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Absence of evidence for chiral Majorana modes in quantum anomalous Hall-superconductor devices

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Science  03 Jan 2020:
Vol. 367, Issue 6473, pp. 64-67
DOI: 10.1126/science.aax6361

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Looking for chiral Majoranas

Chiral Majorana modes have been predicted to exist in heterostructures consisting of a quantum anomalous Hall insulator and a superconductor. Kayyalha et al. fabricated more than 30 such samples and used transport measurements to look for signatures of the Majorana modes. The data indicated that the transport signatures previously thought to be associated with Majorana physics could, in their samples, be explained using a more mundane mechanism.

Science, this issue p. 64

Abstract

A quantum anomalous Hall (QAH) insulator coupled to an s-wave superconductor is predicted to harbor chiral Majorana modes. A recent experiment interprets the half-quantized two-terminal conductance plateau as evidence for these modes in a millimeter-size QAH-niobium hybrid device. However, non-Majorana mechanisms can also generate similar signatures, especially in disordered samples. Here, we studied similar hybrid devices with a well-controlled and transparent interface between the superconductor and the QAH insulator. When the devices are in the QAH state with well-aligned magnetization, the two-terminal conductance is always half-quantized. Our experiment provides a comprehensive understanding of the superconducting proximity effect observed in QAH-superconductor hybrid devices and shows that the half-quantized conductance plateau is unlikely to be induced by chiral Majorana fermions in samples with a highly transparent interface.

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