Towards an ab Initio, description of adsorbate vibrations

This thesis investigates accurate theoretical prediction of anharmonic vibrational frequencies of molecules adsorbed on metal surfaces. Such adsorbed systems are composed of two parts with dierent electronic properties, the adsorbate and the surface. However, most existing quantum mechanical methods are not identically accurate for both parts. Moreover, methods that can accurately describe extended system are very time consuming and signicantly complicates their usage for standard anharmonic calculations.This thesis introduces a fragment method to overcome this difficulty. Within our method an energy correction is computed using high-level ab initio quantum mechanical method by considering an adsorbed molecule separately from the metal surface. The reliability of this approach is demonstrated for two test systems: an acetylene molecule adsorbed on a Cu(001) surface and a thiophene molecule adsorbed on a Au(111) surface. In both cases intra-adsorbate anharmonic frequencies obtained using the fragment method show better agreement with experimental data than the corresponding anharmonic frequencies computed using a standard approach. Moreover, a correlation between the accuracy of the fragment method and the accuracy of the ab initio method used for adsorbed molecule is observed. This correlation provides a way to systematically improve adsorbate frequencies by improving the quality of the potential energy surface used.Finally, for each test systems we established a correlation between the strength of adsorption and the value of the frequencies shift upon adsorption. This allows us to conclude that terthiophene is only weakly adsorbed on a Au(111) surface based on the similarity between the adsorbate and the gas-phase vibrational spectra

Ideal two-dimensional electron systems with a giant Rashba-type spin splitting in real materials: surfaces of bismuth tellurohalides

Spintronics is aimed at active controlling and manipulating the spin degrees of freedom in semiconductor devices. A promising way to achieve this goal is to make use of the tunable Rashba effect that relies on the spin-orbit interaction (SOI) in a two-dimensional (2D) electron system immersed in an inversion-asymmetric environment. The SOI induced spin-splitting of the 2D-electron state provides a basis for many theoretically proposed spintronic devices. However, the lack of semiconductors with large Rashba effect hinders realization of these devices in actual practice. Here we report on a giant Rashba-type spin splitting in 2D electron systems which reside at tellurium-terminated surfaces of bismuth tellurohalides. Among these semiconductors, BiTeCl stands out for its isotropic metallic surface-state band with the Gamma-point energy lying deep inside the bulk band gap. The giant spin-splitting of this band ensures a substantial spin asymmetry of the inelastic mean free path of quasiparticles with different spin orientations.Comment: 12 pages, 5 figure

Verification of Photometric Parallaxes with Gaia DR2 Data

Results of comparison of Gaia DR2 parallaxes with data derived from a combined analysis of 2MASS (Two Micron All-Sky Survey), SDSS (Sloan Digital Sky Survey), GALEX (Galaxy Evolution Explorer), and UKIDSS (UKIRT Infrared Deep Sky Survey) surveys in four selected high-latitude $|b|>48^{\circ}$ sky areas are presented. It is shown that multicolor photometric data from large modern surveys can be used for parameterization of stars closer than 4400 pc and brighter than $g_{SDSS} = 19.^m6$, including estimation of parallax and interstellar extinction value. However, the stellar luminosity class should be properly determined.Comment: 11 pages, 5 figure

Interplay of Topological States on TI/TCI Interfaces

Based on first-principles calculations, we study electronic structure of interfaces between a Z2 topological insulator (TI) SnBi2Te4 and a topological crystalline insulator (TCI) SnTe. We consider two interface models characterized by the different atomic structure on the contact of the SnTe(111) and SnBi2Te4(0001) slabs: the model when two materials are connected without intermixing (abrupt type of interface) and the interface model predicted to be realized at epitaxial immersion growth on topological insulator substrates (smooth interface). We find that a strong potential gradient at the abrupt interface leads to the redistribution of the topological states deeper from the interface plane which prevents the annihilation of the Γ¯ Dirac states, predicted earlier. In contrast, a smooth interface is characterized by minor charge transfer, which promotes the strong interplay between TI and TCI Γ¯ Dirac cones leading to their complete annihilation.The M¯ topologically protected Dirac state of SnTe(111) survives irrespective of the interface structure.This research was funded by Ministry of Education and Science of the Russian Federation (state task No. 0721-2020-0033), the Government research assignment for ISPMS SB RAS, project No. III.23.2.9

Ab initio study of 2DEG at the surface of topological insulator Bi2Te3

By means of ab initio DFT calculation, we analyze the mechanism that drives the formation and evolution of the 2D electron gas (2DEG) states at the surface of Bi2Te3 topological insulator (TI). As it has been proved earlier it is due to an expansion of the van der Waals (vdW) spacing produced by intercalation of adsorbates. We will show that the effect of this expansion, in this particular surface, leads to several intriguing phenomena. On one hand we observe a different dispersion of the Dirac cone with respect to the ideal surface and the formation of Parabolic Bands (PB) below the conduction band and M-shaped bands in the valence band, the latters have been observed recently in photoemission experiments. On the other hand the expansion of the vdW gaps changes the symmetry of the orbitals forming the Dirac cone and therefore producing modifications in the local spin texture. The localization of these new 2DEG-states and the relocalization of the Dirac cone will be studied as well.This work was supported in part by the University of the Basque Country (project no. GVUPV/EHU, grant no. IT36607) and Ministerio de Ciencia e Inovacion (grant no. FIS201019609C0200). Calcula tions were performed on the Arina supercomputer of the Basque Country University.Peer reviewe

First principles calculations of optical properties for oxygen vacancies in binary metal oxides

Using an advanced computational methodology implemented in CP2K, a non-local PBE0-TC-LRC density functional and the recently implemented linear response formulation of the Time-dependent Density Functional Theory equations, we test the interpretation of the optical absorption and photoluminescence signatures attributed by previous experimental and theoretical studies to O-vacancies in two widely used oxides—cubic MgO and monoclinic (m)-HfO2. The results obtained in large periodic cells including up to 1000 atoms emphasize the importance of accurate predictions of defect-induced lattice distortions. They confirm that optical transitions of O-vacancies in 0, +1, and +2 charge states in MgO all have energies close to 5 eV. We test the models of photoluminescence of O-vacancies proposed in the literature. The photoluminescence of V+2O centers in m-HfO2 is predicted to peak at 3.7 eV and originate from radiative tunneling transition between a V+1O center and a self-trapped hole created by the 5.2 eV excitation

New generation of two-dimensional spintronic systems realized by coupling of Rashba and Dirac fermions

Intriguing phenomena and novel physics predicted for two-dimensional (2D) systems formed by electrons in Dirac or Rashba states motivate an active search for new materials or combinations of the already revealed ones. Being very promising ingredients in themselves, interplaying Dirac and Rashba systems can provide a base for next generation of spintronics devices, to a considerabl