Quantum Scars — Stadium Billiard

Semiclassical wavefunctions enhanced along unstable periodic orbits in a chaotic billiard

Eigenstate

(-ℏ²/2m)∇²ψ = Eψ inside Ω
ψ|∂Ω = 0 (Dirichlet BC)
Scar: |ψ|² ↑ near periodic orbit
Quantum scars (Heller 1984): in classically chaotic billiards, certain quantum eigenstates show anomalously large probability density along unstable classical periodic orbits — defying ergodic expectations.