Phonon-roton dispersion of ⁴He; two-fluid model ρ_s(T)/ρ
Landau's two-fluid model explains superfluidity via the phonon-roton excitation spectrum of ⁴He. The roton minimum at k₀ ≈ 1.93 Å⁻¹ with gap Δ ≈ 8.65 K sets the critical velocity v_c = Δ/ℏk₀. The superfluid fraction ρ_s/ρ → 1 as T→0 and vanishes at T_λ = 2.17 K. The spectrum was measured by neutron scattering (Henshaw & Woods 1961).