Evidence for a Strong Topological Insulator Phase in ZrTe_{5}

The complex electronic properties of ZrTe_{5} have recently stimulated in-depth investigations that assigned this material to either a topological insulator or a 3D Dirac semimetal phase. Here we report a comprehensive experimental and theoretical study of both electronic and structural properties of ZrTe_{5}, revealing that the bulk material is a strong topological insulator (STI). By means of angle-resolved photoelectron spectroscopy, we identify at the top of the valence band both a surface and a bulk state. The dispersion of these bands is well captured by ab initio calculations for the STI case, for the specific interlayer distance measured in our x-ray diffraction study. Furthermore, these findings are supported by scanning tunneling spectroscopy revealing the metallic character of the sample surface, thus confirming the strong topological nature of ZrTe_{5}.

Collective Modes and Structural Modulation in Ni-Mn-Ga(Co) Martensite Thin Films Probed by Femtosecond Spectroscopy and Scanning Tunneling Microscopy

The origin of the martensitic transition in the magnetic shape memory alloy Ni-Mn-Ga has been widely discussed. While several studies suggest it is electronically driven, the adaptive martensite model reproduced the peculiar nonharmonic lattice modulation. We used femtosecond spectroscopy to probe the temperature and doping dependence of collective modes, and scanning tunneling microscopy revealed the corresponding static modulations. We show that the martensitic phase can be described by a complex charge-density wave tuned by magnetic ordering and strong electron-lattice coupling.

Electronic structure and imaging contrast of graphene moiré on metals

Realization of graphene moiré superstructures on the surface of 4d and 5d transition metals offers templates with periodically modulated electron density, which is responsible for a number of fascinating effects, including the formation of quantum dots and the site selective adsorption of organic molecules or metal clusters on graphene. Here, applying the combination of scanning probe microscopy/spectroscopy and the density functional theory calculations, we gain a profound insight into the electronic and topographic contributions to the imaging contrast of the epitaxial graphene/Ir(111) system. We show directly that in STM imaging the electronic contribution is prevailing compared to the topographic one. In the force microscopy and spectroscopy experiments we observe a variation of the interaction strength between the tip and high-symmetry places within the graphene moiré supercell, which determine the adsorption sites for molecules or metal clusters on graphene/Ir(111).

The structure and optical properties of ZnO nanocrystals embedded in SiO2 fabricated by radio-frequency sputtering

Zinc oxide (ZnO) nanocrystals (NCs) with high crystalline quality were prepared via radio-frequency magnetron sputtering as a SiO(2)/ZnO/SiO(2) trilayer on Si(100) and Al(2)O(3)(0001) substrates with an intermediate in situ annealing step. Transmission electron microscopy reveals a uniform dispersion of ZnO NCs in the amorphous SiO(2) matrix with typical sizes up to 16 nm with a larger fraction of smaller crystals. The size distribution analysis yields a mean grain size of 5 nm for small particles. Individual ZnO NCs show a well-defined hexagonal close packed wurtzite structure and lattice parameters close to those of bulk ZnO, confirming their high crystalline quality. Mapping of the Zn distribution by means of energy-filtered transmission electron microscopy reveals a strongly non-uniform distribution of Zn within the SiO(2) matrix, corroborating the chemical separation of ZnO NCs from surrounding SiO(2). Optical transmittance measurements confirm the findings of the electron microscopy analysis. The fabrication technique described opens up new possibilities in the preparation of ZnO NCs with high crystalline quality, including growth in monolithic optical cavities without intermediate ex situ fabrication steps.

Rashba effect in the graphene/ni(111) system

We report on angle-resolved photoemission studies of the electronic pi states of high-quality epitaxial graphene layers on a Ni(111) surface. In this system the electron binding energy of the pi states shows a strong dependence on the magnetization reversal of the Ni film. The observed extraordinarily large energy shift up to 225 meV of the graphene-derived pi band peak position for opposite magnetization directions is attributed to a manifestation of the Rashba interaction between spin-polarized electrons in the pi band and the large effective electric field at the graphene/Ni interface. Our findings show that an electron spin in the graphene layer can be manipulated in a controlled way and have important implications for graphene-based spintronic devices.

Magnetite: a search for the half-metallic state

We present a detailed study of the spin-dependent electronic structure of thin epitaxial magnetite films of different crystallographic orientations. Using spin- and angle-resolved photoelectron spectroscopy at room temperature, we determine for epitaxial Fe(3)O(4)(111) films a maximum spin polarization value of -(80 ± 5)% near E(F). The spin-resolved photoelectron spectra for binding energies between 1.5 eV and E(F) show good agreement with the spin-split band structure from density functional theory (DFT) calculations which predict an overall energy gap in the spin-up electron bands in high symmetry directions, thus providing evidence for the half-metallic ferromagnetic state of Fe(3)O(4) in the [111] direction. In the case of the Fe(3)O(4)(100) surface, both the spin-resolved photoelectron spectroscopy experiments and the DFT density of states give evidence for a half-metal to metal transition: the measured spin polarization of about -(55 ± 10)% at E(F) and the theoretical value of -40% are significantly lower than the -100% predicted by local spin density approximation (LSDA) calculations for the bulk magnetite crystal as well as the -(80 ± 5)% obtained for the Fe(3)O(4)(111) films. The experimental findings were corroborated by DFT calculations as due to a surface reconstruction leading to the electronic states in the majority-spin band gap and thus to the reduced spin polarization.