# Zero-energy vortex bound state in the superconducting topological surface state of Fe(Se,Te)

## Abstract

Majorana quasiparticles in condensed matter are important for topological quantum computing1,2,3, but remain elusive. Vortex cores of topological superconductors may accommodate Majorana quasiparticles that appear as the Majorana bound state (MBS) at zero energy4,5. The iron-based superconductor Fe(Se,Te) possesses a superconducting topological surface state6,7,8,9 that was investigated by scanning tunnelling microscopy (STM) studies, which suggest such a zero-energy vortex bound state (ZVBS)10,11. Here we present ultrahigh energy-resolution spectroscopic imaging (SI)–STM to clarify the nature of the vortex bound states in Fe(Se,Te). We found the ZVBS at 0 ± 20 μeV, which constrained its MBS origin, and showed that some vortices host the ZVBS but others do not. We show that the fraction of vortices hosting the ZVBS decreases with increasing magnetic field and that local quenched disorders are not related to the ZVBS. Our observations elucidate the necessary conditions to realize the ZVBS, which paves the way towards controllable Majorana quasiparticles.

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## Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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## Acknowledgements

The authors thank C.-K. Chiu, A. Furusaki, P. A. Lee, D.-H. Lee, Y. Nagai and T. T. Ong for valuable comments and C. J. Butler for a critical reading. This work was partly supported by CREST project JPMJCR16F2 from the Japan Science and Technology Agency, Grants-in-Aid for Scientific Research (KAKENHI) (numbers 17H01141, 16H04024, 19H01843), a Grant-in-Aid for Young Scientists (KAKENHI) (number 19K14661), and Japan China Bilateral Joint Research Project by the Japan Society for the Promotion of Science (JSPS).

## Author information

T.M. carried out the experiments and the data analyses with assistance from Y.K. and T.H. Y.S., S.P., T.T., S.T. and T.S. grew single crystals. T.H. supervised the project. T.M. and T.H. wrote the manuscript.

Correspondence to T. Machida or T. Hanaguri.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–10, Supplementary Notes 1–7 and Supplementary References 1–8.

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Reprints and Permissions

Machida, T., Sun, Y., Pyon, S. et al. Zero-energy vortex bound state in the superconducting topological surface state of Fe(Se,Te). Nat. Mater. 18, 811–815 (2019) doi:10.1038/s41563-019-0397-1

• #### DOI

https://doi.org/10.1038/s41563-019-0397-1

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