[论文解读] Flux-tunable Andreev bound states in hybrid full-shell nanowires

Marco Valentini, Fernando Peñaranda+8 more|arXiv (Cornell University)|Aug 5, 2020

Topological Materials and Phenomena被引用 2

一句话总结

本研究证明，在异质全壳纳米线中通过磁通量调控可明确识别出亚能隙态为Andreev束缚态，具体为由量子点杂质引起的Yu-Shiba-Rusinov态。输运谱学揭示了由量子相变引起的磁通量依赖的零能交叉，且在约一个磁通量子附近，Little-Parks振荡会模拟出类似Majorana的零偏压峰，凸显了在异质系统中进行细致拓扑分类的必要性。

ABSTRACT

Understanding excitations of the Cooper pair condensate in a superconductor is crucial for many applications in quantum information processing. A remarkable example is the possibility of creating topologically-protected non-local qubits based on quasiparticle excitations at no energy cost, so-called Majorana zero modes. Their unambiguous detection has, however, been impeded by the ubiquitous presence of nontopological Andreev bound states pinned to zero energy. It has thus become of utmost importance to find ways to experimentally establish the physical origin of subgap states in a controlled way. Here we show that the magnetic flux tunability of full-shell nanowires, a semiconducting core fully wrapped by a superconducting shell, allows to clearly identify subgap levels as Andreev bound states. Specifically, transport spectroscopy reveals them as Yu-Shiba-Rusinov bound states, resulting from a quantum spin impurity, a quantum dot forming within the tunneling region, that forms Kondo-like singlets with quasiparticles in the superconductor. The magnetic field induces quantum phase transitions, subgap level crossings at zero energy. Apart from the Zeeman effect, the crossings also depend on the Little-Parks modulation of the gap which, in some cases, results in robust zero bias peaks in tunneling conductance near one flux quantum, a feature that could be easily misinterpreted as Majoranas. Our understanding of the complex interplay of different physical effects on the same device, fully supported by theory, offers a starting point for systematic experiments towards an unambiguous topological classification of zero modes in hybrid systems.

研究动机与目标

• 区分异质纳米线器件中拓扑Majorana零能模与非拓扑Andreev束缚态。
• 解决由于非拓扑态诱导的磁通量相关零偏压峰而导致Majorana态误判的挑战。
• 利用磁通量调控建立受控的实验平台，以探究亚能隙态的物理起源。
• 阐明自旋-轨道耦合、Kondo屏蔽与Little-Parks振荡在超导纳米线中的相互作用。

提出的方法

• 采用具有半导体芯和超导壳的全壳纳米线进行输运谱学，探测亚能隙态。
• 通过纳米线施加磁通量，调控超导能隙并诱导量子相变。
• 利用磁通量依赖性区分Zeeman分裂的Andreev束缚态与拓扑Majorana模。
• 分析在不同磁通量和磁场下零能交叉的出现，表明存在量子相变。
• 使用包含量子点杂质Kondo屏蔽和超导近邻效应的理论框架对系统进行建模。
• 识别出间隙中的Little-Parks振荡是产生稳健零偏压电导峰的来源，这些峰会模拟出Majorana特征。

实验结果

研究问题

• RQ1 能否利用磁通量调控在异质纳米线中区分Andreev束缚态与Majorana零能模？
• RQ2 量子点杂质和Kondo屏蔽在全壳纳米线中生成亚能隙态的过程中起何种作用？
• RQ3 磁通量诱导的Little-Parks振荡如何影响零偏压电导峰的出现？
• RQ4 Zeeman分裂与量子相变在亚能隙态中对零能交叉的贡献程度如何？
• RQ5 超导性、自旋-轨道耦合与磁通量之间的相互作用是否会导致Majorana探测中的假阳性结果？

主要发现

• 磁通量调控揭示了亚能隙态中清晰的零能交叉，表明Zeeman分裂与超导配对相互作用驱动的量子相变。
• 亚能隙态被识别为由量子点杂质形成的Yu-Shiba-Rusinov束缚态，这些杂质与超导体中的准粒子形成类似Kondo的单重态。
• Little-Parks振荡调制了超导能隙，并在约一个磁通量子附近产生稳健的零偏压峰，这些峰可能被误认为是Majorana模。
• 观测到的磁通量依赖行为与理论建模完全一致，证实了束缚态的非拓扑起源。
• 磁场与磁通量调控共同揭示了复杂的干涉效应，增加了对拓扑零能模识别的难度。
• 本研究为异质超导器件中零能模的无歧义拓扑分类提供了系统性框架。