Weine Olovsson1, Isao Tanaka1,2, Peter Puschnig3, Claudia Ambrosch-Draxl3
1Department of Materials Science and Engineering, Kyoto University, Sakyo, Kyoto 606-8501, Japan
2Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya 456-8587, Japan
3Chair of Atomistic Modelling and Design of Materials, Montanuniversität Leoben,
Franz-Josef Straße 18, A-8700 Leoben, Austria
X-ray absorption near-edge structure (XANES) is a wide spread spectroscopy which provides chemical information of materials by probing their excited states, or more specifically, the transition probability between core-electrons and unoccupied states. For the insulating materials featuring band gap, excitonic effects may significantly modify the absorption spectra as bound states, core excitons, can form due to the attractive Coulomb interaction.
In order to rigorously take into account the correlated motion of the excited electron and the hole created in the absorption process, we perform first principles Bethe-Salpeter equation (BSE) calculations . The BSE scheme is implemented within the all-electron full-potential linearized augmented planewave (FPLAPW) method . We have made investigations for a number of different materials, considering more shallow core-states. Recently, we demonstrated core-excitonic effects for the Li K-edge (1s) in LiF, LiCl, LiBr, LiI, Li2O and Li2S, together with the Mg L2,3-edge (2p) in MgO [2,3]. It was found that BSE improves the agreement with experiment, as compared with density functional theory (DFT) supercell calculations using a core-hole.
An advantage of the used BSE scheme is that the absorption spectra as well as core-exciton binding energies and wave functions can be obtained directly in the calculations.
 G. Onida et al., Rev. Mod. Phys. 74, 601 (2002),  P. Puschnig and C. Ambrosch-Draxl, Phys.
Rev. B 66, 165105 (2002),  W. Olovsson et al., Phys. Rev. B 79, 041102R (2009),  W. Olovs-
son et al., J. Phys.: Condens. Matter 21, 104205 (2009).
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