  Using NRG, we show that a system containing a quantum impurity (QI), strongly
coupled to a semiconductor (gap $2 \Delta$) and weakly coupled to a metal,
displays a 'reentrant' Kondo stage at low temperatures. The NRG analysis of the
corresponding Single Impurity Anderson Model (SIAM) shows that the reentrant
stage is characterized by a second sequence of SIAM fixed points: free orbital
(FO) > local moment (LM) > strong coupling (SC). In the first stage, the SC
fixed point (Kondo temperature $T_{K1}$) is unstable, while the second stage
exhibits a much lower Kondo temperature $T_{K2}$, associated to a stable SC
fixed point. The results indicate that the reentrant Kondo screening is
associated to an effective SIAM, with an effective repulsion $U_{eff}$. This
low temperature effective SIAM, which we dub as 'reentrant' SIAM, behaves as a
'replica' of the high temperature (bare) SIAM. The intuitive picture that
emerges is that the first Kondo state develops through impurity screening by
semiconducting electrons, while the second Kondo state involves screening by
metallic electrons, once the semiconducting electrons are out of reach to
thermal excitations ($T < \Delta$) and only the metallic spectral weight inside
the gap is available for impurity screening. In addition, we analyze a hybrid
system formed by a QI `sandwiched' between an armchair graphene nanoribbon
(AGNR) and a scanning tunneling microscope (STM) tip, with respective couplings
set to reproduce the generic model described above. The energy gap in the AGNR
can be externally tuned by an electric-field-induced Rashba spin-orbit
interaction. We analyzed this system for realistic parameter values, using NRG,
and concluded that the reentrant SIAM, with its associated second stage Kondo,
is worthy of experimental investigation.
