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  《npj 计算材料学》是在线出版、完全开放获取的国际学术期刊。发表结合计算模拟与设计的材料学一流的研究成果。本刊由中国科学院上海硅酸盐研究所与英国自然出版集团(Nature Publishing Group,NPG)以伙伴关系合作出版。
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Unfolding the complexity of phonon quasi-particle physicsin disordered materials(揭示无序材料中声子准粒子物理的复杂性) 
Sai Mu, Raina J. Olsen, Biswanath Dutta, Lucas Lindsay, German D. Samolyuk, Tom Berlijn, Eliot D. Specht, KeJin, Hongbin Bei, Tilmann Hickel , Bennet C. Larson and George M. Stocks
npj Computational Materials 6:4(2020)
doi:s41524-020-0271-3
Published online:17 January 2020

Abstract| Full Text | PDF OPEN

摘要:准粒子的概念构成了我们对新现微观现象认识的理论基础,而这些新现现象与量子力学多体相互作用有关。然而,无序材料领域的准粒子理论遇到了难以逾越的困难,从而凸显了平均场方法的优势。本研究报告了为力常数显性无序的等原子合金(NiCoNiFeAgPdNiFeCo)采用第一原理声子计算以及非弹性X射线和中子散射测量的结果,并与50年前的重要假设对比,此假设即为描述非对角无序的所有平均场准粒子解的哈密顿量。我们的结果揭示,存在一个巨大的、迄今未被认识的、局部化学环境对物种--分辨的力学常数无序分布的影响,成为影响声子散射的主要因素。这一发现不但找到了分析中的关键问题,对其他元素激发态(如磁合金中的磁子和斯格明子)的研究将产生广泛影响,而且为超低热导率材料的设计提供了重要工具 

Abstract:The concept of quasi-particles forms the theoretical basis of our microscopic understanding of emergent phenomena associated with quantum-mechanical many-body interactions. However, the quasi-particle theory in disordered materials has proven difficult, resulting in the predominance of mean-field solutions. Here, we report first-principles phonon calculations and inelastic X-ray and neutron-scattering measurements on equiatomic alloys (NiCo, NiFe, AgPd, and NiFeCo) with force-constant dominant disorder— confronting a key 50-year-old assumption in the Hamiltonian of all mean-field quasi-particle solutions for off-diagonal disorder. Our results have revealed the presence of a large, and heretofore unrecognized, impact of local chemical environments on the distribution of the species-pair-resolved force-constant disorder that can dominate phonon scattering. This discovery not only identifies a critical analysis issue that has broadimplications for other elementary excitations, such as magnons and skyrmions in magnetic alloys, but also provides an important tool for the design of materials with ultralow thermal conductivities.

Editorial Summary

Complexity of phonon quasi-particle physics: theoretical and experimental results声子准粒子物理的复杂性:理论计算与实验验证

该研究揭示,存在一个巨大的、迄今未被认识的、局部化学环境对物种--分辨的力学常数无序分布的影响,成为影响声子散射的主要因素。来自美国橡树林国家实验室的.George M. Stocks教授,通过第一原理理论结合实验测量研究(对浓缩的、带有很强力常数紊乱却几无质量紊乱的无序合金NiCoNiFeAgPdNiFeCo所作的声子准粒子物理学(色散和线宽)研究),首次解决了这一问题。 值得注意的是,在从头算的超胞声子展开(SPU)模拟,及其与ICPA和实验测量的比较中,考虑到每个单独的AABBAB型物种对,力常数的变化远远超过了通常的全局平均力常数波动。此外,他们的研究结果表明,增强型磁阻的根源,是围绕着单个AABBAB型物种对的局部化学环境所固有的随机变化。因此,以往长期使用的近似方法(用无序材料的准粒子平均-Er场理论的哈密顿量的全局平均值,来代替单个物种对的力常数波动),现在必须重新考虑。

The presence of a large, and heretofore unrecognized, impact of local chemical environments on the distribution of the species-pair-resolved force-constant disorder, which can dominate phonon scattering, is revealed. A team led by Prof..George M. Stocks from the Oak Ridge National Laboratory, USA, addressed this issue for the first time through a combined first-principles theory and experimental measurement investigation of the phonon quasi-particle physics (dispersions and linewidths) of concentrated disordered alloys, NiCo, NiFe, AgPd, and NiFeCo, with strong force-constant disorder but minimal mass disorder. Remarkably, from ab initio supercell phonon-unfolding (SPU) simulations and their comparison with ICPA and experimental measurements, that force-constant fluctuations considering each individual AA, BB, and AB-type species pairs far surpass that of the usual global average force-constant fluctuations.In addition, they have shown that the source of the enhanced magnetoresistance is the inherent random change of single AA, BB, and AB-type pairs. to the local chemical environment. Therefore,, the long-standing approximation of replacing individual-pair force-constant fluctuations with their global averages in the Hamiltonian of quasi-particle mean-field theories for disordered materials must be reconsidered.

The Exotically Stoichiometric Compounds in Al-S System Under High Pressure (高压下新奇的Al-S化合物) 
Sen Shao, Wenji Zhu, Jian Lv, Yanchao Wang, Yue Chen& Yanming Ma
npj Computational Materials 6:11(2020)
doi:s41524-020-0278-9
Published online:04 February 2020

Abstract| Full Text | PDF OPEN

摘要:铝和硫作为地球上丰度较高的元素,在自然界中只能够形成Al2S3化合物。高压可以改变化合物的化学计量比,形成常压下不能形成的化合物,这对发现具有新奇功能特性的新材料至关重要。我们利用CALYPSOAl-S体系在高压下可能存在的稳定化合物进行了系统的搜索,发现了四种新化学计量比的Al-S化合物(AlSAl2SAl3S4AlS2),这些化合物在高压下表现出新奇的物性。例如,在100GPa时,Al3S4是一种潜在的超导体,其超导转变温度为20.9KAl2S是一种新的电子化合物,铝的价电子能够局域在晶格间隙中,形成阴离子。本工作为实验进一步研究Al-S体系的性质提供了一个可行的方向 

Abstract:Aluminum and sulfur, as abundant elements in earth, only form Al2S3 in nature at ambient pressure. It has been realized that the stoichiometry of compounds may change under high pressure, which is crucial in the discovery of novel materials. In this work, we systematically perform structure search for Al-S system under pressure. Four binary compounds of Al-S with exotic stoichiometries of AlS, Al2S, Al3S4 and AlS2 are found at high pressure and show exciting physical properties. Particularly, Al3S4 becomes a superconductor with a predicted superconducting transition temperature Tc of 20.9 K at 100 GPa, while the pressure-induced Al2S becomes an electride, where the valence electrons of aluminum strongly localize in the interstices, acting as anions, at a pressure of 70 GPa. The present work provides a viable direction for further experimental study of the properties of Al-S system.

