An ultrafast symmetry switch in a Weyl semimetal
▲ 作者：Edbert J. Sie、Clara M. Nyby、C. D. Pemmaraju、Su Ji Park、Xijie Wang、Aaron M. Lindenberg， et al
Topological quantum materials exhibit fascinating properties, with important applications for dissipationless electronics and fault-tolerant quantum computers. Manipulating the topological invariants in these materials would allow the development of topological switching applications analogous to switching of transistors. Lattice strain provides the most natural means of tuning these topological invariants because it directly modifies the electron–ion interactions and potentially alters the underlying crystalline symmetry on which the topological properties depend. However, conventional means of applying strain through heteroepitaxial lattice mismatch and dislocations are not extendable to controllable time-varying protocols, which are required in transistors. Integration into a functional device requires the ability to go beyond the robust, topologically protected properties of materials and to manipulate the topology at high speeds. Here we use crystallographic measurements by relativistic electron diffraction to demonstrate that terahertz light pulses can be used to induce terahertz-frequency interlayer shear strain with large strain amplitude in the Weyl semimetal WTe2, leading to a topologically distinct metastable phase. Separate nonlinear optical measurements indicate that this transition isassociated with a symmetry change to a centrosymmetric, topologically trivialphase. We further show that such shear strain provides an ultrafast, energy-efficient way of inducing robust, well separated Weyl points or of annihilating all Weyl points of opposite chirality. This work demonstrates possibilities for ultrafast manipulation of the topological properties of solids and for the development of a topological switch operating at terahertz frequencies.
Scalable energy-efficient magnetoelectric spin–orbit logic
▲ 作者：SasikanthManipatruni、Dmitri E.Nikonov、Chia-ChingLin、Tanay A.Gosavi，et al
文章中，研究者展示了一種制作新型邏輯器件和存儲器件的方法。相比于傳統 CMOS 集成的微處理器，新微處理器能效可以得到顯著增強。
Since the early 1980s, most electronics have relied on the use of complementary metal–oxide–semiconductor (CMOS) transistors. However, the principles of CMOS operation, involving a switchable semiconductor conductance controlled by aninsulating gate, have remained largely unchanged, even as transistors are miniaturized to sizes of 10 nanometres. We investigated what dimensionally scalable logic technology beyond CMOS could provide improvements in efficiency and performance for von Neumann architectures and enable growth in emerging computing such as artifical intelligence. Such a computing technology needs to allow progressive miniaturization, reduce switching energy, improve device interconnection and provide a complete logic and memory family. Here we propose a scalable spintronic logic device that operates via spin–orbit transduction (the coupling of an electron’s angular momentum with its linear momentum) combined with magnetoelectric switching. The device uses advanced quantum materials, especially correlated oxides and topological states of matter, for collective switching and detection. We describe progress in magnetoelectric switching and spin–orbit detection of state, and show that in comparison with CMOS technology our devicehas superior switching energy (by a factor of 10 to 30), lower switching voltage (by a factor of 5) and enhanced logic density (by a factor of 5). Inaddition, its non-volatility enables ultralow standby power, which is critical to modern computing. The properties of our device indicate that the proposed technology could enable the development of multi-generational computing.
Greenland melt drives continuous export of methane from the ice-sheet bed
▲ 作者：Guillaume Lamarche-Gagnon、Barbara Sherwood Lollar、Sandra Arndt, Peer Fietzek、Alexander D. Beaton,et al
Ice sheets are currently ignored in global methane budgets. Although ice sheets have been proposed to contain large reserves of methane that may contribute to a rise in atmospheric methane concentration if released during periods of rapid iceretreat, no data exist on the current methane footprint of ice sheets. Here we find that subglacially produced methane is rapidly driven to the ice margin by the efficient drainage system of a subglacial catchment of the Greenland ice sheet. We report the continuous export of methane-supersaturated waters (CH4(aq)) from the ice-sheet bed during the melt season. Pulses of high CH4(aq) concentration coincide with supraglacially forced subglacial flushing events, confirming a subglacial source and highlighting the influence of melt on methane export. Sustained methane fluxes over the melt season are indicative of subglacial methane reserves that exceed methane export, with an estimated 6.3 tonnes (discharge-weighted mean; range from 2.4 to 11 tonnes) of CH4(aq) transported laterally from the ice-sheet bed. Stable-isotope analyses reveal a microbial origin for methane, probably from a mixture of inorganic and ancient organic carbon buried beneath the ice. We show that subglacial hydrology is crucial for controlling methane fluxes from the ice sheet, with efficient drainage limiting the extent of methane oxidation to about 17 per cent of methane exported. Atmospheric evasion is the main methane sink once run off reaches the ice margin, with estimated diffusive fluxes (4.4 to 28 millimoles of CH4 per square metre per day) rivalling that of major world rivers. Overall, our results indicate that ice sheets overlie extensive, biologically active methanogenic wetlands and that high rates of methane export to the atmospherecan occur via efficient subglacial drainage pathways. Our findings suggest that such environments have been previously underappreciated and should be consideredin Earth’s methane budget.
