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High-resolution studies of the Majorana atomic chain platform

Abstract

Ordered assemblies of magnetic atoms on the surface of conventional superconductors can be used to engineer topological superconducting phases and realize Majorana fermion quasiparticles (MQPs) in a condensed matter setting. Recent experiments have shown that chains of Fe atoms on Pb generically have the required electronic characteristics to form a one-dimensional topological superconductor and have revealed spatially resolved signatures of localized MQPs at the ends of such chains. Here we report higher-resolution measurements of the same atomic chain system performed using a dilution refrigerator scanning tunnelling microscope (STM). With significantly better energy resolution than previous studies, we show that the zero-bias peak (ZBP) in Fe chains has no detectable splitting from hybridization with other states. The measurements also reveal that the ZBP exhibits a distinctive ‘double eye’ spatial pattern on nanometre length scales. Theoretically we show that this is a general consequence of STM measurements of MQPs with substantial spectral weight in the superconducting substrate, a conclusion further supported by measurements of Pb overlayers deposited on top of the Fe chains. Finally, we report experiments performed with superconducting tips in search of the particle–hole symmetric MQP signature expected in such measurements.

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Figure 1: Zero-bias end mode in Fe chains on Pb.
Figure 2: ‘Double eye’ pattern and spatial dependence of the ZBP.
Figure 3: Theoretical model of the zero-bias end mode.
Figure 4: Theoretical model of the ‘double eye’ spatial pattern of the ZBP.
Figure 5: ZBP in Pb overlayer above Fe chain.
Figure 6: Superconducting tip spectra and electron–hole symmetry of the zero-energy end mode.

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Acknowledgements

The work at Princeton has been supported by ONR-N00014-14-1-0330, ONR-N00014-11-1-0635, ONR-N00014-13-10661, NSF-MRSEC programs through the Princeton Center for Complex Materials DMR-142054, NSF-DMR-1104612, NSF-DMR-1420541, NSF EAGER Award NOA-AWD1004957, DOE DE-SC0016239, Simons Investigator Award, Packard Foundation and Schmidt Fund for Innovative Research, and by the Gordon and Betty Moore Foundation as part of EPiQS initiative (GBMF4530). This project was also made possible using the facilities at Princeton Nanoscale Microscopy Laboratory supported by grants through ARO-MURI Program W911NF-12-1-0461, DOE-BES, LPS and ARO-W911NF-1-0606, and Eric and Wendy Schmidt Transformative Technology Fund at Princeton. B.E.F. acknowledges financial support from the Dicke Fellowship. M.T.R. acknowledges support from the NSF Graduate Research Fellowship Program.

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B.E.F., M.T.R. and I.K.D. performed the dilution refrigerator STM measurements. S.J., Y.X. and I.K.D. conducted the measurements on Fe chains capped with Pb overlayers. J.L., Z.W. and B.A.B. performed the theoretical modelling and simulations. All authors contributed to analysing the data and writing the manuscript.

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Correspondence to Ali Yazdani.

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Feldman, B., Randeria, M., Li, J. et al. High-resolution studies of the Majorana atomic chain platform. Nature Phys 13, 286–291 (2017). https://doi.org/10.1038/nphys3947

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