Coherence in the Two Kondo Impurity Problem

We show through a perturbative and an exact calculation using Wilson's renormalization methods that in the problem of two interacting Kondo impurities, on-site potential scattering generates a quantum-tunneling between the two impurities through a marginally relevant operator. The magnitude of this tunneling $V_{12}$ depends on the spin-correlation between the two impurities. For exchange interactions between the moments comparable to the Kondo energy $T_K$, $V_{12}$ is typically much larger than $T_K$. This implies that the heavy-fermion mass in this interesting range is determined by $V_{12}$ rather than $T_K$. The importance of these results for experiments in coupled Kondo dots is also pointed out.