天文学
Astronomy
Observations of fast radio bursts at frequencies down to 400 megahertz
对频率低至400兆赫的快速射电暴的观测
▲ 作者：
CHIME/FRB协作组
▲链接：
https://www.nature.com/articles/s41586-018-0867-7
▲ 摘要：
快速射电暴（FRBs）是高度弥散且持续时间在毫秒级的射电闪光。它可能来自远离银河系的地方。
这一现象在接近1.4千兆赫的射电频率上被发现，并且迄今为止只在高达8千兆赫的情形下被观测到一次。尽管研究人员在低频率下进行了大量搜索，但尚未在700兆赫以下发现过FRBs。
我们在加拿大氢强度测绘实验（CHIME）中利用CHIME/FRB设备，在低至400兆赫的射电频率下对13次FRBs进行了探测。
它们是在望远镜试运行阶段被探测到的，此时望远镜的敏感度和视场尚未完成达到设计规格。多次事件中的辐射被发现低至400兆赫，这是该望远镜敏感范围内的最低射电频率。
▲ Abstract
Fast radio bursts (FRBs) are highly dispersed millisecond-duration radio flashes probably arriving from far outside the Milky Way. This phenomenon was discovered at radio frequencies near 1.4 gigahertz and so far has been observed in one case at as high as 8 gigahertz, but not at below 700 megahertz in spite of substantial searches at low frequencies. Here we report detections of 13 FRBs at radio frequencies as low as 400 megahertz, on the Canadian Hydrogen Intensity Mapping Experiment (CHIME) using the CHIME/FRB instrument. They were detected during a telescope pre-commissioning phase, when the sensitivity and field of view were not yet at design specifications. Emission in multiple events is seen down to 400 megahertz, the lowest radio frequency to which the telescope is sensitive.
A second source of repeating fast radio bursts
重复快速射电暴的另一个来源
▲作者：
The CHIME/FRB协作组
▲链接：
https://www.nature.com/articles/s41586-018-0864-x
▲ 摘要：
重复快速射电暴（FRB）来源——FRB 121102的发现，排除了涉及这个源头的灾难性事件的模型。
迄今为止，尽管有很多最新发现和后续研究，但尚未有其他重复FRB被探测到。这表明，“重复者”可能在FRB群体中非常罕见。
这里，我们报告了对来自FRB 180814.J0422+73的6次重复射电暴的探测结果。FRB 180814.J0422+73是加拿大氢强度测绘实验（CHIME）FRB项目在2018年7月和8月试运行阶段探测到的13次FRBs之一。
这些重复射电暴的来源一致，即来自天空中的单一位置。同时，它们的色散量也相同，约为每立方厘米189秒差距。
▲ Abstract
The discovery of a repeating fast radio burst (FRB) source, FRB 121102, eliminated models involving cataclysmic events for this source. No other repeating FRB has hitherto been detected despite many recent discoveries and follow-ups, suggesting that repeaters may be rare in the FRB population. Here we report the detection of six repeat bursts from FRB 180814.J0422+73, one of the 13 FRBs detected6 by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB project7 during its pre-commissioning phase in July and August 2018. These repeat bursts are consistent with its origin from a single position on the sky, with the same dispersion measure, about 189 parsecs per cubic centimetre.
物理学
Physics
Near-field photonic cooling through control of the chemical potential of photons
通过控制光子化学势的近场光子冷却
▲作者：
Linxiao Zhu, Anthony Fiorino, Dakotah Thompson, et al
▲链接：
https://www.nature.com/articles/s41586-019-0918-8
▲ 摘要：
物质的光子冷却使接触未被探索过的物质状态比如玻色爱因斯坦凝聚体，以及寻找固体制冷的新方法成为可能。
这些光子冷却方法的关键是利用来自激光的低熵相干辐射。这使得冷却过程在热力学上是切实可行的。
我们利用定制制造的纳米级量热装置和光电二极管，实验演示了没有激光情形下的光子冷却。
我们证实，当它们位于彼此的近场中时，即量热装置的平面表面和反向偏置的光电二极管之间的真空间隙大小被缩小至十几纳米时，由于光子在施加的反向偏压下化学势的变化，量热装置的固态冷却可通过光子隧道效应和来自发光二极管的光子发射抑制相结合而得以实现。其中，光子隧道效应增强了光子在纳米尺度孔隙间的传送。
▲ Abstract
Photonic cooling of matter has enabled both access to unexplored states of matter, such as Bose–Einstein condensates, and novel approaches to solid-state refrigeration. Critical to these photonic cooling approaches is the use of low-entropy coherent radiation from lasers, which makes the cooling process thermodynamically feasible. Here we report an experimental demonstration of photonic cooling without laser light using a custom-fabricated nanocalorimetric device and a photodiode. We show that when they are in each other’s near-field—that is, when the size of the vacuum gap between the planar surfaces of the calorimetric device and a reverse-biased photodiode is reduced to tens of nanometres—solid-state cooling of the calorimetric device can be accomplished via a combination of photon tunnelling, which enhances the transport of photons across nanoscale gaps, and suppression of photon emission from the photodiode due to a change in the chemical potential of the photons under an applied reverse bias.
