《科学》（20211210出版）一周论文导读

编译 | 未玖

Science, 10 DECEMBER 2021, VOL 374, ISSUE 6573

《科学》2021年12月10日，第374卷，6573期

物理学Physics

Discovery of segmented Fermi surface induced by Cooper pair momentum

库珀对动量导致的分段费米面

作者：ZHEN ZHU, MICHAŁ PAPAJ, XIAO-ANG NIE, HAO-KE XU, YI-SHENG GU, XU YANG, ET AL.

链接：

https://www.science.org/doi/10.1126/science.abf1077

摘要

一个足够大的超导电流可通过有限库珀对动量引起的准粒子能量的多普勒频移，来关闭超导体中的能隙并产生无能隙准粒子。在这种无能隙超导状态下，零能量准粒子位于正常态费米面的某一段上，而剩余的费米面仍然有能隙。

在超导体二硒化铌（NbSe2）临近效应下，研究组利用准粒子干涉对碲化铋（Bi2Te3）薄膜磁场控制的费米面进行成像。较小的水平磁场诱导一个屏蔽超电流，导致Bi2Te3拓扑表面态的有限动量配对。

研究组确定了不同的干涉模式，证明了分段费米面的无能隙超导状态。该结果揭示了有限库珀对动量对准粒子谱的强烈影响。

Abstract

A sufficiently large supercurrent can close the energy gap in a superconductor and create gapless quasiparticles through the Doppler shift of quasiparticle energy caused by finite Cooper pair momentum. In this gapless superconducting state, zero-energy quasiparticles reside on a segment of the normal-state Fermi surface, whereas the remaining Fermi surface is still gapped. We use quasiparticle interference to image the field-controlled Fermi surface of bismuth telluride (Bi2Te3) thin films under proximity effect from the superconductor niobium diselenide (NbSe2). A small applied in-plane magnetic field induces a screening supercurrent, which leads to finite-momentum pairing on the topological surface states of Bi2Te3. We identify distinct interference patterns that indicate a gapless superconducting state with a segmented Fermi surface. Our results reveal the strong impact of finite Cooper pair momentum on the quasiparticle spectrum.

Time-of-flight 3D imaging through multimode optical fibers

多模光纤飞行时间3D成像

作者：DAAN STELLINGA, DAVID B. PHILLIPS, SIMON PETER MEKHAIL, ADAM SELYEM, SERGEY TURTAEV, TOMÁŠ ČIŽMÁR, ET AL.

链接：

https://www.science.org/doi/10.1126/science.abl3771

摘要

飞行时间三维（3D）成像的应用范围从工业检测覆盖到运动跟踪。通过测量激光脉冲的往返飞行时间来复原深度，通常使用直径几厘米的收集光学器件。

研究组演示了通过总孔径为几百微米的多模光纤进行近视频速率的三维成像，使用与脉冲源同步的波前整形实现像差校正，并以每秒23000点的速度扫描场景。

研究组以大约5赫兹的帧率，对直径50微米、约40厘米长的光纤末端几米以外的移动物体进行成像。该工作为超薄显微内窥镜提供了远场深度分辨能力，有望应用于临床和远程检查场景。

Abstract

Time-of-flight three-dimensional (3D) imaging has applications that range from industrial inspection to motion tracking. Depth is recovered by measuring the round-trip flight time of laser pulses, typically using collection optics of several centimeters in diameter. We demonstrate near–video-rate 3D imaging through multimode fibers with a total aperture of several hundred micrometers. We implement aberration correction using wavefront shaping synchronized with a pulsed source and scan the scene at ~23,000 points per second. We image moving objects several meters beyond the end of an ~40-centimeters-long fiber of 50-micrometer core diameter at frame rates of ~5 hertz. Our work grants far-field depth-resolving capabilities to ultrathin microendoscopes, which we expect to have applications to clinical and remote inspection scenarios.

人工智能Artificial Intelligence

Pushing the frontiers of density functionals by solving the fractional electron problem

解决分数电子问题，推动密度泛函进展

作者：JAMES KIRKPATRICK, BRENDAN MCMORROW, DAVID H. P. TURBAN, ALEXANDER L. GAUNT, JAMES S. SPENCER, ALEXANDER G. D. G. MATTHEWS, ET AL.

链接：

https://www.science.org/doi/10.1126/science.abj6511

摘要

密度泛函理论在量子层面上描述物质，但所有流行的近似理论都会因违反精确泛函的数学性质而产生系统误差。研究组通过在分子数据和带有分数电荷和自旋的虚拟系统上训练神经网络，克服了这一基本限制。

由此产生的泛函DM21（DeepMind 21）正确地描述了人工电荷离域和强关联的典型示例，在主基团原子和分子的全面基准测试中，其表现优于传统泛函。DM21精确地模拟了复杂系统，如氢链、带电DNA碱基对和双自由基过渡态。

对该领域而言更重要的是，由于该方法依赖于不断改进的数据和约束条件，因此它代表了一条通向精确通用泛函的可行途径。

Abstract

Density functional theory describes matter at the quantum level, but all popular approximations suffer from systematic errors that arise from the violation of mathematical properties of the exact functional. We overcame this fundamental limitation by training a neural network on molecular data and on fictitious systems with fractional charge and spin. The resulting functional, DM21 (DeepMind 21), correctly describes typical examples of artificial charge delocalization and strong correlation and performs better than traditional functionals on thorough benchmarks for main-group atoms and molecules. DM21 accurately models complex systems such as hydrogen chains, charged DNA base pairs, and diradical transition states. More crucially for the field, because our methodology relies on data and constraints, which are continually improving, it represents a viable pathway toward the exact universal functional.

