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Unfolding the complexity of phonon quasi-particle physicsin disordered materials(揭示無序材料中聲子準粒子物理的復雜性) 
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:準粒子的概念構成了我們對新現微觀現象認識的理論基礎,而這些新現現象與量子力學多體相互作用有關。然而,無序材料領域的準粒子理論遇到了難以逾越的困難,從而凸顯了平均場方法的優勢。本研究報告了為力常數顯性無序的等原子合金(NiCo、NiFe、AgPdNiFeCo)采用第一原理聲子計算以及非彈性X射線和中子散射測量的結果,并與50年前的重要假設對比,此假設即為描述非對角無序的所有平均場準粒子解的哈密頓量。我們的結果揭示,存在一個巨大的、迄今未被認識的、局部化學環境對物種--分辨的力學常數無序分布的影響,成為影響聲子散射的主要因素。這一發現不但找到了分析中的關鍵問題,對其他元素激發態(如磁合金中的磁子和斯格明子)的研究將產生廣泛影響,而且為超低熱導率材料的設計提供了重要工具。 

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聲子準粒子物理的復雜性:理論計算與實驗驗證

該研究揭示,存在一個巨大的、迄今未被認識的、局部化學環境對物種--分辨的力學常數無序分布的影響,成為影響聲子散射的主要因素。來自美國橡樹林國家實驗室的.George M. Stocks教授,通過第一原理理論結合實驗測量研究(對濃縮的、帶有很強力常數紊亂卻幾無質量紊亂的無序合金NiCo、NiFe、AgPdNiFeCo所作的聲子準粒子物理學(色散和線寬)研究),首次解決了這一問題。 值得注意的是,在從頭算的超胞聲子展開(SPU)模擬,及其與ICPA和實驗測量的比較中,考慮到每個單獨的AA、BBAB型物種對,力常數的變化遠遠超過了通常的全局平均力常數波動。此外,他們的研究結果表明,增強型磁阻的根源,是圍繞著單個AA、BBAB型物種對的局部化學環境所固有的隨機變化。因此,以往長期使用的近似方法(用無序材料的準粒子平均-Er場理論的哈密頓量的全局平均值,來代替單個物種對的力常數波動),現在必須重新考慮。

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 (高壓下新奇的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:鋁和硫作為地球上豐度較高的元素,在自然界中只能夠形成Al2S3化合物。高壓可以改變化合物的化學計量比,形成常壓下不能形成的化合物,這對發現具有新奇功能特性的新材料至關重要。我們利用CALYPSOAl-S體系在高壓下可能存在的穩定化合物進行了系統的搜索,發現了四種新化學計量比的Al-S化合物(AlS、Al2S、Al3S4AlS2),這些化合物在高壓下表現出新奇的物性。例如,在100GPa時,Al3S4是一種潛在的超導體,其超導轉變溫度為20.9K;Al2S是一種新的電子化合物,鋁的價電子能夠局域在晶格間隙中,形成陰離子。本工作為實驗進一步研究Al-S體系的性質提供了一個可行的方向。 

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高壓下材料的奇特性質:多種Al-S化合物的發現

壓力能夠使物質的電荷重新排布,是產生新奇材料的有力工具。二元硫化物在高壓下往往表現出許多有趣的結構和性質。例如,在常壓下,H-S體系中唯一穩定的化合物是具有臭雞蛋氣味的硫化氫氣體。理論結構預測發現H3S、HS2、H2S3等化合物在高壓下也可以穩定存在,并且預測的新型氫化物H3S200GPa時的超導轉變溫度高達203K。最近理論研究工作還發現硫與第二主族元素Be 1:1混合時,會產生奇特的周期調制結構。由于第I與第II主族硫化物在高壓下產生耐人尋味的結構和性質,來自中國吉林大學超硬材料國家重點實驗室王彥超教授團隊進一步研究了SIII主族元素Al在高壓下可能存在的化合物和性質。該研究利用課題組自主研發的結構搜索軟件CALYPSO結合第一性原理計算,預測了四種新的Al-S化合物AlS,Al2S,Al3S4AlS2。其中Al3S4具有超導電性,在100 GPa時的超導轉變溫度為20.9K;該研究還發現在70GPa高壓作用下Al-S可形成穩定的電子化合物Al2S。該研究為實驗研究Al-S體系高壓結構和性質提供了理論支撐。

