  We analyze the crossover from Kondo to weak-link regime by means of a model
of tunable bond impurities in the middle of a spin-1/2 XXZ Heisenberg chain. We
study the Kondo screening cloud and estimate the Kondo length by combining
perturbative renormalization group approach with the exact numerical
calculation of the integrated real-space spin-spin correlation functions. We
show that, when the spin impurity is symmetrically coupled to the two parts of
the chain with realistic values of the Kondo coupling strengths and spin-parity
symmetry is preserved, the Kondo length takes values within the reach of
nowadays experimental technology in ultracold-atom setups. In the case of
non-symmetric Kondo couplings and/or spin parity broken by a nonzero magnetic
field applied to the impurity, we discuss how Kondo screening redistributes
among the chain as a function of the asymmetry in the couplings and map out the
shrinking of the Kondo length when the magnetic field induces a crossover from
Kondo impurity to weak-link physics.
