative distance of 80 μm, has negligible influence on its spin projection. Extrapolating these results to the case of much larger arrays points at a diverse range of potential applications, from quantum information to imaging and sensing.

A new class of materials termed topological insulators have been intensively investigated due to their unique Dirac surface state carrying dissipationless edge spin currents. Recently, it has been theoretically proposed that the three dimensional analogue of this type of band structure, the Weyl Semimetal phase, is materialized in pyrochlore oxides with strong spin-orbit coupling, accompanied by all-in-all-out spin ordering. Here, we report on the fabrication and magnetotransport of Eu 2 Ir 2 O 7 single crystalline thin films. We reveal that one of the two degenerate all-in-all-out domain structures, which are connected by time-reversal operation, can be selectively formed by the polarity of the cooling magnetic field. Once formed, the domain is robust against an oppositely polarised magnetic field, as evidenced by an unusual odd field dependent term in the magnetoresistance and an anomalous term in the Hall resistance. Our findings pave the way for exploring the predicted novel quantum transport phenomenon at the surfaces/interfaces or magnetic domain walls of pyrochlore iridates.

Sm-doped CeO2-δ (Ce0.9Sm0.1O2-δ; SDC) thin films were prepared on Al2O3 (0001) substrates by radio frequency magnetron sputtering. The prepared thin films were preferentially grown along the [111] direction, with the spacing of the (111) plane (d111) expanded by 2.6% to compensate for a lattice mismatch against the substrate. The wet-annealed SDC thin film, with the reduced d111 value, exhibited surface protonic conduction in the low-temperature region below 100 °C. The O1s photoemission spectrum exhibits H2O and OH− peaks on the SDC surface. These results indicate the presence of physisorbed water layers and the generation of protons on the SDC (111) surface with oxygen vacancies. The protons generated on the SDC surface were conducted through a physisorbed water layer by the Grotthuss mechanism.

Electrostatically defined quantum dot arrays offer a compelling platform for quantum computation and simulation. However, tuning up such arrays with existing techniques becomes impractical when going beyond a handful of quantum dots. Here, we present a method for systematically adding quantum dots to an array one dot at a time, in such a way that the number of electrons on previously formed dots is unaffected. The method allows individual control of the number of electrons on each of the dots, as well as of the interdot tunnel rates. We use this technique to tune up a linear array of eight GaAs quantum dots such that they are occupied by one electron each. This new method overcomes a critical bottleneck in scaling up quantum-dot based qubit registers.