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鐵電體GeTeSnTe中的自旋霍爾效應 
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:鐵電Rashba半導體FERSCs)是近年來出現的一類有前途的自旋電子學材料。自旋自由度和極自由度之間的特殊耦合導致了一些特殊的性質,包括拉什巴自旋織構的鐵電開關,展示電子的自旋只需電場來控制即可。這方面的新近實驗研究揭示,GeTeSnTe兩種典型FERSCs中的電荷-自旋相互轉化現象極為相關。本研究通過密度泛函理論計算,探索了這些材料中的自旋霍爾效應(SHE),發現無論是鐵電結構還是順電結構,其中的SHE都很大。我們進一步探討了1)半導體中SHE所需切實可行的摻雜與2)確定FERSCsRashba相關現象的極性畸變二者間的相容性,但二者間的相容性可以被自由電荷載流子抑制。通過對這些材料中驅動鐵電性孤對載流子的分析,我們發現GeTe中的極性位移可以維持在超過1021/cm3的臨界空穴濃度以上,而SnTe中的微小畸變在最低摻雜水平下消失。最后,我們估計了摻雜結構的自旋霍爾角,并證明了自旋霍爾效應確實可在極性相位中實現。我們認為,自旋霍爾效應、Rashba自旋織構和鐵電共存于一種材料中,將有助于設計新型的無磁性ELD的一體化自旋電子器件。 

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鐵電體:自旋霍爾效應

該研究基于密度泛函理論(DFT)的計算而定量評價了低對稱和高對稱結構的自旋霍爾電導率(SHC)。來自美國杜克大學的Stefano Curtarolo和北德克薩斯大學的Jagoda Slawińska共同領導的團隊,研究了Rashba鐵電原型GeTeSnTeSHE。他們首先發現,鐵電相確實可以增強SHE,但順電結構則不能。從自旋Berry曲率的附加貢獻來看,SHCs起源于極性相的自旋分裂電子態。其次,由于在半導體中實現SHE需要摻雜,作者研究了極性畸變隨載流子濃度的演化,并估計了維持低對稱相的摻雜臨界水平。通過對這些材料中驅動鐵電性的孤對載流子的分析,發現GeTe中的極性位移可以維持在1021/cm3以上的臨界空穴濃度,而SnTe中的微小畸變在最低摻雜水平下消失。最后,作者計算了摻雜結構的自旋霍爾角,證明了自旋霍爾效應確實可以在極性相位中實現。自旋霍爾效應、Rashba自旋織構和鐵電共存于一種材料中,將有助于設計新型的無磁性ELD的一體化自旋電子器件。在實際實現SHERashba效應(RE)電控制等應用方面具有廣闊的前景。

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 (重新審視鋰硫電池中的錨定機制:多體效應在抑制穿梭效應中的作用) 
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:本研究采用先進的多體色散(MBD)方法研究了鋰硫電池中可溶鋰硫基團的錨定效應,該效應與臭名昭著的穿梭效應密切相關。與金屬硫化物(硫化亞鐵和硫化錫)的實驗結果相比,發現多體色散方法對于錨定機制的預測比其它范德華力修正方法更為準確。隨后系統地研究了鋰硫電池中兩種典型的錨定材料氟化碳化鈦和摻雜石墨烯體系。發現多體效應對于減弱錨定行為起到了重要作用,尤其當體系中極化很強并且范德華力在錨定行為中起主導作用的時候。該工作加深了對鋰硫電池中錨定機制的理解,也為更加準確篩選抑制穿梭效應的錨定材料提供了新的標準。 

