In a BCS superconductor, the pairing interaction transforms electrons and holes into Bogoliubov quasiparticles: superpositions of electron (probability u²) and hole (probability v²) with u² + v² = 1.
The quasiparticle energy is E(k) = √(ξₖ² + Δ²), where ξₖ = ℏ²k²/2m − μ is the normal-state dispersion and Δ is the superconducting gap. The gap is visible as a minimum at k = kF.
The coherence factors u(k) and v(k) determine the electron/hole character. At k = kF both are 1/√2 — maximum mixing.