密度及氦孔隙率论文文献

狗，形状大小各一；颜色、品种都很丰富；而且生性灵活，是惹人喜爱的动物。 狗的五官也像人般端正，四肢更是健全。它们走起路来，摇摇摆摆的，快活得不得了。一般情况下，狗的长度约八分米，其重量也不会超过35公斤…… 狗毛的颜色五颜六色，品种更是多的不可计数。狗毛的颜色以黑、黄、白为主，还有的黑黄、黄白、黑白相间……据说，狗毛的颜色与其本性有联系的：黑的紧守家门，凶猛如虎；黄的貌似流浪，来去自如；白的纯真活泼，言听计从……根据多年统计，狗的品种约占世界动物总数的四分之一；其中，"沙皮狗"便是我们大沥的名牌狗种。约有500个家庭领养了此种狗…… 狗的生性灵活。世界上几乎没有一只狗是痴痴呆呆，不爱吃也不爱玩的。然而，狗的反应十分敏捷。当看到有老鼠、虫蚁的出现，狗便展示其自身本领，快速地捕捉盯紧的"猎物"；在狗的视线范围内，几乎没有一只"猎物"能够脱离它们的"五指山"。 狗的食量不大，但却很挑剔。狗每一天的食量大约是人类的三分之一，只吃鱼、肉或是其它的骨头，蔬菜、水果大多数都不喜欢吃…… 狗的智力在低等动物界中也是非比寻常的，是高等动物--"人类"的智力的四分之一。通过人工训练的狗能做许多高难度的动作：在低空中极其准确地接往从较高空中掉下来的微细物品；单脚踮地，双手握住人类的手，像和人类打交道般friend…… 狗--人见人爱，聪明伶俐的好动物

块体石料的孔隙体积，可将准备好的干燥试样，放在密封容器内，自试样中抽出空气，在一定真空度下使试样被液体所饱和。完全充满孔隙空间的液体体积等于试样的孔隙体积。碎石的孔隙率可用如前所述可将液态氦或其他介质充入孔隙中，以求得孔隙体积。孔隙率=孔隙体积/总体积孔隙率，指块状材料中孔隙体积与材料在自然状态下总体积的百分比。或散粒状材料堆积体积中,颗粒之间的空隙体积占总体积的比例。孔隙率与多孔介质固体颗粒的形状、结构和排列有关。

编译 | 李言

Nature , 17 March 2022, Volume 603 Issue 7901

《自然》 2022年3月17日，第603卷，7901期

化学 Chemistry

High-resolution laser resonances of antiprotonic helium in superfluid 4He

超流体4He中反质子氦的高分辨率激光共振

作者：Anna Sótér, Hossein Aghai-Khozani et al.

链接：

摘要

在此，我们展示了当一个具有反质子成分的奇异氦原子被嵌入到超流氦中时，它的可见波长谱线保持了亚千兆赫的线宽。当原子周围的液体转变为超流相时，反质子激光共振线宽突然减小。

这就解决了电子和反质子之间自旋-自旋相互作用所产生的超精细结构，相对光谱分辨率为106的两部分，尽管反质子氦位于正常物质原子的密集矩阵中。在激光激发期间，反质子原子的电子壳层保持一个大约40皮米的小半径。

这意味着，其他含有反核的氦原子以及带负电荷的介子和超子，包括在超流氦中形成的奇异夸克，可以通过具有高光谱分辨率的激光光谱学来研究，从而能够确定粒子的质量。清晰的光谱线可以探测到宇宙射线的反质子，也可以搜索到滞留在液氦目标中的反氘核。

Abstract

Here we show that when an exotic helium atom with a constituent antiproton is embedded into superfluid helium, its visible-wavelength spectral line retains a sub-gigahertz linewidth. An abrupt reduction in the linewidth of the antiprotonic laser resonance was observed when the liquid surrounding the atom transitioned into the superfluid phase. This resolved the hyperfine structure arising from the spin–spin interaction between the electron and antiproton with a relative spectral resolution of two parts in 106, even though the antiprotonic helium resided in a dense matrix of normal matter atoms. The electron shell of the antiprotonic atom retains a small radius of approximately 40 picometres during the laser excitation. This implies that other helium atoms containing antinuclei, as well as negatively charged mesons and hyperons that include strange quarks formed in superfluid helium, may be studied by laser spectroscopy with a high spectral resolution, enabling the determination of the particle masses. The sharp spectral lines may enable the detection of cosmic-ray antiprotons or searches for antideuterons that come to rest in liquid helium targets.

