the "double delta approximation" and EPW

[chrisewolf]

Hi all,

I have a short question. I realize when I have a gapped system and put the Fermi level in the gap the phonon self energy (linewidth) is zero, which, as I read in another thread, is due to the two delta-functions which, I think, is also referred to here:

http://epw.org.uk/Documentation/Electron-phononCoupling

The final expression is positive definite and is often referred to as the ``double-delta function'' approximation. This approximation is no longer necessary when using EPW.

Now I am a bit puzzled if the double-delta approximation is always used or not. The MgO example n-dopes the cell enough to move the fermi level into the conduction band which alleviates the zero linewidth but what happens when Ef is actually in the gap?

Thanks everyone who is active on this board - it is a great source of help!

Best,
Chris

[carla.verdi]

Hi,

In the expression for the phonon self-energy you have f_k - f_k+q , that is, the self-energy is nonzero only if the k and k+q states are one above and one below the Fermi level - and if that is in the middle of the gap, and the gap is much larger than the phonon energy, you'll have zero linewidth.

Hope this helps,
Carla