Boson Peak — Vibrational Anomaly in Glass

g(ω)/ω² = g_Debye + g_excess(ω) — Boson peak appears as excess in reduced DOS g(ω)/ω²
Debye prediction: g(ω) = 3Nω²/ω_D³ (crystal). Glass shows anomalous peak at ω_BP ≈ 1–3 meV
Origin: soft quasi-localized modes, heterogeneous elasticity — still debated (2020s)