Fröhlich coupling α determines large vs small polaron; m* = mₑ·f(α)
A polaron forms when an electron polarizes the surrounding ionic lattice, creating a phonon cloud that dresses the electron. Fröhlich coupling α = (e²/ℏ)√(m*/2ℏω_LO)(1/ε∞ − 1/ε₀) quantifies the interaction. For α < 6 (large polaron): m* ≈ mₑ/(1−α/6) (Feynman). For α > ~6 (small polaron): electron self-traps, hopping transport replaces band conduction. High-Tc cuprates and organic semiconductors host small polarons.