N-S interface: electrons retroreflect as holes, Cooper pairs enter superconductor
Click "Fire Electron" to see Andreev process
Bias E/Δ0.50
Gap Δ1.00
Barrier Z1.00
Temperature kT/Δ0.05
Andreev reflection (BTK theory, Blonder-Tinkham-Klapwijk 1982): at an N-S interface, an electron with |E|<Δ cannot propagate as a quasiparticle. Instead it retroreflects as a hole (opposite momentum and spin), injecting a Cooper pair into the superconductor. The BTK conductance G(V) = G₀(1+|a|²-|b|²) where |a|² is Andreev probability. Sub-gap conductance doubles: G(0)/G_N = 2 for Z=0. The barrier Z interpolates from Ohmic contact (Z=0) to tunnel junction (Z≫1).