拓扑近藤绝缘体SmB6中的奇异电子性质

拓扑近藤绝缘体是一种本征的强关联拓扑电子体系,其体能隙来源于近藤关联效应。自2010年拓扑近藤绝缘体的理论概念被提出后,六硼化钐(SmB6) 作为第一种被预测为拓扑近藤绝缘体的材料在这十多年中被多种实验手段反复研究验证,被广泛接受认为是第一种拓扑近藤绝缘体。在这篇综述中,我们回顾了关于SmB6 的一些重要实验结果,比如电输运测量,角分辨光电子能谱(ARPES), 表面形貌分析(STM) 等,并论述了如何通过这些关键的实验证据证实SmB6 的拓扑近藤绝缘物相。同时,我们也展示了SmB6 这一关联电子体系的其他奇异物性,包括中间价态在表面和体内的分离现象,以及量子振荡发现的体振荡信号等等。这些性质表明我们对SmB6 这一材料的理解仍然不充分,其中还有更为丰富的物理值得挖掘。     

Angle-resolved photoemission spectroscopy studies of electron-electron interactions in graphene

Graphene is an emergent research topic that has attracted a huge amount of research interest ever since its experimental demonstration as a two-dimensional realization of Dirac fermions in 2005. In subsequent years, the research on graphene has rapidly expanded its field not only due to the new paradigm to study relativistic high energy physics in a condensed matter, but also due to its potential in the application for next generation devices. Most of the novel phenomena observed so far in graphene are attributed to its low-energy excitations, which is described by those of relativistic Dirac fermions. This article reviews recent progress in angle-resolved photoemission spectroscopy studies of electron-electron interactions in graphene.     

铜氧化物高温超导体的角分辨光电子能谱研究

由于能量和动量分辨率以及实验效率的大幅度提高，角分辨光电子能谱在过去的20年取得了革命性的进展，成为当今凝聚态物理最重要的实验手段之一．文章综述了20年来角分辨光电子能谱在高温超导领域所取得的研究成果，并通过6个例子来阐明如何利用角分辨光电子能谱来理解铜氧化物超导体丰富的相图，以及对应的多体物理现象．     

高温超导体电子结构和超导机理的角分辨光电子能谱研究

超导是一种奇异的宏观量子现象.100多年来,已发现的超导体主要分为两类:以金属或者合金为代表的常规超导体以及以铜氧化物和铁基高温超导体为代表的非常规超导体.常规超导体的超导机理能被BCS超导理论完美解释,但高温超导体的超导机理至今仍未达成共识,已经成为凝聚态物理领域中长期争论且充满挑战的重大科学问题.从实验上揭示非常规超导材料的微观电子结构,是理解其奇异正常态和超导电性机理、建立新理论的前提和基础.角分辨光电子能谱技术,由于可以实现对材料中电子的能量、动量和自旋的直接测量,在高温超导研究中发挥了重要的作用.本文综述了我们利用角分辨光电子能谱技术在铜氧化物和铁基高温超导体电子结构和超导机理研究中取得的一些进展,主要包括母体的电子结构、正常态的非费米液体行为、超导态的能带和超导能隙结构以及多体相互作用等.这些结果为理解铜氧化物和铁基高温超导体的物性及超导机理提供了重要的信息.     

Magnetically confined states and transport property on the surface of a topological insulator

We study the electronic structure and transport for a quasi-one-dimensional channel constructed via two ferromagnetic (FM) stripes on the surface of a three-dimensional (3D) topological insulator (TI) in parallel (P) or antiparallel (AP) magnetization configuration along the vertical z$z$-direction. We demonstrate that the confined states which are localized inside the channel always exist due to the magnetic potential confinement. Interestingly, the channel is metallic because of the existence of a topologically protected gapless chiral edge mode in the case of AP configuration. The asymmetric spatial-distribution of both electron probability density and in-plane spin polarization for the confined states implies that in the case of P configuration there exists a chiral state near the channel edge owing to the Hamiltonian inversion symmetry broken in real space, while the distributions in AP case are always symmetry since the inversion symmetry is still kept. Furthermore, the transmission probability and the spatial-dependent distributions of charge and spin along a narrow–wide–narrow channel on the surface with P configuration confinement are also calculated, from which a fully in-plane spin-polarized electron output is achieved. Along with the mathematical analysis we provide an intuitive, topological understanding of these effects.     

