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

Abstract

In this work, we study the physics of a magnetic impurity coupled to several conduction band structures (metallic band, pseudo-gap systems and semiconductors with finite gap). However, the main focus is to explain the behavior of a system comprising a quantum impurity, strongly coupled to a semiconductor (with gap 2 ) and weakly coupled to a metal. Using the Numerical Renormalization Group (NRG) and Anderson’s poor man’s scaling, we show that this system (Impurity+metal-semiconductor hybrid contact), displays a reentrant Kondo stage as one gradually lowers the temperature. The 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 in the lower temperature, the Kondo screening exhibits a much lower Kondo temperature TK2, associated to a stable SC fixed point. The results clearly suggest that the reentrant Kondo screening is associated to an effective SIAM, with an effective Hubbard Ueff, whose value is clearly identifiable in the impurity’s local density of states. This reentrant SIAM, or effective SIAM, at temperatures below the gap, behaves as a replica of the high temperature SIAM. We show this in our results, and more specifically, in the NRG flow diagram (obtained through NRG). The second stage RG flow, whose FO fixed point emerges for T < TK1, 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 stage involves screening by metallic electrons, once the semiconducting electrons are out of reach to thermal excitations (T < ) and only the metallic (low) spectral weight inside the gap is available for impurity screening. For all parameter ranges analyzed, we find through the NRG results that TK2 TK1. Last, we analyze a hybrid system formed by a quantum impurity ‘sandwiched’ between an armchair graphene nanoribbon (AGNR) and a scanning tunneling microscope (STM). In this system, the energy gap (2 ) 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, and the second stage Kondo, is worthy of experimental investigation.