¦ van der Waals gap Septuple layer J § J || T 1/2 J 0,1 || J 0,2 || J 0,3 || J 0,4 ||

The electronic structure of Bi(001) ultrathin films (thickness > or =7 bilayers) on Si(111)-7x7 was studied by angle-resolved photoemission spectroscopy and first-principles calculations. In contrast with the semimetallic nature of bulk Bi, both the experiment and theory demonstrate the metallic character of the films with the Fermi surface formed by spin-orbit-split surface states (SSs) showing little thickness dependence. Below the Fermi level, we clearly detected quantum well states (QWSs) at the M point, which were surprisingly found to be non-spin-orbit split; the films are "electronically symmetric" despite the obvious structural nonequivalence of the top and bottom interfaces. We found that the SSs hybridize with the QWSs near M and lose their spin-orbit-split character.

Electron dynamics in the bulk and at the surface of solid materials are well known to play a key role in a variety of physical and chemical phenomena. In this article we describe the main aspects of the interaction of low-energy electrons with solids, and report extensive calculations of inelastic lifetimes of both low-energy electrons in bulk materials and image-potential states at metal surfaces. New calculations of inelastic lifetimes in a homogeneous electron gas are presented, by using various well-known representations of the electronic response of the medium. Band-structure calculations, which have been recently carried out by the authors and collaborators, are reviewed, and future work is addressed. q

A topological insulator is a state of quantum matter that, while being an insulator in the bulk, hosts topologically protected electronic states at the surface. These states open the opportunity to realize a number of new applications in spintronics and quantum computing. To take advantage of their peculiar properties, topological insulators should be tuned in such a way that ideal and isolated Dirac cones are located within the topological transport regime without any scattering channels. Here we report ab-initio calculations, spin-resolved photoemission and scanning tunnelling microscopy experiments that demonstrate that the conducting states can effectively tuned within the concept of a homologous series that is formed by the binary chalcogenides (Bi 2 Te 3 , Bi 2 se 3 and sb 2 Te 3 ), with the addition of a third element of the group IV.

The electronic structure of ultrathin Bi͑001͒ films on Si͑111͒-7 ϫ 7 was studied by spin and angle-resolved photoemission spectroscopy. We directly observed a clear momentum-dependent spin splitting and polarization of the surface-state bands. The spin structure was antisymmetric with respect to the ⌫ point as predicted by theory, and the obtained in-plane spin polarization was as high as ±0.5. The qualitative features of the observed spin polarization are discussed in comparison with the spin-polarized band structure obtained by first-principles calculations. DOI: 10.1103/PhysRevB.76.153305 PACS number͑s͒: 79.60.Bm, 68.35.Ϫp, 73.20.Ϫr, 85.75.Ϫd Spintronics, which aims at the utilization of the spin degree of freedom, has attracted wide interest due to its potential in realizing new functionalities in electronic devices. 1Spin manipulation is the key factor in spintronics, and the conventional style was to develop novel ferromagnetic materials.2 Recently, on the other hand, it was found that spin-split two-dimensional electron gases can be formed in asymmetric quantum wells controlled by an electric field even for nonmagnetic materials.3 This is called the Rashba effect, which is a combined effect of the spin-orbit interaction and structural inversion asymmetry ͑SIA͒. 4At the crystal surface, the same effect occurs and spinorbit split band structures have been found for Au͑111͒ ͑Refs. 5 and 6͒ and W͑110͒-H ͑Ref. 7͒ surfaces. This splitting is caused by the spin-orbit coupling Hamiltonian, H soc = ͑ប /4m e 2 c 2 ͒ ជ · ٌ͑V ϫ p ជ͒, where ជ is the spin of electrons, V the one-electron potential, and p ជ the momentum. 8 Usually in the bulk, the time-reversal symmetry ͓E͑k ជ , ↑ ͒ = E͑−k ជ , ↓ ͔͒ and the space-inversion symmetry ͓E͑k ជ , ↑ ͒ = E͑−k ជ , ↑ ͔͒ lead to the Kramers degeneracy. However, at the crystal surface, due to the SIA in the surface-normal direction, the degeneracy will be lifted. The spin orientation of such states is perpendicular both to the momentum p ជ and to the electric field ٌV, meaning an in-plane spin polarization antisymmetric about k ជ = 0, as the electric field is perpendicular to the surface. 8Bismuth ͑Bi͒ is a very heavy element and its electronic structure is highly influenced by the spin-orbit interaction.9 It was recently shown from angle-resolved photoemission spectroscopy ͑ARPES͒ measurements that the surface states of Bi crystals are highly metallic in contrast to the semimetallic nature of bulk Bi ͑Fig. 1͒. [10][11][12][13] Additionally, it was suggested by ab initio calculations that they will show large Rashba splitting due to the significant spin-orbit coupling.12,13 Furthermore, in a recent theoretical study, 14 two-dimensional Bi bilayers were predicted to show the quantum spin Hall ͑QSH͒ effect, and it was said that these surface states may have some relations with the edge modes that characterize the QSH phase. The spin property of the highly metallic surface states of semimetallic Bi films also has importance in application to spintronics. However, although some insi...

BiTeI has a layered and noncentrosymmetric structure where strong spin-orbit interaction leads to a giant Rashba spin splitting in the bulk bands. We present direct measurements of the bulk band structure obtained with soft x-ray angle-resolved photoemission (ARPES), revealing the three-dimensional Fermi surface. The observed spindle torus shape bears the potential for a topological transition in the bulk by hole doping. Moreover, the bulk electronic structure is clearly disentangled from the two-dimensional surface electronic structure by means of high-resolution and spin-resolved ARPES measurements in the ultraviolet regime. All findings are supported by ab initio calculations.

The electronic band structure of a material can acquire interesting topological properties in the presence of a magnetic field or as a result of the spin-orbit coupling [1][2][3] . We study graphene on Ir, with Pb monolayer islands intercalated between the graphene sheet and the Ir surface. Although the graphene layer is structurally una ected by the presence of the Pb islands, its electronic properties change markedly, with regularly spaced resonances appearing. We interpret these resonances as the e ect of a strong and spatially modulated spin-orbit coupling, induced in graphene by the Pb monolayer. As well as confined electronic states, the electronic spectrum has a series of gaps with non-trivial topological properties, resembling a realization of the quantum spin Hall e ect proposed by Bernevig and Zhang 4

Inducing magnetism into topological insulators is intriguing for utilizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) for technological applications. While most studies have focused on doping magnetic impurities to open a gap at the surface-state Dirac point, many undesirable effects have been reported to appear in some cases that makes it difficult to determine whether the gap opening is due to the time-reversal symmetry breaking or not. Furthermore, the realization of the QAHE has been limited to low temperatures. Here we have succeeded in generating a massive Dirac cone in a MnBi2Se4/Bi2Se3 heterostructure, which was fabricated by self-assembling a MnBi2Se4 layer on top of the Bi2Se3 surface as a result of the codeposition of Mn and Se. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the fabricated MnBi2Se4/Bi2Se3 heterostructure shows ferromagnetism up to room temperature and a clear Dirac cone gap opening of ∼100 meV without any other significant changes in the rest of the band structure. It can be considered as a result of the direct interaction of the surface Dirac cone and the magnetic layer rather than a magnetic proximity effect. This spontaneously formed self-assembled heterostructure with a massive Dirac spectrum, characterized by a nontrivial Chern number C = −1, has a potential to realize the QAHE at significantly higher temperatures than reported up to now and can serve as a platform for developing future “topotronics” devices.