二维拓扑材料的新进展——纯平锡烯中存在大的拓扑能隙

<正>近年来,得益于拓扑物理理论和二维材料制备的迅速发展,以量子自旋霍尔绝缘体为代表的二维拓扑材料的研究受到热切关注~([1, 2])。早在2005年前后,理论表明在二维材料体系如石墨烯~([3])和HgTe量子阱体系~([4])中由于自旋轨道耦合作用而存在拓扑量子自旋霍尔效应。然而石墨烯中的碳是轻元素,自旋轨道作用非常微弱,所以其拓扑     

二维范德瓦耳斯半导体莫尔超晶格实验研究进展

在二维范德瓦耳斯材料中,可以通过转角及晶格失配构造周期性的莫尔超晶格.自从实验上在“魔角”石墨烯系统中观察到关联绝缘体态和超导电性以来,利用各种二维范德瓦耳斯材料构造莫尔超晶格并研究其中的新奇量子物态成为了凝聚态物理研究的热点和前沿问题.本文主要综述了最近几年在二维半导体过渡金属硫族化合物莫尔超晶格系统中的相关实验进展.在该系统中实现电子“平带”不依赖于特定魔角,实验上,一系列的关联电子物态和拓扑电子物态被相继发现和证实.进一步的理论和实验研究有望在该系统中揭示更多的受电子关联作用和拓扑物理共同支配的新奇量子物态.     

二维有机拓扑绝缘体的研究进展

新材料的发现促进了科学与技术的进步.拓扑绝缘体是近期材料领域新的研究热点,相关研究的进一步深入,不仅加深了人们对材料物理性质的理解,也为其在自旋电子学和量子计算机等领域的潜在应用提供了有价值的参考.近年来,理论工作预测了一系列由金属和有机物构筑的二维有机拓扑绝缘体,本文主要介绍六角对称的金属有机晶格与Kagome金属有机晶格两类典型的二维有机拓扑绝缘体的研究进展,其中重点介绍了理论预测的氰基配位二维本征有机拓扑绝缘体.除了理论计算方面的工作,还简要介绍了关于二维有机拓扑绝缘体材料合成方面的实验工作.二维有机拓扑绝缘体的理论与实验研究不仅拓展了拓扑绝缘体的研究体系,还为寻找新的拓扑绝缘体材料提供了思路.     

单层二维量子自旋霍尔绝缘体1T'-WTe2研究进展

量子自旋霍尔效应通常存在于二维拓扑绝缘体中,其具有受拓扑保护的无耗散螺旋边界态. 2014年,理论预言单层1T’相过渡金属硫族化合物是一类新型的二维量子自旋霍尔绝缘体.其中,以单层1T’-WTe₂为代表的材料体系具有原子结构稳定、体带隙显著、拓扑性质易于调控等许多独特的优势,对低功耗自旋电子器件的发展具有重要的意义.本文总结了单层1T’-WTe₂在实验上的最新进展,包括基于分子束外延生长的材料制备,螺旋边界态的探测及其对磁场的响应,掺杂、应力等手段在单层1T’-WTe₂中诱导出的新奇量子物态等.也对单层1T’-WTe₂未来可能的应用前景进行了展望.     

石墨烯中新奇量子物态的研究

石墨烯特殊的晶格结构和能带结构赋予了它独特的电学性质. 近年来, 分数量子霍尔态、 魔角石墨烯中的 关联绝缘体态和超导态等现象的发现不断证明着石墨烯是一种理想的二维模型体系, 可用于实现一系列新奇的量子物态, 对石墨烯中新奇量子物态的探测和调控也一直是凝聚态物理领域的前沿研究热点之一. 本文将系统地介绍近年来石墨烯中对称性破缺量子物态的研究进展, 包括平带中强关联量子物态的研究以及谷赝自旋调控的研究, 并介绍一种在纳米尺度、 单电子精度上探测二维材料体系简并度及对称性破缺态的普适方法, 希望为相关领域的研究人员提供参考和借鉴.     

二维材料中的氢隧穿研究进展

石墨烯、石墨烯衍生物以及类石墨烯材料通常具有致密的网状晶格结构,研宄表明这类材料对分子、原子和离子具有根强的阻挡性.然而对于不同形态的氢粒子(原子、离子、氢气分乎)是否能够隧穿二维材料仍然存在很多科学争议,并已成为目前科学研究的一个热点.本文综述了氢隧穿二维材料的研究进展,介绍了不同结构氢粒子隧穿二维材料体系的特点,阐述了氢粒子隧穿不同质量石墨烯和类石墨烯材料时所需要逾越的势垒高度,并对比了其跃迁的难度.讨论了从二维材料本身出发,降低氢隧穿势垒大小和改变环境对氢隧穿过程的影响,实现氢粒子隧穿二维材料.最后展望了氢隧穿二维材料在实际应用中可能存在的问题及未来的研究方向.     

