  Co single atom junctions on copper surfaces are studied by scanning tunneling
microscopy and ab-initio calculations. The Kondo temperature of single cobalt
atoms on the Cu(111) surface has been measured at various tip-sample distances
ranging from tunneling to the point contact regime. The experiments show a
constant Kondo temperature for a whole range of tip-substrate distances
consistently with the predicted energy position of the spin-polarized d-levels
of Co. This is in striking difference to experiments on Co/Cu(100) junctions,
where a substantial increase of the Kondo temperature has been found. Our
calculations reveal that the different behavior of the Co adatoms on the two Cu
surfaces originates from the interplay between the structural relaxations and
the electronic properties in the near-contact regime.