Editorial Summary

Materials under pressure: novel Al-S compounds高压下材料的奇特性质:多种Al-S化合物的发现

压力能够使物质的电荷重新排布,是产生新奇材料的有力工具。二元硫化物在高压下往往表现出许多有趣的结构和性质。例如,在常压下,H-S体系中唯一稳定的化合物是具有臭鸡蛋气味的硫化氢气体。理论结构预测发现H3SHS2H2S3等化合物在高压下也可以稳定存在,并且预测的新型氢化物H3S200GPa时的超导转变温度高达203K。最近理论研究工作还发现硫与第二主族元素Be 1:1混合时,会产生奇特的周期调制结构。由于第I与第II主族硫化物在高压下产生耐人寻味的结构和性质,来自中国吉林大学超硬材料国家重点实验室王彦超教授团队进一步研究了SIII主族元素Al在高压下可能存在的化合物和性质。该研究利用课题组自主研发的结构搜索软件CALYPSO结合第一性原理计算,预测了四种新的Al-S化合物AlSAl2SAl3S4AlS2。其中Al3S4具有超导电性,在100 GPa时的超导转变温度为20.9K;该研究还发现在70GPa高压作用下Al-S可形成稳定的电子化合物Al2S该研究为实验研究Al-S体系高压结构和性质提供了理论支撑

It is well-known that pressure is considered as a powerful tool to rearrange electrons and create new exotic materials.In this context, binary sulfides are typically systems showing intriguing structure and properties under compression. For example, H2S with distinctive odour of rotten eggs is the only stable compound in H-S system at ambient conditions. Several novel compounds of H3S, HS2 and H2S3 have been predicted under pressures and H3S shows remarkably high superconducting Tc of 203 K. Furthermore, a modulated structure has been discovered when sulfur is mixed with the IIA group element Be in stoichiometry of 1:1. The intriguing structures and properties of group IA and IIA sulfides motivate researchers to further investigate binary mixtures between S and group IIIA element Al under pressures. A team led by Prof. Yanchao Wang from the State Key Laboratory of Superhard Materials, Jilin University, China, studied the potential compounds and properties of S and Al under pressure using their in-house-developed structure search software of CALYPSO. Four new aluminum sulfide compounds of AlS, Al2S, Al3S4 and AlS2are predicted. Al3S4is a superconductorwith an estimated value of Tc around 20.7 K at 100 GPa and Al2S, as a new electride, is stable above 70 GPa. This study provides implications for further experimental exploration of Al-S system under high pressure.

Spin Hall effect in prototype Rashba ferroelectrics GeTe and SnTe原型Rashba铁电体GeTeSnTe中的自旋霍尔效应 
Haihang Wang, Priya Gopal, Silvia Picozzi, Stefano Curtarolo, Marco Buongiorno Nardelli and Jagoda Slawińska
npj Computational Materials 6:7(2020)
doi:s41524-020-0274-0
Published online:24 January 2020

Abstract| Full Text | PDF OPEN

摘要:铁电Rashba半导体FERSCs)是近年来出现的一类有前途的自旋电子学材料。自旋自由度和极自由度之间的特殊耦合导致了一些特殊的性质,包括拉什巴自旋织构的铁电开关,展示电子的自旋只需电场来控制即可。这方面的新近实验研究揭示,GeTeSnTe两种典型FERSCs中的电荷-自旋相互转化现象极为相关。本研究通过密度泛函理论计算,探索了这些材料中的自旋霍尔效应(SHE),发现无论是铁电结构还是顺电结构,其中的SHE都很大。我们进一步探讨了1)半导体中SHE所需切实可行的掺杂与2)确定FERSCsRashba相关现象的极性畸变二者间的相容性,但二者间的相容性可以被自由电荷载流子抑制。通过对这些材料中驱动铁电性孤对载流子的分析,我们发现GeTe中的极性位移可以维持在超过1021/cm3的临界空穴浓度以上,而SnTe中的微小畸变在最低掺杂水平下消失。最后,我们估计了掺杂结构的自旋霍尔角,并证明了自旋霍尔效应确实可在极性相位中实现。我们认为,自旋霍尔效应、Rashba自旋织构和铁电共存于一种材料中,将有助于设计新型的无磁性ELD的一体化自旋电子器件 

Abstract:Ferroelectric Rashba semiconductors (FERSCs) have recently emerged as a promising class of spintronics materials. The peculiar coupling between spin and polar degrees of freedom responsible for several exceptional properties, including ferroelectric switching of Rashba spin texture, suggests that the electron’s spin could be controlled by using only electric fields. In this regard, recent experimental studies revealing charge-to-spin interconversion phenomena in two prototypical FERSCs, GeTe and SnTe, appear extremely relevant. Here, by employing density functional theory calculations, we investigate spin Hall effect (SHE) in these materials and show that it can be large either in ferroelectric or paraelectric structure. We further explore the compatibility between doping required for the practical realization of SHE in semiconductors and polar distortions which determine Rashba-related phenomena in FERSCs, but which could be suppressed by free charge carriers. Based on the analysis of the lone pairs which drive ferroelectricity in these materials, we have found that the polar displacements in GeTe can be sustained up to a critical hole concentration of over ~1021/cm3, while the tiny distortions in SnTe vanish at a minimal level of doping. Finally, we have estimated spin Hall angles for doped structures and demonstrated that the spin Hall effect could be indeed achieved in a polar phase. We believe that the confirmation of spin Hall effect, Rashba spin textures and ferroelectricity coexisting in one material will be helpful for design of novel all-in-one spintronics devices operating without magnetic fields.