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南京理工大學劉偉教授:鋰硫電池錨定材料的篩選

在鋰硫電池電極中,如果極化很強并且范德華力在錨定行為中占主導作用時,必須考慮多體效應才能得到準確的理論預測結果。本工作由南京理工大學材料學院納米異構材料中心的劉偉教授和李爽副教授等人共同完成。他們發現在金屬硫化物體系中(硫化亞鐵和硫化錫),只有多體色散(MBD)方法的模擬結果與實驗結果一致,而其它包含范德華力的計算方法得出的結果都與實驗結果不完全相符。他們還系統研究了兩種典型的錨定材料,一種是多層原子的氟化碳化鈦,一種是單層氮摻雜石墨烯。研究結果表明在氟化碳化鈦體系中,包含多體效應的多體色散方法得到的模擬結果會顯著減弱錨定作用,但是在氮摻雜石墨烯體系中基本沒有差別。這一發現說明多體效應在多原子層極化體系中的重要地位。這表明基于多體色散方法的密度泛函理論模擬不僅可以加深對鋰硫電池中錨定機制的理解,還可以為篩選錨定材料提供更加準確的標準。

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(多單元蒙特卡洛方法進行相位預測) 
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:我們提出了一種多單元蒙特卡洛算法,或(MC)2,用于預測化學復雜晶體系統的穩定相。該算法利用多個單元來表示可能的相,同時消除了先前算法中的尺寸和濃度的限制。自由原子在單元之間的遷移是通過杠桿規則的應用而實現的,而杠桿規則中設定的一個摩爾比在整個模擬過程中實際上控制了每個單元的百分比,從而使(MC)2成為能成功模擬結晶固體共存相的首個算法。應用該方法的過程中,所有能量都是由密度泛函理論直接計算出來的。我們通過成功預測已知二進制系統的穩定相位驗證了這一方法。然后,我們在四元高熵合金中應用了該方法。該方法特別適用于預測無相圖的多組分系統中的穩定相。 

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多單元蒙特卡洛方法:多相預測

本研究介紹了一種新蒙特卡洛(MC)算法,(MC)2方法,來預測多組分合金中的穩定相和相分率。來自美國俄亥俄州立大學材料科學與工程系的Maryam Ghazisaeidi教授領導的團隊,使用了一種新算法,該算法消除了材料固定尺寸的限制,同時受益于多單元MC的概念,為每個單元分配摩爾比,以控制其在整個系統中的百分比。摩爾比由“杠桿規則”確定,因此系統的總組成是恒定的。與Gibbs集成MC模擬中的原子轉移想法相反,此處通過更改一個或多個單元中原子的種類來實現隨機原子的轉移,作者稱之為“翻轉”運動。翻轉會改變每個單元中的成分,從而改變整個摩爾比集,這等效于一組隨機原子在單元之間的轉移。雖然局部翻轉已經足夠,但偶爾進行的全局翻轉可以加速收斂,而不會失去通用性。正如作者證明的那樣,每個收斂的(MC)2運行都針對模擬的溫度和成分標識的混溶區域或相關相界。(MC)2中的算法利用了多個單元的并行計算優勢,并提供了識別相關相位和相位邊界的獨特功能,無需任何可能的相位先驗知識。該算法能夠預測化學復雜的晶體系統,既可預測多個相,又可預測單個相中不同相的組成和結構。作者認為,這是第一種也是唯一一種可以僅從一個初始成分捕獲相界而無需內插中間成分的方法。

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 (機器學習指導高熵合金的相設計評估與探索) 
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)和復雜成分合金(CCA)近期因具有卓越的機械和物理性能引起人們極大的研究興趣。盡管已有許多有用的HEACCA的報道,但用來指導合金篩選的相設計規則尚不明確。本研究采用不同機器學習(ML)算法對當前學術界常用的設計規則作了嚴格的評估?;谌斯ど窠浘W絡算法,我們能夠從ML建模中提取出敏感性矩陣,由此可以定量評估如何調整設計參數以形成特定的相,如固溶體、金屬間化合物或非晶相等。此外,在基于ML建模開展HEACCA相設計時,我們探索使用了一組全新的、擴展的新設計參數。為了驗證ML指導的設計規則,我們基于Fe-Cr-Ni-Zr-Cu體系進行了各種實驗并設計了一系列合金。實驗結果與預測相當吻合,表明基于ML技術有望發展HEAsCCAs設計的有用工具。 

