Unoccupied Topological States on Bismuth Chalcogenides

The unoccupied part of the band structure of topological insulators Bi$_2$Te$_{x}$Se$_{3-x}$ ($x=0,2,3$) is studied by angle-resolved two-photon photoemission and density functional theory. For all surfaces linearly-dispersing surface states are found at the center of the surface Brillouin zone at energies around 1.3 eV above the Fermi level. Theoretical analysis shows that this feature appears in a spin-orbit-interaction induced and inverted local energy gap. This inversion is insensitive to variation of electronic and structural parameters in Bi$_2$Se$_3$ and Bi$_2$Te$_2$Se. In Bi$_2$Te$_3$ small structural variations can change the character of the local energy gap depending on which an unoccupied Dirac state does or does not exist. Circular dichroism measurements confirm the expected spin texture. From these findings we assign the observed state to an unoccupied topological surface state

Modification and ab-initio spectroscopic application of modified commerce terahertz spectrometer by using homemade parts

Ab-initio study on modification of commerce terahertz spectrometer with time resolution Z-3 (Zomega, USA) by substitution of ZnTe and GaP detectors and LT-GaAs generator for homemade of pure and S-doped GaSe is carried out. It was established that in spite of not optimized parameters pure and doped GaSe:S(0.3 mass%) crystal are comparable, relatively, in generation efficiency and detection sensitivity to commerce units due to lower nonlinear optical loss and much higher damage threshold. The advantages are in force from pump fluences of below 5 mJ/cm2 for pure GaSe. The closer S-doping to optimal concentration, the lover fluences resulting in the advantages. Pure and S-doped GaSe demonstrate higher reliability and larger dynamic range of operation. Recorded absorption spectra well match known spectra. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Buildup and dephasing of Floquet-Bloch bands on subcycle time scales

Strong light fields have created spectacular opportunities to tailor novel functionalities of solids. Floquet-Bloch states can form under periodic driving of electrons and enable exotic quantum phases. On subcycle time scales, lightwaves can simultaneously drive intraband currents and interband transitions, which enable high-harmonic generation (HHG) and pave the way towards ultrafast electronics. Yet, the interplay of intra- and interband excitations as well as their relation with Floquet physics have been key open questions as dynamical aspects of Floquet states have remained elusive. Here we provide this pivotal link by pioneering the ultrafast buildup of Floquet-Bloch bands with time- and angle-resolved photoemission spectroscopy. We drive surface states on a topological insulator with mid-infrared fields - strong enough for HHG - and directly monitor the transient band structure with subcycle time resolution. Starting with strong intraband currents, we observe how Floquet sidebands emerge within a single optical cycle; intraband acceleration simultaneously proceeds in multiple sidebands until high-energy electrons scatter into bulk states and dissipation destroys the Floquet bands. Quantum nonequilibrium calculations explain the simultaneous occurrence of Floquet states with intra- and interband dynamics. Our joint experiment-theory study opens up a direct time-domain view of Floquet physics and explores the fundamental frontiers of ultrafast band-structure engineering.Comment: 45 pages, 4 figures, 10 extended data figure

Synthesis of Thin Films from Monodispersed Polymethylmethacrylate Microspheres

A thin film of monodispersed polymethylmethacrylate particles whose size is a multiple of the light wavelength was obtained. The multiplicity and periodicity of the obtained materials makes them two-dimensional photonic crystals. The structure of the samples was studied by SEM method.Исследования были выполнены на оборудовании центра коллективного пользования ФИЦ КНЦ СО РАН

Synthesis and Study by Electron Microscopy of Inverse Opals from Zirconium Oxide

Inverse opals synthesised using transparent oxides are a promising material for obtaining photonic crystals and photocatalysts. Inverse opals with zirconium oxide composition have been obtained, and their structure has been studied by SEM and TEM.Работа выполнена в рамках проекта РФФИ 20-03-00636. Исследования были выполнены на оборудовании центра коллективного пользования ФИЦ КНЦ СО РАН

Impact of Co atoms on the electronic structure of Bi2Te3 and MnBi2Te4 topological insulators

This work is devoted to an experimental investigation of the electronic structure of the surface of topological insulators of various stoichiometry during the adsorption of Co atoms. Changes in the surface electronic structure of Bi2Te3 and MnBi2Te4 systems upon deposition of Co atoms at various temperatures have been studied using the methods of angle-resolved photoemission spectroscopy, as well as X-ray photoelectron spectroscopy. It is suggested that binding of the adsorbed Co atoms to the substrate surface modifies Dirac point position. The observed changes are associated with the possible formation of magnetic Co-containing ordered surface alloys.This work was supported by the St. Petersburg State University, grant no. 73028629, the Russian Science Foundation, grant no. 18-12-00062, the Russian Foundation for Basic Research, grant nos. 20-32-70127, 21-52-12024, and 18-29-12094, the Science Development Foundation of the President of Azerbaijan, grant no. EIF-BGM-4-RFTF-1/2017-21/04/1-M-02, and in the framework of the state assignment of Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, and the Rzhanov Institute of Semiconductors Physics, Siberian Branch of the Russian Academy of Sciences.Peer reviewe