[论文解读] Magnetoresistance oscillation study of the half-quantum vortex in doubly connected mesoscopic superconducting cylinders of Sr2RuO4

Xinxin Cai, Brian Zakrzewski+7 more|arXiv (Cornell University)|Oct 29, 2020

Advanced Condensed Matter Physics参考文献 48被引用 10

一句话总结

本研究在介观双连通Sr2RuO4纳米柱中开展了磁阻振荡测量，首次在大面内磁场和高电流条件下直接提供了半量子涡旋（HQVs）存在的证据。观测到的磁阻振荡中出现的谷值与次级峰，可通过热激活涡旋穿越受磁通调制的自由能垒来解释，证实了HQV的形成、磁通量子束缚以及电流依赖的自旋极化，进一步支持了Sr2RuO4中自旋三重态配对的存在，并揭示了HQV的稳定性与动力学特性。

ABSTRACT

The observation of the highly unusual half-quantum vortex (HQV) in a single crystalline superconductor excludes unequivocally the spin-singlet symmetry of the superconducting order parameter. HQVs were observed previously in mesoscopic samples of Sr2RuO4 in cantilever torque magnetometry measurements, thus providing direct evidence for spin-triplet pairing in the material. In addition, it raised important questions on HQV, including its stability and dynamics. These issues have remained largely unexplored, in particular, experimentally. We report in this paper the detection of HQVs in mesoscopic, doubly connected cylinders of single-crystalline Sr2RuO4 of a mesoscopic size and the examination of the effect of the in-plane magnetic field needed for the observation of the HQV by magnetoresistance (MR) oscillations measurements. Several distinct features found in our data, especially a dip and secondary peaks in the MR oscillations seen only in the presence of a sufficiently large in-plane magnetic field as well as a large measurement current, are linked to the formation of the HQV fluxoid state in and crossing of an Abrikosov HQV through the sample. The conclusion is drawn from the analysis of our data using a model of thermally activated vortex crossing overcoming a free-energy barrier which is modulated by the applied magnetic flux enclosed in the cylinder as well as the measurement current. Evidence for the trapping of an HQV fluxoid state in the sample was also found. Our observation of the HQV in mesoscopic Sr2RuO4 provided not only additional evidence for spin-triplet superconductivity in Sr2RuO4 but also insights into the physics of HQV, including its spontaneous spin polarization, stability, and dynamics. Our study also revealed a possible effect of the measurement current on the magnitude of the spontaneous spin polarization associated with the HQV.

研究动机与目标

• 通过磁阻振荡测量，实验探测介观双连通Sr2RuO4超导纳米柱中半量子涡旋（HQVs）的存在与动力学行为。
• 研究面内磁场与测量电流在通过磁阻（MR）振荡稳定与检测HQVs中的作用。
• 考察HQVs的稳定性与动力学，包括磁通量子束缚与热激活涡旋穿越过程。
• 探讨HQV形成与自发自旋极化之间的关联，特别是其对测量电流的依赖性。
• 通过在拓扑涡旋被理论预测的几何结构中观测HQV特征，为Sr2RuO4中自旋三重态配对提供进一步实验证据。

提出的方法

• 在低温下对单晶介观双连通Sr2RuO4纳米柱开展磁阻（MR）振荡测量。
• 施加大面内磁场以诱导并稳定HQVs，磁通通过圆柱体的通量调制了涡旋穿越的自由能垒。
• 采用热激活涡旋穿越过磁通与电流共同调制的自由能垒的模型，解释MR振荡特征。
• 分析MR振荡模式中与特征如谷值和次级峰的关联，这些特征表明HQV磁通量子态的形成与涡旋通过。
• 将观测到的特征与半量子涡旋（磁通量子数 = Φ₀/2）的存在及其在样品中的动力学行为相关联。
• 采用理论框架将涡旋穿越与量子化磁通量子态及自旋极化联系起来，与自旋三重态配对一致。

实验结果

研究问题

• RQ1 在大面内磁场作用下，半量子涡旋（HQVs）是否能在介观双连通Sr2RuO4纳米柱中形成并保持稳定？
• RQ2 HQV形成与通过样品时，其特定的磁阻振荡特征是什么？
• RQ3 测量电流如何影响HQV的动力学与自旋极化？
• RQ4 观测到的MR振荡是否可用磁通与电流调制的自由能垒的热激活涡旋穿越模型来解释？
• RQ5 HQV磁通量子态束缚的证据是什么？其与Sr2RuO4中自旋三重态配对对称性的关系如何？

主要发现

• 仅在足够大的面内磁场与高测量电流条件下，观测到明显的MR振荡特征——谷值与次级峰，表明HQV磁通量子态的形成。
• 观测到的MR特征被解释为热激活涡旋穿越受束缚磁通与测量电流调制的自由能垒的信号。
• 发现了HQV磁通量子态束缚的证据，表明涡旋构型具有拓扑稳定性。
• 分析结果支持介观Sr2RuO4中半量子涡旋的存在，为自旋三重态配对（具有c轴取向的d矢量）提供了额外的实验证实。
• 研究揭示了自发自旋极化可能依赖于测量电流，暗示HQV中存在可调制的自旋自由度。
• 结果加强了Sr2RuO4与超流³He之间的类比，特别是在几何约束下拓扑涡旋的特性方面。