Yambo Community Forum

Dear developers:
I have a question about the exciton binding energy of Si.The BSE absorption peak of bulk si is about 3.5eV for fisrt peak and 4.3 eV for the second,this value agree well with experiment.But its GW gap should less than 3.5eV (I have not calculated this, the experiment value is about 1.2eV , I think the GW value should about 1.2eV more or less), and the exciton binding energy=GW gap-BSE peak,so, the exciton binding of bulk Si is negative?? Is my opinion wrong??

Thanks!

Yes, the values your are supposing to be the GW gap is wrong.
If you don't want to calculate it, just check some literature:

M.S. Hybertsen and S. Louie, PRL 55, 1418 (1985)
M.S. Hybertsen and S. Louie, PRB 34, 5390 (1986)
R.W. Godby, M. Schlueter, L.J. Sham, PRB 37, 10159 (1988)

The Silicon gap, at LDA level, is the double of what you are supposing to be.

Cheers,
Daniele

PS: in case excitation peaks are higher in energy than photoemission gap, the excitation is not bound!

Dear Shudong,

The minimal gap in Si is indeed about 1.2 eV, as you mentioned. However it is indirect
Optical transitions do not "connect" different k-points. So you have to compare with the direct gap of Si, that I believe is above 3 eV.

Regards,
m