# Half-integer level shift of vortex bound states in an iron-based superconductor

## Abstract

Vortices in topological superconductors may host Majorana zero modes (MZMs), which have been proposed as the building blocks of fault-tolerant topological quantum computers. Recently, a new single-material platform with the potential for realizing MZMs has been discovered in iron-based superconductors, without involving hybrid semiconductor–superconductor structures. Here, we report a detailed scanning tunnelling spectroscopy study of a FeTe0.55Se0.45 single crystal and show that this material hosts two distinct classes of vortex. These differ by a half-integer level shift in the energy spectra of the vortex bound states. This level shift is directly tied to the presence or absence of a zero-bias conductance peak and also alters the ratios of higher energy levels from integer to half-odd-integer. Our model calculations fully reproduce the spectra of these two types of vortex bound state, suggesting the presence of regions with and without topological surface states, which coexist within the same crystal. Our findings provide strong evidence for the presence of MZMs in FeTe0.55Se0.45 and establish it as an excellent platform for further studies.

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## Data availability

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

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## Acknowledgements

We thank N.F. Yuan, C.-K. Chiu, C. Schrade, S.-S. Qin and R.-X. Zhang for helpful discussions and F.-Z. Yang, G.-J. Qian for technical assistance. The work at IOP is supported by grants from the Ministry of Science and Technology of China (2015CB921000, 2015CB921300 and 2016YFA0202300), the National Natural Science Foundation of China (11234014, 11574371 and 61390501), and the Chinese Academy of Sciences (XDB28000000 and XDB07000000). L.F. and G.D.G are supported by US DOE (DE-SC0010526 and DE-SC0012704, respectively). J.S. and R.D.Z. are supported by the Center for Emergent Superconductivity, an EFRC funded by the US DOE.

## Author information

H.D. and H.-J.G. designed the experiments. S.Z., L.C., H.C. and Y.X. performed the STM experiments with assistance from L.K., W.L., D.W., P.F. and S.D. M.P., H.I. and L.F. provided theoretical models and simulations. J.S., R.Z. and G.D.G. provided samples. L.K., S.Z. and H.D. analysed experiment data with input from all other authors. L.K., M.P. and S.Z. plotted figures with input from all other authors. All authors participated in writing the manuscript. H.D., H.-J.G. and L.F. supervised the project.

Correspondence to Liang Fu or Hong-Jun Gao or Hong Ding.

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Peer review information: Nature Physics thanks Peter Wahl and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

Additional theoretical and experimental details, Supplementary Figs. 1–11 and refs. 1–41.

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