# Phase separation and magnetic order in K-doped iron selenide superconductor

## Abstract

The newly discovered alkali-doped iron selenide superconductors1,2 not only reach a superconducting transition temperature as high as 32 K, but also exhibit unique characteristics that are absent from other iron-based superconductors, such as antiferromagnetically ordered insulating phases3,4, extremely high Néel transition temperatures5,6 and the presence of Fe vacancies and ordering7,8,9,10. These features have generated considerable excitement as well as confusion, regarding the delicate interplay between Fe vacancies, magnetism and superconductivity11,12,13. Here we report on molecular beam epitaxy growth of high-quality KxFe2−ySe2 thin films and in situ low-temperature scanning tunnelling microscope measurement of their atomic and electronic structures. We demonstrate that a KxFe2−ySe2 sample contains two distinct phases: an insulating phase with well-defined order of Fe vacancies, and a superconducting KFe2Se2 phase containing no Fe vacancies. An individual Fe vacancy can locally destroy superconductivity in a similar way to a magnetic impurity in conventional superconductors. Measurement of the magnetic-field dependence of the Fe-vacancy-induced bound states reveals a magnetically related bipartite order in the tetragonal iron lattice. These findings elucidate the existing controversies on this new superconductor and provide atomistic information on the interplay between magnetism and superconductivity in iron-based superconductors.

## Access optionsAccess options

from\$8.99

All prices are NET prices.

## References

1. 1

Guo, J. et al. Superconductivity in the iron selenide KxFe2Se2 (0≤x≤1.0). Phys. Rev. B 82, 180520(R) (2010).

2. 2

Wang, A. F. et al. Superconductivity at 32 K in single-crystalline RbxFe2−ySe2 . Phys. Rev. B 83, 060512(R) (2011).

3. 3

Fang, M-H. et al. Fe-based superconductivity with Tc=31 K bordering an antiferromagnetic insulator in (Tl,K)FexSe2 . Europhys. Lett. 94, 27009 (2011).

4. 4

Chen, Z. G. et al. Infrared spectrum and its implications for the electronic structure of the semiconducting iron selenide K0.83Fe1.53Se2 . Phys. Rev. B 83, 220507(R) (2011).

5. 5

Bao, W. et al. A novel large moment antiferromagnetic order in K0.8Fe1.6Se2 superconductor. Chin. Phys. Lett. 28, 086104 (2011).

6. 6

Pomjakushin, V. Yu. et al. Iron-vacancy superstructure and possible room-temperature antiferromagnetic order in superconducting CsyFe2−xSe2 . Phys. Rev. B 83, 144410 (2011).

7. 7

Wang, Z. et al. Microstructure and ordering of iron vacancies in the superconductor system KyFexSe2 as seen via transmission electron microscopy. Phys. Rev. B 83, 140505(R) (2011).

8. 8

Zavalij, P. et al. Structure of vacancy-ordered single-crystalline superconducting potassium iron selenide. Phys. Rev. B 83, 132509 (2011).

9. 9

Yan, X-W., Gao, M., Lu, Z-Y. & Xiang, T. Ternary iron selenide K0.8Fe1.6Se2 is an antiferromagnetic semiconductor. Phys. Rev. B 83, 233205 (2011).

10. 10

Ricci, A. et al. Nanoscale phase separation in the iron chalcogenide superconductor K0.8Fe1.6Se2 as seen via scanning nanofocused X-ray diffraction. Phys. Rev. B 84, 060511(R) (2011).

11. 11

Shermadini, Z. et al. Coexistence of magnetism and superconductivity in the iron-based compound Cs0.8(FeSe0.98)2 . Phys. Rev. Lett. 106, 117602 (2011).

12. 12

Bao, W. et al. Vacancy tuned magnetic high- Tc superconductor KxFe2−x/2Se2. Preprint at http://arxiv.org/abs/1102.3674 (2011).

13. 13

Shen, B. et al. Intrinsic percolative superconductivity in KxFe2−ySe2 single crystals. Europhys. Lett. 96, 37010 (2011).

14. 14

Song, C-L. et al. Direct observation of nodes and twofold symmetry in FeSe superconductor. Science 332, 1410–1413 (2011).

15. 15

Zhang, Y. et al. Nodeless superconducting gap in AxFe2Se2(A=K,Cs) revealed by angle-resolved photoemission spectroscopy. Nature Mater. 10, 273–277 (2011).

16. 16

Qian, T. et al. Absence of a holelike Fermi surface for the iron-based K0.8Fe1.7Se2 superconductor revealed by angle-resolved photoemission spectroscopy. Phys. Rev. Lett. 106, 187001 (2011).

