Magnetic impurity coupled to interacting conduction electrons

We consider a magnetic impurity which interacts by hybridization with a system of weakly correlated electrons and determine the energy of the ground state by means of a 1/${\mathit{N}}_{\mathit{f}}$ expansion. The correlations among the conduction electrons are described by a Hubbard Hamiltonian and are treated to the lowest order in the interaction strength. We find that their effect on the Kondo temperature, ${\mathit{T}}_{\mathit{K}}$, in the Kondo limit is twofold: first, the position of the impurity level is shifted due to the reduction of charge fluctuations, which reduces ${\mathit{T}}_{\mathit{K}}$. Secondly, the bare Kondo exchange coupling is enhanced as spin fluctuations are enlarged. In total, ${\mathit{T}}_{\mathit{K}}$ increases. Both corrections require intermediate states beyond the standard Varma-Yafet ansatz. This shows that the Hubbard interaction does not just provide quasiparticles, which hybridize with the impurity, but also renormalizes the Kondo coupling. © 1996 The American Physical Society.