《自然》（20190829出版）一周论文导读

翻译 | 冯维维

Nature, 29 August 2019, Volume 572 Issue 7771

《自然》 2019年8月29日 第572卷7771期

物理学 Physics

Non-line-of-sight imaging using phasor-field virtual wave optics

使用相量场虚拟波光学非视线成像

▲ 作者：Xiaochun Liu, Ibón Guillén, Marco La Manna, Ji Hyun Nam, Andreas Velten, etc.

▲ 链接：

https://www.nature.com/articles/s41586-019-1461-3

▲ 摘要：

论文通过引入虚波场，即相量场，将非视距成像问题转化为衍射波传播问题。

在传统的视线成像系统中，利用数学算子对波的传播进行建模，可以从原始的飞行时间数据中对非视线场景进行成像。

这一方法产生了一类新的成像算法，模拟了视线摄像机的功能。

为了演示新技术，作者推导了三种成像算法，它们是根据三种不同的视线系统建模的。这些算法依赖于求解波衍射积分，即瑞利—索末菲衍射积分。

快速求解瑞利—索末菲衍射及其近似解是一种有效的方法。

作者演示了具有强多重散射和环境光、任意材料、大深度范围和遮挡的复杂场景的非视线成像。

该方法在不显式反转光传输模型的情况下处理这些具有挑战性的情况。

作者认为该方法有助于开发非视线成像的潜力，并促进相关应用的发展，而不局限于实验室条件。

▲ Abstract

Here we show that the problem of non-line-of-sight imaging can also be formulated as one of diffractive wave propagation, by introducing a virtual wave field that we term the phasor field. Non-line-of-sight scenes can be imaged from raw time-of-flight data by applying the mathematical operators that model wave propagation in a conventional line-of-sight imaging system. Our method yields a new class of imaging algorithms that mimic the capabilities of line-of-sight cameras. To demonstrate our technique, we derive three imaging algorithms, modelled after three different line-of-sight systems. These algorithms rely on solving a wave diffraction integral, namely the Rayleigh–Sommerfeld diffraction integral. Fast solutions to Rayleigh–Sommerfeld diffraction and its approximations are readily available, benefiting our method. We demonstrate non-line-of-sight imaging of complex scenes with strong multiple scattering and ambient light, arbitrary materials, large depth range and occlusions. Our method handles these challenging cases without explicitly inverting a light-transport model. We believe that our approach will help to unlock the potential of non-line-of-sight imaging and promote the development of relevant applications not restricted to laboratory conditions.

Thermal conductance of single-molecule junctions

单分子结的导热系数

▲ 作者：Longji Cui, Sunghoon Hur, Jan C. Klöckner, Wonho Jeong, Edgar Meyhofer, etc.

▲ 链接：

https://www.nature.com/articles/s41586-019-1420-z

▲ 摘要：

作者展示了之前开发的用于研究单金属原子结热导率的皮瓦分辨率扫描探针，如果结合时间平均测量方案来提高信噪比，也可以用于量化更低的单分子连接的热导率。

作者在含有2到10个碳原子的典型金—烷二硫醇—金结上进行的实验证实，导热系数是与分子长度无关的第一近似值，这与详细的从头开始模拟一致。

作者预计该方法将使在许多其他一维系统中，如短分子和聚合物链的热输运的系统探索成为可能，对于这些一维系统，热导的计算预测在实验上仍然无法实现。

▲ Abstract

Here we show that picowatt-resolution scanning probes previously developed to study the thermal conductance of single-metal-atom junctions, when used in conjunction with a time-averaging measurement scheme to increase the signal-to-noise ratio, also allow quantification of the much lower thermal conductance of single-molecule junctions.

Our experiments on prototypical Au–alkanedithiol–Au junctions containing two to ten carbon atoms confirm that thermal conductance is to a first approximation independent of molecular length, consistent with detailed ab initio simulations. We anticipate that our approach will enable systematic exploration of thermal transport in many other one-dimensional systems, such as short molecules and polymer chains, for which computational predictions of thermal conductance have remained experimentally inaccessible.