Flat-surface-assisted and self-regulated oxidation resistance of Cu(111)

平坦表面和自调节抗氧化性能的铜薄膜材料Cu(111)

作者：Su Jae Kim, Yong In Kim, Bipin Lamichhane et al.

链接：

摘要

氧化会降低铜的性能，而铜的性能对其使用至关重要，尤其是在半导体工业和电光应用中。在此，我们展示半永久抗氧化的铜薄膜制备，因为它们由平坦的表面组成，只有偶发单原子台阶。第一性原理计算证实，单原子台阶边缘与平坦表面一样不受氧的影响，并且一旦达到50%氧面心立方FCC表面位置覆盖率，氧原子表面吸附就被抑制。这些综合效应解释了超平摊铜表面特殊的抗氧化性。

Abstract

Oxidation can deteriorate the properties of copper that are critical for its use, particularly in the semiconductor industry and electro-optics applications. Here we report the fabrication of copper thin films that are semi-permanently oxidation resistant because they consist of flat surfaces with only occasional mono-atomic steps. First-principles calculations confirm that mono-atomic step edges are as impervious to oxygen as flat surfaces and that surface adsorption of O atoms is suppressed once an oxygen face-centred cubic (fcc) surface site coverage of 50% has been reached. These combined effects explain the exceptional oxidation resistance of ultraflat Cu surfaces.

物理学 Physics

Unbiasing fermionic quantum Monte Carlo with a quantum computer

无偏的费米子量子蒙特卡罗与量子计算机

作者：William J. Huggins, Bryan A. O’Gorman, Nicholas C. Rubin, David R. Reichman, Ryan Babbush & Joonho Lee

链接：

摘要

用约束来控制费米符号问题保证了量子蒙特卡罗计算（QMC）的效率，但代价是由于经典计算的灵活性有限而可能存在显著的偏差。

在此，我们提出了一种将QMC与量子计算相结合的方法来减少这种偏差。我们的方案在实验中实现了，我们使用了多达16个量子位来进行无偏约束QMC计算，这些计算是在多达120个轨道的化学系统上进行的。

这些实验代表了在量子计算机的帮助下进行的最大的化学模拟，同时实现了与最先进的经典方法竞争的精度，而不需要负担的错误缓解。

与流行的变分量子本征求解器相比，我们的量子-经典混合计算模型为电子结构问题提供了另一种实现实际量子优势的途径，而不需要非常精确的基态波函数的制备和测量。

Abstract

Controlling the fermionic sign problem with constraints ensures the efficiency of QMC at the expense of potentially significant biases owing to the limited flexibility of classical computation. Here we propose an approach that combines constrained QMC with quantum computation to reduce such biases. We implement our scheme experimentally using up to 16 qubits to unbias constrained QMC calculations performed on chemical systems with as many as 120 orbitals. These experiments represent the largest chemistry simulations performed with the help of quantum computers, while achieving accuracy that is competitive with state-of-the-art classical methods without burdensome error mitigation. Compared with the popular variational quantum eigensolver, our hybrid quantum-classical computational model offers an alternative path towards achieving a practical quantum advantage for the electronic structure problem without demanding exceedingly accurate preparation and measurement of the ground-state wavefunction.

HighlWind dispersal of battery-free wireless devices

风扩散的无电池无线设备

作者：Vikram Iyer, Hans Gaensbauer, Thomas L. Daniel & Shyamnath Gollakota

链接：

摘要

植物覆盖了地球陆地的很大一部分，尽管大多数物种都不能运动。为了运输繁殖后代，许多植物进化出了利用风传播种子的机制。

例如，蒲公英的种子有一种刚毛状的丝状结构，可以降低其末端速度，并在种子飘向地面时帮助其定位。受此启发，我们在此展示无电池无线的风扩散传感设备。我们的毫米级设备重量为30毫克，基于一个灵活的基板上，使用可编程的、现成的部件，为各种传感和计算应用提供可扩展性和灵活性。

该系统使用轻型太阳能电池和能量收集电路供电，电路对低光照和可变光照条件非常稳定，并有一个反向散射通信链路，可以实现数据传输。

为了实现太阳能收集所必需的大面积分散和垂直降落，我们开发了蒲公英启发的薄膜多孔结构，其终端速度为 米/秒，空气动力学稳定性，垂直降落的概率超过95%。户外环境实验的结果表明，这些设备可以在轻柔到正常的微风中移动50-100米。

最后，在自然系统中，个别种子形态的差异导致一些种子落得更近，而另一些则传播得更远。我们采用了类似的方法，并展示了我们如何调节结构的孔隙度和直径，以实现设备间的分散变化。

