Histogram at the Superconducting gap

[krishnendu]

Dear Forum Members,

I have a question regarding the histograms that indicate the number of states on the Fermi surface with that superconducting gap energy. For example, below is the figure from EPW documentation / Tutorial / Superconductivity / MgB2:

Superconducting magnesium diboride — EPW documentation.png (143.02 KiB) Viewed 64 times

I too got this type of histogram from plotting nb.imag_aniso_gap0_013.50 from an epw exercise.

However, in Superconductivity lecture we note that there are no valence electron density of state in the superconducting gap. For example see below the slide from EPW 2024, Lecture wednesday 2, Superconductors and Migdal-Eliashberg theory by Roxana Margine.

Screenshot 2025-09-25 at 14-30-35 Wed.2.Margine.pdf.png (65.75 KiB) Viewed 64 times

So, from the Superconductors theory there should not be any valence electron state in the superconducting gap. But from the attached Tutorial Figure we see that there are states in the superconducting gap. Can you kindly describe to me so that I can get rid of the confusion.

I think the Histogram is indicating the superconducting boson condensate states below the Fermi level. And there is a range of the gap for which the superconducting boson condensate states exist. Is that an possible description ?

Thanking you,
Best regards,
Krishnendu