拓扑近藤绝缘体

拓扑近藤绝缘体自10年前被提出后,很快成为第一个被实验证实的强关联拓扑材料。文章回顾了拓扑近藤绝缘体六硼化钐中的重要实验结果,并进一步分析了该材料中的关键科学问题和对未来研究的展望。     

Ce 基重费米子材料的电子态研究与维度调控

重费米子材料作为一类典型的强关联电子体系,蕴含着非常规超导、奇异金属、量子临界、 磁有序、重电子态、关联拓扑态等新奇的量子态,而4f 电子在其中扮演着重要的作用。随着高分 辨角分辨光电子能谱和薄膜生长技术的发展,精确探测重费米子材料中4f 电子在能量/动量空间 的色散和谱权重成为了可能,这为从微观上理解这类材料中的电子关联效应和新奇量子现象提供 了重要的基础。本论文总结了几个典型的重费米子单晶和薄膜体系的电子态研究,包括Ce-115 体 系、CeCu2Si2、CeRh6Ge4 以及单晶 Ce 膜等。这些结果为理解重费米子体系中重电子态的形成 和温度演化、近藤杂化的能带/动量依赖、重电子能带与超导的关系、近藤效应与磁性和其它量子 态的竞争、4f 电子的维度调控等重要物理问题提供了谱学证据。     

5d过渡金属氧化物—Weyl半金属奇异量子物性研究

绝缘体系统的拓扑量子行为是当前物理学领域的焦点。那么没有能隙的金属体系是否也有拓扑非平庸行为呢?答案是肯定的,5d过渡金属元素既有着大的自旋轨道耦合,又有着不可忽略的电子关联作用,由其构成的材料体系因而具有奇异的量子物性。本文先简介人们最近关注的几类5d材料,着重讨论烧绿石结构铱(Ir)氧化物(A2Ir2O7,其中A=Y或稀土元素)。我们通过第一性原理计算结合有效模型成功地预言了这一大类磁阻挫材料的基态磁构型,并预言A2Ir2O7是Weyl半金属。我们发现和拓扑绝缘体一样,作为一种新型拓扑量子态,Weyl半金属态也具有其拓扑性质:Weyl点是受拓扑保护稳定的;Weyl半金属有着受拓扑保护的表面态,即非闭合的费米面(Fermi arc);它对外场的响应也由其拓扑性质决定(只与Weyl点的位置有关,和能带的细节无关)。我们进一步预言尖晶石结构锇(Os)氧化物(AOs2O4,其中A=Ca或Sr)是有着特别磁电响应的Axion绝缘体。最后我们讨论了电子关联、自旋轨道耦合、磁序结构对钙态矿结构锇氧化物的影响,进一步确定NaOsO3是由磁序导致金属–绝缘体相变的Slater绝缘体。     

拓扑绝缘体/超导体异质结中的近邻效应

拓扑超导体自身具有对量子退相干天然的免疫性以及可编织性,这使得它在现代量子计算领域中受到了越来越多的重视,并且成为了下一代计算技术中最有希望的候选者之一。由于拓扑超导态在固有拓扑超导体中相当罕见,因此,当前大部分实验上的工作主要集中在由 s 波超导体与拓扑绝缘体之间通过近邻效应所诱导的拓扑超导体上。本论文中,我们回顾了基于拓扑绝缘体/超导体异质结的拓扑超导体的研究进展。在理论上,Fu 和 Kane 提出,通过近邻效应将 s 波超导体的能隙引入到拓扑绝缘体,可以诱导出拓扑超导电性。在实验上,我们也回顾了一些不同体系中的拓扑超导近邻效应的研究进展。文章的第一部分,我们介绍了一些异质结,包括：三维拓扑绝缘体 Bi2Se3和 Bi2Se3 与 s 波超导体NbSe2 以及 d 波超导体 Bi2Sr2CaCu2O8+δ 的异质结,拓扑绝缘体 Sn1−xPbxTe 与 Pb 的异质结,二维拓扑绝缘体 WTe2 与NbSe2 的异质结。此外,还介绍了 TiBiSe2 在 Pb 上的拓扑绝缘近邻效应。另一部分中,我们对基于拓扑绝缘体的约瑟夫森结进行了回顾,包括著名的基于 Fu-Kane 体系的拓扑绝缘体约瑟夫森结,以及基于约瑟夫森结的超导量子干涉器件。     

拓扑量子体系中的长波光电探测

拓扑绝缘体是一类体态绝缘而表面态具有金属特性的材料,它作为一种全新的量子物质,引起了人们对新型拓扑相的关注。寻找自然界中新的拓扑材料已经成为近十几年来凝聚态物理中的研究热点。继拓扑绝缘体之后,能带结构的拓扑分类又被扩展到了金属体系,出现了另一类具有特殊电子结构的拓扑材料,即拓扑半金属。     

