ELECTRON-PHONON COUPLING

In metals, electrons couple to lattice vibrations (phonons). Strong coupling (large λ) leads to Cooper pairing and superconductivity via BCS theory. T_c ≈ 1.13·ω_D·exp(−1/λ). The Fröhlich Hamiltonian H = Σ εk c†k ck + Σ ωq a†q aq + Σ g(q) c†k+q ck (aq + a†−q). Lattice distortion lowers energy when two electrons interact via exchanged phonons.