翻譯 | 李言
Science, 03 January 2020, Vol 367, Issue 6473
《科學》2020年1月3日，第367卷，6473期
物理學Physics
Absence of evidence for chiral Majorana modes in quantum anomalous Hall-superconductor devices
量子反常霍爾超導體中馬約拉納手性模式的存在缺乏證據
作者：Morteza Kayyalha, Di Xiao, Ruoxi Zhang, Jaeho Shin, Jue Jiang, Fei Wang, Yi-Fan Zhao, et al
鏈接：
https://science.sciencemag.org/content/367/6473/64
摘要：
量子反常霍爾絕緣體與s波超導體結合被預言可以產生馬約拉納手性模式。
近期的一項實驗將半整數量子化兩端法電導平臺解釋為馬約拉納手性模式在毫米級量子反常霍爾超導樣品上存在的證據。
然而，非馬約拉納機制也可以產生相似的結果，尤其是在無序樣品中。在此，我們研究了在量子反常霍爾絕緣體與超導體之間有易於控制且透明表面的相似複合樣品。
當樣品處於量子反常霍爾狀態且有序磁化時，總能觀測到半整數量子化兩端法電導平臺。我們的實驗為在量子反常霍爾超導樣品中觀測到的超導鄰近效應提供了全面的認識，並表明在具有高透明性的表面的樣品中，手性馬約阿納費米子不太可能導致半整數量子化電導平臺。
Abstract
A quantum anomalous Hall (QAH) insulator coupled to an s-wave superconductor is predicted to harbor chiral Majorana modes. A recent experiment interprets the half-quantized two-terminal conductance plateau as evidence for these modes in a millimeter-size QAH-niobium hybrid device. However, non-Majorana mechanisms can also generate similar signatures, especially in disordered samples. Here, we studied similar hybrid devices with a well-controlled and transparent interface between the superconductor and the QAH insulator. When the devices are in the QAH state with well-aligned magnetization, the two-terminal conductance is always half-quantized. Our experiment provides a comprehensive understanding of the superconducting proximity effect observed in QAH-superconductor hybrid devices and shows that the half-quantized conductance plateau is unlikely to be induced by chiral Majorana fermions in samples with a highly transparent interface.
Evidence for dispersing 1D Majorana channels in an iron-based superconductor
鐵基超導體中一維馬約拉納通道分散的證據
作者：Zhenyu Wang, Jorge Olivares Rodriguez, Lin Jiao, Sean Howard, Martin Graham, G. D. Gu, Taylor L. Hughes, Dirk K. Morr, Vidya Madhavan
鏈接：
https://science.sciencemag.org/content/367/6473/104
摘要：
在凝聚態物理學中將馬約拉納費米子作為準粒子激發來實現的可能性令人興奮。大多數研究都專注於馬約拉納束縛態；然而，馬約拉納態線性色散的傳播也被預測了。
在此，我們報告了在鐵基超導體FeSe0.45Te0.55中晶體疇壁（DWs）的掃描隧道光譜測量結果。通過移動了半個單位晶胞的晶格結構，我們定位了疇壁。
這些疇壁在超導能隙中具有有限的平坦態密度，這是一維線性色散模式的標誌。在不處於拓撲階段的相關超導體FeSe的疇壁中則沒有這種特徵。我們的綜合數據與在近似拓撲材料中π相移疇壁分散馬約拉納態的觀察結果一致。
Abstract
The possible realization of Majorana fermions as quasiparticle excitations in condensed-matter physics has created much excitement. Most studies have focused on Majorana bound states; however, propagating Majorana states with linear dispersion have also been predicted. Here, we report scanning scanning tunneling spectroscopic measurements of crystalline domain walls (DWs) in FeSe0.45Te0.55. We located DWs across which the lattice structure shifts by half a unit cell. These DWs have a finite, flat density of states inside the superconducting gap, which is a hallmark of linearly dispersing modes in one dimension. This signature is absent in DWs in the related superconductor, FeSe, which is not in the topological phase. Our combined data are consistent with the observation of dispersing Majorana states at a π-phase shift DW in a proximitized topological material.
