Emergence of a Kondo singlet state with the Kondo temperature well beyond 1,000K in the proton-embedded electron gas: Possible route to high-Tc superconductivity

Hydrogen in metals has attracted much attention for a long time from both basic scientific and technological points of view. Its electronic state has been investigated in terms of a proton embedded in the electron gas mostly by the local density approximation (LDA) to the density functional theory. At high electronic densities, it is well described by a bare proton H^+ screened by metallic electrons (charge resonance), while at low densities two electrons are localized at the proton site to form a closed-shell negative ion H^- protected from surrounding metallic electrons by the Pauli exclusion principle. However, no details are known about the transition from H^+ to H^- in the intermediate-density region. Here, by accurately determining the ground-state electron distribution n(r) by the combination of LDA and diffusion Monte Carlo simulations with the total electron number up to 170, we obtain a complete picture of the transition, in particular, a sharp transition from short-range H^+ screening charge resonance to long-range Kondo-like spin-singlet resonance, the emergence of which is confirmed by the presence of an anomalous Friedel oscillation characteristic to the Kondo singlet state with the Kondo temperature T_K well beyond 1,000K. This study not only reveals interesting competition between charge and spin resonances, enriching the century-old paradigm of metallic screening to a point charge, but also discovers a long-sought novel high-T_K system, opening an unexpected route to room-temperature superconductivity in a Kondo lattice made of protons.