材料科学Materials Science

Elemental electrical switch enabling phase segregation–free operation

单元素电子开关实现无相分离操作

作者：JIABIN SHEN, SHUJING JIA, NANNAN SHI, QINGQIN GE, TAMIHIRO GOTOH, SHILONG LV, ET AL.

链接：

https://www.science.org/doi/10.1126/science.abi6332

摘要

非易失性相变存储器已成功商业化，但若想进一步将密度缩放到10纳米以下，则存储单元和相关垂直堆叠的双端接入开关需要在成分和结构上更均质的材料。

选择开关大多为非晶硫系双向阈值开关（OTS），在非晶态下运行的非线性电流响应高于阈值电压。然而，它们目前被所使用的四价或更多价硫属化合物成分所引入的化学复杂性所影响。

研究组提出了一种单元素碲（Te）易失性开关，具有较大的驱动电流密度（ 11兆安/平方厘米）的，约103开/关电流比，开关速度快于20纳秒。低关断电流源于Te-电极界面存在大约0.95电子伏肖特基势垒，而纯Te的瞬态电压脉冲诱导的晶-液熔融转变导致高开断电流。

该研究发现的单元素电子开关可能有助于实现更密集的存储芯片。

Abstract

Nonvolatile phase-change memory has been successfully commercialized, but further density scaling below 10 nanometers requires compositionally and structurally homogeneous materials for both the memory cell and the associated vertically stacked two-terminal access switch. The selector switches are mostly amorphous-chalcogenide Ovonic threshold switches (OTSs), operating with a nonlinear current response above a threshold voltage in the amorphous state. However, they currently suffer from the chemical complexity introduced by the quaternary or even more perse chalcogenide compositions used. We present a single-element tellurium (Te) volatile switch with a large ( 11 megaamperes per square centimeter) drive current density, ~103 ON/OFF current ratio, and faster than 20 nanosecond switching speed. The low OFF current arises from the existence of a ~0.95–electron volt Schottky barrier at the Te–electrode interface, whereas a transient, voltage pulse–induced crystal-liquid melting transition of the pure Te leads to a high ON current. Our discovery of a single-element electrical switch may help realize denser memory chips.

Detection of graphene’s pergent orbital diamagnetism at the Dirac point

在狄拉克点探测石墨烯的轨道抗磁性

作者：J. VALLEJO BUSTAMANTE, N. J. WU, C. FERMON, M. PANNETIER-LECOEUR, T. WAKAMURA, K. WATANABE, ET AL.

链接：

https://www.science.org/doi/10.1126/science.abf9396

摘要

石墨烯的电子性质在过去十年间得到了广泛研究。然而，未掺杂石墨烯的奇异轨道磁性，即石墨烯电子波函数特征贝里相的基本特性，在单层中的测量一直颇具挑战性。

使用高灵敏度巨磁电阻（GMR）传感器，研究组测量了封装在氮化硼晶体之间的单层石墨烯的栅极电压依赖磁化强度。该信号在狄拉克点显示出一个抗磁峰，其磁场和温度依赖性与长期以来的理论预测一致。

该研究提供了一种新方法，用于监测贝里相位奇点，以及探索库仑相互作用、应变或莫尔势综合效应产生的相关态。

Abstract

The electronic properties of graphene have been intensively investigated over the past decade. However, the singular orbital magnetism of undoped graphene, a fundamental signature of the characteristic Berry phase of graphene’s electronic wave functions, has been challenging to measure in a single flake. Using a highly sensitive giant magnetoresistance (GMR) sensor, we have measured the gate voltage–dependent magnetization of a single graphene monolayer encapsulated between boron nitride crystals. The signal exhibits a diamagnetic peak at the Dirac point whose magnetic field and temperature dependences agree with long-standing theoretical predictions. Our measurements offer a means to monitor Berry phase singularities and explore correlated states generated by the combined effects of Coulomb interactions, strain, or moiré potentials.

地球科学Earth Science

Multidimensional tropical forest recovery

多维热带森林恢复

作者：LOURENS POORTER, DYLAN CRAVEN, CATARINA C. JAKOVAC, MASHA T. VAN DER SANDE, LUCY AMISSAH, FRANS BONGERS, ET AL.

链接：

https://www.science.org/doi/10.1126/science.abh3629

摘要

由于森林砍伐，热带森林迅速消失，但它们有望在废弃土地上自然再生。研究组分析了12个森林属性在次生演替过程中如何恢复，以及它们的恢复如何通过热带地区的77个次生林相互关联。

热带森林对低强度土地利用具有很强的恢复力；20年后，森林属性达到其原本成长值的78%（33-100%）。土壤（<10年）和植物功能（<25年）最快恢复到原本成长值的90%，结构和物种多样性（25-60年）恢复速度居中，生物量和物种组成恢复最慢（>120年）。

网络分析显示了三个独立的属性恢复集群，分别与结构、物种多样性和物种组成有关。研究结果表明，次生林应被视为一种低成本的自然解决途径，以恢复生态系统、缓解气候变化和保护生物多样性。

Abstract

Tropical forests disappear rapidly because of deforestation, yet they have the potential to regrow naturally on abandoned lands. We analyze how 12 forest attributes recover during secondary succession and how their recovery is interrelated using 77 sites across the tropics. Tropical forests are highly resilient to low-intensity land use; after 20 years, forest attributes attain 78% (33 to 100%) of their old-growth values. Recovery to 90% of old-growth values is fastest for soil (<1 decade) and plant functioning (<2.5 decades), intermediate for structure and species persity (2.5 to 6 decades), and slowest for biomass and species composition (>12 decades). Network analysis shows three independent clusters of attribute recovery, related to structure, species persity, and species composition. Secondary forests should be embraced as a low-cost, natural solution for ecosystem restoration, climate change mitigation, and biopersity conservation.