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Type-II iron-based superconductors (Fe-SCs), the alkali-metal-intercalated iron selenide Ax Fe2−y Se2 (A = K, Tl, Rb, etc.) with a superconducting transition temperature of 32 K, exhibit unique properties such as high Néel temperature, Fe-vacancies ordering, antiferromagnetically ordered insulating state in the phase diagram, and mesoscopic phase separation in the superconducting materials. In particular, the electronic and structural phase separation in these systems has attracted intensive attention since it provides a platform to unveil the insulating parent phase of type-II Fe-SCs that mimics the Mott parent phase in cuprates. In this work, we use spatial- and angle-resolved photoemission spectroscopy to study the electronic structure of superconducting Kx Fe2−y Se2. We observe clear electronic phase separation of Kx Fe2−y Se2 into metallic islands and insulating matrix, showing different K and Fe concentrations. While the metallic islands show strongly dispersive bands near the Fermi level, the insulating phase shows an energy gap up to 700 meV and a nearly flat band around 700 meV below the Fermi energy, consistent with previous experimental and theoretical results on the superconducting K1−x Fe2Se2 (122 phase) and Fe-vacancy ordered K0.8Fe1.6Se2 (245 phase), respectively. Our results not only provide important insights into the mysterious composition of phase-separated superconducting and insulating phases of Kx Fe2−y Se2, but also present their intrinsic electronic structures, which will shed light on the comprehension of the unique physics in type-II Fe-SCs.

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Publication history

Received: 04 July 2020
Revised: 03 September 2020
Accepted: 15 September 2020
Published: 01 March 2021
Issue date: March 2021

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© The Author(s) 2020

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 11427903, 11774109 and 11674229), the National Key R&D Program of China (Nos. 2017YFA0304600 and 2017YFA0305400), and EPSRC Platform Grant (No. EP/ M020517/1). L. X. Y. acknowledges the support from Tsinghua University Initiative Scientific Research Program.

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Email: nanores@tup.tsinghua.edu.cn

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