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機器學習:指導高熵合金相結構設計

本研究開發了機器學習模型用于指導高熵合金的相結構設計。來自中國香港城市大學工程學院力學工程系的楊勇團隊,基于人工神經網絡等三種不同的算法開發了機器學習模型,并采用601組多元合金數據集訓練了模型?;谠撃P?,他們定量評估了文獻中已有的高熵合金相結構的設計規則,并探索提出了一組全新的設計參數。這些新參數與多組元系統的勢能圖波動相關聯,從而大大提高了機器學習模型的準確性。為了驗證模型的可靠性,他們基于Fe-Cr-Ni-Zr-Cu多元體系開展了一系列實驗,包括鑄造、熔融紡絲和共濺射等,并設計出了一系列新型合金,實驗結果與理論預測高度吻合。該研究表明,基于機器學習技術有望發展高熵或多組元合金設計的新工具。

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(固體內高次諧波產生于自旋極化缺陷) 
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:氣體中高次諧波的產生使我們能夠以前所未有的分辨率探測原子和分子中的阿秒電子動力學。這些技術最初是為原子和分子氣體開發的,如果擴展到固體研究,則需要了解其物理學的重要方面,這在理論上曾得到了部分解決。本研究利用含時密度泛函理論探索了空位對單層六角氮化硼高次諧波發射的影響。我們展示了這些真實的自旋極化缺陷如何改變了諧波發射,并演示了無損固體和缺陷固體之間諧波的重要區別。特別地,我們發現自旋極化點缺陷對不同自旋通道的影響是不同的。此外,波函數的局部化、缺陷的幾何結構以及電子-電子的相互作用都是描述固體內缺陷產生高次諧波的關鍵因素。 

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自旋極化缺陷的高次諧波:缺陷探測

該研究揭示了自旋極化缺陷的高次諧波(HHG)對固體中自旋極化缺陷進行成像的可能性。來自印度理工學院Gopal Dixit教授以及德國馬克斯·普朗克物質結構與動力學研究所的Angel Rubio領導的科研團隊,基于含時密度泛函理論(TD-DFT)方法,研究了空位對單層六角氮化硼高次諧波發射的影響并展示了這些真實自旋極化缺陷如何影響諧波發射以及存在于原始固體和缺陷固體之間的諧波的重要區別。人們可能認為,具有硼原子空位或具有氮原子空位的h-BN會表現出相似的HHG光譜,因其都是去除了h-BN中的單個原子。然而情況并非如此,因為從帶隙狀態可明顯看出硼和氮的空位會導致質子的電子結構不同。人們曾發現一旦從h-BN中除去硼或氮原子,該系統就會在空位附近以非零磁矩旋轉自旋極化。結果,他們發現對于每個自旋通道和每個空位,由缺陷引起的帶隙狀態是不同的。根據缺陷狀態的順序和占據率,這些結果很大程度上取決于自旋??梢?,通過分析自旋極化譜可理解缺陷狀態,其結果與自旋極化能帶結構一致。該研究建立了缺陷狀態在固體強場動力學中的作用。這一結果為進一步研究二維和擴展系統中的強場電子動力學,特別是涉及孤立缺陷的強場電子動力學開辟了有趣的前景。

