Emergence of a Kondo Singlet State with TK≈ 2000K in the Proton-Embedded ElectronGas: Route to High-Tc Superconductivity
Takada GroupPhysics in heavy fermion superconductors has been
understood by the concept of quantum criticality in aKondo lattice and the spin-fluctuation mechanism isbelieved to be responsible for superconductivity, as
inferred from the strong correlation between Tc andTK. More specifically, Tc is of the order of 0.1TK, as
shown in Fig. 1, from which we can conceive an ideathat high-Tc superconductivity will be obtained if wecan discover a Kondo system with very high TK. In
fact, the recently-discovered plutonium compounds
such as PuCoGa5 with Tc=18.5K and TK≈260K may
be regarded as a successful realization of this idea.
Thus we should make further pursuit of this idea bysearching for a new class of Kondo systems with TK
higher than 1000K. Theoretically, this search can bedone by the first-principles quantitative determinationof TK for the composite system of an impurity atom
embedded in a metal.

Fig. 1: Superconducting transition temperature Tc versus
Kondo temperature TK (a characteristic energy scale for spin
fluctuations) in heavy fermion superconductors.

As first suggested by Debye and Hückel, an atomic
nucleus charge +Ze in a metal is screened byaccumulation of metallic electrons which is regarded
as a charge resonance and well described by the linear
response theory. This concept of metallic screening isprevailing for a century, but because Z is not
infinitesimally small, we need to consider nonlinear
effects in the screening, including the contribution
from spin fluctuations. The spin contribution will beenhanced, if Z is an odd integer, such as Z=1 (case ofa proton), in which a spin-polarized bound statemight appear at the impurity atom.
With the above basic scientific issues in mind, we
have concerned with the problem of hydrogen
impurity in metals which attracts long attention froma technological point of view, such as hydrogenstorage in solids, sensor applications, and catalysis.
Its electronic state has been investigated in terms of a

proton immersed into the electron gas since 1970s,
but no serious attention has been paid so far to thespin resonance effect.
We have studied this system with changing theelectron density in a wide range by employing thediffusion Monte Carlo simulations with the total
electron number N up to 170 to accurately obtain theground-state electronic distribution n(r) and found a
sharp transition at rs≈2 from short-range H+ screening
charge resonance to long-range Kondo singlet
resonance, the emergence of which is confirmed by
the presence of an oscillation-period-shortenedFriedel oscillation characteristic to the Kondo singlet
state with TK well beyond 1000K for rs in the range
3-8. Another transition to the closed-shell Hconfinement state occurs at rs≈12.5 (see Fig. 2).

Fig. 2: Phase diagram for sharp sequential transitions among(a) charge resonance, (b) spin-singlet resonance, and (c)closed-shell H- ion confinement states in the proton-embedded
electron gas with N the total number of electrons and rs theelectron density parameter to specify the electron gas.

This study not only reveals interesting competition
between charge and spin resonances, enriching thecentury-old paradigm of metallic screening to a point
charge, but also discovers a long-sought high-TK
system, opening a new and unexpected route toroom-temperature superconductivity in a Kondo
lattice made of protons. The metal hydrides, usuallyused for secondary batteries, are promising
candidates, if the metallic electron density is suitably
arranged and the lattice constant is properly tuned toexhibit quantum criticality in the Kondo lattice.
Reference[1] Y. Takada, R. Maezono, and K. Yoshizawa, Phys.
Rev. B 92, 155140 (2015).
Authors
Y. Takada, R. Maezonoa, and K. Yoshizawaaa Japan Advanced Institute of Science and Technology