Electron → reflected hole + Cooper pair at N-S interface
At a normal metal–superconductor (N-S) interface, an incoming electron with energy E < Δ cannot enter as a quasiparticle (no available states). Instead it is Andreev reflected: a retroreflected hole travels back into N, and a Cooper pair enters S. The BTK theory (Blonder-Tinkham-Klapwijk 1982) gives conductance G(eV) in terms of interface transparency Z. For Z=0: G(eV<Δ)=2G_N (doubled by Andreev); for Z→∞: G(eV<Δ)→0.