  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.