Editorial Summary

Prototype Rashba ferroelectrics: Spin Hall effect原型Rashba铁电体:自旋霍尔效应

该研究基于密度泛函理论(DFT)的计算而定量评价了低对称和高对称结构的自旋霍尔电导率(SHC)。来自美国杜克大学的Stefano Curtarolo和北德克萨斯大学的Jagoda Slawińska共同领导的团队,研究了Rashba铁电原型GeTeSnTeSHE。他们首先发现,铁电相确实可以增强SHE,但顺电结构则不能。从自旋Berry曲率的附加贡献来看,SHCs起源于极性相的自旋分裂电子态。其次,由于在半导体中实现SHE需要掺杂,作者研究了极性畸变随载流子浓度的演化,并估计了维持低对称相的掺杂临界水平。通过对这些材料中驱动铁电性的孤对载流子的分析,发现GeTe中的极性位移可以维持在1021/cm3以上的临界空穴浓度,而SnTe中的微小畸变在最低掺杂水平下消失。最后,作者计算了掺杂结构的自旋霍尔角,证明了自旋霍尔效应确实可以在极性相位中实现。自旋霍尔效应、Rashba自旋织构和铁电共存于一种材料中,将有助于设计新型的无磁性ELD的一体化自旋电子器件。在实际实现SHERashba效应(RE)电控制等应用方面具有广阔的前景

Spin Hall conductivities (SHCs) for low- and high-symmetry structures of both materials are quantitatively estimated by calculations based on density functional theory (DFT). A team co-led by Stefano Curtarolo from Duke University, and Jagoda Slawińska from the University of North Texas, USA, studied SHE in prototype Rashba ferroelectrics GeTe and SnTe. First, they have unveiled that the ferroelectric phase could indeed enhance the SHE as compared with the paraelectric structure. They have interpreted this effect in terms of additional contributions to spin Berry curvature, originating from spin-splitted electronic states in the polar phase. Second, since the realization of SHE in semiconductors requires doping, the authors have studied the evolution of polar distortion with respect to the charge carrier concentration and estimated critical levels of doping that can sustain the low- symmetry phase. Finally, the authors have calculated spin Hall angles for doped structures and explored their potential for practical realization of SHE and electric control of Rashba effect (RE) at the same time.

Revisiting the anchoring behavior in lithium-sulfur batteries:many-body effect on the suppression of shuttle effect (重新审视锂硫电池中的锚定机制:多体效应在抑制穿梭效应中的作用) 
Min Fang, Xinyi Liu, Ji-Chang Ren, Sha Yang, GuirongSu, Qin Fang, Jianzhong Lai, Shuang Li & Wei Liu
npj Computational Materials 6:8(2020)
doi:s41524-020-0273-1
Published online:30 January 2020

Abstract| Full Text | PDF OPEN

摘要:本研究采用先进的多体色散(MBD)方法研究了锂硫电池中可溶锂硫基团的锚定效应,该效应与臭名昭著的穿梭效应密切相关。与金属硫化物(硫化亚铁和硫化锡)的实验结果相比,发现多体色散方法对于锚定机制的预测比其它范德华力修正方法更为准确。随后系统地研究了锂硫电池中两种典型的锚定材料氟化碳化钛和掺杂石墨烯体系。发现多体效应对于减弱锚定行为起到了重要作用,尤其当体系中极化很强并且范德华力在锚定行为中起主导作用的时候。该工作加深了对锂硫电池中锚定机制的理解,也为更加准确筛选抑制穿梭效应的锚定材料提供了新的标准 

Abstract:We apply the state-of-the-art many-body dispersion (MBD) method to study the anchoring behavior in lithium-sulfur (Li–S) batteries, which is closely related to the notorious “shuttle effect”. Based on the experimental results of metal sulfides (FeS and SnS2), we find that the MBD method gives a more accurate prediction of anchoring mechanism compared with other van der Waals (vdW) inclusive methods. We systematically investigate the anchoring mechanism of two prototypal anchoring materials—Ti2CF2 and doped-graphene systems. The many-body effect is found to play an important role on the reduction of anchoring behaviors, especially when the systems have large polarization and the vdW interactions predominate the anchoring behavior. Our work deepens the fundamental understanding of the anchoring mechanism, and provides a more accurate criterion for screening anchoring materials for suppressing the shuttle effect.

Editorial Summary

Anchoring materials in Li-S batteries: many-body dispersion method南京理工大学刘伟教授:锂硫电池锚定材料的筛选

在锂硫电池电极中,如果极化很强并且范德华力在锚定行为中占主导作用时,必须考虑多体效应才能得到准确的理论预测结果。本工作由南京理工大学材料学院纳米异构材料中心的刘伟教授和李爽副教授等人共同完成。他们发现在金属硫化物体系中(硫化亚铁和硫化锡),只有多体色散(MBD)方法的模拟结果与实验结果一致,而其它包含范德华力的计算方法得出的结果都与实验结果不完全相符。他们还系统研究了两种典型的锚定材料,一种是多层原子的氟化碳化钛,一种是单层氮掺杂石墨烯。研究结果表明在氟化碳化钛体系中,包含多体效应的多体色散方法得到的模拟结果会显著减弱锚定作用,但是在氮掺杂石墨烯体系中基本没有差别。这一发现说明多体效应在多原子层极化体系中的重要地位。这表明基于多体色散方法的密度泛函理论模拟不仅可以加深对锂硫电池中锚定机制的理解,还可以为筛选锚定材料提供更加准确的标准

For obtaining the accurate anchoring mechanismin Li-S batteries, the many-body effect must be considered if electrodes have large polarization and the vdW interactions predominate the anchoring behavior.Prof. Wei Liu and Associate Prof. Shuang Li from the Nano and Heterogeneous Materials Center, School of Materials Science and Technology, Nanjing University of Science and Technology, found that in systems of mental sulfides (FeS and SnS2), the predicted results from the many-body dispersion (MBD) method correspond with the experimental observation. However, the results from other van der Waals (vdW) inclusive methods cannot completely explain the experimental observation.They systematically investigated the anchoring mechanism of two prototypal anchoring materials—Ti2CF2 (the same as metal sulfides with large polarization) and N-dopedgraphene (with little polarization) systems. The many-body effect is found to play an important role on the reduction of anchoring behaviors in Ti2CF2 systems and there is no difference in N-doped graphene systems, indicated that the significance of the many-body effect in strong polarization systems. Hence, the density of functional theory simulation based on the many-body dispersion method not only deepens fundamental understanding of the anchoring mechanism, but also provides a more accurate criterion for screening anchoring materials for suppressing the shuttle effect.