Itinerant to relocalized transition of f electrons in the Kondo insulator CeRu4Sn6

The three-dimensional electronic structure and the nature of Ce 4f electrons of the Kondo insulator CeRu₄Sn₆ are investigated by angle-resolved photoemission spectroscopy, utilizing tunable photon energies. Our results reveal (i) the three-dimensional k-space nature of the Fermi surface, (ii) the localized-to-itinerant transition of f electrons occurs at a much high temperature than the hybridization gap opening temperature, and (iii) the “relocalization” of itinerant f-electrons below 25 K, which could be the precursor to the establishment of magnetic order.     

Angle-resolved photoemission spectroscopy study on iron-based superconductors

Angle-resolved photoemission spectroscopy （ARPES） has played an important role in determining the band structure and the superconducting gap structure of iron-based superconductors. In this paper, from the ARPES perspective, we briefly review the main results from our group in recent years on the iron-based superconductors and their parent compounds, and depict our current understanding on the antiferromagnetism and superconductivity in these materials.     

Angle resolved photoemission study of surface states on the Pt(997) vicinal surface

One-dimensional atomic chains can be synthesized on stepped surfaces and the electronic structure of the high vicinal surface plays an essential role in determining the physical properties of atomic chains grown on top of it. We have applied surface analysis techniques to study the surface of a Pt(997) single crystal. The STM image of the surface showed that the surface was uniform with a well defined distance between the terraces. Angle resolved photoemission spectroscopy (ARPES) was used to characterize the electronic states of the Pt(997) surface, and confinement of electrons with wave vector perpendicular to the step direction was observed.     

Angle-resolved photoemission study of no chemisorption on Ag(111) surface

An angle-resolved photoemission study using synchrotron radiation has been carried out to investigate the adsorption of NO on Ag(111). At 150 K, NO is adsorbed molecularly on the Ag(111) surface. The NO molecule is found to have an upright orientation from a study using incidence angle dependent measurements and symmetry selection rules. The upright orientation is also confirmed from the study of the shape resonance of the 5σ level. Photon induced dissociation of the chemisorbed NO was observed and is briefly discussed.     

Angle-resolved photoemission study of the Li-induced structures of the Si(001) surface

We have investigated dispersion relations of surface bands induced by Li adsorption on the Si(001) surface at room temperature by Angle-Resolved PhotoE-mission Spectroscopy (ARPES). The characteristic spectral changes in the Energy Distribution Curves (EDCs) with increasing Li coverage implicate that Li adatoms occupy on-top sites of the dimer-forming silicon atoms resulting in 0.5 and 1.0 monolayers for the completion of the first two Li-induced ordered (2×2): Li and (2×1): Li phase, respectively. The unique upward dispersions of the surface bands for these Li-induced structures are ascribed to the size effect of a Li adatom. We further discuss a mechanism which might account for the metallie peak found at the early stage of Li adsorption.     

角分辨光电子能谱仪在f电子特性研究中的应用

重费米子体系是凝聚态物理研究中的热点和难点,其低的能量尺度和奇异物理特性吸引着研究者的注意力。文章首先简要介绍了重费米子体系的基本概念,随后着重介绍了角分辨光电子能谱技术(ARPES)在重费米子体系中的应用。主要包括以下几个方面内容：f电子与传导电子的杂化特征、f电子的杂化与磁性、f电子杂化与量子临界、超导的关系以及掺杂对杂化的影响等。最后介绍了目前ARPES技术在重费米子体系应用中存在的几个问题,并提出相应解决办法。     

