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

机译：耦合到金属-半导体混合接触的量子杂质的折返近藤效应

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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 7". 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 ≈ Δ＜ 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 ＜ Δ ) 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 (2Δ) 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.
机译：使用数值归一化组（NRG）和安德森穷人的缩放比例，我们显示出一个包含量子杂质（QI）的系统，该系统强耦合到半导体（带隙2Δ），弱耦合到金属，显示出可重入的近藤阶段一个逐渐降低温度7英寸。通过杂质的热力学和光谱特性，对相应的单个杂质安德森模型（SIAM）的NRG分析表明，折返阶段的特征是第二个SIAM固定点序列，即自由轨道（FO）→局部力矩（LM）→强耦合（SC）。在高温阶段，SC固定点（具有近藤温度T_（K1））是不稳定的，而较低温度的近藤屏蔽显示出与稳定的SC固定点相关的近藤温度T_（K2）更低，结果清楚地表明，折返的近藤筛选与有效的SIAM相关，有效的Hubbard排斥力U_（eff），其值可明确确定在杂质的局部态密度中是有效的。我们称其为折返型SIAM的这种低温有效SIAM表现为高温（裸）SIAM的复制品。一旦RG从不稳定的第一级SC固定点流出，第二级RG流（通过NRG获得）的FO固定点出现在T≈Δ＜T_（K1）处。从我们的分析中得出的直观图景是，第一个近藤状态通过半导体电子进行杂质筛选而形成，而第二个Kondo状态涉及通过金属电子进行杂质筛选，一旦半导体电子无法达到热激发（T ＜Δ）且间隙中只有金属（低）光谱重可用于杂质筛选。此开关意味着第一个近藤云比第二个云小得多，因为NRG结果表明，对于所分析的所有参数范围，T_（K2）＜＜ T_（K1）。最后但并非最不重要的一点是，我们分析了由QI夹在扶手椅石墨烯纳米带（AGNR）和扫描隧道显微镜（STM）尖端（AGNR + QI + STM系统）之间的QI组成的混合系统，重现上述通用模型。 AGNR中的能隙（2Δ）可以通过电场感应的Rashba自旋轨道相互作用从外部进行调节。我们使用NRG分析了该系统的实际参数值，并得出结论：可重入的SIAM及其相关的第二阶段Kondo，值得进行实验研究。

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  • 来源
    《Physical review》 |2020年第12期|125115.1-125115.13|共13页
  • 作者

    G. Diniz; G. S. Diniz; G. B. Martins; E. Vernek;

  • 作者单位

    Instituto de Física Universidade Federal de Uberlândia Uberlândia Minas Gerais 38400-902 Brazil;

    Curso de Física Universidade Federal de Jataí Jataí GO 75801-615 Brazil;

    Instituto de Física Universidade Federal de Uberlândia Uberlândia Minas Gerais 38400-902 Brazil Department of Physics and Astronomy and Nanoscale and Quantum Phenomena Institute Ohio University Athens Ohio 45701-2979 USA;

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