CaIrO3/YBa2Cu3O7异质结的PLD生长及超导邻近效应研究

马约拉纳费米子对于实现可容错量子计算具有重要意义。拓扑超导体有望承载这种神秘的马约拉纳束缚态，因此成为了凝聚态物理和材料科学领域最重要的研究领域之一。已有相关研究表明人们可以通过制备拓扑材料/超导体异质结结构，利用超导邻近效应在拓扑材料中诱导出拓扑超导相。

在本论文中，我们系统研究了拓扑半金属CaIrO₃（CIO）/高温超导体YBa₂Cu₃O₇（YBCO）异质结体系的超导邻近效应，以期在CIO中诱导出超导。为此，我们进行了以下实验工作：优化脉冲激光沉积技术的生长条件，然后使用该技术成功生长出CIO/YBCO异质结薄膜样品；使用电子束曝光和离子刻蚀技术，制备出用于研究超导邻近效应的特定微纳器件；在温度低至100mK，磁场高达14T的稀释制冷机中细致测量器件的输运特性。

从测量到的器件电阻-温度曲线来看，YBCO层在25.5K以下变得超导。随着温度继续降低至15.7K以下，原本平坦的微分电阻谱在零偏置电流附近出现了下降，表明在超导YBCO和非超导CIO的界面处发生了安德烈夫反射。更有趣的是，随着温度进一步降至500mK以下，在微分电阻谱中开始出现一个完整的微分电阻谷。同时电阻-温度曲线在529mK以下也观察到了电阻的急剧下降。实验还发现该微分电阻谷对外部磁场敏感，随着磁场的增加而迅速缩小。这些现象表明，CIO层在超导邻近效应的影响下已经变得超导，并且正是超导CIO和金属电极的界面处发生的安德烈夫反射导致了500mK以下出现的微分微分电阻谷。此外，我们还利用BCS理论拟合了实验结果，得到CIO在零温时通过超导邻近效应诱导产生的超导带隙为5.1μeV。

我们的工作成功证明了可以使用d波超导体YBCO通过超导邻近效应在拓扑半金属CIO中诱导出超导，从而为我们继续探索CIO/YBCO异质结中可能的拓扑超导及其它有趣的物理机制打下了坚实的基础。

Majorana fermion has been shown to be of great importance to the r­ealization of fault-tolerant quantum computation. Topological superconduc­t­ors are expected to host such mysterious Majorana bound states and thus have attracted intense research interest in condensed matter physics and m­aterial science. Previous studies have revealed that one possible way to search for topological superconductivity is to prepare topological materials/superconductor heterostructures. Due to superconducting proximity effect, the intriguing topological superconducting phase might be induced in the t­opological materials.

In this work, we have systematically studied the superconducting pro­ximity effect in heterostructures composed of topological semimetal CaIrO₃(CIO) and high-temperature superconductor YBa₂Cu₃O₇(YBCO), aiming t­o induce superconductivity in CIO. In order to do so, we have carried o­ut t­he following experimental works: Optimize the growth conditions of t­h­e pulsed laser deposition technique and then use this technique to successf­u­l­­ly synthesize the CIO/YBCO epitaxial heterostructures; Collectively imp­l­ement the e-beam lithography and ion etching techniques to fabricate de­v­ices specially designed for studying the superconductivity proximity effe­c­t; Measure the transport properties of the device in a dilution refrigerat­o­r with temperatures down to 100mK and magnetic fields up to 5T.

From the measured resistance-temperature curve of the device, the YBCO layer becomes superconducting below 25.5K. As the temperature continues to decrease below 15.7K, the originally flat differential resistance spectrum develops a dip in the vicinity of zero bias, indicating the occurrence of Andreev reflection at the interface between superconducting YBCO and non-superconducting CIO. More interestingly, with temperature further decreasing below 500mK, there begins to appear a well-formed differential resistance valley in the differential resistance spectra. Meanwhile, a sharp decrease of resistance is also noticed below 529mK in the resistance-temperature curve. The valley is found sensitive to the external magnetic field, shrinking quickly with increasing magnetic fields. These phenomena lead us to believe that the CIO layer has become superconducting due to the superconducting proximity effect and it is the Andreev reflection occurring at the interface between superconducting CIO and electrodes that gives rise to the valley structure below 500mK. We also use the BCS theory to fit the experimental results and obtain the induced zero-temperature superconducting gap of CIO to be 5.1μeV.

Our work successfully demonstrates that one can use the d-wave YBCO to induce superconductivity in topological semimetal CIO by the superconducting proximity effect, thus providing a solid foundation for us to continue to explore the possible topological superconductivity and other interesting physical phenomena in CIO/YBCO heterostructures.

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