拓扑半金属材料的单晶生长研究进展

拓扑半金属已经成为凝聚态物理研究的一个热点领域,这类材料的单晶生长是研究其物理性质的基础.目前,对于拓扑材料的研究已经形成了以理论计算为指引,对潜在的拓扑材料进行单晶制备,并结合物性测量对理论预言加以验证的科研合作方式.在这种科研团队合作中,单晶生长起衔接作用.本文介绍了近年来拓扑半金属材料单晶生长方法,涵盖了拓扑Dirac半金属、Weyl半金属、Node-Line半金属以及其他打破常规分类的拓扑绝缘体及拓扑半金属材料等,并针对各个材料,详细总结了其生长方法.     

基于磁激元效应的石墨烯-金属纳米结构近红外吸收研究

石墨烯作为一种单层碳原子二维材料,在可见光和近红外波段吸收率只有2.3%左右,这限制了石墨烯在光电探测、光电调制等领域的应用.本文基于纳米超材料结构的磁激元共振效应,设计了一种金属-绝缘层-金属-石墨烯混合二维浅光栅结构,通过设计混合二维浅光栅结构尺寸来改变石墨烯化学势,实现了石墨烯在近红外波段的吸收增强和调制.利用有限元仿真和等效电路模型,系统地分析了非正入射、结构参数和石墨烯化学势对吸收特性的影响.研究结果表明,混合二维浅光栅结构的磁激元共振效应可以明显提升石墨烯在近红外波段的吸收率,并且对入射角度和极化方向不敏感.在特定结构参数下,混合二维浅光栅结构在1480nm处吸收率达到了85%,其中石墨烯的吸收率为55%,提升了24倍;通过调控石墨烯化学势从0.1eV增大到1.0eV,分别实现了不同结构尺寸下54.8%,50.3%,46.8%的反射率调制深度.     

有限温度张量重正化群方法及其在阻挫量子磁性研究中的应用

阻挫量子磁体中的新奇物态与效应是凝聚态物理研究的重要前沿方向,因其与高温超导、拓扑量子计算等的密切联系,近年来吸引了人们浓厚的研究兴趣。实验上,阻挫自旋液体候选材料的 新进展层出不穷,人们系统地研究了若干三角晶格、笼目晶格和六角Kitaev 阻挫磁体等材料,发 现其在一定条件下展现出自旋液体态的特征,但澄清其中的量子物态是充满挑战的量子多体问题。 作者最近的工作指出,可以从有限温度张量重正化群多体计算入手,开展热力学性质的精确计算 与分析,确定阻挫磁体的微观自旋模型,做出进一步理论预言并开展实验验证,从而建立量子磁性 系统的多体计算精确研究方案。有限温度张量重正化群方法是计算大尺寸二维阻挫量子自旋模型 有限温度性质的有力工具,在本文中作者首先介绍新近发展的系列张量重正化群方法,包括线性 和指数张量重正化群等。随后,作者讨论有限温度张量方法在三角晶格量子伊辛磁体TmMgGaO4 和六角晶格Kitaev 磁体α-RuCl3 的微观自旋模型中的具体应用：通过高精度和全面的多体计算, 揭示出其中存在演生U(1) 对称性与拓扑相变,以及高场量子自旋液体态等新颖的结论,这些理 论预言也陆续被实验所证实。通过上述实例,作者展示了有限温度张量重正化群计算方法在自旋 液体候选材料研究中的应用价值,并期待这些方法能在强关联量子物质研究中发挥重要作用。     

自旋轨道耦合视角下的二维狄拉克材料

石墨烯的发现引发了科学界探索新奇二维狄拉克材料的热潮。这类具有狄拉克线性能带色散特征的二维材料被认为是发展新物理,实现超高速、低能耗新型纳米器件的理想平台。文章首先回顾二维狄拉克材料研究的源起,然后从自旋轨道耦合作用的视角重点介绍四类代表性的二维狄拉克材料的研究进展,最后对相关领域进行总结和展望。     

Two-dimensional silicon-carbon hybrids with a honeycomb lattice: New family for two-dimensional photovoltaic materials

We predict a series of new two-dimensional(2D) inorganic materials made of silicon and carbon elements(2D SixC1?x) based on density functional theory. Our calculations on optimized structure, phonon dispersion, and finite temperature molecular dynamics confirm the stability of 2D SixC1?x sheets in a two-dimensional, graphene-like, honeycomb lattice. The electronic band gaps vary from zero to 2.5 e V as the ratio x changes in 2D SixC1?x changes, suggesting a versatile electronic structure in these sheets. Interestingly, among these structures Si0.25C0.75 and Si0.75C0.25 with graphene-like superlattices are semimetals with zero band gap as their ? and ?* bands cross linearly at the Fermi level. Atomic structural searches based on particle-swarm optimization show that the ordered 2D SixC1?x structures are energetically favorable. Optical absorption calculations demonstrate that the 2D silicon-carbon hybrid materials have strong photoabsorption in visible light region, which hold promising potential in photovoltaic applications. Such unique electronic and optical properties in 2D SixC1?x have profound implications in nanoelectronic and photovoltaic device applications.     