Abstract

Plants cover a large fraction of the Earth’s land mass despite most species having limited to no mobility. To transport their propagules, many plants have evolved mechanisms to disperse their seeds using the wind. A dandelion seed, for example, has a bristly filament structure that decreases its terminal velocity and helps orient the seed as it wafts to the ground. Inspired by this, we demonstrate wind dispersal of battery-free wireless sensing devices. Our millimetre-scale devices weigh 30 milligrams and are designed on a flexible substrate using programmable, off-the-shelf parts to enable scalability and flexibility for various sensing and computing applications. The system is powered using lightweight solar cells and an energy harvesting circuit that is robust to low and variable light conditions, and has a backscatter communication link that enables data transmission. To achieve the wide-area dispersal and upright landing that is necessary for solar power harvesting, we developed dandelion-inspired, thin-film porous structures that achieve a terminal velocity of    metres per second and aerodynamic stability with a probability of upright landing of over 95%. Our results in outdoor environments demonstrate that these devices can travel 50–100 metres in gentle to moderate breeze. Finally, in natural systems, variance in inpidual seed morphology causes some seeds to fall closer and others to travel farther. We adopt a similar approach and show how we can modulate the porosity and diameter of the structures to achieve dispersal variation across devices.

地球科学 Geoscience

Limited increases in savanna carbon stocks over decades of fire suppression

几十年没有火灾的稀树草原碳储量的增长有限

作者：Yong Zhou, Jenia Singh, John R. Butnor, Corli Coetsee, Peter B. Boucher, Madelon F. Case, Evan G. Hockridge, Andrew B. Davies & A. Carla Staver

链接：

摘要

在此，我们提供了一个首次直接评估一个60年没有火灾的非洲湿地大草原整个生态系统碳响应

我们发现灭火增加整个生态系统碳储存只有 12%（平均值 标准错误），即使树木覆盖增加了 ，相应的总收益为 Mg C ha 1 平均约为 Mg C ha 1 year 1 ，低于之前的设想。

经常被烧毁的稀树草原地下有大量的碳，特别是在生物量和深层土壤中。这些地下水库在造林或灭火计划中没有得到充分考虑，但这可能意味着热带稀树草原的十年封存潜力是微不足道的，特别是与随之而来的生物多样性和功能损失相比。

Abstract

Here we provide one of the first direct estimates of whole-ecosystem carbon response to more than 60 years of fire exclusion in a mesic African savanna. We found that fire suppression increased whole-ecosystem carbon storage by only    12% (mean   standard error), even though tree cover increased by    , corresponding to total gains of     Mg C ha 1 at an average of about     Mg C ha 1  year 1 , more than an order of magnitude lower than previously assumed. Frequently burned savannas had substantial belowground carbon, especially in biomass and deep soils. These belowground reservoirs are not fully considered in afforestation or fire-suppression schemes but may mean that the decadal sequestration potential of savannas is negligible, especially weighed against concomitant losses of biopersity and function.

化学 Chemistry

New land-use-change emissions indicate a declining CO2 airborne fraction

新的土地利用变化排放表明空气中二氧化碳的比例在下降

作者：Margreet J. E. van Marle, Dave van Wees, Richard A. Houghton, Robert D. Field, Jan Verbesselt & Guido. R. van der Werf

链接：

摘要

在此，我们使用可见性数据在关键的森林砍伐区构建了一个土地利用与土地覆盖变化（LULCC）排放数据集。这些可见性观测与火灾排放相关，而火灾排放又与LULCC相关。虽然这是间接的，但它提供了一个长期一致的LULCC排放数据集，表明自1958年开始CO2浓度测量以来，热带森林砍伐排放大幅增加。

到目前为止，这些排放被认为是相对稳定的，导致空气中的比例不断增加。然而，我们的结果表明，自1959年以来，大气中CO2含量下降了 decade 1。这表明，陆地-海洋联合汇的增长速度至少与人为排放的增长速度一样快。

Abstract

Here we construct a LULCC emissions dataset using visibility data in key deforestation zones. These visibility observations are a proxy for fire emissions, which are — in turn — related to LULCC. Although indirect, this provides a long-term consistent dataset of LULCC emissions, showing that tropical deforestation emissions increased substantially ( Pg C decade 1) since the start of CO2 concentration measurements in 1958. So far, these emissions were thought to be relatively stable, leading to an increasing airborne fraction. Our results, however, indicate that the CO2 airborne fraction has decreased by     decade 1 since 1959. This suggests that the combined land–ocean sink has been able to grow at least as fast as anthropogenic emissions.