Mapping orbital changes upon electron transfer with tunnelling microscopy on insulators
利用隧道显微术描绘绝缘体电子转移的轨道变化
▲作者：
Laerte L. Patera, Fabian Queck, Philipp Scheuerer & Jascha Repp
▲链接：
https://www.nature.com/articles/s41586-019-0910-3
▲ 摘要：
电子转移在包括光合作用、燃烧和腐蚀在内的很多化学反应中起到了关键作用。
不过，尽管氧化还原状态转换改变了所涉及的分子电子结构，但在单分子水平描绘这些变化具有挑战性。
这里，我们证实，当使控制导电原子力显微镜尖端和基板之间的电子隧道效应的电压脉冲和显微镜尖端的振荡同步时,我们能在非导电性基板上开展隧道效应试验并因此将孤立分子的轨道结构描述为其氧化还原状态的函数。
▲ Abstract
Electron transfer plays a crucial part in many chemical reactions, including photosynthesis, combustion and corrosion. But even though redox-state transitions change the electronic structure of the molecules involved, mapping these changes at the single-molecule level is challenging. Here we show that when synchronizing voltage pulses that steer electron tunnelling between a conductive atomic force microscope tip and a substrate with the oscillation of the tip, we can perform tunnelling experiments on non-conductive substrates and thereby map the orbital structure of isolated molecules as a function of their redox state.
生物学
Biology
Interleukin-22 protects intestinal stem cells against genotoxic stress
白介素-22保护肠干细胞对抗基因毒性压力
▲作者：
Konrad Gronke, Pedro P. Hernández, Jakob Zimmermann, et al
▲链接：
https://www.nature.com/articles/s41586-019-0899-7
▲ 摘要：
环境遗传毒性因素对将宿主生物体同环境分开的屏障表面的上皮细胞的基因组完整性带来了挑战。
上皮干细胞的基因组完整性由进化上保守的分子响应通路——DNA损伤应答（DDR）来维持。
这里，我们证实，3型先天淋巴细胞（ILC3）和γδ T细胞产生的细胞因子白介素-22（IL-22)是DDR机制在肠上皮干细胞中的重要调控者。
利用使结肠上皮干细胞中的IL-22受体分散激活成为可能的新小鼠模型，我们证实，IL-22是DNA损伤后DDR有效启动所必需的。
▲ Abstract
Environmental genotoxic factors pose a challenge to the genomic integrity of epithelial cells at barrier surfaces that separate host organisms from the environment. Genome integrity in epithelial stem cells is maintained by an evolutionarily conserved cellular response pathway, the DNA damage response (DDR). Here we show that the cytokine interleukin-22 (IL-22), produced by group 3 innate lymphoid cells (ILC3) and γδ T cells, is an important regulator of the DDR machinery in intestinal epithelial stem cells. Using a new mouse model that enables sporadic inactivation of the IL-22 receptor in colon epithelial stem cells, we demonstrate that IL-22 is required for effective initiation of the DDR following DNA damage.
Discrete attractor dynamics underlies persistent activity in the frontal cortex
离散吸引子动力学是额皮质持续行为的基础
▲作者：
Hidehiko K. Inagaki, Lorenzo Fontolan, Sandro Romani & Karel Svoboda
▲链接：
https://www.nature.com/articles/s41586-019-0919-7
▲ 摘要：
短期记忆将在时间上分开的事件联系起来，比如过去的感觉和未来的行动。这种记忆同包括选择性持续行为在内、可维持在几秒钟内的缓慢神经动力学存在关联。
在一项要求短期记忆的延迟响应任务中，小鼠前外侧运动皮质（ALM）的神经元表现出指示未来行动的持续行为。为确定这一持续行为背后的原理，我们将细胞内和细胞外电生理学同光发生扰动和网络建模相结合。
我们证实，在延迟期，ALM神经元的活动朝对特定运动指令作出响应的离散终点移动。我们的结果证实，离散的吸引子动力学是同动作计划相关的短期记忆的基础。
▲ Abstract
Short-term memories link events separated in time, such as past sensation and future actions. Short-term memories are correlated with slow neural dynamics, including selective persistent activity, which can be maintained over seconds. In a delayed response task that requires short-term memory, neurons in the mouse anterior lateral motor cortex (ALM) show persistent activity that instructs future actions. To determine the principles that underlie this persistent activity, here we combined intracellular and extracellular electrophysiology with optogenetic perturbations and network modelling. We show that during the delay epoch, the activity of ALM neurons moved towards discrete end points that correspond to specific movement directions. Our results show that discrete attractor dynamics underlie short-term memory related to motor planning.
（宗华）
