O. Heckmann

Crystallographic structure of cobalt films on Cu(001): elastic deformation to a tetragonal structure

Abstract The stable structure of cobalt is hexagonal closed-packed (hcp), but cobalt can be stabilized in a distorted face-centered cubic structure (fcc) by epitaxy on Cu(001). The deviation from the isotropic fcc structure is determined by surface-extended X-ray absorption fine structure (EXAFS). A polarization dependent first shell analysis of the EXAFS spectra shows that the Co/Cu(001) films have a face-centered tetragonal structure (fct): the mean nearest-neighbour distance parallel to the surface is 2.55A(same value as in bulk copper) and the interlayer bond length is 2.50A: the films are in perfect epitaxy on copper (001) with a contraction of the lattice parameter perpendicular to the surface of 4%, in agreement with the continuum elasticity theory. A constant tetragonalization is observed for films of 2 to 15 monolayers. A simulation of the EXAFS spectra is also done using the FEFF code. In order to simulate the polarization dependence of the experimental spectra, we have introduced in this theoretical calculation a polarization dependence of the paths (single scattering and multiple scattering ones) based on a plane wave approximation. This simple model gives a good agreement between calculated and experimental polarization dependent EXAFS spectra both on bulk hcp cobalt and on the face-centered tetragonal films Co/Cu(001); this agreement confirms the structure determined from the first shell analysis.

EXAFS study of the crystallographic structure of cobalt thin films on Pt(111)

Abstract The growth and structure of cobalt on a platinum (111) single crystal is studied using X-ray absorption spectroscopy at the cobalt K-edge. A polarisation dependent analysis of the EXAFS spectra shows first neighbours distances in Co Pt (111) films identical to those in bulk cobalt for thicknesses between 2 and 10 monolayers. The full shell spectra polarisation dependence indicates the hexagonal stacking of the thicker film. Influences of annealing and recovering by platinum on the structure of the cobalt films are also described.

Resonant photoemission and XMCD on Mn-based systems

Abstract We report results obtained by resonant photoemission (RPE) and X-ray magnetic circular dichroism (XMCD) in absorption with photon energies around the L 2/3 edges of manganese. The samples contain manganese in different environments and details about the dynamics of electronic structure on one side, magnetic structure on the other side are being discussed. We will present studies on the following samples: submonolayers of Mn on Cu(100), Mn in InAs and thin films of the manganese perovskite LSMO.

Quasi 1D structures at the Bi/InAs(100) surface

Thin Bi films are interesting candidates for spintronic applications due to a large spin-orbit splitting that, combined with the loss of inversion symmetry at the surface, results in a band structure that is not spin-degenerate. In recent years, applications for topological insulators based on Bi and Bi alloys have as well attracted much attention. Here we present ARPES studies of Bi/InAs(100) interface. Bismuth deposition followed by annealing of the surface results in the formation of one full Bi monolayer decorated by Bi-nanolines. We found that the building up of the interface does affect the electronic structure of the substrate. As a consequence of weak interaction, bismuth states are placed in the gaps of the electronic structure of InAs(100). We observe a strong resonance of the Bi electronic states close to the Fermi level; its intensity depends on the photon energy and the photon polarization. These states show nearly no dispersion when measured perpendicular to the nanolines, confirming their one-dimensionality.

Electronic structure of organic titanium bis-phthalocyanine on InAs(001)4

We investigated the electronic structure of ultra thin interface of organic titanium bis-phthalocyanine TiPc 2 deposited on InAs(001)(4 x 2)c-(8 x 2) clean surface, by means of high-resolution core-levels and valence band (VB) photoelectron spectroscopies. In4d, As3d, and Cls core levels were measured for different TiPc 2 thicknesses up to 1 monolayer. Surface core level shifts were found on both In4d andAs3d core levels. These shifts originate from the interaction between 4 x 2-c(8 x 2) InAs reconstruction and organic molecules suggesting the presence of strong bounds. C1 core level exhibits a line shape attributed to the C-C and C-N bonds. The VB spectroscopy shows dramatic changes related to the interaction of the TiPc 2 molecule with the InAs related surface states.