Spectral Function A(k,ω) — Retarded Green's Function & Self-Energy

The retarded Green's function G^R(k,ω) = 1/(ω − ε_k − Σ(ω) + iη). The spectral function A(k,ω) = −2 Im G^R measures the quantum state density. Self-energy Σ shifts and broadens quasiparticle peaks.