  Motivated by recent STM experiments, we explore the magnetic field induced
Kondo effect that takes place at symmetry protected level crossings in finite
Co adatom chains. We argue that the effective two-level system realized at a
level crossing acts as an extended impurity coupled to the conduction electrons
of the substrate by a distribution of Kondo couplings at the sites of the
chain. Using auxiliary-field quantum Monte Carlo simulations, which
quantitatively reproduce the field dependence of the zero-bias signal, we show
that a proper Kondo resonance is present at the sites where the effective Kondo
coupling dominates. Our modeling and numerical simulations provide a
theoretical basis for the interpretation of the STM spectrum in terms of level
crossings of the Co adatom chains.
