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# Detecting and distinguishing Majorana zero modes with the scanning tunnelling microscope

## Abstract

One of the most exciting areas of research in quantum condensed matter physics is the push to create topologically protected qubits using non-Abelian anyons. The focus of these efforts has been Majorana zero modes (MZMs), which are predicted to emerge as localized zero-energy states at the ends of 1D topological superconductors. A key role in the search for experimental signatures of these quasiparticles has been played by the scanning tunnelling microscope (STM). The power of high-resolution STM techniques is perhaps best illustrated by their application in identifying MZMs in 1D chains of magnetic atoms on the surface of a superconductor. In this platform, STM spectroscopic mapping has demonstrated the localized nature of MZM zero-energy excitations at the ends of such chains, and experiments with superconducting and magnetic STM tips have been used to uniquely distinguish them from trivial edge modes. Beyond the atomic chains, STM has also uncovered signatures of MZMs in 2D materials and topological surface and boundary states, when they are subjected to the superconducting proximity effect. Looking ahead, future STM experiments may be able to demonstrate the non-Abelian statistics of MZMs.

## Key points

• Majorana zero modes (MZMs) are non-Abelian anyons that hold promise for facilitating topologically protected quantum computation. They can emerge as localized zero-energy states at the end of 1D topological superconductors.

• Scanning tunnelling microscopy (STM), with its ability to map the surface topography and probe the local electronic properties of samples with high spectral resolution, is particularly well suited to visualize MZMs on the atomic scale.

• STM experiments have demonstrated the presence of MZMs as localized end states of Fe chains on a Pb surface. Combining superconductivity with spin–orbit coupling and ferromagnetism, this model system realizes the Kitaev model for 1D topological superconductivity.

• High-resolution spectroscopy with the STM can explore various concepts for topological superconductivity and visualize the presence of localized zero-energy states across a plurality of material platforms, such as topological surface and boundary modes.

• Measurements using functional STM tips can probe other properties of zero-energy states, such as their spin signature. Through this capacity, these experiments are uniquely suited to distinguish topological from trivial zero-energy states.

• Future experiments with the STM on chains of magnetic atoms have the potential to demonstrate manipulation and braiding of MZMs, an important step towards realizing topologically protected quantum computation.

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## Acknowledgements

The authors thank A. Bernevig, J. Li, S. Nadj-Perge, I. Drozdov, S. Joen, B. Feldman, M. Randeria and Z. Wang for many years of collaboration on the topics covered in this Review. B.J. acknowledges support from the Alexander-von-Humboldt foundation through a postdoctoral fellowship. A.Y. acknowledges support from the Office of Naval Research grant ONR-N00014-17-1-2784, Gordon and Betty Moore Foundation as part of EPiQS initiative (GBMF 9469), the US National Science Foundation’s NSF-MRSEC programmes through the Princeton Center for Complex Materials NSF-DMR-2011750 grant, and NSF-DMR-1904442.

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Jäck, B., Xie, Y. & Yazdani, A. Detecting and distinguishing Majorana zero modes with the scanning tunnelling microscope. Nat Rev Phys 3, 541–554 (2021). https://doi.org/10.1038/s42254-021-00328-z

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• DOI: https://doi.org/10.1038/s42254-021-00328-z