A
single photonic cavity with two independent physical synthetic dimensions
作者：Avik Dutt, Qian Lin, Luqi Yuan, Momchil Minkov, Meng Xiao, Shanhui Fan
鏈接：
https://science.sciencemag.org/content/367/6473/59
摘要：
因其為實現有效的規範勢和拓撲物理學提供了一個多樣的平臺，合成維度的概念激發了從冷原子物理到光子學許多科學領域的興趣。
以前的實驗已經通過一個合成維度增強了現實空間維度。在這項研究中，我們將兩個獨立合成維度賦予單個環諧振器。
我們的系統由一個短時調製的環諧振器組成，該諧振器具有順時針和逆時針模式之間的空間耦合，為光子在環形內傳播創造了一個沿著頻率和贗自旋自由度的合成霍爾梯子晶格。
我們觀察到在合成維度上進行的多種物理現象，包括有效的自旋軌道耦合、磁場、自旋動量鎖定以及拓撲單向邊界流的特徵。我們的實驗證明，高維度物理效應可以通過利用多個同時合成維的概念進行研究。
Abstract
The concept of synthetic dimensions has generated interest in many branches of science, ranging from ultracold atomic physics to photonics, as it provides a versatile platform for realizing effective gauge potentials and topological physics. Previous experiments have augmented the real-space dimensionality by one additional physical synthetic dimension. In this study, we endow a single ring resonator with two independent physical synthetic dimensions. Our system consists of a temporally modulated ring resonator with spatial coupling between the clockwise and counterclockwise modes, creating a synthetic Hall ladder along the frequency and pseudospin degrees of freedom for photons propagating in the ring. We observe a wide variety of physics, including effective spin-orbit coupling, magnetic fields, spin-momentum locking, a Meissner-to-vortex phase transition, and signatures of topological chiral one-way edge currents, completely in synthetic dimensions. Our experiments demonstrate that higher-dimensional physics can be studied in simple systems by leveraging the concept of multiple simultaneous synthetic dimensions.
Topological mechanics of knots and tangles
結和纏結的拓撲力學
作者：Vishal P. Patil, Joseph D. Sandt, Mathias Kolle, Jörn Dunkel
鏈接：
https://science.sciencemag.org/content/367/6473/71
摘要：
結在生物和物理系統動力學中扮演著重要的角色，從DNA到湍流等離子體，以及攀登、編織、航海和外科手術。
儘管已經過了幾個世紀的研究，但人們對彈性結的拓撲結構和力學之間微妙的相互作用仍然知之甚少。
在此，我們結合光力學實驗理論和模擬來分析打結纖維在機械變形下的顏色變化。利用一個長距離鐵磁自旋系統類比，我們確定了簡單的拓撲計數規則來預測結和纏結的相對機械穩定性，這與常用的攀登和航行彎道的模擬和實驗相一致。
我們的結果突出了扭轉和扭曲在打結過程中的重要性，併為複雜纏結系統的控制提供了指導。
Abstract
Knots play a fundamental role in the dynamics of biological and physical systems, from DNA to turbulent plasmas, as well as in climbing, weaving, sailing, and surgery. Despite having been studied for centuries, the subtle interplay between topology and mechanics in elastic knots remains poorly understood. Here, we combined optomechanical experiments with theory and simulations to analyze knotted fibers that change their color under mechanical deformations. Exploiting an analogy with long-range ferromagnetic spin systems, we identified simple topological counting rules to predict the relative mechanical stability of knots and tangles, in agreement with simulations and experiments for commonly used climbing and sailing bends. Our results highlight the importance of twist and writhe in unknotting processes, providing guidance for the control of systems with complex entanglements.
On-chip integrated laser-driven particle accelerator
芯片上集成的激光驅動粒子加速器
作者：Neil V. Sapra, Ki Youl Yang, Dries Vercruysse, Kenneth J. Leedle, Dylan S. Black, R. Joel England, Logan Su, Rahul, et al
鏈接：
https://science.sciencemag.org/content/367/6473/79
摘要：
粒子加速器是科學和工業中必不可少的工具。然而，常規射頻加速器的尺寸和成本限制了該技術的實用性和可用範圍。
介質激光加速器（DLAs）通過使用可見或近紅外脈衝激光驅動加速器納米結構，提供了一種經濟有效的解決方案，使尺寸縮小了104倍。目前介質激光加速器的實現依賴於自由空間激光直接入射到加速結構上，限制了這項技術的可擴展性和可積性。
我們提出了一個波導集成DLA的實驗演示，該設計採用了光子反設計方法。通過比較測量的電子能譜和粒子跟蹤模擬，我們推斷在30微米以上的最大能量增益為0.915千電子伏特，對應於每米30.5兆電子伏特的加速度梯度。芯片加速器為完全集成的超大電子伏特級DLA提供了可能性。
Abstract
Particle accelerators represent an indispensable tool in science and industry. However, the size and cost of conventional radio-frequency accelerators limit the utility and reach of this technology. Dielectric laser accelerators (DLAs) provide a compact and cost-effective solution to this problem by driving accelerator nanostructures with visible or near-infrared pulsed lasers, resulting in a 104 reduction of scale. Current implementations of DLAs rely on free-space lasers directly incident on the accelerating structures, limiting the scalability and integrability of this technology. We present an experimental demonstration of a waveguide-integrated DLA that was designed using a photonic inverse-design approach. By comparing the measured electron energy spectra with particle-tracking simulations, we infer a maximum energy gain of 0.915 kilo–electron volts over 30 micrometers, corresponding to an acceleration gradient of 30.5 mega–electron volts per meter. On-chip acceleration provides the possibility for a completely integrated mega–electron volt-scale DLA.
