Fundamental Limits on the Electron‐Phonon Coupling and Superconducting Tc

Abstract Fundamental upper bounds on the electron‐phonon interaction strength and superconducting transition temperature T c in metals are established based on the intrinsic instability of the equilibrium between electrons and the crystal lattice under strong interaction. This instability explains why observed electron‐phonon coupling constants are limited to λ ≲ 4. The theory also accounts for the mechanism of metastable superconductivity with enhanced T c , which emerges near the instability threshold. Based on theoretical analysis and comparison with experimental data, room‐temperature phonon‐mediated superconductivity is found to be feasible exclusively in hydrogen compounds.