Multi-cell Monte Carlo method for phase prediction(多单元蒙特卡洛方法进行相位预测) 
Changning NiuYou RaoWolfgang Windl & Maryam Ghazisaeidi
npj Computational Materials 5:120(2019)
doi:s41524-019-0259-z
Published online:10 December 2019

Abstract| Full Text | PDF OPEN

摘要:我们提出了一种多单元蒙特卡洛算法,或(MC)2,用于预测化学复杂晶体系统的稳定相。该算法利用多个单元来表示可能的相,同时消除了先前算法中的尺寸和浓度的限制。自由原子在单元之间的迁移是通过杠杆规则的应用而实现的,而杠杆规则中设定的一个摩尔比在整个模拟过程中实际上控制了每个单元的百分比,从而使(MC)2成为能成功模拟结晶固体共存相的首个算法。应用该方法的过程中,所有能量都是由密度泛函理论直接计算出来的。我们通过成功预测已知二进制系统的稳定相位验证了这一方法。然后,我们在四元高熵合金中应用了该方法。该方法特别适用于预测无相图的多组分系统中的稳定相 

Abstract:We propose a Multi-Cell Monte Carlo algorithm, or (MC)2, for predicting stable phases in chemically complex crystalline systems. This algorithm takes advantage of multiple cells to represent possible phases, while eliminating the size and concentration restrictions in the previous counterparts.Free atomic transfer among cells is achieved via the application of the lever rule, where an assigned molar ratio virtually controls the percentage of each cell in the overall simulation, making (MC)2the first successful algorithm for simulating phase coexistence in crystalline solids.During the application of this method, all energies are directly computed via density functional theory calculations. We test the method by successful prediction of the stable phases of known binary systems. We then apply the method to a quaternary high-entropy alloy. The method is particularly powerful in predicting stable phases of multicomponent systems, for which phase diagrams do not exist.

Editorial Summary

Multi-cell Monte Carlo method: phase prediction多单元蒙特卡洛方法:多相预测

本研究介绍了一种新蒙特卡洛(MC)算法,(MC)2方法,来预测多组分合金中的稳定相和相分率。来自美国俄亥俄州立大学材料科学与工程系的Maryam Ghazisaeidi教授领导的团队,使用了一种新算法,该算法消除了材料固定尺寸的限制,同时受益于多单元MC的概念,为每个单元分配摩尔比,以控制其在整个系统中的百分比。摩尔比由“杠杆规则”确定,因此系统的总组成是恒定的。与Gibbs集成MC模拟中的原子转移想法相反,此处通过更改一个或多个单元中原子的种类来实现随机原子的转移,作者称之为“翻转”运动。翻转会改变每个单元中的成分,从而改变整个摩尔比集,这等效于一组随机原子在单元之间的转移。虽然局部翻转已经足够,但偶尔进行的全局翻转可以加速收敛,而不会失去通用性。正如作者证明的那样,每个收敛的(MC)2运行都针对模拟的温度和成分标识的混溶区域或相关相界。(MC)2中的算法利用了多个单元的并行计算优势,并提供了识别相关相位和相位边界的独特功能,无需任何可能的相位先验知识。该算法能够预测化学复杂的晶体系统,既可预测多个相,又可预测单个相中不同相的组成和结构。作者认为,这是第一种也是唯一一种可以仅从一个初始成分捕获相界而无需内插中间成分的方法

TheMulti-Cell Monte Carlomethod, or (MC)2algorithm, is introduced to predict both stable phases and phase fractions in multicomponent alloys. Ateam led by Maryam Ghazisaeidi from the Department of Materials Science and Engineering, The Ohio State University, USA,used a new algorithm that eliminates the fixed size restriction while profiting from the concept of multi-cell MC. Each cell was assigned a molar ratio which controls its percentage in the total system. The molar ratios were determined by the ‘lever rule’ so that the total composition of the system was constant. In contrast to the atom-transfer idea in a Gibbs ensemble MC simulation, the transfer of random atoms was achieved by changing the species of an atom in one or more cells, which the authors called a ‘flip’ move. A flip move changes the composition in each cell and thus the overall set of molar ratios, equivalent to the transfer of a group of random atoms among the cells. While local flips are sufficient, occasional global flips accelerate convergence without any loss of generality. They demonstrated, each converged (MC)2run identifies either a region of miscibility, or the relevant phase boundaries for the simulated temperature and composition. The algorithm in (MC)2takes advantage of parallel computations of multiple cells and provides the unique capability to identify relevant phases and phase boundaries without any prior knowledge of possible phases. The(MC)2algorithm is capable of predicting for a chemically complex crystalline system both the coexistence of multiple phases and the composition and structure of the different phases in a single run. The authors believe that this is the first and only method that can capture the phase boundary from only one initial composition, without the need to interpolate intermediate compositions.

Machine learning guided appraisal and exploration of phase design for high entropy alloys (机器学习指导高熵合金的相设计评估与探索) 
Ziqing ZhouYeju ZhouQuanfeng HeZhaoyi DingFucheng Li & Yong Yang
npj Computational Materials 5:128(2019)
doi:s41524-019-0265-1
Published online:20 December 2019

Abstract| Full Text | PDF OPEN

摘要:高熵合金(HEA)和复杂成分合金(CCA)近期因具有卓越的机械和物理性能引起人们极大的研究兴趣。尽管已有许多有用的HEACCA的报道,但用来指导合金筛选的相设计规则尚不明确。本研究采用不同机器学习(ML)算法对当前学术界常用的设计规则作了严格的评估。基于人工神经网络算法,我们能够从ML建模中提取出敏感性矩阵,由此可以定量评估如何调整设计参数以形成特定的相,如固溶体、金属间化合物或非晶相等。此外,在基于ML建模开展HEACCA相设计时,我们探索使用了一组全新的、扩展的新设计参数。为了验证ML指导的设计规则,我们基于Fe-Cr-Ni-Zr-Cu体系进行了各种实验并设计了一系列合金。实验结果与预测相当吻合,表明基于ML技术有望发展HEAsCCAs设计的有用工具 