材料科學
Materials Science
Oriented attachment induces fivefold twins by forming and decomposing high-energy grain boundaries定向附著通過形成和分解高能晶界產生五重孿晶
作者：Miao Song, Gang Zhou, Ning Lu, Jaewon Lee, Elias Nakouzi, Hao Wang, Dongsheng Li
鏈接：
https://science.sciencemag.org/content/367/6473/40
天然和合成納米粒子組成的五重孿晶疇具有獨特的性質。但人們對五重孿晶納米粒子的組成機制瞭解甚少。
我們使用原位高分辨透射電子顯微鏡結合分子動力學模擬來證明，通過重複定向連接~3納米金、鉑和鈀納米顆粒，可以產生五重孿晶。我們發現了兩種不同的形成五重孿晶納米粒子的機制，這些機制是由應變的積累和消除所驅動的。
伴隨著晶界的分解，形成了一個特殊的、淨應變為零的雙晶系。這些觀察使我們能夠對雙晶形成過程完成一個定量的描述。該機制為在廣泛中控制大量材料的雙晶結構和形態提供了指導。
Abstract
Natural and synthetic nanoparticles composed of fivefold twinned crystal domains have distinct properties. The formation mechanism of these fivefold twinned nanoparticles is poorly understood. We used in situ high-resolution transmission electron microscopy combined with molecular dynamics simulations to demonstrate that fivefold twinning occurs through repeated oriented attachment of ~3-nanometer gold, platinum, and palladium nanoparticles. We discovered two different mechanisms for forming fivefold twinned nanoparticles that are driven by the accumulation and elimination of strain. This was accompanied by decomposition of grain boundaries and the formation of a special class of twins with a net strain of zero. These observations allowed us to develop a quantitative picture of the twinning process. The mechanisms provide guidance for controlling twin structures and morphologies across a wide range of materials.
考古學Archaeology
Cooked starchy rhizomes in Africa 170 thousand years ago
科學家在非洲發現17萬年前的煮熟澱粉根狀莖
作者：Lyn Wadley, Lucinda Backwell, Francesco d’Errico, Christine Sievers
鏈接：
https://science.sciencemag.org/content/367/6473/87
摘要：
毫無疑問，古人會食用植物碳水化合物，但澱粉類地下芽植物卻很少能在考古學中被發現。
我們發現了至少17萬年前的早期人類利用開發地下芽植物的證據。通過古代和現代根莖的形態和解剖學比較，我們認為，來自南非邊境洞穴中的燒焦根狀莖為Hypoxis L. 屬。Hypoxis angustifolia Lam.（可能的分類群）大量生長在撒哈拉以南的非洲和阿拉伯也門相對水資源豐富的地區。
在那些地區以及更北的區域，Hypoxis的根莖可以為流動群體提供可靠和常見的碳水化合物來源。
Abstract
Plant carbohydrates were undoubtedly consumed in antiquity, yet starchy geophytes were seldom preserved archaeologically. We report evidence for geophyte exploitation by early humans from at least 170,000 years ago. Charred rhizomes from Border Cave, South Africa, were identified to the genus Hypoxis L. by comparing the morphology and anatomy of ancient and modern rhizomes. Hypoxis angustifolia Lam., the likely taxon, proliferates in relatively well-watered areas of sub-Saharan Africa and in Yemen, Arabia. In those areas and possibly farther north during moist periods, Hypoxis rhizomes would have provided reliable and familiar carbohydrate sources for mobile groups.