Fundamental Limits on the Electron-Phonon Coupling and Superconducting T_c

Area of Science:

• Condensed Matter Physics
• Materials Science

Background:

• Electron-phonon interactions are crucial for superconductivity in metals.
• Observed electron-phonon coupling constants (λ) are empirically limited, suggesting underlying fundamental constraints.

Purpose of the Study:

• To establish fundamental upper bounds on electron-phonon interaction strength and superconducting transition temperature (T_c) in metals.
• To explain the observed limitations of λ and the mechanism for enhanced T_c.

Main Methods:

• Theoretical analysis of electron-lattice equilibrium under strong interactions.
• Investigating the role of intrinsic instability.
• Comparison with existing experimental data on superconductivity.

Main Results:

• Established fundamental upper bounds for electron-phonon interaction strength (λ ≲ 4) due to intrinsic lattice instability.
• Identified a mechanism for metastable superconductivity with enhanced T_c near the instability threshold.
• Theoretical analysis indicates room-temperature phonon-mediated superconductivity is exclusively feasible in hydrogen compounds.

Conclusions:

• Intrinsic instability of electron-lattice equilibrium fundamentally limits electron-phonon coupling in metals.
• Metastable superconductivity offers a pathway to higher T_c near this limit.
• Hydrogen compounds are the only viable candidates for achieving room-temperature phonon-mediated superconductivity.