Dear Ghadiyali Thank you for your reply. I know it is a rather high doping level, but it is intentional. I understand that with such a high value it is possible that the whole thing is unstable.
My doubt is wether these strange results are due to an instability or due to some convergence issue. I performed another test, with the same system, but now with a bigger cell, leading to a doping concentration of 6.25%, obtaining a similar result, with three completely negative optical modes. To me, the main question here is to understand wether these results are due to a real instability, or this may be because of some mistake I did. Thank you for your attention Fábio Costa
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