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1 High thermal conductivity in cubic boron arsenide crystals 2018-07-07             

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The high density of heat generated in power electronics and optoelectronic devices is a critical bottleneck in their application. New, high thermally-conducting materials are needed to effectively dissipate heat and thereby enable enhanced performance of power controls, solid-state lighting, communication, and security systems. We report our experimental discovery of high thermal conductivity of 1000 ± 90 W/m/K at room temperature in cubic boron arsenide (BAs) grown through modified chemical vapor transport technique. Such thermal conductivity is a factor of 3 higher than that of silicon carbide and surpassed only by diamond and the basal plane value of graphite. This work establishes BAs as the first realization of a new class of ultrahigh thermal conductivity materials predicted by a recent theory, and a potential revolutionary thermal management material.

2 Unusual high thermal conductivity in boron arsenide bulk crystals 2018-07-07             

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Conventional theory predicts that ultrahigh lattice thermal conductivity can only occur in crystals composed of strongly-bonded light elements, and that it is limited by anharmonic three-phonon processes. We report experimental evidence that is a departure from these long-held criteria. We measured a local room-temperature thermal conductivity exceeding 1000 W m–1 K–1 and an average bulk value reaching 900 W m–1 K–1 in bulk boron arsenide (BAs) crystals, where boron and arsenic are light and heavy elements, respectively. The high values are consistent with a proposal for phonon band engineering and can only be explained with higher order phonon processes. These findings yield new insight into the physics of heat conduction in solids and show BAs to be the first known semiconductor with ultrahigh thermal conductivity.

3 A liquid phase of synapsin and lipid vesicles 2018-07-07             

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Neurotransmitter containing synaptic vesicles (SVs) form tight clusters at synapses. These clusters act as a reservoir from which SVs are drawn for exocytosis during sustained activity. Several components associated with synaptic vesicles likely to help forming such clusters have been reported, including synapsin. Here we found that synapsin can form a distinct liquid phase in an aqueous environment. Other scaffolding proteins could co-assemble into this condensate, but were not necessary for its formation. Importantly, the synapsin phase could capture small lipid vesicles. The synapsin phase rapidly disassembled upon phosphorylation by calcium/calmodulin-dependent protein kinase II (CaMKII), mimicking the dispersion of synapsin 1 that occurs at presynaptic sites upon stimulation. Thus, principles of liquid-liquid phase separation may apply to the clustering of SVs at synapses.

4 Juno observations of spot structures and a split tail in Io-induced aurorae on Jupiter 2018-07-07             

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Jupiter’s aurorae are produced in its upper atmosphere when incoming high-energy electrons precipitate along the planet's magnetic field lines. A northern and a southern main auroral oval are visible, surrounded by small emission features associated with the Galilean moons. We present infrared observations, obtained with the Juno spacecraft, showing that in the case of Io, this emission exhibits a swirling pattern that is similar in appearance to a von Kármán vortex street. Well downstream of the main auroral spots the extended tail is split in two. Both of Ganymede’s footprints also appear as a pair of emission features, which may provide a remote measure of Ganymede’s magnetosphere. These features suggest that magnetohydrodynamic interaction between Jupiter and its moon is more complex than previously anticipated.

5 Experimental observation of high thermal conductivity in boron arsenide 2018-07-07             

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Improving thermal management of small scale devices requires developing materials with high thermal conductivities. The semiconductor boron arsenide (BAs) is an attractive target due to ab initio calculation indicating single crystals have an ultrahigh thermal conductivity. We synthesized BAs single crystals with undetectable defects, and measured a room temperature thermal conductivity of 1300 W/mK. Our spectroscopy study in conjunction with atomistic theory reveals that the unique band structure of BAs allows for very long phonon mean free paths and strong high-order anharmonicity through the four-phonon process. The single-crystal BAs has better thermal properties than other metals and semiconductors. Our study establishes BAs as a benchmark material for thermal management applications, and exemplifies the power of combing experiments and ab initio theory in new materials discovery.

