Abstract
Type-II iron-based superconductors (Fe-SCs), the alkali-metal-intercalated iron selenide AxFe2−ySe2 (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 KxFe2−ySe2. We observe clear electronic phase separation of KxFe2−ySe2 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−xFe2Se2 (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 KxFe2−ySe2, but also present their intrinsic electronic structures, which will shed light on the comprehension of the unique physics in type-II Fe-SCs.
Article PDF
Similar content being viewed by others
Avoid common mistakes on your manuscript.
References
Guo, J. G.; Jin, S. F.; Wang, G.; Wang, S. C.; Zhu, K. X.; Zhou, T. T.; He, M.; Chen, X. L. Superconductivity in the iron selenide KxFe2Se2 (0 ≤ x ≤ 1.0). Phys. Rev. B 2010, 82, 180520(R).
Ying, J. J.; Wang, X. F.; Luo, X. G; Wang, A. F.; Zhang, M.; Yan, Y. J.; Xiang, Z. J.; Liu, R. H.; Cheng, P.; Ye, G. J. et al. Superconductivity and magnetic properties of single crystals of K0.75Fe1.66Se2 and Cs0.81Fe1.61Se2. Phys. Rev. B 2011, 83, 212502.
Mou, D. X.; Zhao, L.; Zhou, X. J. Structural, magnetic and electronic properties of the iron-chalcogenide AxFe2−ySe2 (A = K, Cs, Rb, and Tl, etc.) superconductors. Front. Phys. 2011, 6, 410–428.
Dagotto, E. Colloquium: The unexpected properties of alkali metal iron selenide superconductors. Rev. Mod. Phys. 2013, 85, 849–867.
Bao, W. Structure, magnetic order and excitations in the 245 family of Fe-based superconductors. J. Phys. Condens. Matter 2015, 27, 023201.
Krzton-Maziopa, A.; Svitlyk, V.; Pomjakushina, E.; Puzniak, R.; Conder, K. Superconductivity in alkali metal intercalated iron selenides. J. Phys. Condens. Matter 2016, 28, 293002.
Wu, M. K.; Wu, P. M.; Wen, Y. C.; Wang, M. J.; Lin, P. H.; Lee, W. C.; Chen, T. K.; Chang, C. C. An overview of the Fe-chalcogenide superconductors. J. Phys. D: Appl. Phys. 2015, 48, 323001.
Bao, W.; Huang, Q. Z.; Chen, G. F.; Wang, D. M.; He, J. B.; Qiu, Y. M. A novel large moment antiferromagnetic order in K0.8Fe1.6Se2 superconductor. Chin. Phys. Lett. 2011, 28, 086104.
Yan, Y. J.; Zhang, M.; Wang, A. F.; Ying, J. J.; Li, Z. Y.; Qin, W.; Luo, X. G.; Li, J. Q.; Hu, J. P.; Chen, X. H. Electronic and magnetic phase diagram in KxFe2−ySe2 superconductors. Sci. Rep. 2012, 2, 212.
Ding, X. X.; Fang, D. L.; Wang, Z. Y.; Yang, H.; Liu, J. Z.; Deng, Q.; Ma, G. B.; Meng, C.; Hu, Y. H.; Wen, H. H. Influence of microstructure on superconductivity in KxFe2−ySe2 and evidence for a new parent phase K2F7Se8. Nat. Commun. 2013, 4, 1897.
Zhang, Y.; Yang, L. X.; Xu, M.; Ye, Z. R.; Chen, F.; He, C.; Xu, H. C.; Jiang, J.; Xie, B. P.; Ying, J. J. et al. Nodeless superconducting gap in AxFe2Se2 (A = K, Cs) revealed by angle-resolved photoemission spectroscopy. Nat. Mater. 2011, 10, 273–277.
Qian, T.; Wang, X. P.; Jin, W. C.; Zhang, P.; Richard, P.; Xu, G; Dai, X.; Fang, Z.; Guo, J. G.; Chen, X. L. 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. 2011, 106, 187001.
Mou, D. X.; Liu, S. Y.; Jia, X. W.; He, J. F.; Peng, Y. Y.; Zhao, L.; Yu, L.; Liu, G. D.; He, S. L.; Dong, X. L. et al. Distinct fermi surface topology and nodeless superconducting gap in a (Tl0.58Rb0.42)Fe1.72Se2 superconductor. Phys. Rev. Lett. 2011, 106, 107001.
Fang, M. H.; Wang, H. D.; Dong, C. H.; Li, Z. J.; Feng, C. M.; Chen, J.; Yuan, H. Q. Fe-based superconductivity with Tc = 31 K bordering an antiferromagnetic insulator in (Tl,K)FexSe2. EPL 2011, 94, 27009.
Tsurkan, V.; Deisenhofer, J.; Günther, A.; Krug von Nidda, H. A.; Widmann, S.; Loidl, A. Anisotropic magnetism, superconductivity, and the phase diagram of Rb1−xFe2−ySe2. Phys. Rev. B 2011, 84, 144520.
Ksenofontov, V.; Wortmann, G.; Medvedev, S. A.; Tsurkan, V.; Deisenhofer, J.; Loidl, A.; Felser, C. Phase separation in superconducting and antiferromagnetic Rb0.8Fe1.6Se2 probed by Mössbauer spectroscopy. Phys. Rev. B 2011, 84, 180508(R).
