ANDREEV REFLECTION

Normal metal | Superconductor interface

Energy E / Δ 0.5

Interface barrier Z 0.2

Gap Δ (meV) 1.0

CONDUCTANCE

A(E) Andreev prob.—

B(E) Normal refl.—

G/G_N (subgap)—

Regime—

BTK: A(E)=|u₀v₀/(u₀²+Z²(u₀²-v₀²))|²

An electron incident below Δ cannot enter the SC as a quasiparticle. Instead, it retroreflects as a hole (Cooper pair transferred). For Z=0 (ideal interface), G = 2G_N for E<Δ. This doubles the conductance — a direct probe of pairing.