C. Sorg

Measuring the Kernel of Time-Dependent Density Functional Theory with X-Ray Absorption Spectroscopy of 3d Transition Metals

The 2p-3d core-hole interaction in the L2.3 absorption spectra of the transition metals is treated within time-dependent density functional theory. A simple three-level model explains the origin of the strong deviations from the one-particle branching ratio and yields matrix elements of the unknown exchange-correlation kernel directly from experiment.

XMCD Analysis Beyond Standard Procedures

We show that the standard analysis procedures as sum rule application and multipole-moment analysis for XMCD spectra can fail for magnetic samples of present interest. Two examples will be given: 1) The induced magnetic moments in ultrathin films of the light 3d elements Ti, V and Cr at the interface to Fe cannot be determined by the XMCD sum rule or multipole-moment analysis at the L2,3 edges. This is due to correlation effects which result in the deviation of the intensity ratio (branching ratio) from its statistical value. To address this point we established a double-pole approximation within time-dependent density functional theory. 2) The analysis of the L2,3 XMCD of rare earth elements is not only hampered by the appearance of electric quadrupolar contributions (E2) in addition to the dipolar contributions (E1). Even after separation of the two, one determines the wrong sign of the induced 5d moment by the sum rules. This originates from the spin dependence of the transition matrix elements. To tackle these difficulties we compare the experimental spectra to ab initio calculations of the entire isotropic XAS and the dichroic signal. ©2007 American Institute of Physics

A systematic study of embedded atom EXAFS: the (2 × 1)O/Cu(110) reconstruction as an ideal prototype system

A systematic analysis of the embedded atom EXAFS (AXAFS) effect is presented. This effect is explained by the backscattering of the photoelectron at interstitial charge densities. The reconstructed (2 × 1)O/Cu(110) system is an ideal prototype system in which to study the angular dependence of the AXAFS because oxygen–copper rows are formed resulting in a C2 symmetry. The scattering potential is non-spherical because of the high directionality of the O–Cu bonds. The high signal-to-noise ratio of the experimental data enables us to clearly identify the AXAFS contribution. Here we present the definite angular dependence of the experimental AXAFS for the first time, giving a unique opportunity to measure the anisotropy of the local embedded atom potential. The angular dependence demonstrates that the AXAFS effect cannot be mimicked by multi-electron excitations or experimental artifacts for this system. We compare our experiments to theoretical calculations within the muffin-tin approximation and show that future full-potential calculations are needed to model the angular dependence determined.

Comment on "magnetic phase transition in Co/Cu/Ni/Cu(100) and Co/Fe/Ni/Cu(100)".

A Comment on the Letter by C. Won et al., Phys. Rev. Lett. 91, 147202 (2003). The authors of the Letter offer a Reply.

XMCD of Oxygen Adsorbates on Fe, Co, and Ni Monolayers

Oxygen is used as a surfactant to improve the layer‐by‐layer growth of ultrathin ferromagnetic Fe, Co, and Ni films. During this process 0.5 monolayers of oxygen ions are floating on top of the ferromagnetic film. XMCD enables us to study the magnetism of such films at their L2,3 edges and in the same experiment the induced magnetic moment of the oxygen at the K edge. In addition, NEXAFS at the O K edge is used to investigate the bonding of the adsorbate to the 3d metal. The experimental findings are combined with DFT calculations to reveal the crystallographic and electronic structure as well as the magnetic properties of Fe, Co, and Ni monolayers grown with oxygen surfactant.