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 (多層WTe2中的鐵電非線性反?;魻栃?/span>) 
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:在垂直于縱向電流并施加磁場(如外部磁場或內部磁化)的材料中可以建立橫向電壓,這就是經典的霍爾效應,這種現象屬于時間反演對稱破缺。然而,這種對稱性約束可以在非線性范圍內得到馳豫,從而在時間反演不變的材料中實現非線性反?;魻栯娏鳌@是一個探索不足的領域,超越了經典的線性霍爾效應,具有令人興奮的新機遇。本研究使用群論和第一性原理,證明了在時間反演不變的多層WTe2中,非線性反?;魻栯娏髟诔?/span>1T'單層以外的WTe2奇數層中出現,而在偶數層中保持不變,表現出顯著的鐵電非線性反?;魻栃?。鐵電非線性反?;魻栃倪@種奇-偶層振蕩,是由于在奇-偶層WTe2中有明顯的鐵電變換而引起的貝里曲率偶極子反演和位移偶極子反演的存在-不存在而引起的。我們的研究不僅平等地對待Berry曲率偶極子和頻移偶極子,以說明能帶內和能帶間對非線性反?;魻栃呢暙I,而且還將Berry曲率偶極子和頻移偶極子確立為非中心對稱材料的新序參量。目前的發現表明,鐵電金屬和Weyl半金屬有可能為非線性量子電子學的發展,提供前所未有的機會。 

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多層WTe2:鐵電非線性反?;魻栃?/span>

該研究使用第一性原理計算和群論理論分析,研究了雙層和三層WTe2中的非線性反?;魻栃?/span>NAHE),更重要的是揭示了三層WTe2中鐵電NAHE(即,NAHE的鐵電轉換)的潛在微觀起源。美國德克薩斯州農工大學材料科學與工程系的Hua WangXiaofeng Qian發現,盡管雙層和三層WTe2都顯示出具有相似電極化的鐵電躍遷,但它們在NAHE中的表現卻大不相同。在三層結構中,由于在兩個鐵電態的有效反演操作下,Berry曲率偶極子(BCD)和位移偶極子(SD)出現反演,非線性反?;魻栯娏髟阼F電切換時會反轉方向。相比之下,雙層WTe2中的兩個鐵電態通過不翻轉BCD / SD的滑行平面操作,有效地關聯起來,因此其非線性反?;魻栯娏髟阼F電切換時不會翻轉。此外,NAHE有望在三層WTe2的順電態下消失,但對于雙層情況下的順電態保持非平凡。以上結論適用于任何偶數和奇數層WTe2(單層1T'WTe2除外,因其為中心對稱,且二階NAHE消失),只要兩個相反的鐵電態與雙層和三層WTe2的關系相同即可。本研究介紹的理論方法也可應用于諸如Weyl半金屬等其他材料。 

更重要的是,該研究提示BCDSD可作為非中心對稱材料的新序參量,這為基于BCD/SD和鐵電有序耦合研究非線性多鐵性開辟了可能性。鐵電金屬可能是有益的,因其消失的帶隙不僅能為帶內帶來非線性反?;魻栯娏鳎ㄟ@在半導體/絕緣體中根本沒有),而且還因降低了非線性帶間過程的帶隙,而顯著增強帶間非線性反?;魻栯娏?。如,從雙層和三層WTe2帶間線性光電效應計算出的非線性反?;魻栯娏?,比鐵電GeS的高約一個數量級。此外,鐵電NAHE為直接讀出鐵電態提供了一種簡便的方法,與垂直鐵電寫入相結合,可實現非線性多鐵性存儲器。而且,獨特的鐵電轉化途徑有望為實現Weyl節點的非阿貝爾互惠編織提供新途徑。因此,作者的發現揭開了一個超出經典線性霍爾效應和常規鐵電學的尚未開發的新領域,為使用鐵電金屬和Weyl半金屬基于鐵電NAHE的非線性量子電子學提供了令人興奮的新機遇

