I. A. Nechaev

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 Γ-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.

Evidence for a direct band gap in the topological insulator Bi 2Se3 from theory and experiment

We gratefully acknowledge financial support from the Lundbeck foundation, the VILLUM foundation, the Danish National Research Foundation, the University of the Basque Country UPV/EHU (Grant No. GIC07-IT-366-07), the Departamento de Educacion del Gobierno Vasco, and the Spanish Ministerio de Ciencia e Innovacion (Grant No. FIS2010-19609-C02-01).

Rashba split surface states in BiTeBr

Within density functional theory, we study the bulk band structure and surface states of BiTeBr. We consider both ordered and disordered phases, which differ in atomic order in the Te–Br sublattice. On the basis of relativistic ab initio calculations, we show that the ordered BiTeBr is energetically preferable as compared with the disordered one. We demonstrate that both Te- and Br-terminated surfaces of the ordered BiTeBr hold surface states with a giant spin–orbit splitting. The Te-terminated surface-state spin splitting has Rashba-type behavior with the coupling parameter αR ~ 2 eVA.

Energy shift and wave function overlap of metal-organic interface states

The properties of Shockley-type interface states between

Many-body effects on the Rashba-type spin splitting in bulk bismuth tellurohalides

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Giant Rashba-type spin splitting at polar surfaces of BiTeI

-conjugated organic molecular layers and metal surfaces are investigated by time-resolved two-photon photoemission experiments and density functional theory. For perylene- and naphthalene-tetracarboxylic acid dianhydride (PTCDA and NTCDA) adsorbed on Ag(111), a common mechanism of formation of the interface state from the partly occupied surface state of the bare Ag(111) is revealed. The energy position is found to be strongly dependent on the distance of the molecular carbon rings from the metal and their surface density. Bending of the carboxyl groups enhances the molecular overlap of the interface state.

New generation of two-dimensional spintronic systems realized by coupling of Rashba and Dirac fermions

We report on many-body corrections to one-electron energy spectra of bulk bismuth tellurohalides—materials that exhibit a giant Rashba-type spin splitting of the band-gap edge states. We show that the corrections obtained in the one-shot GW approximation noticeably modify the spin-orbit-induced spin splitting evaluated within density functional theory. We demonstrate that taking into account many-body effects is crucial to interpret the available experimental data.

Quasiparticle band gap in the topological insulator Bi2Te3

On the basis of relativistic ab initio calculations, we show that both Te- and I-terminated surfaces of the polar layered semiconductor BiTeI hold surface states with a giant Rashba-type spin splitting. The Te-terminated surface state has nearly isotropic free-electron-like dispersion with a positive effective mass, which along with the giant spin splitting makes BiTeI fulfilling the requirements demanded by many semiconductor-spintronics applications. The I-terminated surface state with its negative effective-mass dispersion reproduces nicely the situation with the Rashba-split surface state on surfaces of noble-metal based surface alloys. The crucial advantage of BiTeI as compared with the surface alloys is the location of the I-terminated surface state in a quite wide band gap.

Inelastic decay of electrons in Shockley-type metal-organic interface states

Intriguing phenomena and novel physics predicted for two-dimensional (2D) systems formed by electrons in Dirac or Rashba states motivate an active search for new materials or combinations of the already revealed ones. Being very promising ingredients in themselves, interplaying Dirac and Rashba systems can provide a base for next generation of spintronics devices, to a considerable extent, by mixing their striking properties or by improving technically significant characteristics of each other. Here, we demonstrate that in BiTeI@PbSb2Te4 composed of a BiTeI trilayer on top of the topological insulator (TI) PbSb2Te4 weakly- and strongly-coupled Dirac-Rashba hybrid systems are realized. The coupling strength depends on both interface hexagonal stacking and trilayer-stacking order. The weakly-coupled system can serve as a prototype to examine, e.g., plasmonic excitations, frictional drag, spin-polarized transport, and charge-spin separation effect in multilayer helical metals. In the strongly-coupled regime, within ~100 meV energy interval of the bulk TI projected bandgap a helical state substituting for the TI surface state appears. This new state is characterized by a larger momentum, similar velocity, and strong localization within BiTeI. We anticipate that our findings pave the way for designing a new type of spintronics devices based on Rashba-Dirac coupled systems.

Spin-helical Dirac states in graphene induced by polar-substrate surfaces with giant spin-orbit interaction: a new platform for spintronics

We also acknowledge partial support from the Basque Country Government, Departamento de Educacion, Universidades e Investigacion (Grant No. IT-366-07), and the Spanish Ministerio de Ciencia e Innovacion (Grant No. FIS2010-19609-C02- 00), and the Ministry of Education and Science of Russian Federation (Grant No. 2.8575.2013).