拓扑狄拉克半金属

<正>拓扑狄拉克半金属是一种全新的奇特拓扑量子材料。这种材料的体电子形成了三维的狄拉克锥结构,所以可以看作是"三维的石墨烯"。另外,由于这种材料的电子结构具有非平庸的拓扑性质,它也有和拓扑绝缘体类似的表面态。这些独特的电子结构在最近的高分辨光电子谱实验中得到了证实。封面图中是Na3Bi,第一个被实验证实的三维拓扑狄拉克半金属的能带结构:内部的线     

强关联电子系统中的热输运测量

当考虑电子间的库伦排斥相互作用,以及电荷、自旋和轨道之间的相互耦合时,诸多超越 了近自由电子框架的新奇量子态涌现而出,如非常规超导态和量子自旋液体等。对这些新奇物态 的认知不仅会拓展现有的知识框架,也有望引发新一轮的量子科技革命。因此,对强关联物理的 研究是当下凝聚态物理领域的前沿课题。铜基高温超导体的母体是一种莫特绝缘体,在传统的能 带论之下被预言为金属态。然而电子间的强关联行为使得它表现出绝缘体的性质。由于莫特绝缘 体中库伦相互作用致使能隙打开并冻结其中的电荷自由度,所以在该体系中难以开展电输运性质 的测量研究。作为一种对于元激发（不仅包括电子,还包括磁振子、自旋子等）敏感的探针,热输 运测量在强关联电子系统的研究中发挥着重要的作用。本文回顾了近些年在非常规超导、重费米 子系统和量子自旋液体研究中一些有趣的纵向热输运性质的研究成果,并与我们近期发表的运用 横向热导率测量热霍尔现象的综述文章相互补充。      

Li(Na)AuS体系拓扑绝缘体材料的能带结构

在拓扑领域中发现可以通过大数据搜索拓扑绝缘体,使得此领域对材料的探索转变为对材料性质的研究.半Heusler合金体系是非平庸拓扑绝缘体材料的重要载体.通过全势线性缀加平面波方法计算Li(Na)AuS体系拓扑绝缘体材料的能带结构.采用各种关联泛函计算LiAuS的平衡晶格常数,发现得到的能带图均为具有反带结构的拓扑绝缘体,而且打开了自然带隙.较小的单轴应力破坏立方结构后也破坏了此类拓扑绝缘体的自然带隙,通过施加单轴拉应力直到四方结构的平衡位置时,系统带隙值约为0.2 eV,这与立方结构平衡位置得到的带隙结果一致.运用同族元素替代的手段,实现了在保证材料拓扑绝缘体性质的同时,不改变立方结构,在体系的平衡晶格常数下使得材料的带隙打开,从而提高了实验合成拓扑绝缘体材料的可行性.     

拓扑近藤半金属

拓扑物态是凝聚态物理近年来最重要的研究领域之一。随着研究的不断深入,对拓扑物态的研究逐渐从弱关联材料体系拓展到了强关联材料体系。文章梳理了近年来对拓扑近藤半金属的相关研究,介绍了其中的理论模型、计算方法和一些候选材料及实验研究,并对该方向未来的发展做了展望。     

拓扑绝缘体表面态的STM研究

文章主要介绍了利用扫描隧道显微镜对拓扑绝缘体表面态进行的一系列研究工作,包括拓扑绝缘体表面态的电子驻波以及拓扑表面态的朗道量子化现象.这些工作对于拓扑绝缘体基本性质的确立以及深入理解具有十分重要的意义.     

Magnetic Properties of the Topological Kondo Insulator SmB6: Localized Magnetic Moments and Pauli Paramagnetism

JETP Letters - The magnetic properties of the topological Kondo insulator SmB6 are studied in the temperature range T &lt; 10 K in an applied magnetic field up to 5 T. The analysis of the...     

Magnetic and topological transitions in three-dimensional topological Kondo insulator

Using an extended slave-boson method,we draw a global phase diagram summarizing both magnetic phases and paramagnetic(PM) topological insulators(TIs) in a three-dimensional topological Kondo insulator(TKI). By including electron hopping(EH) up to the third neighbors, we identify four strong TI(STI) phases and two weak TI(WTI) phases. Then, the PM phase diagrams characterizing topological transitions between these TIs are depicted as functions of EH,f-electron energy level,and hybridization constant. We also find an insulator-metal transition from an STI phase that has surface Fermi rings and spin textures in qualitative agreement with the TKI candidate SmBs. In the weak hybridization regime, antiferromagnetic(AF) order naturally arises in the phase diagrams. Depending on how the magnetic boundary crosses the PM topological transition lines,AF phases are classified into the AF topological insulator(AFTI) and the non-topological AF insulator, according to their Z_2 indices. In two small regions of parameter space, two distinct topological transition processes between AF phases occur, leading to two types of AFTIs showing distinguishable surface dispersions around their Dirac points.     

