The former TGM3-beamline of BESSYI was rebuilt at the 1.5 GeV storage ring facility DELTA at the University of Dortmund, Germany. This dipole beamline provides synchrotron radiation between 6 and 200 eV for photoelectron spectroscopy and Fermi-surface-mapping experiments. The former beamline design was kept, especially the optical layout. The vacuum system was completely redesigned, except the mirror and slit chambers. For the beamlinesetup at DELTA, the focusing mirror is mounted inside the radiation wall shielding, therefore a motorisation of the mirror movements was necessary. The thermal load on the optical elements at DELTA is much higher than the previous load at BESSYI because of the higher electron beam energy in the storage ring. Therefore a watercooling of the first mirror had to be integrated and a complete new design of the chamber for the focusing mirror was necessary. In order to integrate the monochromator into the beamline control system, the monochromator control system had to be replaced. First commissioning of the beamline will be performed in 2006.

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.

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.

The authors investigated the formation of periodic subwavelength structures, so-called nanogratings, in the volume of fused silica. These self-organized structures emerge upon irradiation with ultrashort laser pulses, undergoing three distinct stages of growth from randomly distributed nanostructures to extended domains with uniform periodicity. The experiments revealed that the cumulative action of subsequent laser pulses is mediated by dangling-bond type defects. On shorter time scales, transient self trapped excitons may significantly enhance the formation process. Nanogratings exhibit an extremely large temperature stability up to 1150 °C. In combination with the possibility to precisely tune their form birefringence, nanogratings provide a powerful tool to realize, thermally stable complex phase elements.