  It is well established that a correlated quantum impurity embedded in a
metallic host can form the many-body Kondo state with itinerant electrons due
to the effective antiferromagnetic coupling. Such effect is manifested
spectroscopically by a narrow Abrikosov-Suhl peak appearing at the Fermi level
below a characteristic temperature T_K. Recent experiments using nanoscopic
heterojunctions where the correlated quantum impurities (dots) are coupled to
superconducting reservoirs revealed that the Kondo-type correlations are
substantially weaker because: i) the single-particle states of superconductors
are depleted around the Fermi level and ii) the on-dot pairing (proximity
effect) competes with the spin ordering. Within the Anderson impurity scenario
we study here influence of such induced on-dot paring on the exchange
interaction adopting the continuous unitary transformation, which goes beyond
the perturbative framework. Our analytical and numerical results show strong
detrimental influence of the electron pairing on the effective
antiferromagnetic coupling thereby suppressing the Kondo temperature in
agreement with the experimental observations.
