  Heavy fermion quantum criticality is an extremely rich domain of research
which represents a framework to understand strange metals as a consequence of a
Kondo breakdown transition. Here we provide an experimental realization of such
systems in terms of organic radicals on a metallic surface. The ground state of
organic radicals is a Kramer's doublet that can be modeled by a spin 1/2 degree
of freedom. Using on-surface synthesis and scanning tunneling microscopy (STM)
tip manipulation, one can controllably engineer and characterize chains of
organic radicals on a Au(111) surface. The spatial-resolved differential
conductance reveals site-dependent low-energy excitations, which support the
picture of emergent many-body Kondo physics. Using quantum Monte Carlo
simulations, we show that a Kondo lattice model of spin chains on a metallic
surface reproduces accurately the experimental results. This allows us to
interpret the experimental results in terms of a heavy fermion metal, below the
coherence temperature. We foresee that the tunability of these systems will
pave the way to realize quantum simulators of heavy fermion criticality.