Abstract:High entropy alloys (HEAs) and compositionally complex alloys (CCAs) have recently attracted great research interest because of their remarkable mechanical and physical properties. Although many useful HEAs or CCAs were reported, the rules of phase design, if there are any, which could guide alloy screening are still an open issue. In this work, we made a critical appraisal of the existing design rules commonly used by the academic community with different machine learning (ML) algorithms. Based on the artificial neural network algorithm, we were able to derive and extract a sensitivity matrix from the ML modeling, which enabled the quantitative assessment of how to tune a design parameter for the formation of a certain phase, such as solid solution, intermetallic, or amorphous phase.Furthermore, we explored the use of an extended set of new design parameters, which had not been considered before, for phase design in HEAs or CCAs with the ML modeling.To verify our ML-guided design rule, we performed various experiments and designed a series of alloys out of the Fe-Cr-Ni-Zr-Cu system. The outcomes of our experiments agree reasonably well with our predictions, which suggests that the ML-based techniques could be a useful tool in the future design of HEAs or CCAs.

Editorial Summary

High entropy alloys: phase design by machine learning机器学习:指导高熵合金相结构设计

本研究开发了机器学习模型用于指导高熵合金的相结构设计。来自中国香港城市大学工程学院力学工程系的杨勇团队,基于人工神经网络等三种不同的算法开发了机器学习模型,并采用601组多元合金数据集训练了模型。基于该模型,他们定量评估了文献中已有的高熵合金相结构的设计规则,并探索提出了一组全新的设计参数。这些新参数与多组元系统的势能图波动相关联,从而大大提高了机器学习模型的准确性。为了验证模型的可靠性,他们基于Fe-Cr-Ni-Zr-Cu多元体系开展了一系列实验,包括铸造、熔融纺丝和共溅射等,并设计出了一系列新型合金,实验结果与理论预测高度吻合。该研究表明,基于机器学习技术有望发展高熵或多组元合金设计的新工具

Machine learning (ML) models has been developed to guide the phase design of high entropy alloys. A team led by Prof. Yong Yang from the Department of Mechanical Engineering, College of Engineering, City University of Hong Kong, developed ML models based on three various algorithms, e.g. artificial neural network toquantitatively assess the design parameter for the formation of a certain phase in multi-component system. A data set containing 601 multi-component alloys was used to train the model. With the trained models, they made appraisal of existing designing parameters of High entropy alloys in literatures, and also proposed a set of new parameters. As these new parametersare associated with the fluctuation of potential energy landscape of a multicomponent system, theygreatly improve the accuracy of the ML modeling.To verify the ML modeling, a series of experiments was further carried out in Fe-Cr-Ni-Zr-Cu system, which generally agree well with the prediction. This research suggests the great potential of ML-based techniques as useful tools in the design of high entropy alloys.

High-harmonic generation from spin-polarised defects in solids(固体内高次谐波产生于自旋极化缺陷) 
M. S. Mrudul, Nicolas Tancogne-Dejean, Angel Rubio and Gopal Dixit
npj Computational Materials 6:10(2020)
doi:s41524-020-0275-z
Published online:31 January 2020

Abstract| Full Text | PDF OPEN

摘要:气体中高次谐波的产生使我们能够以前所未有的分辨率探测原子和分子中的阿秒电子动力学。这些技术最初是为原子和分子气体开发的,如果扩展到固体研究,则需要了解其物理学的重要方面,这在理论上曾得到了部分解决。本研究利用含时密度泛函理论探索了空位对单层六角氮化硼高次谐波发射的影响。我们展示了这些真实的自旋极化缺陷如何改变了谐波发射,并演示了无损固体和缺陷固体之间谐波的重要区别。特别地,我们发现自旋极化点缺陷对不同自旋通道的影响是不同的。此外,波函数的局部化、缺陷的几何结构以及电子-电子的相互作用都是描述固体内缺陷产生高次谐波的关键因素 

Abstract:The generation of high-order harmonics in gases enabled to probe the attosecond electron dynamics in atoms and molecules with unprecedented resolution. Extending these techniques to solids, which were originally developed for atomic and molecular gases, requires a fundamental understanding of the physics that has been partially addressed theoretically. Here, we employ time- dependent density-functional theory to investigate how the electron dynamics resulting in high-harmonic emission in monolayer hexagonal boron nitride is affected by the presence of vacancies. We show how these realistic spin-polarised defects modify the harmonic emission and demonstrate that important differences exist between harmonics from a pristine solid and a defected solid. In particular, we found that the different spin channels are affected differently by the presence of the spin-polarised point defect. Moreover, the localisation of the wavefunction, the geometry of the defect, and the electron–electron interaction are all crucial ingredients to describe high-harmonic generation in defected solids.

Editorial Summary

Higher Harmonic Generation in Solid: Spin Polarized Defects自旋极化缺陷的高次谐波:缺陷探测

该研究揭示了自旋极化缺陷的高次谐波(HHG)对固体中自旋极化缺陷进行成像的可能性。来自印度理工学院Gopal Dixit教授以及德国马克斯·普朗克物质结构与动力学研究所的Angel Rubio领导的科研团队,基于含时密度泛函理论(TD-DFT)方法,研究了空位对单层六角氮化硼高次谐波发射的影响并展示了这些真实自旋极化缺陷如何影响谐波发射以及存在于原始固体和缺陷固体之间的谐波的重要区别。人们可能认为,具有硼原子空位或具有氮原子空位的h-BN会表现出相似的HHG光谱,因其都是去除了h-BN中的单个原子。然而情况并非如此,因为从带隙状态可明显看出硼和氮的空位会导致质子的电子结构不同。人们曾发现一旦从h-BN中除去硼或氮原子,该系统就会在空位附近以非零磁矩旋转自旋极化。结果,他们发现对于每个自旋通道和每个空位,由缺陷引起的带隙状态是不同的。根据缺陷状态的顺序和占据率,这些结果很大程度上取决于自旋。可见,通过分析自旋极化谱可理解缺陷状态,其结果与自旋极化能带结构一致。该研究建立了缺陷状态在固体强场动力学中的作用。这一结果为进一步研究二维和扩展系统中的强场电子动力学,特别是涉及孤立缺陷的强场电子动力学开辟了有趣的前景

