E. V. Chulkov

Strong ferromagnetism at the surface of an antiferromagnet caused by buried magnetic moments

Carrying a large, pure spin magnetic moment of 7 μB per atom in the half-filled 4f shell, divalent europium is an outstanding element for assembling novel magnetic devices in which a two-dimensional electron gas may be polarized due to exchange interaction with an underlying magnetically-active Eu layer. Here we show that the Si-Rh-Si surface trilayer of the antiferromagnet EuRh2Si2 bears a surface state, which exhibits an unexpected and large spin splitting controllable by temperature. The splitting sets in below ~32.5 K, well above the ordering temperature of the Eu 4f moments (~24.5 K) in the bulk, indicating a larger ordering temperature in the topmost Eu layers. The driving force for the itinerant ferromagnetism at the surface is the aforementioned exchange interaction. Such a splitting may also be induced into states of functional surface layers deposited onto the surface of EuRh2Si2 or similarly ordered magnetic materials with metallic or semiconducting properties.

Theory of surface plasmons and surface-plasmon polaritons

Collective electronic excitations at metal surfaces are well known to play a key role in a wide spectrum of science, ranging from physics and materials science to biology. Here we focus on a theoretical description of the many-body dynamical electronic response of solids, which underlines the existence of various collective electronic excitations at metal surfaces, such as the conventional surface plasmon, multipole plasmons and the recently predicted acoustic surface plasmon. We also review existing calculations, experimental measurements and applications.

Image potential states on metal surfaces: binding energies and wave functions

We present self-consistent pseudopotential calculations of both surface and image potential states on simple metal surfaces: Li(110), Na(110), Be(0001), Mg(0001), Al(100), and Al(111). The local density approximation (LDA) is used to describe the one-electron potential inside the film and in the surface region. In the vacuum space (at z>z im ) the LDA potential is replaced by the image potential. A one-dimensional potential proposed recently is constructed for 14 simple and noble metal surfaces. By using this model potential we study wave functions and binding energies of the image states and also image plane position trends for these metal surfaces.

Theory of inelastic lifetimes of low-energy electrons in metals

Abstract 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.

Role of Bulk and Surface Phonons in the Decay of Metal Surface States

We present a comprehensive theoretical investigation of the electron-phonon contribution to the lifetime broadening of the surface states on Cu(111) and Ag(111), in comparison with high-resolution photoemission results. The calculations, including electron and phonon states of the bulk and the surface, resolve the relative importance of the Rayleigh mode, being dominant for the lifetime at small hole binding energies. Including the electron-electron interaction, the theoretical results are in excellent agreement with the measured binding energy and temperature dependent lifetime broadening.

Atom-specific spin mapping and buried topological states in a homologous series of topological insulators

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.

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.

Disentanglement of Surface and Bulk Rashba Spin Splittings in Noncentrosymmetric BiTeI

BiTeI has a layered and non-centrosymmetric structure where strong spin-orbit interaction leads to a giant spin splitting in the bulk bands. Here we present high-resolution angle-resolved photoemission (ARPES) data in the UV and soft x-ray regime that clearly disentangle the surface from the bulk electronic structure. Spin-resolved UV-ARPES measurements on opposite, nonequivalent surfaces show identical spin structures, thus clarifying the surface state character. Soft x-ray ARPES data clearly reveal the spindle-torus shape of the bulk Fermi surface, induced by the spin-orbit interaction. PACS numbers: 71.20.Nr, 71.70.Ej, 79.60.Bm 1 ar X iv :1 20 4. 21 96 v1 [ co nd -m at .m tr lsc i] 1 0 A pr 2 01 2 The breaking of inversion symmetry and its influence on the spin structure of surface states under action of spin–orbit interaction (SOI) has been extensively studied in recent years [1, 2]. The main finding is that the surface states become spin-split according to the Rashba model [3] resulting in two spin-polarized concentric Fermi contours. The lack of inversion symmetry in the bulk crystal structure is expected to induce a spin splitting with a more complex bandand spin-structure. Combined with strong SOI the Fermi surface can take the shape of a torus [4]. For non-centrosymmetric superconductors such as for example CePt3Si [5] this peculiar band structure is expected to result in topologically protected spin polarized edge states reminiscent of Majorana modes [6]. Recently, an ARPES and spin-resolved ARPES study by Ishizaka et al. [7] proposed that the semiconductor BiTeI features a very large spin-splitting, arising from the broken inversion symmetry in the crystal bulk and a strong SOI. Theoretical work based on the perturbative k ·p formalism linked the unusually large spin splitting in BiTeI to the negative crystal field splitting of the top valence bands [8]. Optical transition measurements [9] are in accordance with the giant bulk spin-splitting of the gap defining valence and conduction bands predicted by first principle calculations [7, 8]. In addition it was shown in recent theoretical work that BiTeI can become a topological insulator under action of hydrostatic pressure [10], and thus is closely related to non-centrosymmetric topological superconductors. The present study provides first band mapping of a system without bulk inversion symmetry and giant SOI by the example of BiTeI, featuring a three-dimensional Rashba splitting of the bulk bands. Further it is shown that the Rashba-split state observed for this material in the UV photon energy regime is not a quantum well state [7] but rather a surface state, using a simple symmetry argument based on spin-resolved ARPES (SARPES) measurements, which is confirmed by first principle calculations. All measurements were performed at the Swiss Light Source of the Paul-Scherrer-Institut. The SARPES data was measured with the Mott polarimeter at the COPHEE endstation [11] of the Surface and Interface Spectroscopy beamline at a photon energy of 24 eV. The spin-integrated data at photon energies 20-63 eV were taken at the high-resolution ARPES endstation at the same beamline. The soft x-ray ARPES data were taken at the SX-ARPES endstation of the ADRESS beamline at photon energies of 310-850 eV. All spin-integrated measurements were performed at a sample temperature of 11 K and a base pressure lower than 10−10 mbar, the SARPES data was taken at 20 K.

Potential energy landscape for hot electrons in periodically nanostructured graphene.

We explore the spatial variations of the unoccupied electronic states of graphene epitaxially grown on Ru(0001) and observed three unexpected features: the first graphene image state is split in energy; unlike all other image states, the split state does not follow the local work function modulation, and a new interfacial state at +3  eV appears on some areas of the surface. First-principles calculations explain the observations and permit us to conclude that the system behaves as a self-organized periodic array of quantum dots.

Surface Scattering via Bulk Continuum States in the 3D Topological Insulator Bi2Se3

We have performed scanning tunneling microscopy and differential tunneling conductance (dI/dV) mapping for the surface of the three-dimensional topological insulator Bi(2)Se(3). The fast Fourier transformation applied to the dI/dV image shows an electron interference pattern near Dirac node despite the general belief that the backscattering is well suppressed in the bulk energy gap region. The comparison of the present experimental result with theoretical surface and bulk band structures shows that the electron interference occurs through the scattering between the surface states near the Dirac node and the bulk continuum states.