Reentrant Kondo effect for a quantum impurity coupled to a metal-semiconductor hybrid contact

Using the numerical renormalization group (NRG) and Anderson's poor man's scaling, we show that a system containing a quantum impurity (QI), strongly coupled to a semiconductor (with gap 2Δ) and weakly coupled to a metal, displays a reentrant Kondo stage as one gradually lowers the temperature T. The NRG analysis of the corresponding single impurity Anderson model (SIAM), through the impurity's thermodynamic and spectral properties, shows that the reentrant stage is characterized by a second sequence of SIAM fixed points, viz., free orbital (FO) → local moment (LM) → strong coupling (SC). In the higher-temperature stage, the SC fixed point (with a Kondo temperature TK1) is unstable, while the lower-temperature Kondo screening exhibits a much lower Kondo temperature TK2, associated to a stable SC fixed point. The results clearly indicate that the reentrant Kondo screening is associated to an effective SIAM, with an effective Hubbard repulsion Ueff, whose value is clearly identifiable in the impurity's local density of states. This low-temperature effective SIAM, which we dub as reentrant SIAM, behaves as a replica of the high-temperature (bare) SIAM. The second-stage RG flow (obtained through NRG), whose FO fixed point emerges for T≈Δ