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arXiv 2209.09599cond-mat.supr-con
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Yu-Shiba-Rusinov 束缚态之间的隧穿过程

Tunneling processes between Yu-Shiba-Rusinov bound states

A. Villas, R. L. Klees, G. Morrás, H. Huang, C. R. Ast, G. Rastelli, W. Belzig, J. C. Cuevas

AI总结:

本文基于Anderson模型与非平衡格林函数技术,全面研究了两个磁杂质耦合超导电极系统中YSR束缚态间的隧穿过程,成功解释了能隙内电流峰实验现象,并预测了高透明度结中由YSR态介导的多重Andreev反射及负微分电导。

摘要:

最近的实验报道了原子尺度下 Yu-Shiba-Rusinov (YSR) 束缚态之间的隧穿现象。这些实验是借助扫描隧道显微镜实现的,其中超导针尖被磁杂质功能化,用于探测沉积在超导基底上的另一个磁杂质。通过这种方式,首次实现了对单个超导束缚态之间自旋依赖输运的研究。受这些实验启发,我们在此对两个磁杂质耦合到超导电极的系统中的 YSR 束缚态隧穿过程进行了全面的理论研究。我们的理论基于结合了具有破缺自旋简并的 Anderson 模型来描述杂质,并采用非平衡格林函数技术来计算电流-电压特性。这种结合使我们能够描述任意杂质间隧穿耦合强度下的自旋依赖输运。我们首先关注隧穿区域,并表明我们的理论自然地解释了实验观察到的现象,即由于两个杂质中 YSR 态之间的直接隧穿和热隧穿,在能隙内区域出现电流峰。然后,我们详细研究了透明度逐渐增加的结的情况,这尚未在实验上被探索过,并预测了由 YSR 态介导的多种(多重)Andreev 反射的发生,这些反射在能隙内电流中产生了非常丰富的结构。特别是,我们预测了涉及不同杂质中 YSR 态的多重 Andreev 反射的发生。这些过程在单杂质结中没有类似物,它们表现为能隙内电压下具有负微分电导的电流峰。

英文摘要:

Very recent experiments have reported the tunneling between Yu-Shiba-Rusinov (YSR) bound states at the atomic scale. These experiments have been realized with the help of a scanning tunneling microscope where a superconducting tip is functionalized with a magnetic impurity and is used to probe another magnetic impurity deposited on a superconducting substrate. In this way it has become possible to study for the first time the spin-dependent transport between individual superconducting bound states. Motivated by these experiments, we present here a comprehensive theoretical study of the tunneling processes between YSR bound states in a system in which two magnetic impurities are coupled to superconducting leads. Our theory is based on a combination of an Anderson model with broken spin degeneracy to describe the impurities and nonequilibrium Green's function techniques to compute the current-voltage characteristics. This combination allows us to describe the spin-dependent transport for an arbitrary strength of the tunnel coupling between the impurities. We first focus on the tunnel regime and show that our theory naturally explains the experimental observations of the appearance of current peaks in the subgap region due to both the direct and thermal tunneling between the YSR states in both impurities. Then, we study in detail the case of junctions with increasing transparency, which has not been experimentally explored yet, and predict the occurrence of a large variety of (multiple) Andreev reflections mediated by YSR states that give rise to a very rich structure in the subgap current. In particular, we predict the occurrence of multiple Andreev reflections that involve YSR states in different impurities. These processes have no analogue in single-impurity junctions and they are manifested as current peaks with negative differential conductance for subgap voltages.

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