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(受金屬-半導體界面上光-物質相互作用控制的電子轉移) 
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:由金屬納米團簇和半導體組成的異質結構的光激發伴隨光催化和光學器件的興起而得到廣泛研究。異質結構的光電功能源自局部表面等離子體激元共振,其可以引起電子和共振能量轉移。雖然眾所周知,金屬納米團簇和半導體之間的光誘導電子相互作用是引起共振能量轉移的原因,但是與光誘導電子相互作用相關的電子轉移尚未被討論。在本文中,我們使用原始的第一性原理計算方法(特別地處理了光-物質相互作用)闡明了銀納米團簇/ TiO2 異質結構的光激發動力學過程。結果表明,光誘導的銀-TiO2 電子相互作用直接導致從銀納米團簇轉移到TiO2 層地激發態電子,而沒有通過銀納米團簇的導帶。 

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金屬-半導體界面:光-物質相互作用

該研究采用原始的第一性原理計算方法闡明了由銀納米團簇和TiO2層組成的異質結構中的光激發電子轉移,該方法明確解釋了光-物質相互作用。來自日本分子科學研究所的Kenji Iida  Masashi Noda,發現激發的電子從銀納米團簇轉移到TiO2層,卻沒有穿過銀的導帶。在銀-TiO2界面處的光誘導電子相互作用是電子直接轉移的根源,其機理與常規局部表面等離子體激元共振LSPR誘導的電子轉移顯著不同。雖然該研究提出的機制雖然是作者們新提出的,但他們認為直接電子轉移也可能在之前的實驗研究中就已發生,只是沒有引起重視。這是由于這一現象不需要奇異的界面電子態——而僅需要使銀納米簇與TiO2層之間的距離短于范德華接觸距離即可。根據這個相當寬松的要求,除銀TiO2系統之外的金屬/半導體異質結構也可能引起類似的直接電子轉移。根據作者揭示的規律,積極開發金屬/半導體異質結構,很可能提高光能轉換效率。在這方面,該研究提供了可用于設計由金屬/半導體異質結構組成的光學功能材料的一般概念。為進一步改善這類材料的功能,還需要更深入地了解界面區域原子尺度的結構。因此,他們的第一原理方法將獲得廣泛應用,如,根據界面處的缺陷和摻雜物來改變光學功能。

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 (基于深度學習的機械剝離2D晶體的質量篩查) 
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

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摘要:二維晶體由于具有低維特性及與體材料截然不同的性能,在工程、物理、化學、藥學和生物學等各領域引起了越來越多的興趣。在制備二維晶體的各種技術中,機械剝離不論對于實際應用和基礎研究都十分重要的一種。但是,在襯底上機械剝離出的晶體通常含有較厚的薄片,手動篩找和清除這些薄片費時費力,限制了原子厚度的二維晶體和范德華異質結的高通量制備。本研究提出了一種基于深度學習的方法,可以根據光學顯微鏡圖像區分和識別原子層薄片的厚度。我們設計了基于U-Net的神經網絡,該神經網絡只使用少量實際圖像的數據進行訓練就可成功區分單層和雙層MoS2和石墨烯,成功率可達70%-80%,這對于無人眼監督的單、雙層薄片的首輪篩選過程具有實用價值。本研究結果表明,很大一部分的實驗室手工工作將可能被基于AI的系統所取代,從而提高生產率。 

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機械剝離的二維晶體:基于深度學習的篩選

該研究介紹了一種通過深度神經網絡自動劃分和識別二維晶體厚度的通用技術。日本理研大學高級情報項目中心(AIP)的齋藤雄(Yu Saito)和津田康治(Koji Tsuda)共同領導的團隊,通過構建由卷積U-Net組成的框架,分別從不到2430 MoS2和石墨烯的光學顯微圖像中重構了二維晶體的圖像。交叉驗證得分和正確率檢測發現,U-Net生成的數據正確率約為70%-80%,與人類非專家水平相當。這意味著他們構建的神經網絡能以實用精確度,在第一輪篩選過程中區分出Si/SiO2襯底上的MoS2和石墨烯的單層、雙層和其他更厚的薄片,以便能在進一步的傳輸/光學實驗之前進行挑選。該研究為探索基于AI的大規??焖僦圃?/span>2D材料和范德華異質結的新方法,開辟了新的途徑。

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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