# Multiple topological states in iron-based superconductors

## Abstract

Topological materials and unconventional iron-based superconductors are both important areas of study but, to date, relatively little overlap has been identified between these two fields. However, the combination of topological bands and superconductivity promises the manifestation of exotic superconducting states, including Majorana fermions, the central component of topological quantum computation. Here, using laser-based, spin-resolved and angle-resolved photoemission spectroscopy and density functional theory calculations, we have identified both topological insulator and Dirac semimetal states near the Fermi energy in different iron-based superconducting compounds. Carrier doping can tune these topologically non-trivial bands to the Fermi energy, potentially allowing access to several different superconducting topological states in the same material. These results reveal the generic coexistence of superconductivity and multiple topological states in iron-based superconductors, indicating that this broad class of materials is a promising platform for high-temperature topological superconductivity.

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## Data availability

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

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## Acknowledgements

We acknowledge K. Asakawa, A. Harasawa, Y. Hesagawa, D. Hirai, Z. Hiroi, K. Ishizaka, N. Mitsuishi, M. Sakano and Y. Yoshida for experimental assistance. This work was supported by the Photon and Quantum Basic Research Coordinated Development Program from MEXT, JSPS (KAKENHI Grant Nos. 25220707, JP17H02922, JP16K17755 and 17H01141), the Grants-in-Aid for Scientific Research on Innovative Areas ‘Topological Material Science’, JSPS (grant no. JP15H05855), the Chinese Academy of Sciences (XDB28000000) and the Ministry of Science and Technology of China (2015CB921300). The work in Brookhaven is supported by the Office of Science, US Department of Energy under contract no. DE-SC0012704 and the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science. The work in Würzburg is supported by ERC-StG-TOPOLECTRICS-336012, DFG-SFB 1170 and DFG-SPP 1666.

## Author information

Authors

### Contributions

P.Z. performed the ARPES measurements on Li(Fe,Co)As and analysed the data with help from K.Y., T. Kondo and S.S.. X. Wu, J.H. and R.T. performed the DFT calculations. G.D., X. Wang and C.J. synthesized the Li(Fe,Co)As samples. P.Z. performed the ARPES measurements on Fe(Te,Se) and analysed the data with help from Y.I., K.Y., C.B., K. Kuroda, T. Kondo, K.O., K.S., S.W., K.M., T.O., H.D. and S.S.. P.Z., Y.K. and K. Kindo performed the magnetoresistance measurements on Fe(Te,Se). Z.W., X.Wu, R.T., T. Kawakami and M.S. performed the theoretical calculations on Fe(Te,Se). G.D.G., Y.S. and T.T. synthesized the Fe(Te,Se) samples. All authors discussed the manuscript. P.Z. and S.S. supervised the whole project.

### Corresponding authors

Correspondence to Peng Zhang or Shik Shin.

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### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary Figures 1–8, Supplementary References 1–9, and additional mathematical derivations

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Zhang, P., Wang, Z., Wu, X. et al. Multiple topological states in iron-based superconductors. Nature Phys 15, 41–47 (2019). https://doi.org/10.1038/s41567-018-0280-z

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