6 DNA-induced liquid phase condensation of cGAS activates innate immune signaling 2018-07-07             

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The binding of DNA to cyclic GMP-AMP synthase (cGAS) leads to the production of the secondary messenger cyclic GMP-AMP (cGAMP), which activates innate immune responses. Here, we show that DNA binding to cGAS robustly induced the formation of liquidlike droplets in which cGAS was activated. The disordered and positively charged cGAS N terminus enhanced cGAS-DNA phase separation by increasing the valencies of DNA binding. Long DNA was more efficient in promoting cGAS liquid phase separation and cGAS enzyme activity than short DNA. Moreover, free zinc ion enhanced cGAS enzyme activity both in vitro and in cells by promoting cGAS-DNA phase separation. These results demonstrated that the DNA-induced phase transition of cGAS promotes cGAMP production and innate immune signaling.

7 Dirac electrons in a dodecagonal graphene quasicrystal 2018-06-30             

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Quantum states of quasiparticles in solids are dictated by symmetry. We experimentally demonstrate Dirac electrons in a two-dimensional quasicrystal without translational symmetry. A dodecagonal quasicrystalline order was realized by epitaxial growth of twisted bilayer graphene rotated exactly 30°. The graphene quasicrystal was grown up to a millimeter scale on an SiC (0001) surface while maintaining the single rotation angle over an entire sample and was successfully isolated from a substrate, demonstrating its structural and chemical stability under ambient conditions. Multiple Dirac cones replicated with the 12-fold rotational symmetry were observed in angle-resolved photoemission spectra, which revealed anomalous strong interlayer coupling with quasi-periodicity. Our study provides a way to explore physical properties of relativistic fermions with controllable quasicrystalline orders.

8 Emergence of coexisting ordered states in active matter systems 2018-06-30             

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Active systems can produce a far greater variety of ordered patterns than conventional equilibrium systems. Especially, transitions between disorder and either polar- or nematically-ordered phases have been predicted and observed in two-dimensional active systems. However, coexistence between phases of different types of order has not been reported. We demonstrate the emergence of dynamic coexistence of ordered states with fluctuating nematic and polar symmetry in an actomyosin motility assay. Combining experiments with agent-based simulations, we identify sufficiently weak interactions that lack a clear alignment symmetry as a prerequisite for coexistence. Thus, the symmetry of macroscopic order becomes an emergent and dynamic property of the active system. These results provide a pathway in which living systems can express different types of order by using identical building blocks.

9 Structural basis for the recognition of Sonic Hedgehog by human Patched1 2018-06-30             

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The Hedgehog (Hh) pathway involved in development and regeneration is activated by the extracellular binding of Hh to the membrane receptor Patched (Ptch). We report the cryo-EM structures of human Ptch1 alone and in complex with the N-terminal domain of human Sonic hedgehog (ShhN) at resolutions of 3.9 Å and 3.6 Å, respectively. Ptch1 comprises two interacting extracellular domains ECD1 and ECD2 and twelve transmembrane segments (TMs), with TMs 2-6 constituting the sterol-sensing domain (SSD). Two steroid-shaped densities are resolved in both structures, one enclosed by ECD1/2, and the other on the membrane-facing cavity of SSD. Structure-guided mutational analysis shows that interaction between ShhN and Ptch1 is steroid-dependent. The structure of a steroid binding-deficient Ptch1 mutant displays pronounced conformational rearrangements.

10 Cryo-EM structure of a mitochondrial calcium uniporter 2018-06-30             

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Calcium transport plays an important role in regulating mitochondrial physiology and pathophysiology. The mitochondrial calcium uniporter (MCU) is a calcium-selective ion channel that is the primary mediator for calcium uptake into the mitochondrial matrix. Here we present the cryo-electron microscopy structure of the full-length MCU from Neurospora crassa to an overall resolution of ~3.7 Å. Our structure reveals a tetrameric architecture, with the soluble and transmembrane domains adopting different symmetric arrangements within the channel. The conserved W-D---E-P-V-T-Y sequence motif of MCU pore forms a selectivity filter comprising two acidic rings separated by one helical turn along the central axis of the channel pore. The structure combined with mutagenesis gives insight into the basis of calcium recognition.

 
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