17. 17

Zhao, L. et al. Common Fermi-surface topology and nodeless superconducting gap of K0.68Fe1.79Se2 and (Tl0.45K0.34)Fe1.84Se2 superconductors revealed via angle-resolved photoemission. Phys. Rev. B 83, 140508(R) (2011).

18. 18

Wang, X-P. et al. Strong nodeless pairing on separate electron Fermi surface sheets in (Tl,K)Fe1.78Se2 probed by ARPES. Europhys. Lett. 93, 57001 (2011).

19. 19

Mun, E. D. et al. Anisotropic Hc2 of K0.8Fe1.76Se2 determined up to 60 T. Phys. Rev. B 83, 100514(R) (2011).

20. 20

Gao, Z. et al. Upper fields and critical current density of K0.58Fe1.56Se2 single crystals grown by one step technique. Preprint at http://arxiv.org/abs/1103.2904 (2011).

21. 21

Torchetti, D. A. et al. 77Se NMR investigation of the KxFe2−ySe2 high- Tc superconductor (Tc=33 K). Phys. Rev. B 83, 104508 (2011).

22. 22

Johnston, D. C. The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides. Adv. Phys. 59, 803–1061 (2010).

23. 23

Fischer, Ø. et al. Scanning tunneling spectroscopy of high-temperature superconductors. Rev. Mod. Phys. 79, 353–419 (2007).

24. 24

Yazdani, A. et al. Probing the local effects of magnetic impurities on superconductivity. Science 275, 1767–1770 (1997).

25. 25

Ji, S-H. et al. High-resolution scanning tunneling spectroscopy of magnetic impurity induced bound states in the superconducting gap of Pb thin films. Phys. Rev. Lett. 100, 226801 (2008).

26. 26

Balatsky, A. V., Vekhter, I. & Zhu, J-X. Impurity-induced states in conventional and unconventional superconductors. Rev. Mod. Phys. 78, 373–433 (2006).

27. 27

Salkola, M. I., Balatsky, A. V. & Schrieffer, J. R. Spectral properties of quasiparticle excitations induced by magnetic moments in superconductors. Phys. Rev. B 55, 12648–12661 (1997).

## Acknowledgements

We thank Y. G. Zhao for discussions. The work was financially supported by the National Science Foundation and Ministry of Science and Technology of China.

## Author information

W.L., H.D., P.D., K.C. and C.S. carried out the experiments; K.H., L.W., X.M. and J-P.H. analysed the data; X.C. and Q-K.X. designed and coordinated the experiments; X.C. wrote the paper. All authors discussed the results and commented on the manuscript.

Correspondence to Xi Chen or Qi-Kun Xue.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

## Supplementary information

### Supplementary Information

Supplementary Information (PDF 712 kb)

## Rights and permissions

Reprints and Permissions

• #### DOI

https://doi.org/10.1038/nphys2155

• ### Superconductivity in solid-state synthesized (Li,Fe)OHFeSe by tuning Fe vacancies in FeSe layer

• G. B. Hu
• , N. Z. Wang
• , M. Z. Shi
• , F. B. Meng
• , C. Shang
• , L. K. Ma
• , X. G. Luo
•  & X. H. Chen

Physical Review Materials (2019)

• ### Imaging the local electronic and magnetic properties of intrinsically phase separated Rb x Fe2–y Se2 superconductor using scanning microscopy techniques

• P Dudin
• , D Herriott
• , T Davies
• , A Krzton-Maziopa
• , E Pomjakushina
• , K Conder
• , C Cacho
• , J R Yates
•  & S C Speller

Superconductor Science and Technology (2019)

• ### 2D Metallic Transitional Metal Dichalcogenides for Electrochemical Hydrogen Evolution

• Yahuan Huan
• , Jianping Shi
• , Guanchao Zhao
• , Xiaoqin Yan
•  & Yanfeng Zhang

Energy Technology (2019)

• ### 3D visualizations of nanoscale phase separation and ultrafast dynamic correlation between phases in (Na0.32K0.68)0.95Fe1.75Se2

• P. C. Cheng
• , W. Y. Tzeng
• , Y. J. Chu
• , C. W. Luo
• , A. A. Zhukov
• , J. Whittaker
• , J.-Y. Lin
• , K. H. Wu
• , J. Y. Juang
• , M. Liu
• , I. V. Morozov
• , A. I. Boltalin
•  & A. N. Vasiliev

Physical Review Materials (2019)

• ### Enhancement of thermoelectric properties by fluorine doping in LaO1−xFxBiPbS3

• Yi Yu
• , Chunchang Wang
• , Qiuju Li
• , Chao Cheng
• , Shuting Wang
•  & Changjin Zhang

Ceramics International (2019)