Absence of Evidence for Chiral Majorana Modes in Quantum Anomalous Hall-Superconductor Devices*

A quantum anomalous Hall (QAH) insulator coupled to an s-wave superconductor is predicted to harbor chiral Majorana fermions, excitations that could serve as topological qubits. A recent experiment interprets the half-quantized two-terminal conductance plateau as evidence for these excitations in a millimeter-size QAH-Nb hybrid device. However, non-Majorana mechanisms can also generate similar signatures, especially in disordered samples. In this talk, I will introduce our recent progress in this direction. We first studied the contact transparency between the QAH insulator and Nb layers using Andreev reflection spectroscopy measurements. In the QAH-Nb devices with high interface transparency, we found that the two-terminal conductance across the QAH insulator-Nb hybrid device is always half-quantized throughout the entire magnetic field range with well-aligned magnetization. Our systematic measurements and careful analysis of the data showed that the half-quantized two-terminal conductance plateau observed in such a QAH-Nb device is unlikely to arise from chiral Majorana fermion excitations.