Ideal two-dimensional electron systems with a giant Rashba-type spin splitting in real materials: Surfaces of bismuth tellurohalides

Spintronics is aimed at actively controlling and manipulating the spin degrees of freedom in semiconductor devices. A promising way to achieve this goal is to make use of the tunable Rashba effect that relies on the spin-orbit interaction in a two-dimensional electron system immersed in an inversion-asymmetric environment. The spin-orbit-induced spin splitting of the two-dimensional electron state provides a basis for many theoretically proposed spintronic devices. However, the lack of semiconductors with large Rashba effect hinders realization of these devices in actual practice. Here we report on a giant Rashba-type spin splitting in two-dimensional electron systems that reside at tellurium-terminated surfaces of bismuth tellurohalides. Among these semiconductors, BiTeCl stands out for its isotropic metallic surface-state band with the Γ̄-point energy lying deep inside the bulk band gap. The giant spin splitting of this band ensures a substantial spin asymmetry of the inelastic mean free path of quasiparticles with different spin orientations. © 2012 American Physical Society.We acknowledge partial support by the University of the Basque Country (Project No. GV-UPV/EHU, Grant No. IT-366-07) and Ministerio de Ciencia e Inovación (Grant No. FIS2010-19609-C02-00).Peer Reviewe

Role of surface passivation in the formation of Dirac states at polar surfaces of topological crystalline insulators: The case of SnTe(111)

We present ab initio density functional theory (DFT) calculation results for electronic and spin structures of both the Te- and Sn-terminated SnTe(111) polar surfaces. Rocksalt narrow-gap semiconductor SnTe belongs to the recently discovered class of topological crystalline insulators in which the topological nature of surface electronic states arises from the crystal symmetry combined with band inversion at the L point. We demonstrate that in contrast to earlier model calculations only trivial spin-split states propagating over the entire two-dimensional Brillouin zone emerge at the SnTe(111) surfaces owing to the surface potential effect which destroys weakly protected topological states. We show that the surface passivation eradicates the trivial surface states and recovers the even number of the helical spin-polarized topological Dirac cones centered at the Γ¯¯¯ and M¯¯¯ points prescribed for the topological crystalline insulator by the crystal symmetry.We acknowledge partial support from the Basque Country government, Departamento de Educacion, Universidades e Investigacion (Grant No. IT-366-07), the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2010-19609-C02-00), the Ministry of Education and Science of the Russian Federation (Grant No. 2.8575.2013), and the Russian Foundation for Basic Research (Grant No. 13-02-12110-ofi-m).Peer Reviewe

Structure and oscillatory multilayer relaxation of the bismuth (100) surface

13 páginas, 6 figuras, 1 tabla.-- et al.We present a combined experimental and theoretical study of the surface structure of single crystal Bi(100) via scanning tunneling microscopy (STM), low-energy electron diffraction intensity versus energy (LEED-IV) analysis and density functional theory (DFT). We find that the surface is unreconstructed and shows an unusually large oscillatory multilayer relaxation down to the sixth layer. This unexpected behavior will be explained by a novel mechanism related to the deeply penetrating electronic surface states. STM reveals wide (100) terraces, which are separated by two-layer high steps in which the shorter of the two interlayer spacings is terminating this surface, consistent with the LEED structural analysis and DFT.This work was supported by the American National Science Foundation DMR-0134933 and the Petroleum Research Fund under grant no. 46323-AC5, the Danish National Science Foundation, the Basque Country Government and the University of the Basque Country.Peer reviewe

Surface- and edge-states in ultrathin Bi–Sb films

13 páginas, 7 figuras.Employing first-principles calculations, we studied the electronic structure of ultrathin Bi–Sb films, focusing on the appearance of surface or edge states that are topologically protected. Our calculations show that in ordered structures the Bi–Sb bonds are quite strong, forming well-defined double layers that contain both elements.We find surface states appearing on the (111) surface of a thin film of layerwise ordered Bi–Sb compound, while thin films in (110) orientation are insulating. In the gap of this insulator, edge states can be found in a (110)-oriented ribbon in the A17 (black phosphorus) structure. While these states are strongly spin polarized, their topological properties are found to be trivial. In all structures, we investigate the influence of spin–orbit coupling and analyze spin polarization of the states at the boundaries of the material.The financial support of the Deutsche Forschungsgemeinschaft (grant no. BI823/1-1).Peer reviewe

Electronic and crystal structure of the Pt(111)-(√3 × √3)R30°-K system

We present the density functional calculation results for K adsorption on Pt(111) in a (√3 × √3)R30° structure. The site preference, surface relaxation, work function, and electron structure of the system are analyzed. The hcp hollow position is found to be the most favorable for K adsorption. The calculated surface relaxations and adsorption geometry are in agreement with available experimental data. It is demonstrated that the K adsorption leads to the disappearance of a number of platinum surface and resonance states in the energy region above −2 eV and to the appearance of new platinum surface features, as well as bands that are significantly localized at the adsorbate. It is found that the K adsorption 1) transforms the Shockley surface state lying in the bulk band gap near the Γ¯ point on the clean Pt surface into the state localized at the K adlayer and 2) pushes this state up in energy by about 0.17 eV relative to the bottom of the bulk band gap. It is shown that the Rashba spin-orbit strength parameter in this state is surprisingly greater than the respective parameter in the Shockley surface state on Au(111) and Bi(111).We acknowledge the Tomsk State University competitiveness programme (Project No. 8.1.01.2017) and partial support by the Saint Petersburg State University (Project No. 15.61.202.2015). Y.M.K. acknowledges the Russian Foundation for Basic Research (Project No. 15-02-02717-a) and the Fundamental Research Program of the State Academies of Sciences for 2013–2020. E.V.C. acknowledges the Spanish Ministry of Science and Innovation (Grant No. FIS2016-75862-P).Peer Reviewe