重费米子化合物 CemMnIn3m+2n(m = 1, 2, 3; n = 0, 1, 2; M = Co,Rh,Ir,Pt 及Pd) 的角分辨光电子能谱研究

重费米子材料以其新奇多变的宏观性质,复杂而难以理解的微观物理过程而受到广泛的关注,长期是凝聚态物理研究的重点。角分辨光电子能谱作为一种能够直接探测材料电子结构的实验手段,随着近年来实验技术的高度发展,能量和动量分辨率得到了极大的提高,能够有力地探测到强关联材料中更加精细的电子结构。使用角分辨光电子能谱探测重费米子材料的电子结构,为理解其各种物理过程提供强有力的实验证据,进而推进重费米子理论的发展。本文回顾 Ce 基重费米子材料 CemMnIn3m+2n (m = 1; 2; 3;n = 0; 1; 2;M =Co, Rh, Ir, Pt, Pd) 系列化合物的角分辨光电子能谱研究,包括电子结构的维度、近藤共振、f 电子与导带电子的杂化和电子结构随温度的变化等。     

X-ray photoemission studies of the electronic structure of single-crystalline CdO(100)

The electronic structure of single-crystalline CdO is investigated using X-ray photoemission spectroscopy. The surface is prepared by in situ Ar⁺ ion bombardment and annealing (IBA). The core level spectra before and after the IBA are presented, and reveal a dramatic reduction in the surface contamination. The semi-core Cd 4d level and valence band region following the IBA are also shown. The surface Fermi level is pinned 1.3±0.10 eV above the valence band maximum.     

Semiconductor–metal–semiconductor transition: valence band photoemission study of Ag/Si(1 1 1) surfaces

The , and 6×6 phases of Ag/Si(1 1 1) have been studied by angle-resolved photoemission and low-energy electron diffraction. The Ag/Si(1 1 1) surface has an intrinsic semiconducting character with two fully occupied, dispersing surface state bands. We find that only one of the additional surface bands on the surface is metallic in contrast to the two metallic bands discussed in the literature. On the 6×6 surface, the partially occupied surface band of the surface seems to be absent, resulting in a gap of about 0.2 eV with respect to the Fermi-level.     

Analysis on surface charging of insulator prior to flashover in vacuum

Prior to the flashover across an insulator in vacuum, the insulator surface is usually charged. It is of great importance to investigate the charging phenomena for better understanding the flashover characteristics. It is considered that there are two kinds of mechanisms closely related to the surface charging of insulator i.e., the secondary electron emission occurred over an insulator, and while employing a perfect electrode contact ahead of electron emission into vacuum, the charge injection and accumulation occurred inside the surface layer of an insulator. Based on the rigorous analysis of the kinetic processes of both primary and secondary electrons, the related surface charges were theoretically deduced. Involving the detrapping of charges trapped and the recombination of charges injected, the charging process due to charge injection and accumulation was analyzed. Some formulas were given to express the density of surface charges.     

Impurity-induced states on the surface of a three-dimensional Weyl semimetal

We study theoretically the features of impurity-induced states on the surface of a three-dimensional Weyl semimetal in this work. For calculating the impurity-induced local density of states based on T-matrix formulation, we found that for different Weyl semimetal phases the behaviors of a local impurity exhibit distinguishable prominent features for the surface Fermi arc states. Due to two opposite-directional and -chirality surface currents for a surface, a bound state appears at the unitary limit of scattering intensity near the impurity site. Then the resonance condition for different Weyl semimetal phases and scattering intensity is investigated. Our results can be used to identify distinctive topological phases of Weyl semimetal. Furthermore, the relevance of topological nodal-point and -line systems is discussed. Some relation between our theoretical results and current experimental scheme are also discussed.     

Electronic structure of heavy fermion system CePt_2In_7 from angle-resolved photoemission spectroscopy

We have carried out high-resolution angle-resolved photoemission measurements on the Ce-based heavy fermion compound CePt_2In_7 that exhibits stronger two-dimensional character than the prototypical heavy fermion system CeCoIn_5.Multiple Fermi surface sheets and a complex band structure are clearly resolved. We have also performed detailed band structure calculations on CePt_2In_7. The good agreement found between our measurements and the calculations suggests that the band renormalization effect is rather weak in CePt_2In_7. A comparison of the common features of the electronic structure of CePt_2In_7 and CeCoIn_5 indicates that CeCoIn_5 shows a much stronger band renormalization effect than CePt_2In_7. These results provide new information for understanding the heavy fermion behaviors and unconventional superconductivity in Ce-based heavy fermion systems.