反铁磁半金属 EuCd2Pn2（Pn = As,Sb）中磁交换诱导的外尔态

由于丰富的拓扑量子效应及巨大的潜在应用价值,拓扑材料逐渐成为凝聚态物理前沿的研究材料体系。其中,作为与石墨烯具有相似电子结构的材料,三维拓扑半金属吸引了越来越多的研究兴趣。目前已知的拓扑半金属大多为非磁性的,而磁性拓扑半金属数量有限,与非磁性拓扑半金属相比较,研究开展的还比较少。磁性与拓扑之间的相互作用能够导致非常规的物理性质,如反常霍尔效应甚至量子反常霍尔效应等。此外,在一些具有特殊磁结构的拓扑半金属中,施加外磁场能够调制其自旋结构,从而影响其拓扑能带结构。在该综述中,笔者将详细介绍利用外磁场在 EuCd2Pn2 (Pn = As, Sb) 反铁磁半金属材料中通过调制自旋结构从而改变晶体结构对称性来诱导拓扑相变。此外,笔者也将简单介绍包括 GdPtBi 和 MnBi2Te4 在内的几个相关材料。该综述中讨论的外磁场调控的磁交换诱导的拓扑相变不仅有望应用于拓扑器件,也有助于为理解磁性与拓扑态之间的紧密关联提供新的线索,对于设计新的磁性拓扑材料有启发意义。综述最后,笔者对发展磁性拓扑半金属做了一些简单展望。     

拓扑材料中的超导

近来,人们在凝聚态体系中发现了由拓扑不变量定义的物相,其中最重要的有拓扑绝缘体、拓扑半金属和拓扑超导体等.这些物相的拓扑性质由非平凡的拓扑数描述,相应的材料被称为拓扑材料,具有诸多新奇的物理特性.其中拓扑超导体由于边界上有满足非阿贝尔统计的Majorana零能模,成为实现拓扑量子计算的主要候选材料.除了探索本征的拓扑超导体外,由于拓扑性质上的相似性,在不超导的拓扑材料中调制出超导自然成为了实现拓扑超导的重要手段.目前,人们发展了栅极调制、掺杂、高压、近邻效应调制和硬针尖点接触等多种技术,已经成功地在许多拓扑绝缘体和半金属中诱导出了超导,并对超导的拓扑性和Majorana零能模进行了研究.本文回顾了本征拓扑超导候选材料,以及拓扑绝缘体和半金属中诱导出超导的代表性工作,评述了不同实验手段的优势和缺陷、分析了其超导拓扑性的证据,并提出展望.     

Graphene-like physics in optical lattices

Graphene has attracted enormous attention over the past years in condensed matter physics. The most interesting feature of graphene is that its low-energy excitations are relativistic Dirac fermions. Such feature is the origin of many topological properties in graphene-like physics. On the other hand, ultracold quantum gas trapped in an optical lattice has become a unique setting for quantum simulation of condensed matter physics. Here, we mainly review our recent work on quantum simulation of graphene-like physics with ultracold atoms trapped in a honeycomb or square optical lattice, including the simulation of Dirac fermions and quantum Hall effect with and without Landau levels. We also present the related experimental advances.     

Study on ultrasound-assisted liquid-phase exfoliation for preparing graphene-like molybdenum disulfide nanosheets

Nanomaterials with graphene-like structures have many excellent properties different from bulk materials and have become one of the most popular international frontiers in recent years. In this paper, graphene-like molybdenum disulfide materials are prepared by ultrasonic exfoliation method assisted with 1-Dodecanethiol. Double solvent ultrasonic exfoliation is realized by introducing chloroform. In addition, the optimal preparation conditions for the preparation of graphene-based molybdenum disulfide are investigated from the aspects of ultrasonic processing time and volume ratio of 1-Dodecanethiol and chloroform. Ultrasonic cleaning instrument is used in the experiment, it’s type is KQ3200E (40KHz, 150 W). Results show that the prepared graphene-like molybdenum disulfide has the highest concentration when the volume ratios of 1-Dodecanethiol and chloroform is 1:1 and ultrasonic processing time is 12 h. The dispersion of graphene-like molybdenum disulfide in low-boiling organic solvents, such as chloroform, tetrahydrofuran (THF), isopropanol (IPA), acetone, acetonitrile (CH₃CN) and ethanol, is realized by solvent exchange method. Characterized by transmission electron microscopy, a relatively thin sheet material is obtained. In addition, the latest methods for preparing graphene-like molybdenum disulfide nanosheet are discussed in view of micromechanical exfoliation, lithium ion intercalation, chemical vapor deposition and chemical synthesis, the advantages and limitations of various preparation methods are compared, the applications of graphene-like molybdenum disulfide nanosheets are also investigated.     