Finite temperature physics of 1D topological Kondo insulator: Stable Haldane phase,emergent energy scale and beyond

In recent years, interacting topological insulators have emerged as new frontiers in condensed matter physics, and the hotly studied topological Kondo insulator (TKI) is one of such prototypes. Although its zero-temperature ground-state has been widely investigated, the finite temperature physics on TKI is largely unknown. Here, we explore the finite temperature properties in a simplified model for TKI, namely the one-dimensional p-wave periodic Anderson model, with numerically exact determinant quantum Monte Carlo simulation. It is found that the topological Haldane phase established for groundstate is still stable against small thermal fluctuation and its characteristic edge magnetization develops at low temperature. Such facts emphasize the robustness of (symmetry-protected) topological order against temperature effect, which always exists at real physical world. Moreover, we use the saturated low-T spin structure factor and the \frac{1}{T}-law of susceptibility to detect the free edge spin moment, interestingly the low-temperature upturn behavior of the latter one is similar to experimental finding in SmB₆ at T<50 K. It implies that similar physical mechanism may work both for idealized models and realistic correlated electron materials. We have also identified an emergent energy scale T_cr, which signals a crossover into interesting low-T regime and seems to be the expected Ruderman–Kittel–Kasuya–Yosida coupling. Finally, the collective Kondo screening effect has been examined and it is heavily reduced at boundary, which may give a fruitful playground for novel physics beyond the wellestablished Haldane phase and topological band insulators.     

High-pressure ground state of SmB6: electronic conduction and long range magnetic order

High-pressure 149Sm nuclear forward scattering of synchrotron radiation and specific heat measurements have been performed on the intermediate valent Kondo insulator SmB6. The results show that at a critical pressure p(c) approximately = 6 GPa, where the charge gap closes, a first order transition occurs to a magnetically ordered state, which shows typical features of trivalent samarium compounds. The similarity with SmS stresses the role of local correlations and gives important insight into the debate on the local or itinerant character of the f electrons in heavy fermion systems.     

Photoexcited electron dynamics in Kondo insulators and heavy fermions

We have studied the photoexcited carrier relaxation dynamics in the Kondo insulator SmB6 and the heavy fermion metal YbAgCu4 as a function of temperature and excitation level. The dynamic response is found to be both strongly temperature dependent and nonlinear. The data are analyzed with a Rothwarf-Taylor bottleneck model, where the dynamics are governed by the presence of a narrow gap in the density of states near the Fermi level. The remarkable agreement with the model suggests that carrier relaxation in a broad class of heavy electron systems (both metals and insulators) is governed by the presence of a (weakly temperature dependent) hybridization gap.     

Angle-resolved photoemission spectroscopy study of the (101) surface of the Kondo insulator CeNiSn

The electronic structure of CeNiSn, which is considered a possible topological Kondo insulator, has been investigated by employing synchrotron radiation excited angle-resolved photoemission spectroscopy (ARPES). We have found that the easy cleavage plane in CeNiSn is (101), for which we have investigated the Fermi surface (FS) and band structures. The measured FS and ARPES for the (101) plane are described well by the calculated FS and band structures, obtained from the DFT calculations. The measured ARPES bands and photon energy map show that the metallic states crossing the Fermi level have the 3D nature, casting a negative suspicion for the existence of the topological surface states of the 2D character in CeNiSn. The Ce 4f Kondo resonance peak is observed in Ce 4d → 4f resonant photoemission spectroscopy, suggesting the importance of the Ce 4f electrons in determining the temperature-dependent topological electronic structure of CeNiSn.     

Electronic properties of SnTe-class topological crystalline insulator materials

The rise of topological insulators in recent years has broken new ground both in the conceptual cognition of condensed matter physics and the promising revolution of the electronic devices.It also stimulates the explorations of more topological states of matter.Topological crystalline insulator is a new topological phase,which combines the electronic topology and crystal symmetry together.In this article,we review the recent progress in the studies of SnTe-class topological crystalline insulator materials.Starting from the topological identifications in the aspects of the bulk topology,surface states calculations,and experimental observations,we present the electronic properties of topological crystalline insulators under various perturbations,including native defect,chemical doping,strain,and thickness-dependent confinement effects,and then discuss their unique quantum transport properties,such as valley-selective filtering and helicity-resolved functionalities for Dirac fermions.The rich properties and high tunability make SnTe-class materials promising candidates for novel quantum devices.