  We show that a self-assembled phase of potassium (K) doped single-layer
para-sexiphenyl (PSP) film on gold substrate is an excellent platform for
studying the two-impurity Kondo model. On K-doped PSP molecules well separated
from others, we find a Kondo resonance peak near EF with a Kondo temperature of
about 30 K. The Kondo resonance peak splits when another K-doped PSP molecule
is present in the vicinity, and the splitting gradually increases with the
decreased inter-molecular distance, with no signs of phase transition. Our data
demonstrate how a Kondo singlet state gradually evolves into an
antiferromagnetic singlet state due to the competition between Kondo screening
and antiferromagnetic RKKY coupling, as described in the two-impurity Kondo
model. Intriguingly, the antiferromagnetic singlet is destroyed quickly upon
increasing temperature and transforms back to a Kondo singlet well below the
Kondo temperature. Our data provide a comprehensive picture and quantitative
constraints on related theories and calculations of two-impurity Kondo model.

Strongly Correlated Electrons