Graphene-like Be_3X_2(X=C,Si, Ge,Sn):A new family of two-dimensional topological insulators

Using first-principle calculations, we predict a new family of stable two-dimensional(2 D) topological insulators(TI),monolayer Be_3 X_2(X = C,Si, Ge, Sn) with honeycomb Kagome lattice. Based on the configuration of Be_3 C_2, which has been reported to be a 2 D Dirac material, we construct the other three 2 D materials and confirm their stability according to their chemical bonding properties and phonon-dispersion relationships. Because of their tiny spin-orbit coupling(SOC)gaps, Be_3 C_2 and Be_3 Si_2 are 2 D Dirac materials with high Fermi velocity at the same order of magnitude as that of graphene.For Be3 Ge2 and Be_3 Sn_2,the SOC gaps are 1.5 meV and 11.7 meV, and their topological nontrivial properties are also confirmed by their semi-infinite Dirac edge states. Our findings not only extend the family of 2 D Dirac materials, but also open an avenue to track new 2 DTI.     

Magnetic and topological properties in hydrogenated transition metal dichalcogenide monolayers

Two-dimensional (2D) transition metal dichalcogenide (TMD) monolayers have currently been of immense interest in materials research because of their versatility, and tunable electronic and magnetic properties. In this study, we systematically studied the electronic and magnetic properties in pristine and hydrogenated 1T, 1T’, and 2H TMD monolayers. We found Group IV (Ti, Zr, and Hf), VI (Cr, Mo, and W), and X (Ni, Pd, and Pt) pristine TMD monolayers, respectively, mostly adopted 1T, 2H, and 1T as their stable structures, except for WTe₂ which exhibits 1T’. The stable 1T’ structure only exists for pristine WTe₂ and it had been identified as a topological insulator with a band gap of 0.11 eV. Upon hydrogenation, a structural phase transition occurred from 1T to 2H in Group IV, while for Group X, the stable structure remained 1T. For Group VI, the stable phase transitioned from 1T to 2H or 1T’ phases. Moreover, we found nineteen 2D magnetic materials through hydrogenation. Finally, further exploration of band topologies under hybrid functional calculations revealed that four of these identified magnetic monolayer structures exhibit quantum anomalous Hall effect. Our findings show that hydrogenated TMDs provide a new ground in searching for materials which have the potential for spintronics applications.     

Recent progress in quantum chemistry of hetero[8]circulenes

ABSTRACT

This mini-review presents recent advances in theory of electronic and spectral properties of hetero[8]circulenes used as promising fluorescent emitters for organic light-emitting diodes. Special attention is paid to the possibility of their further functionalisation into one-dimensional and two-dimensional (2D) materials. Such materials are predicted to be useful ambipolar organic semiconductors showing high charge carrier mobility. The porous structure of hetero[8]circulene-based nano-arrays can also provide suitable hydrogen storage materials, biomimetic-type nanopores and ionic channels. They serve as a good example of the density functional theory application for design of stable 2D structures, which extends the family of graphene-like materials.     

Progress in functional studies of transition metal borides

In recent years, transition metal borides (TMBs) have attracted much attention because they are considered as potential superhard materials and have more abundant crystal structures compared with traditional superhard materials. So far, however, no superhard materials have been found in TMBs. A large number of structures and potential new properties in TMBs are induced by the various hybridization ways of boron atoms and the high valence electrons of transition metals, which provide many possibilities for its application. And most TMBs have layered structures, which make TMBs have the potential to be a two-dimensional (2D) material. The 2D materials have novel properties, but the research on 2D TMBs is still nearly blank. In this paper, the research progress of TMBs is summarized involving structure, mechanical properties, and multifunctional properties. The strong covalent bonds of boron atoms in TMBs can form one-dimensional, two-dimensional, and three-dimensional substructures, and the multiple electron transfer between transition metal and boron leads to a variety of chemical bonds in TMBs, which are the keys to obtain high hardness and multifunctional properties of TMBs. Further research on the multifunctional properties of TMBs, such as superconductors, catalysts, and high hardness ferromagnetic materials, is of great significance to the discovery of new multifunctional hard materials.