Flowing uphill: From single- to multi-channel Kondo effect due to weak charging energies in the leads

We study a model proposed recently in which a small quantum dot is coupled symmetrically to several large quantum dots characterized by a charging energy $E_c$. Even if $E_c$ is much smaller than the Kondo temperature $ T_K$, the long-ranged interactions destabilize the single-channel Kondo effect and induce a flow towards a multi-channel Kondo fixed point associated with a {\em rise} of the impurity entropy with decreasing temperature. Such an ``uphill flow'' implies a {\em negative} impurity specific heat, in contrast to all systems with local interactions. An exact solution found for a large number of channels allows us to capture this physics and to predict transport properties.