  Magnetic atoms on a superconductor give rise to Yu-Shiba-Rusinov (YSR) states
within the superconducting energy gap. A spin chain of magnetic adatoms on an
s-wave superconductor may lead to topological superconductivity accompanied by
the emergence of Majorana modes at the chain ends. For their usage in quantum
computation, it is a prerequisite to artificially assemble the chains and
control the exchange couplings between the spins in the chain and in the
substrate. Here, using a scanning tunneling microscope tip, we demonstrate
engineering of the energy levels of the YSR states by placing interstitial Fe
atoms in close proximity to adsorbed Fe atoms on an oxidized Ta surface. Based
on this prototype platform, we show that the interaction within a long chain
can be strengthened by linking the adsorbed Fe atoms with the interstitial
ones. Our work adds an important step towards the controlled design and
manipulation of Majorana end states.
