Dear all users of QE, I have recently started using XSpectra package to calculate an X-ray absorption spectrum (XANES to be more specific) for GaMnAs material K-edge of Mn. Currently, I work with the simplest model: cubic structure, 8 atoms in total: 1 Mn, 3 Ga and 4 As, no core-holes, etc.
After setting up the model, I encountered a feature in the spectrum, which I am puzzled with: It seems that the spectrum is more-or-less OK near fermi level (I can cut occupied states). Then, from 0 eV up to about 15-17 eV, the spectrum has correct positions of 2 peaks at around 9 and 13.5 eV (the ratio between them is 1.0, however, as oppose to the experimental value of 1.2; still I can leave with that and maybe correct it by changing gamma from 1.6 to slightly higher values). However, after reaching the maximum at 13.5 eV, the spectrum falls down steadily (like 1/x) and relatively quickly. The maximum value at 13.5 eV was 0.0013 and at 40 eV it is less than 0.0001. The experimental data instead would oscillate around value 0.0012 (which is a usual behavior for a spectrum). I tried several things (just in case): - I increased number of k-points in SCF calculations; - I increased number of k-points in XSpectra calculations; - I increased number of bands in SCF calculations; In all these cases the spectra were the same (in numerical sense, of course)!!! I also changed the gamma coefficient from very low (0.01), to reasonable values (0.7 and 1.6), to very high (5). The gamma coefficient affects the calculations as expected, but could not ?lift up? the tail at all. At the very high value the spectrum becomes broad one with one maximum only, i.e. as expected, but the tail keeps falling down... So, my question is whether anybody experienced such an effect before and what should I do in order to correct it. Thank you in advance for any hints/comments! Yevgen Melikhov Lab of X-ray and Electron Microscopy Research Institute of Physics, PAS Warsaw, Poland -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20140710/95f1f569/attachment.html