地球物理 Geophysics

Increased shear in the North Atlantic upper-level jet stream over the past four decades

过去四十年北大西洋高空急流的切变增加

▲ 作者：Simon H. Lee, Paul D. Williams & Thomas H. A. Frame

▲ 链接：

https://www.nature.com/articles/s41586-019-1465-z

▲ 摘要：

作者研究表明，尽管自1979年观测卫星时代开始以来，北大西洋极地急流的纬向风速（250百帕斯卡）一直没有改变，但根据三个不同的再分析数据集，垂直剪切增加了15%（其范围为11%至17%）。

研究进一步表明，这一趋势是由于上层子午温度梯度增强引起的热风响应。研究结果表明，气候变化对北大西洋急流的影响可能比之前认为的更大。

垂直切变的增加与气候变化带来的由切变驱动的晴空湍流的增强是一致的，这将会给飞机造成一个湍流更多的飞行环境，影响繁忙的跨大西洋航空走廊的飞行。

这项研究的结论是，气候变化和变率对高空急流的影响部分被传统的风速而不是风切变所掩盖。

▲ Abstract

Here we show that, although the zonal wind speed in the North Atlantic polar jet stream at 250 hectopascals has not changed since the start of the observational satellite era in 1979, the vertical shear has increased by 15 per cent (with a range of 11–17 per cent) according to three different reanalysis datasets. We further show that this trend is attributable to the thermal wind response to the enhanced upper-level meridional temperature gradient. Our results indicate that climate change may be having a larger impact on the North Atlantic jet stream than previously thought. The increased vertical shear is consistent with the intensification of shear-driven clear-air turbulence expected from climate change, which will affect aviation in the busy transatlantic flight corridor by creating a more turbulent flying environment for aircraft. We conclude that the effects of climate change and variability on the upper-level jet stream are being partly obscured by the traditional focus on wind speed rather than wind shear.

Seismic velocities of CaSiO3 perovskite can explain LLSVPs in Earth’s lower mantle

钙钛矿的地震速度可以解释地球下地幔大低剪切速度区

▲ 作者：A. R. Thomson, W. A. Crichton, J. M. R. Muir, D. P. Dobson, S. A. Hunt, etc.

▲ 链接：

https://www.nature.com/articles/s41586-019-1483-x

▲ 摘要：

作者同时测量了钙钛矿样品的晶体结构和剪切波速和压缩波速，并对该材料的绝热体和剪切模量提供了直接的约束。

他们观察到，在较高的温度下，钛与钙钛矿的结合稳定了正方结构，材料的剪切模量大大低于计算或热力学数据预测的值。

结合文献资料和外推，这一研究结果表明，俯冲洋壳在整个下地幔中表现为低地震速度异常。

特别是，作者发现大的低剪切速度区（LLSVPs）与再生海洋地壳的中度富集是一致的，而中地幔的不连续性可以用含钛钙钛矿中的四方立方相变来解释。

▲ Abstract

Here we simultaneously measure the crystal structure and the shear-wave and compressional-wave velocities of samples of CaSiO3 perovskite, and provide direct constraints on the adiabatic bulk and shear moduli of this material. We observe that incorporation of titanium into CaSiO3 perovskite stabilizes the tetragonal structure at higher temperatures, and that the material’s shear modulus is substantially lower than is predicted by computations or thermodynamic datasets. When combined with literature data and extrapolated, our results suggest that subducted oceanic crust will be visible as low-seismic-velocity anomalies throughout the lower mantle. In particular, we show that large low-shear-velocity provinces (LLSVPs) are consistent with moderate enrichment of recycled oceanic crust, and mid-mantle discontinuities can be explained by a tetragonal–cubic phase transition in Ti-bearing CaSiO3 perovskite.

化学 Chemistry

Efficient molecular doping of polymeric semiconductors driven by anion exchange

由阴离子交换驱动的聚合物半导体的有效分子掺杂

▲ 作者：Yu Yamashita, Junto Tsurumi, Masahiro Ohno, Shun Watanabe, etc.

▲ 链接：

https://www.nature.com/articles/s41586-019-1504-9

▲ 摘要：

作者描述了一个不同的过程——“阴离子交换”可能会提高掺杂水平。

该过程是由离子液体溶剂介导的，可以描述为传统的小P型掺杂阴离子与离子液体提供的第二个阴离子的有效瞬时交换。

在传统的二进制亲水系统引入优化离子盐（离子液体溶剂），可以克服马库斯理论描述的氧化还原电势的限制，并允许阴离子交换效率接近100%。

因此，可以达到几乎每个单体单元一个电荷的掺杂水平。

这种掺杂水平的提高、稳定性的提高和优异的输运性能表明，离子交换掺杂几乎可以无限选择离子盐，可以成为实现先进分子电子学的有力工具。

▲ Abstract

Here we describe a different process—which we term ‘anion exchange’—that might offer improved doping levels. This process is mediated by an ionic liquid solvent and can be pictured as the effective instantaneous exchange of a conventional small p-type dopant anion with a second anion provided by an ionic liquid. The introduction of optimized ionic salt (the ionic liquid solvent) into a conventional binary donor–acceptor system can overcome the redox potential limitations described by Marcus theory, and allows an anion-exchange efficiency of nearly 100 per cent. As a result, doping levels of up to almost one charge per monomer unit can be achieved. This demonstration of increased doping levels, increased stability and excellent transport properties shows that anion-exchange doping, which can use an almost infinite selection of ionic salts, could be a powerful tool for the realization of advanced molecular electronics.

