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

Article

关键词: NUMERICAL RENORMALIZATION-GROUP;    MAGNETIC IMPURITY;    ELECTRONIC-PROPERTIES;    MONTE-CARLO;    ANDERSON;    GRAPHENE;    SYSTEMS;    GAP;   

DOI  :  10.1103/PhysRevB.101.125115

来源: SCIE

【 摘 要 】

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 2 Delta) 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 T-K1) is unstable, while the lower-temperature Kondo screening exhibits a much lower Kondo temperature T-K2, 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 U-eff, 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 approximate to Delta < T-K1, takes over once the RG flows away from the unstable first-stage SC fixed point. The intuitive picture that emerges from our analysis 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 < Alpha) and only the metallic (low) spectral weight inside the gap is available for impurity screening. This switch implies that the first Kondo cloud is much smaller than the second since the NRG results show that, for all parameter ranges analyzed, T-K2 << T-K1. Last, but not least, we analyze a hybrid system formed by a QI sandwiched between an armchair graphene nanoribbon (AGNR) and a scanning tunneling microscope (STM) tip (an AGNR + QI + STM system), with respective couplings set to reproduce the generic model described above. The energy gap (2 Delta) 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.

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