Low-energy collective electronic excitations in Pd metal

A theoretical study of collective electronic excitations in Pd at low-energy (from 0 to ∼3eV) domain is reported. The calculations were performed with full inclusion of the electron band structure obtained within self-consistent pseudopotential approach. We show that the presence in Pd of two kinds of carriers (in s‐p and d bands) at the Fermi level produces dramatic modification of the excitation spectra in this energy range in comparison with free-electron-like model prediction. In particular, at small momenta a peculiar plasmon mode with characteristic sound-like dispersion—an acoustic plasmon—is predicted to exist in this material. This mode has strong directional dependence on the momentum transfer: whereas for momenta along the ⟨100⟩ and ⟨111⟩ symmetry directions it arises as a single mode up to ∼1eV, along the ⟨110⟩ direction two acoustic modes (one of which disperses up to ∼2.5eV) with different slope exist. As in many metallic systems, e.g., in transition metals, there are energy bands with large difference in the Fermi velocities, we expect that the existence of such plasmon mode must be a rather general phenomenon. Additionally, present calculations reveal other well-defined features in the energy-loss spectra in this low-energy range due to numerous interband transitions.We acknowledge the partial support from the University of the Basque Country -Grant No. GIC07IT36607, the Departamento de Educación del Gobierno Vasco, and the Spanish Ministerio de Ciencia y Tecnología -MCyT -Grant No. FIS200766711C0101.Peer reviewe

Quantum-well-induced giant spin-orbit splitting

4 páginas, 3 figuras.-- PACS numbers: 73.21.Fg, 71.70.Ej, 79.60.Dp.-- et al.We report on the observation of a giant spin-orbit splitting of quantum-well states in the unoccupied electronic structure of a Bi monolayer on Cu(111). Up to now, Rashba-type splittings of this size have been reported exclusively for surface states in a partial band gap. With these quantum-well states we have experimentally identified a second class of states that show a huge spin-orbit splitting. First-principles electronic structure calculations show that the origin of the spin-orbit splitting is due to the perpendicular potential at the surface and interface of the ultrathin Bi film. This finding allows for the direct possibility to tailor spin-orbit splitting by means of thin-film nanofabrication.This work was supported by the DFG GRK 792 and the DFG SFB/TRR49, the UPV/EHU (Grant No. GIC07IT36607), the Departamento de Educación del Gobierno Vasco, and the Spanish MCyT (Grant No. FIS200766711C0101).Peer reviewe

Influence of hydrogen absorption on low-energy electronic collective excitations in palladium

A theoretical study of electronic excitation spectra in Pd and PdH is reported. The calculations were performed with full inclusion of the electron band structure obtained within self-consistent pseudopotential approach. We demonstrate that the complicated Pd electronic structure at the Fermi level is reflected in a numerous peak structure of the excitation spectra. The evolution of the energy-loss spectrum with momentum and its anisotropy are analyzed. Strong modification of the excitation spectra upon hydrogen absorption is found. We also study the role of intra- and interband transitions in the formation of dominating plasmon peak both in pure Pd and PdH. In Pd, this peak is mainly determined by intraband transitions. The downward shift of this peak from ∼7.2eV in pure Pd to ∼4.2eV in PdH is mainly explained by interband transitions from occupied Pdd bands to unoccupied hydrogen-modified sp states in the 7–13eV energy range.We acknowledge the partial support from the University of the Basque Country 9/UPV 00206.215-13639/2001, the Departamento de Educación del Gobierno Vasco, and the Spanish Ministerio de Ciencia y Tecnología MCyT FIS 2004-06490-C03-01.Peer reviewe

Electronic structure of FeTi(110) thin film

The electronic structure of the FeTi(110) surface has been calculated theoretically using the self-consistent film linearized augmented-plane-wave method. As a result, surface states both above and below the Fermi level have been found. Most of occupied surface states are localized at the atoms of iron, but unoccupied ones are generated by the titanium states mainly. Moreover, there are a few surface states formed by both iron and titanium atoms. The high density of surface states at EF is found. The trend to a surface segregation is discussed. The calculated work function is close to that for the clean α-Fe(110) surface but significantly exceeds the corresponding value for titanium surface.Peer reviewe

Ternary thallium-based semimetal chalcogenides Tl-V-VI2 as a new class of three-dimensional topological insulators

5 páginas, 4 figuras.The results of the theoretical investigation of the bulk and surface electronic structures of Tl–V–VI2 com pounds, where V is the Bi or Sb semimetal and VI is the Se or Te chalcogen, are reported. It has been shown that these compounds are three-dimensional topological insulators. Both a topologically protected surface state, which forms a Dirac cone at the gamma point, and occupied surface states, which are localized in the band gap, are present on the surface of these compounds.Peer reviewe