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[论文解读] Phase Asymmetry of Andreev Spectra From Cooper-Pair Momentum

Abhishek Banerjee, Max Geier|arXiv (Cornell University)|Jan 5, 2023
Physics of Superconductivity and Magnetism被引用 5
一句话总结

该论文展示了在InAs/Al超导体-正常态-超导体约瑟夫森结中，由于轨道耦合引起的有限动量库珀配对，在平面磁场作用下诱导出相位不对称的Andreev束缚态谱。该不对称性在0.15 T时达到峰值，且在无磁场时完全消失，为无需自旋-轨道或Zeeman耦合的非互易超流响应提供了证据。

ABSTRACT

In analogy to conventional semiconductor diodes, the Josephson diode exhibits superconducting properties that are asymmetric in applied bias. The effect has been investigated in number of systems recently, and requires a combination of broken time-reversal and inversion symmetries. We demonstrate a dual of the usual Josephson diode effect, a nonreciprocal response of Andreev bound states to a superconducting phase difference across the normal region of a superconductor-normal-superconductor Josephson junction, fabricated using an epitaxial InAs/Al heterostructure. Phase asymmetry of the subgap Andreev spectrum is absent in the absence of in-plane magnetic field and reaches a maximum at 0.15 T applied in the plane of the junction transverse to the current direction. We interpret the phase diode effect in this system as resulting from finite-momentum Cooper pairing due to orbital coupling to the in-plane magnetic field, without invoking Zeeman or spin-orbit coupling.

研究动机与目标

  • 研究在不依赖自旋-轨道或Zeeman耦合条件下的非互易Andreev束缚态响应。
  • 确定平面磁场下约瑟夫森结亚能隙谱中相位不对称性的起源。
  • 确立仅通过轨道耦合即可诱导有限动量库珀配对，从而产生非互易超流。
  • 通过三端口谱学实验验证相位不对称Andreev束缚态在整个结区的分布范围。

提出的方法

  • 制备了具有100 nm宽正常区和1.8 µm超导引线的平面InAs/Al外延约瑟夫森结。
  • 通过局域和非局域隧穿谱学测量，绘制了作为超导相位差和平面磁场函数的Andreev束缚态谱。
  • 利用三端口电导测量，确认了相位不对称态在整个结区的体态特性。
  • 采用包含轨道耦合至磁场的哈密顿量进行理论建模，引入了近邻诱导的相位梯度。
  • 利用材料参数（g因子、自旋-轨道耦合、有效质量）推导的参数模拟了能谱和二极管效率。
  • 对有无电感的情况进行建模，以评估动能电感在调节相位响应中的作用。
Figure 1: Device schematic and micrograph (a) Schematic of a planar Josephson junction device consisting of two superconducting leads (blue) of width $w_{s}=1.8\,\mu$ m in epitaxial contact with the underlying semiconductor (brown). The normal region between the two leads is of width $w_{n}=100$ nm,
Figure 1: Device schematic and micrograph (a) Schematic of a planar Josephson junction device consisting of two superconducting leads (blue) of width $w_{s}=1.8\,\mu$ m in epitaxial contact with the underlying semiconductor (brown). The normal region between the two leads is of width $w_{n}=100$ nm,

实验结果

研究问题

  • RQ1能否在无自旋-轨道或Zeeman耦合条件下，在约瑟夫森结中诱导出相位不对称的Andreev束缚态？
  • RQ2轨道耦合至平面磁场在产生有限动量库珀配对中起什么作用？
  • RQ3随着平面磁场的增加，Andreev谱中的相位不对称性如何演化？
  • RQ4非互易Andreev束缚态是局域在结的边缘还是分布在正常区整个区域？
  • RQ5在此系统中，仅通过轨道耦合可实现的最大二极管效率是多少？

主要发现

  • Andreev束缚态谱的相位不对称性仅在平面磁场下出现，并在0.15 T时达到最大值。
  • 相位不对称谱在整个结的体相中均可观察到，而非仅限于边缘，这一点通过三端口电导谱学得到证实。
  • 在零磁场下，Andreev谱在相位反转下保持对称，证实了不存在本征不对称性。
  • 观察到的相位不对称性归因于轨道耦合至平面磁场诱导的有限动量库珀配对，而非Zeeman或自旋-轨道效应。
  • 理论建模成功再现了实验观测到的相位不对称性和二极管效率，峰值效率出现在轨道相位梯度与超导能隙匹配的磁场附近。
  • 系统在局域和非局域电导中均表现出非互易响应，一个通量瓣的一端具有较小的能隙，另一端具有较大的能隙，证实了方向性超流。
Figure 2: Nonreciprocal Andreev bound state spectrum. (a)–(e) Differential conductance measured in Device 1 as a function of $B_{\perp}$ at different value of $B_{\parallel}$ . At $B_{\parallel}=$ 0, the Andreev bound state spectrum is phase-symmetric within each flux lobe. For non-zero $B_{\paralle
Figure 2: Nonreciprocal Andreev bound state spectrum. (a)–(e) Differential conductance measured in Device 1 as a function of $B_{\perp}$ at different value of $B_{\parallel}$ . At $B_{\parallel}=$ 0, the Andreev bound state spectrum is phase-symmetric within each flux lobe. For non-zero $B_{\paralle

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