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Absence of evidence for chiral Majorana modes in quantum anomalous Hall-superconductor devices
Science ( IF 44.7 ) Pub Date : 2020-01-02 , DOI: 10.1126/science.aax6361
Morteza Kayyalha 1 , Di Xiao 1 , Ruoxi Zhang 1 , Jaeho Shin 1 , Jue Jiang 1 , Fei Wang 1 , Yi-Fan Zhao 1 , Run Xiao 1 , Ling Zhang 1 , Kajetan M Fijalkowski 2, 3 , Pankaj Mandal 2, 3 , Martin Winnerlein 2, 3 , Charles Gould 2, 3 , Qi Li 1 , Laurens W Molenkamp 2, 3 , Moses H W Chan 1 , Nitin Samarth 1 , Cui-Zu Chang 1
Affiliation  

A quantum anomalous Hall (QAH) insulator coupled to an s-wave superconductor is predicted to harbor a topological superconducting phase, the elementary excitations of which (i.e. Majorana fermions) can form topological qubits upon non-Abelian braiding operations. A recent transport experiment interprets the half-quantized two-terminal conductance plateau as the presence of chiral Majorana fermions in a millimeter-size QAH-Nb hybrid structure. However, there are concerns about this interpretation because non-Majorana mechanisms can also generate similar signatures, especially in a disordered QAH system. Here, we fabricated QAH-Nb hybrid structures and studied the QAH-Nb contact transparency and its effect on the corresponding two-terminal conductance. When the QAH film is tuned to the metallic regime by electric gating, we observed a sharp zero-bias enhancement in the differential conductance, up to 80% at zero magnetic field. This large enhancement suggests high probability of Andreev reflection and transparent interface between the magnetic topological insulator (TI) and Nb layers. When the magnetic TI film is in the QAH state with well-aligned magnetization, we found that the two-terminal conductance is always half-quantized. Our experiment provides a comprehensive understanding of the superconducting proximity effect observed in QAH-superconductor hybrid structures and shows that the half-quantized conductance plateau is unlikely to be induced by chiral Majorana fermions.

中文翻译:

缺乏量子反常霍尔超导体器件中手性马约拉纳模式的证据

预测耦合到 s 波超导体的量子反常霍尔 (QAH) 绝缘体具有拓扑超导相,其基本激发(即马约拉纳费米子)可以在非阿贝尔编织操作中形成拓扑量子位。最近的一项传输实验将半量化的两端电导平台解释为毫米大小的 QAH-Nb 混合结构中存在手性马约拉纳费米子。然而,这种解释令人担忧,因为非 Majorana 机制也可以产生类似的特征,尤其是在无序的 QAH 系统中。在这里,我们制造了 QAH-Nb 混合结构并研究了 QAH-Nb 接触透明度及其对相应两端电导的影响。当通过电门控将 QAH 薄膜调谐到金属状态时,我们观察到微分电导的零偏置急剧增强,在零磁场下高达 80%。这种大的增强表明磁性拓扑绝缘体 (TI) 和 Nb 层之间的 Andreev 反射和透明界面的可能性很高。当磁性 TI 薄膜处于 QAH 状态且磁化良好时,我们发现两端电导总是半量化的。我们的实验提供了对 QAH-超导体混合结构中观察到的超导邻近效应的全面理解,并表明半量子化电导平台不太可能由手性马约拉纳费米子引起。这种大的增强表明磁性拓扑绝缘体 (TI) 和 Nb 层之间的 Andreev 反射和透明界面的可能性很高。当磁性 TI 薄膜处于 QAH 状态且磁化良好时,我们发现两端电导总是半量化的。我们的实验提供了对 QAH-超导体混合结构中观察到的超导邻近效应的全面理解,并表明半量子化电导平台不太可能由手性马约拉纳费米子引起。这种大的增强表明磁性拓扑绝缘体 (TI) 和 Nb 层之间的 Andreev 反射和透明界面的可能性很高。当磁性 TI 薄膜处于 QAH 状态且磁化良好时,我们发现两端电导总是半量化的。我们的实验提供了对 QAH-超导体混合结构中观察到的超导邻近效应的全面理解,并表明半量子化电导平台不太可能由手性马约拉纳费米子引起。
更新日期:2020-01-02
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