The possibility of imaging spin polarized defects in solids by higher harmonic generation (HHG) is demonstrated by a theoretical model, A team co-led by Gopal Dixit from the Indian Institute of technology in Indian and Angel Rubio of Max Planck Institute of matter structure and dynamics in Germany, reviealed the effect of vacancy on the high-order harmonic emission of single-layer hexagonal boron nitride using time-dependent density functional theory (TD-DFT). In simple terms, one may assume that h-BN with a boron atom vacancy or with a nitrogen atom vacancy would exhibit similar HHG spectra since a single atom from h-BN has been removed. However, this is not the case as boron and nitrogen vacancies lead to qualitatively different electronic structures, and this is apparent from their corresponding gap states. It has been found that once an atom is removed from h-BN, either boron or nitrogen, the system becomes spin-polarised with a non-zero magnetic moment near the vacancy. As a consequence, the defect-induced gap states are found to be different for each spin channel and for each vacancy. These contributions are strongly spin-dependent, according to the ordering and occupancy of the defect states. Altogether, the role of the defect states can be understood by analysing the spin-polarised spectra, and the findings are in accordance with the spin polarised band-structure. This research establishes one aspect of the role of defect states in strong-field dynamics in solids. Their work opens up interesting perspectives for further studies on strong-field electron dynamics in two-dimensional and extended systems, especially involving isolated defects.

Ferroelectric nonlinear anomalous Hall effect in few-layer WTe2 (多层WTe2中的铁电非线性反常霍尔效应) 
Hua Wang & Xiaofeng Qian
npj Computational Materials 5:119(2019)
doi:s41524-019-0257-1
Published online:06 December 2019

Abstract| Full Text | PDF OPEN

摘要:在垂直于纵向电流并施加磁场(如外部磁场或内部磁化)的材料中可以建立横向电压,这就是经典的霍尔效应,这种现象属于时间反演对称破缺。然而,这种对称性约束可以在非线性范围内得到驰豫,从而在时间反演不变的材料中实现非线性反常霍尔电流——这是一个探索不足的领域,超越了经典的线性霍尔效应,具有令人兴奋的新机遇。本研究使用群论和第一性原理,证明了在时间反演不变的多层WTe2中,非线性反常霍尔电流在除1T'单层以外的WTe2奇数层中出现,而在偶数层中保持不变,表现出显著的铁电非线性反常霍尔效应。铁电非线性反常霍尔效应的这种奇-偶层振荡,是由于在奇-偶层WTe2中有明显的铁电变换而引起的贝里曲率偶极子反演和位移偶极子反演的存在-不存在而引起的。我们的研究不仅平等地对待Berry曲率偶极子和频移偶极子,以说明能带内和能带间对非线性反常霍尔效应的贡献,而且还将Berry曲率偶极子和频移偶极子确立为非中心对称材料的新序参量。目前的发现表明,铁电金属和Weyl半金属有可能为非线性量子电子学的发展,提供前所未有的机会 

Abstract:Under broken time reversal symmetry such as in the presence of external magnetic field or internal magnetization, a transverse voltage can be established in materials perpendicular to both longitudinal current and applied magnetic field, known as classical Hall effect.However, this symmetry constraint can be relaxed in the nonlinear regime, thereby enabling nonlinear anomalous Hall current in time-reversal invariant materials – an underexplored realm with exciting new opportunities beyond classical linear Hall effect.Here, using group theory and first-principles theory, we demonstrate a remarkable ferroelectric nonlinear anomalous Hall effect in time-reversal invariant few-layer WTe2 where nonlinear anomalous Hall current switches in odd-layer WTe2 except 1T′ monolayer while remaining invariant in even-layer WTe2 upon ferroelectric transition. This even-odd oscillation of ferroelectric nonlinear anomalous Hall effect was found to originate from the absence and presence of Berry curvature dipole reversal and shift dipole reversal due to distinct ferroelectric transformation in even and odd-layer WTe2.Our work not only treats Berry curvature dipole and shift dipole on an equal footing to account for intraband and interband contributions to nonlinear anomalous Hall effect, but also establishes Berry curvature dipole and shift dipole as new order parameters for noncentrosymmetric materials.The present findings suggest that ferroelectric metals and Weyl semimetals may offer unprecedented opportunities for the development of nonlinear quantum electronics.

Editorial Summary

Nonlinear anomalous Hall effect: few-layer WTe2多层WTe2:铁电非线性反常霍尔效应

该研究使用第一性原理计算和群论理论分析,研究了双层和三层WTe2中的非线性反常霍尔效应(NAHE),更重要的是揭示了三层WTe2中铁电NAHE(即,NAHE的铁电转换)的潜在微观起源。美国德克萨斯州农工大学材料科学与工程系的Hua WangXiaofeng Qian发现,尽管双层和三层WTe2都显示出具有相似电极化的铁电跃迁,但它们在NAHE中的表现却大不相同。在三层结构中,由于在两个铁电态的有效反演操作下,Berry曲率偶极子(BCD)和位移偶极子(SD)出现反演,非线性反常霍尔电流在铁电切换时会反转方向。相比之下,双层WTe2中的两个铁电态通过不翻转BCD / SD的滑行平面操作,有效地关联起来,因此其非线性反常霍尔电流在铁电切换时不会翻转。此外,NAHE有望在三层WTe2的顺电态下消失,但对于双层情况下的顺电态保持非平凡。以上结论适用于任何偶数和奇数层WTe2(单层1T'WTe2除外,因其为中心对称,且二阶NAHE消失),只要两个相反的铁电态与双层和三层WTe2的关系相同即可。本研究介绍的理论方法也可应用于诸如Weyl半金属等其他材料。 

