Victor A. Rogalev

Disentangling bulk and surface Rashba effects in ferroelectric α -GeTe

1Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 2New Technologies-Research Center University of West Bohemia, Plzeň, Czech Republic 3Department of Chemistry, Ludwig Maximillian University, 81377 Munich, Germany 4Department of Condensed Matter Physics, Charles University in Prague, Ke Karlovu 5, 121 16 Praha 2, Czech Republic 5Physik-Institut, Universität Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland 6Institute of condensed matter physics, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland 7Institut für Halbleiter-und Festkörperphysik, Johannes Kepler Universität, A-4040 Linz, Austria 8SwissFEL, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

Band structure of the EuO/Si interface: justification for silicon spintronics

Semiconductor spintronics provides a framework for hybrid devices combining logic, communication and storage, circumventing limitations of the current electronics, especially with respect to the energy efficiency. Enormous efforts have been invested worldwide into the development of spintronics based on Si, the mainstream semiconductor platform. Notwithstanding remarkable pace, Si spintronics still experiences a technological bottleneck – creation of significant spin polarization in nonmagnetic Si. An emerging approach based on direct electrical spin injection from a ferromagnetic semiconductor – EuO being the prime choice – avoids problems inherent to metallic injectors. The functionality of the EuO/Si spin contact is controlled by the interface band alignment. To be competitive with charge electronics, the EuO/Si interface should exhibit a band offset which falls within the 0.5–2 eV range. We employ a soft-X-ray ARPES technique, using synchrotron radiation with photon energies around 1 keV, to probe the electronic structure of the buried EuO/Si interface with momentum resolution and chemical specificity. The band structure reveals a conduction band offset of 1.0 eV attesting the technological potential of the EuO/Si system.

Fermi surface and effective masses in photoemission response of the (Ba 1−x K x )Fe 2 As 2 superconductor

The angle-resolved photoemission spectra of the superconductor (Ba1−xKx)Fe2As2 have been investigated accounting coherently for spin-orbit coupling, disorder and electron correlation effects in the valence bands combined with final state, matrix element and surface effects. Our results explain the previously obscured origins of all salient features of the ARPES response of this paradigm pnictide compound and reveal the origin of the Lifshitz transition. Comparison of calculated ARPES spectra with the underlying DMFT band structure shows an important impact of final state effects, which result for three-dimensional states in a deviation of the ARPES spectra from the true spectral function. In particular, the apparent effective mass enhancement seen in the ARPES response is not an entirely intrinsic property of the quasiparticle valence bands but may have a significant extrinsic contribution from the photoemission process and thus differ from its true value. Because this effect is more pronounced for low photoexcitation energies, soft-X-ray ARPES delivers more accurate values of the mass enhancement due to a sharp definition of the 3D electron momentum. To demonstrate this effect in addition to the theoretical study, we show here new state of the art soft-X-ray and polarisation dependent ARPES measurments.

High-energy electronic interaction in the 3d band of high-temperature iron-based superconductors

D. V. Evtushinsky, A. N. Yaresko, V. B. Zabolotnyy, J. Maletz, T. K. Kim, A. A. Kordyuk, 4 M. S. Viazovska, M. Roslova, 6 I. Morozov, 6 R. Beck, S. Wurmehl, 7 H. Berger, B. Büchner, 7 and S. V. Borisenko Institute for Solid State Research, IFW Dresden, P.O.Box 270116, D-01171 Dresden, Germany Max-Planck-Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany Diamond Light Source Ltd., Didcot, Oxfordshire, OX11 0DE, United Kingdom Institute of Metal Physics of National Academy of Sciences of Ukraine, 03142 Kyiv, Ukraine Humboldt University of Berlin, Rudower Chaussee 25, 12489 Berlin Moscow State University, 119991 Moscow, Russia Institut für Festkörperphysik, Technische Universität Dresden, D-01171 Dresden, Germany Institut de Physique Applique, Ecole Politechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland

Topological surface state of α−Sn on InSb(001) as studied by photoemission

We report on the electronic structure of the elemental topological semimetal alpha-Sn on InSb(001). High-resolution angle-resolved photoemission data allow us to observe the topological surface state (TSS) that is degenerate with the bulk band structure and show that the former is unaffected by different surface reconstructions. An unintentional p-type doping of the as-grown films was compensated by deposition of potassium or tellurium after the growth, thereby shifting the Dirac point of the surface state below the Fermi level. We showthat, while having the potential to break time-reversal symmetry, iron impurities with a coverage of up to 0.25 monolayers do not have any further impact on the surface state beyond that of K or Te. Furthermore, we have measured the spin-momentum locking of electrons from the TSS by means of spin-resolved photoemission. Our results show that the spin vector lies fully in-plane, but it also has a finite radial component. Finally, we analyze the decay of photoholes introduced in the photoemission process, and by this gain insight into the many-body interactions in the system. Surprisingly, we extract quasiparticle lifetimes comparable to other topological materials where the TSS is located within a bulk band gap. We argue that the main decay of photoholes is caused by intraband scattering, while scattering into bulk states is suppressed due to different orbital symmetries of bulk and surface states.

Double band inversion in α -Sn: Appearance of topological surface states and the role of orbital composition

The electronic structure of \graySn(001) thin films strained compressively in-plane was studied both experimentally and theoretically. A new topological surface state (TSS) located entirely within the gapless projected bulk bands is revealed by \textit{ab initio}-based tight-binding calculations as well as directly accessed by soft X-ray angle-resolved photoemission. The topological character of this state, which is a surface resonance, is confirmed by unravelling the band inversion and by calculating the topological invariants. In agreement with experiment, electronic structure calculations show the maximum density of states in the subsurface region, while the already established TSS near the Fermi level is strongly localized at the surface. Such varied behavior is explained by the differences in orbital composition between the specific TSS and its associated bulk states, respectively. This provides an orbital protection mechanism for topological states against mixing with the background of bulk bands.