I'm studying superconductivity and BCS theory. Looking at the BCS ground state construction, I see two things that seem conflicting
- Occupation of Cooper pairs in BCS ground state (look at the figure below from Ibach)
- The fact that the only electrons in the region $\pm \Delta$ about $E_F$ contribute to BCS ground state
So where did other electron (electrons not in the $\pm \Delta$ region) go?