更重要的是,该研究提示BCDSD可作为非中心对称材料的新序参量,这为基于BCD/SD和铁电有序耦合研究非线性多铁性开辟了可能性。铁电金属可能是有益的,因其消失的带隙不仅能为带内带来非线性反常霍尔电流(这在半导体/绝缘体中根本没有),而且还因降低了非线性带间过程的带隙,而显著增强带间非线性反常霍尔电流。如,从双层和三层WTe2带间线性光电效应计算出的非线性反常霍尔电流,比铁电GeS的高约一个数量级。此外,铁电NAHE为直接读出铁电态提供了一种简便的方法,与垂直铁电写入相结合,可实现非线性多铁性存储器。而且,独特的铁电转化途径有望为实现Weyl节点的非阿贝尔互惠编织提供新途径。因此,作者的发现揭开了一个超出经典线性霍尔效应和常规铁电学的尚未开发的新领域,为使用铁电金属和Weyl半金属基于铁电NAHE的非线性量子电子学提供了令人兴奋的新机遇

The nonlinear anomalous Hall effect(NAHE) in bilayer and trilayer WTe2 and, more importantly, the underlying microscopic origin of FNAHE (i.e., ferroelectric switching of NAHE) in trilayer WTe2, is investigated byusing first-principles calculations and group theoretical analyses.Hua Wang andXiaofeng Qian from the Department of Materials Science and Engineering, Texas A&M University, USA, found that, although both bilayer and trilayer WTe2 exhibit ferroelectric transition with similar electric polarization, they behave very differently in NAHE.In the trilayer case, the nonlinear anomalous Hall current flips direction upon ferroelectric switching due to the reversal of Berry curvature dipole (BCD) and shift dipole (SD) under an effective inversion operation of the two ferroelectric states.In contrast, the two ferroelectric states in bilayer WTe2 are related effectively by a glide plane operation which does not flip the BCD/SD, thus its nonlinear anomalous Hall current will not flip upon ferroelectric switching.In addition, NAHE is expected to vanish in the PE state of trilayer WTe2, but remains nontrivial for the PE state of the bilayer case.These conclusions are applicable to any even and odd layer WTe2 (except monolayer 1T′ WTe2 as it is centrosymmetric with vanishing second order NAHE) as long as the two opposite ferroelectric states have the same relationship as the bilayer and trilayer case.The theoretical approaches can also be applied to other materials such as Weyl semimetals.  

More importantly, their results imply that BCD and SD can serve as new order parameters for noncentrosymmetric materials, which opens up the possibility to explore nonlinear multiferroicity based on the coupling of BCD/SD and ferroelectric order. Ferroelectric metals may be advantageous as their vanishing bandgap will not only bring intraband contributions to nonlinear anomalous Hall current that is absent in semiconductors/insulators, but also significantly enhance the interband contributions due to the reduced gap of nonlinear interband processes.For example, the calculated nonlinear anomalous Hall current from interband linear photogalvanic effect in bilayer and trilayer WTe2 is about one order of magnitude higher than that in ferroelectric GeS.Moreover, ferroelectric NAHE provides a facile approach for direct readout of ferroelectric states, which, combined with vertical ferroelectric writing, may allow for realizing nonlinear multiferroic memory.In addition, the distinct ferroelectric transformation pathway may provide potential routes to realizing non-abelian reciprocal braiding of Weyl nodes.The present findings therefore reveal an underexplored realm beyond classical linear Hall effect and conventional ferroelectrics with exciting new opportunities for ferroelectric NAHE-based nonlinear quantum electronics using ferroelectric metals and Weyl semimetals

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Electron transfer governed by light–matter interaction at metal–semiconductor interface(受金属-半导体界面上光-物质相互作用控制的电子转移) 
Kenji Iida & Masashi Noda
npj Computational Materials 6:5(2020)
doi:s41524-019-0269-x
Published online:17 January 2020

Abstract| Full Text | PDF OPEN

摘要:由金属纳米团簇和半导体组成的异质结构的光激发伴随光催化和光学器件的兴起而得到广泛研究。异质结构的光电功能源自局部表面等离子体激元共振,其可以引起电子和共振能量转移。虽然众所周知,金属纳米团簇和半导体之间的光诱导电子相互作用是引起共振能量转移的原因,但是与光诱导电子相互作用相关的电子转移尚未被讨论。在本文中,我们使用原始的第一性原理计算方法(特别地处理了光-物质相互作用)阐明了银纳米团簇/ TiO2 异质结构的光激发动力学过程。结果表明,光诱导的银-TiO2 电子相互作用直接导致从银纳米团簇转移到TiO2 层地激发态电子,而没有通过银纳米团簇的导带 

Abstract:The photoexcitation of heterostructures consisting of metallic nanoclusters and a semiconductor has been extensively investigated in relation to interests in photocatalysis and optical devices. The optoelectronic functions of the heterostructures originate from localized surface plasmon resonance, which can induce electron and resonance energy transfers. While it is well known that photoinduced electronic interaction between a metallic nanocluster and a semiconductor is responsible for the resonance energy transfer, the electron transfer associated with the photoinduced electronic interaction has not been discussed. In this paper, we elucidate the photoexcitation dynamics of a silver nanocluster/TiO2 heterostructure using an original first-principles computational approach that explicitly deals with light–matter interactions. It is shown that the photoinduced silver–TiO2 electronic interaction causes excited electrons to be directly transferred from the silver nanocluster to the TiO2 layer without passing through the conduction band of the silver nanocluster.

Editorial Summary

Metal–semiconductor interface: light–matter interaction金属-半导体界面:光-物质相互作用

该研究采用原始的第一性原理计算方法阐明了由银纳米团簇和TiO2层组成的异质结构中的光激发电子转移,该方法明确解释了光-物质相互作用。来自日本分子科学研究所的Kenji Iida  Masashi Noda,发现激发的电子从银纳米团簇转移到TiO2层,却没有穿过银的导带。在银-TiO2界面处的光诱导电子相互作用是电子直接转移的根源,其机理与常规局部表面等离子体激元共振LSPR诱导的电子转移显著不同。虽然该研究提出的机制虽然是作者们新提出的,但他们认为直接电子转移也可能在之前的实验研究中就已发生,只是没有引起重视。这是由于这一现象不需要奇异的界面电子态——而仅需要使银纳米簇与TiO2层之间的距离短于范德华接触距离即可。根据这个相当宽松的要求,除银TiO2系统之外的金属/半导体异质结构也可能引起类似的直接电子转移。根据作者揭示的规律,积极开发金属/半导体异质结构,很可能提高光能转换效率。在这方面,该研究提供了可用于设计由金属/半导体异质结构组成的光学功能材料的一般概念。为进一步改善这类材料的功能,还需要更深入地了解界面区域原子尺度的结构。因此,他们的第一原理方法将获得广泛应用,如,根据界面处的缺陷和掺杂物来改变光学功能