材料学 Material science

Modern microprocessor built from complementary carbon nanotube transistors

互补的碳纳米管晶体管构成的现代微处理器

▲ 作者：Gage Hills, Christian Lau, Andrew Wright, Samuel Fuller, Max M. Shulaker, etc.

▲ 链接：

https://www.nature.com/articles/s41586-019-1493-8

▲ 摘要：

基于碳纳米管场效应晶体管（CNFET）的数字电路有望带来巨大的节能效益，但由于无法完美地控制碳纳米管固有的纳米级缺陷和可变性，阻碍了超大规模集成系统的实现。

作者克服了这些挑战，展示了完全建立在CNFET之上的超越硅的微处理器。

这款16位微处理器基于RISC-V指令集，在16位数据和地址上运行标准的32位指令，包括超过14000个互补的金属氧化物半导体CNFET，并使用行业标准的设计流程和过程进行设计和制造。

作者提出了一种碳纳米管的制造方法，一套综合处理和设计技术，以克服整个晶圆片基板宏观尺度上的纳米级缺陷。

这项工作在实验上验证了一种很有前途的超硅电子系统。

▲ Abstract

Carbon nanotube field-effect transistor (CNFET)-based digital circuits promise substantial energy-efficiency benefits, but the inability to perfectly control intrinsic nanoscale defects and variability in carbon nanotubes has precluded the realization of very-large-scale integrated systems. Here we overcome these challenges to demonstrate a beyond-silicon microprocessor built entirely from CNFETs. This 16-bit microprocessor is based on the RISC-V instruction set, runs standard 32-bit instructions on 16-bit data and addresses, comprises more than 14,000 complementary metal–oxide–semiconductor CNFETs and is designed and fabricated using industry-standard design flows and processes. We propose a manufacturing methodology for carbon nanotubes, a set of combined processing and design techniques for overcoming nanoscale imperfections at macroscopic scales across full wafer substrates. This work experimentally validates a promising path towards practical beyond-silicon electronic systems.

生态学 Ecology

Climate change and overfishing increase neurotoxicant in marine predators

气候变化和过度捕捞增加了海洋食肉动物的神经毒性

▲ 作者：Amina T. Schartup, Colin P. Thackray, Alex Hanke, Elsie M. Sunderland, etc.

▲ 链接：

https://www.nature.com/articles/s41586-019-1468-9

▲ 摘要：

作者使用了30多年的数据和生态系统模型表明，在20世纪70年代至21世纪头10年期间，由于过度捕捞引发的饮食变化，大西洋鳕鱼中的甲基汞浓度增加了高达23%。

他们的模型还预测，大西洋蓝鳍金枪鱼组织中甲基汞浓度预计将增加56%，这是由于海水温度在1969年的最低点和最近的峰值水平之间升高，这与2017年的观测结果一致。

这一估计的组织甲基汞的增加超过了在1990年代末和2000年代由于海水甲基汞浓度下降而实现的22%的模型减少。

最近报告的全球人为汞排放的平稳表明，海洋变暖和渔业管理计划将是未来海洋捕食者汞浓度的主要驱动因素。

▲ Abstract

Here we use more than 30 years of data and ecosystem modelling to show that MeHg concentrations in Atlantic cod (Gadus morhua) increased by up to 23% between the 1970s and 2000s as a result of dietary shifts initiated by overfishing. Our model also predicts an estimated 56% increase in tissue MeHg concentrations in Atlantic bluefin tuna (Thunnus thynnus) due to increases in seawater temperature between a low point in 1969 and recent peak levels—which is consistent with 2017 observations. This estimated increase in tissue MeHg exceeds the modelled 22% reduction that was achieved in the late 1990s and 2000s as a result of decreased seawater MeHg concentrations. The recently reported plateau in global anthropogenic Hg emissions suggests that ocean warming and fisheries management programmes will be major drivers of future MeHg concentrations in marine predators.

合作事宜：market@stimes.cn

投稿事宜：tougao@stimes.cn