  How many magnetic moments periodically arranged on a metallic surface are
needed to generate a coherent Kondo lattice behavior? We investigate this
fundamental issue within the particle-hole symmetric Kondo lattice model using
quantum Monte Carlo simulations. Extra magnetic atoms forming closed shells
around the initial impurity induce a fast splitting of the Kondo resonance at
the inner shells which signals the formation of composite heavy-fermion bands.
The onset of the hybridization gap matches well the enhancement of
antiferromagnetic spin correlations in the plane perpendicular to the applied
magnetic field, a genuine feature of the coherent Kondo lattice. In contrast,
the outermost shell remains dominated by a local Kondo physics with spectral
features resembling the single-impurity behavior.
