Sven Döring

Electronic structure, surface morphology, and topologically protected surface states of Sb2Te3 thin films grown on Si(111)

We have performed a combined spectroscopy and microscopy study on surfaces of Sb2Te3/Si(111) thin films exposed to air and annealed under ultra-high vacuum conditions. Scanning tunneling microscopy images, with atomic resolution present in most areas of such processed surfaces, show a significant amount of impurities and defects. Scanning tunneling spectroscopy reveals the bulk band gap of ∼170 meV centered ∼65 meV above the Fermi level. This intrinsic p-type doping behavior is confirmed by high-resolution angle-resolved photoemission spectra, which show the dispersions of the lower Dirac cone and the spectral weight of the bulk valence bands crossing the Fermi level. Spin-polarized photoemission revealed up to ∼15% in-plane spin polarization for photoelectrons related to the topologically protected Dirac cone states near the Fermi level, and up to ∼40% for several states at higher binding energies. The results are interpreted using ab initio electronic structure simulations and confirm the robustness of ...

Realization of a vertical topological p-n junction in epitaxial Sb2Te3/Bi2Te3 heterostructures.

Three-dimensional (3D) topological insulators are a new state of quantum matter, which exhibits both a bulk band structure with an insulating energy gap as well as metallic spin-polarized Dirac fermion states when interfaced with a topologically trivial material. There have been various attempts to tune the Dirac point to a desired energetic position for exploring its unusual quantum properties. Here we show a direct experimental proof by angle-resolved photoemission of the realization of a vertical topological p–n junction made of a heterostructure of two different binary 3D TI materials Bi2Te3 and Sb2Te3 epitaxially grown on Si(111). We demonstrate that the chemical potential is tunable by about 200 meV when decreasing the upper Sb2Te3 layer thickness from 25 to 6 quintuple layers without applying any external bias. These results make it realistic to observe the topological exciton condensate and pave the way for exploring other exotic quantum phenomena in the near future.

Enhanced ferrimagnetism in auxetic NiFe2O4 in the crossover to the ultrathin lm limit

The competition of charge, spin and orbital degreesof freedom in complex oxides leads to intriguing phys-ical phenomena, including ferromagnetism, ferroelectri-city or multiferroicity [1]. Fertilized by the continuouslyadvancing art of oxide growth, the controlled synthesisof high-quality oxide heterostructures now approaches amonolayer-precision [2]. Designing electronic propertiesin ultrathin oxide lms and interfaces thereby opens uproutes to explore novel nanoelectronic functionalities forapplications.In the context of spin-based electronics, oxides fea-turing both magnetic and insulating properties reveala highly e ective spin lter e ect, where spin-polarizedelectron currents are generated by a spin-dependent tun-nelling process. Up to 100% spin ltering has beendemonstrated in magnetic oxides with low Curie tem-perature T

Bi1Te1 is a dual topological insulator

Markus Eschbach, ∗ Martin Lanius, ∗ Chengwang Niu, ∗ Ewa M lyńczak, 2 Pika Gospodarič, Jens Kellner, Peter Schüffelgen, Mathias Gehlmann, Sven Döring, Elmar Neumann, Martina Luysberg, Gregor Mussler, Lukasz Plucinski, † Markus Morgenstern, Detlev Grützmacher, Gustav Bihlmayer, Stefan Blügel, and Claus M. Schneider Peter Grünberg Institute and JARA-FIT, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Krakow, Poland II. Institute of Physics B and JARA-FIT, RWTH Aachen University, 52074 Aachen, Germany Peter Grünberg Institute and Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany (Dated: May 2, 2016)New three-dimensional (3D) topological phases can emerge in superlattices containing constituents of known two-dimensional topologies. Here we demonstrate that stoichiometric Bi1Te1, which is a natural superlattice of alternating two Bi2Te3 quintuple layers and one Bi bilayer, is a dual 3D topological insulator where a weak topological insulator phase and topological crystalline insulator phase appear simultaneously. By density functional theory, we find indices (0;001) and a non-zero mirror Chern number. We have synthesized Bi1Te1 by molecular beam epitaxy and found evidence for its topological crystalline and weak topological character by spin- and angle-resolved photoemission spectroscopy. The dual topology opens the possibility to gap the differently protected metallic surface states on different surfaces independently by breaking the respective symmetries, for example, by magnetic field on one surface and by strain on another surface.

Quasi 2D electronic states with high spin-polarization in centrosymmetric MoS2 bulk crystals.

Time reversal dictates that nonmagnetic, centrosymmetric crystals cannot be spin-polarized as a whole. However, it has been recently shown that the electronic structure in these crystals can in fact show regions of high spin-polarization, as long as it is probed locally in real and in reciprocal space. In this article we present the first observation of this type of compensated polarization in MoS2 bulk crystals. Using spin- and angle-resolved photoemission spectroscopy (ARPES), we directly observed a spin-polarization of more than 65% for distinct valleys in the electronic band structure. By additionally evaluating the probing depth of our method, we find that these valence band states at the point in the Brillouin zone are close to fully polarized for the individual atomic trilayers of MoS2, which is confirmed by our density functional theory calculations. Furthermore, we show that this spin-layer locking leads to the observation of highly spin-polarized bands in ARPES since these states are almost completely confined within two dimensions. Our findings prove that these highly desired properties of MoS2 can be accessed without thinning it down to the monolayer limit.

Hard x-ray photoemission spectroscopy on the trilayer system MgO/Au/Fe using standing-wave excitation

The trilayer system MgO/Au monolayer/Fe was investigated by hard x-ray photoemission experiments in combination with the standing-wave technique. The insertion of the Au layer into the Fe/MgO tunnel junction provides an additional handle to influence the properties of the interface. The recently explored method of standing-wave excited hard x-ray photoemission was used to investigate both the structural properties and chemical states of the interfacial layers in one experiment. The results show that the Au monolayer does not grow as a closed layer, but intermixes strongly with the Fe below. This behaviour results in a very sharp interface between the Au/Fe and the MgO layer on top. However, the XPS spectra show no hint for a formation of FeO at the interface.

Thermal stability of ultrathin ZrO{sub 2} films and structure determination of ZrSi{sub 2} islands on Si(100)

The temperature dependence of ultrathin

Determination of layer-resolved composition, magnetization, and electronic structure of an Fe/MgO tunnel junction by standing-wave core and valence photoemission

{\text{ZrO}}_{2}

Spin-polarization limit in

films on clean

Bi_{2}Te_{3}

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