Photoexcited electron transfer in heterostructures consisting of a silver nanocluster and a TiO2 layer is elucidated using an original first-principles computational approach, which explicitly deals with light–matter interactions. Kenji Iida and Masashi Noda from the Institute for Molecular Science in Japan found that excited electrons transfer from the silver nanocluster to the TiO2 layer without passing through the conduction band of silver. The photoinduced electronic interaction at the silver–TiO2 interface is responsible for direct electron transfer with a mechanism that is significantly different from the conventional localized surface plasmon resonance -induced electron transfer. While the proposed mechanism is new, they believe that the direct electron transfer would have occurred in previous experimental studies without being recognized. This is because no singular interfacial electronic state is needed—it is only necessary to make the distance between a silver nanocluster and a TiO2 layer shorter than the van der Waals contact distance. According to this rather lax requirement, analogous direct electron transfer would be induced in metal/semiconductor heterostructures other than silver/TiO2 systems. By actively developing metal/semiconductor heterostructures on the basis of their insights, the efficiency of light-energy conversion should be improved. In this regard, their study provides a general concept that can be applied in the design of optically functional materials consisting of metal/semiconductor heterostructures. For further improving the functionality of these materials, it would be required to obtain deeper atomic-scale insights into the interfacial region. To obtain the insights, their first-principles approach is highly useful because of the wide applicability, for example, to the variation in optical functions depending on defects and contaminants at the interface.

Deep-learning-based quality filtering of mechanically exfoliated 2D crystals (基于深度学习的机械剥离2D晶体的质量筛查) 
Yu Saito, Kento ShinKei TerayamaShaan DesaiMasaru OngaYuji NakagawaYuki M. ItahashiYoshihiro IwasaMakoto Yamada & Koji Tsuda
npj Computational Materials 5:124(2019)
doi:s41524-019-0262-4
Published online:17 December 2019

Abstract| Full Text | PDF OPEN

摘要:二维晶体由于具有低维特性及与体材料截然不同的性能,在工程、物理、化学、药学和生物学等各领域引起了越来越多的兴趣。在制备二维晶体的各种技术中,机械剥离不论对于实际应用和基础研究都十分重要的一种。但是,在衬底上机械剥离出的晶体通常含有较厚的薄片,手动筛找和清除这些薄片费时费力,限制了原子厚度的二维晶体和范德华异质结的高通量制备。本研究提出了一种基于深度学习的方法,可以根据光学显微镜图像区分和识别原子层薄片的厚度。我们设计了基于U-Net的神经网络,该神经网络只使用少量实际图像的数据进行训练就可成功区分单层和双层MoS2和石墨烯,成功率可达70%-80%,这对于无人眼监督的单、双层薄片的首轮筛选过程具有实用价值。本研究结果表明,很大一部分的实验室手工工作将可能被基于AI的系统所取代,从而提高生产率 

Abstract:Two-dimensional (2D) crystals are attracting growing interest in various research fields such as engineering, physics, chemistry, pharmacy, and biology owing to their low dimensionality and dramatic change of properties compared to the bulk counter parts. Among the various techniques used to manufacture 2D crystals, mechanical exfoliation has been essential to practical applications and fundamental research.However, mechanically exfoliated crystals on substrates contain relatively thick flakes that must be found and removed manually, limiting high-throughput manufacturing of atomic 2D crystals and van der Waals heterostructures.Here, we present a deep-learning-based method to segment and identify the thickness of atomic layer flakes from optical microscopy images. Through carefully designing a neural network based on U-Net, we found that our neural network based on U-net trained only with the data based on realistically small number of images successfully distinguish monolayer and bilayer MoS2 and graphene with a success rate of 70–80%, which is a practical value in the first screening process for choosing monolayer and bilayer flakes of all flakes on substrates without human eye. The remarkable results highlight the possibility that a large fraction of manual laboratory work can be replaced by AI-based systems, boosting productivity.

Editorial Summary

Deep-learning-based quality filtering: pick out right 2D crystals机械剥离的二维晶体:基于深度学习的筛选

该研究介绍了一种通过深度神经网络自动划分和识别二维晶体厚度的通用技术。日本理研大学高级情报项目中心(AIP)的斋藤雄(Yu Saito)和津田康治(Koji Tsuda)共同领导的团队,通过构建由卷积U-Net组成的框架,分别从不到2430 MoS2和石墨烯的光学显微图像中重构了二维晶体的图像。交叉验证得分和正确率检测发现,U-Net生成的数据正确率约为70%-80%,与人类非专家水平相当。这意味着他们构建的神经网络能以实用精确度,在第一轮筛选过程中区分出Si/SiO2衬底上的MoS2和石墨烯的单层、双层和其他更厚的薄片,以便能在进一步的传输/光学实验之前进行挑选。该研究为探索基于AI的大规模快速制造2D材料和范德华异质结的新方法,开辟了新的途径

A versatile technique to autonomously segment and identify the thickness of 2D crystals via a deep-neural network is introduced.A team co-led by Yu Saito and Koji Tsuda from the RIKEN Center for Advanced Intelligence Project (AIP), Japan, by constructing an architecture consisting of convolutions, U-Net,reproduced the images of 2D crystals from the less than 24 and 30 OM images of MoS2 and graphene, respectively, and found that both the cross-validation score and the accuracy rate of generated data through U-Net is around 70–80 percent, which is comparable with non-expert human level. This means that their neural network can distinguish monolayer, bilayer and other thicker flakes of MoS2 and graphene on Si/SiO2 substrates with the practical accuracy in the first screening process for searching desirable before further transport/optical experiments. The present study highlights that deep-neural networks have great potential as a new tool for quickly and autonomously segmenting and identifying atomic-layer thickness of various 2D crystals and opens a new way for AI-based quick exploration for manufacturing 2D materials and van der Waals heterostructures in large scale.

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