Mechanism of high-temperature superconductivity in compressed H₂-molecular-type hydride

The discovery of compressed atomic-type hydrides offers a promising avenue toward achieving room-temperature superconductivity, but it necessitates extremely high pressures to completely dissociate hydrogen molecules to release free electrons. Here, we report a remarkable finding of compressed H₂-molecular-type hydride CaH₁₄ exhibiting an unusual transition temperature (T_c) of 204.0 kelvin. The peculiarity of its electronic structure lies in the pronounced emergence of near-free electrons, which manifest metallic bonding, but molecular hydrogen fragments persist. This finding indicates that the necessary condition for superconducting transition is forming the Fermi sea with Cooper pairs rather than the monatomic hydrogen. Notably, the formation mechanism of free electrons can be effectively explained by the finite-depth potential wells model. Intriguingly, this H₂-molecular-type hydride can downgrade the required pressure to 80 gigapascal while maintaining a high T_c of 84 kelvin, well above the liquid-nitrogen temperature. Our study has established a high-temperature superconducting paradigm and opened the prospect for achieving high-T_c superconductors in H₂-molecular-type hydrides at low pressure.