Capture of nebular gases during Earth’s accretion is preserved in deep-mantle neon
▲ 作者：Curtis D. Williams、Sujoy Mukhopadhyay
Evidence for the capture of nebular gases by planetary interiors would place important constraints on models of planet formation. These constraints include accretion timescales, thermal evolution, volatile compositions and planetary redox states. Retention of nebular gases by planetary interiors also constrains the dynamics of outgassing and volatile loss associated with the assembly and ensuing evolution of terrestrial planets. But evidence for such gases in Earth’s interior remains controversia. The ratio of the two primordial neon isotopes, 20Ne/22Ne, is significantly different for the three potential sources of Earth’s volatiles: nebular gas, solar-wind-irradiated material and CI chondrites. Therefore, the 20Ne/22Ne ratio is a powerful tool for assessingthe source of volatiles in Earth’s interior. Here we present neon is otope measurements from deep mantle plumes that reveal 20Ne/22Ne ratios of up to 13.03 ± 0.04 (2 standard deviations). These ratiosare demonstrably higher than those for solar-wind-irradiated material and CI chondrites, requiring the presence of nebular neon in the deep mantle. Furthermore, we determine a 20Ne/22Ne ratio for the primordial plume mantle of 13.23 ± 0.22 (2 standard deviations), which is indistinguishable from the nebular ratio, providing robust evidence for a reservoir of nebular gas preserved in the deep mantle today. The acquisition of nebular gases requires planetary embryos to grow to sufficiently large mass before the dissipation of the protoplanetary disk. Our observations also indicate distinct 20Ne/22Ne ratios between deep mantle plumes and mid-ocean-ridge basalts, which is best explained by addition of a chondritic component to the shallower mantle during the main phase of Earth’s accretion and by subsequent recycling of seawater-derived neon in platetectonic processes.
Identifying the pathways required for coping behaviours associated with sustained pain
▲ 作者：Tianwen Huang、Shing-Hong Lin、Nathalie M. Malewicz、Qiufu Ma , et al
Animals and humans display two types of response to noxious stimuli. The first includes reflexive defensive responses that prevent or limit injury; a well-known example of these responses is the quick withdrawal of one’s hand upon touching a hot object. When the first-line response fails to prevent tissue damage (for example, a finger is burnt), the resulting pain invokes a second-line coping response—such as licking the injured area to soothe suffering. However, the underlying neural circuits that drive these two strings of behaviour remain poorly understood. Here we show in mice that spinal neurons marked by coexpression of TAC1Cre and LBX1Flpo drive coping responses associated with pain. Ablation of these spinal neurons led to the loss of both persistent licking and conditioned aversion evoked by stimuli (including skin pinching and burn injury) that—in humans—produce sustained pain, without affecting any of the reflexive defensive reactions that we tested. This selective in difference to sustained pain resembles the phenotype seen in humans with lesions of medial thalamic nuclei. Consistently, spinal TAC1-lineage neuronsare connected to medial thalamic nuclei by direct projections and via indirect routes through the superior lateral parabrachial nuclei. Furthermore, the anatomical and functional segregation observed at the spinal level also applies to primary sensory neurons. For example, in response to noxious mechanical stimuli, MRGPRD- and TRPV1-positive nociceptors are required to elicit reflexive and coping responses, respectively. Our study therefore reveals a fundamental subdivision within the cutaneous somatosensory system, and challenges the validity of using reflexive defensive responses to measure sustained pain.
Structure of Plasmodium falciparum Rh5–CyRPA–Ripr invasion complex
▲ 作者：WilsonWong、RickHuang、SebastienMenant、ZhihengYu、AlanF. Cowman, et al
Plasmodium falciparum causes the severe form of malaria that has high levels of mortality in humans. Blood-stage merozoites of P. falciparum invade erythrocytes, and this requires interactions between multiple ligands from the parasite and receptors in hosts. These interactions include the binding of the Rh5–CyRPA–Ripr complex with the erythrocyte receptor basigin1, which is an essential step for entry into human erythrocytes. Here we show thatthe Rh5–CyRPA–Ripr complex binds the erythrocyte cell line JK-1 significantlybetter than does Rh5 alone, and that this binding occurs through the insertion of Rh5 and Ripr into host membranes as a complex with high molecular weight. We report a cryo-electron microscopy structure of the Rh5–CyRPA–Ripr complex at subnanometre resolution, which reveals the organization of this essential invasion complex and the mode of interactions between members of the complex, and shows that CyRPA is a critical mediator of complex assembly. Our structure identifies blades 4–6 of the β-propeller of CyRPA as contact sites for Rh5 and Ripr. The limited contacts between Rh5–CyRPA and CyRPA–Ripr are consistent with the dissociation of Rh5 and Ripr from CyRPA for membrane insertion. A comparision of the crystal structure of Rh5–basigin with the cryo-electron microscopy structure of Rh5–CyRPA–Ripr suggests that Rh5 and Ripr are positioned parallel to the erythrocyte membrane before membrane insertion. This provides information on the function of this complex, and thereby provides insights into invasion by P.falciparum.
Loss of ADAR1 in tumours overcomesresistance to immune checkpoint blockade
▲ 作者：Jeffrey J. Ishizuka、Robert T. Manguso、W. Nicholas Haining, etal
Most patients with cancer either do not respond to immune checkpoint blockade or develop resistance to it, often because of acquired mutations that impair antigen presentation. Here we show that loss of function of the RNA-editing enzyme ADAR1 in tumour cells profoundly sensitizes tumours to immunotherapy and overcomes resistance to checkpoint blockade. In the absence of ADAR1, A-to-I editing of interferon-inducible RNA species is reduced, leading to double-stranded RNA ligand sensing by PKR and MDA5; this results in growth inhibition and tumour inflammation, respectively. Loss of ADAR1 overcomes resistance to PD-1 checkpoint blockade caused by inactivation of antigen presentation by tumour cells. Thus, effective anti-tumour immunity is constrained by inhibitory checkpoints such as ADAR1 that limit the sensing of innate ligands. The induction of sufficient inflammation in tumours that are sensitized to interferon can bypass the therapeutic requirement for CD8+ T cell recognition of cancer cells and may provide a general strategy to overcome immunotherapy resistance.