Ricci, A.; Poccia, N.; Campi, G.; Joseph, B.; Arrighetti, G.; Barba, L.; Reynolds, M.; Burghammer, M.; Takeya, H.; Mizuguchi, Y. et al. Nanoscale phase separation in the iron chalcogenide superconductor K0.8Fe1.6Se2 as seen via scanning nanofocused X-ray diffraction. Phys. Rev. B 2011, 84, 060511.
Charnukha, A.; Cvitkovic, A.; Prokscha, T.; Pröpper, D.; Ocelic, N.; Suter, A.; Salman, Z.; Morenzoni, E.; Deisenhofer, J.; Tsurkan, V. et al. Nanoscale layering of antiferromagnetic and superconducting phases in Rb2Fe4Se5 single crystals. Phys. Rev. Lett. 2012, 109, 017003.
Li, W.; Ding, H.; Deng, P.; Chang, K.; Song, C. L.; He, K.; Wang, L. L.; Ma, X. C.; Hu, J. P.; Chen, X. et al. Phase separation and magnetic order in K-doped iron selenide superconductor. Nat. Phys. 2012, 8, 126–130.
Carr, S. V.; Louca, D.; Siewenie, J.; Huang, Q.; Wang, A. F.; Chen, X. H.; Dai, P. C. Structure and composition of the superconducting phase in alkali iron selenide KyFe1.6+xSe2. Phys. Rev. B 2014, 89, 134509.
Speller, S. C.; Dudin, P.; Fitzgerald, S.; Hughes, G. M.; Kruska, K.; Britton, T. B.; Krzton-Maziopa, A.; Pomjakushina, E.; Conder, K.; Barinov, A. et al. High-resolution characterization of microstructural evolution in RbxFe2−ySe2 crystals on annealing. Phys. Rev. B 2014, 90, 024520.
Liu, Y.; Xing, Q. F.; Straszheim, W. E.; Marshman, J.; Pedersen, P.; McLaughlin, R.; Lograsso, T. A. Formation mechanism of superconducting phase and its three-dimensional architecture in pseudo-single-crystal KxFe2−ySe2. Phys. Rev. B 2016, 93, 064509.
Li, W.; Ding, H.; Li, Z.; Deng, P.; Chang, K.; He, K.; Ji, S. H.; Wang, L. L.; Ma, X. C.; Hu, J. P. et al. KFe2Se2 is the parent compound of K-doped iron selenide superconductors. Phys. Rev. Lett. 2012, 109, 057003.
Zhao, J.; Cao, H. B.; Bourret-Courchesne, E.; Lee, D. H.; Birgeneau, R. J. Neutron-diffraction measurements of an antiferromagnetic semiconducting phase in the vicinity of the high-temperature superconducting state of KxFe2@#ySe2. Phys. Rev. Lett. 2012, 109, 267003.
Zhou, Y.; Xu, D. H.; Zhang, F. C.; Chen, W. Q. Theory for superconductivity in (Tl,K)FexSe2 as a doped Mott insulator. EPL 2011, 95, 17003.
Cao, C.; Dai, J. H. Electronic structure and Mott localization of iron-deficient TlFe1.5Se2 with superstructures. Phys. Rev. B 2011, 83, 193104.
Yu, R.; Zhu, J. X.; Si, Q. M. Mott transition in modulated lattices and parent insulator of (K,Tl)yFexSe2 Superconductors. Phys. Rev. Lett. 2011, 106, 186401.
Chen, F.; Xu, M.; Ge, Q. Q.; Zhang, Y.; Ye, Z. R.; Yang, L. X.; Jiang, J.; Xie, B. P.; Che, R. C.; Zhang, M. et al. Electronic identification of the parental phases and mesoscopic phase separation of KxFe2−ySe2 superconductors. Phys. Rev. X 2011, 1, 021020.
Xu, M.; Ge, Q. Q.; Peng, R.; Ye, Z. R.; Jiang, J.; Chen, F.; Shen, X. P.; Xie, B. P.; Zhang, Y.; Wang, A. F. et al. Evidence for an s-wave superconducting gap in KxFe2−ySe2 from angle-resolved photoemission. Phys. Rev. B 2012, 85, 220504(R).
Yi, M.; Lu, D. H.; Yu, R.; Riggs, S. C.; Chu, J. H.; Lv, B.; Liu, Z. K.; Lu, M.; Cui, Y. T.; Hashimoto, M. et al. Observation of temperature-induced crossover to an orbital-selective mott phase in AxFe2−ySe2 (A = K, Rb) superconductors. Phys. Rev. Lett. 2013, 110, 067003.
Maletz, J.; Zabolotnyy, V. B.; Evtushinsky, D. V.; Yaresko, A. N.; Kordyuk, A. A.; Shermadini, Z.; Luetkens, H.; Sedlak, K.; Khasanov, R.; Amato, A. et al. Photoemission and muon spin relaxation spectroscopy of the iron-based Rb0.77Fe1.61Se2 superconductor: Crucial role of the cigar-shaped Fermi surface. Phys. Rev. B 2013, 88, 134501.
Wang, X. P.; Qian, T.; Richard, P.; Zhang, P.; Dong, J.; Wang, H. D.; Dong, C. H.; Fang, M. H.; Ding, H. Strong nodeless pairing on separate electron Fermi surface sheets in (Tl,K)Fe1.78Se2 probed by ARPES. EPL 2011, 93, 57001.
Bendele, M.; Barinov, A.; Joseph, B.; Innocenti, D.; Iadecola, A.; Bianconi, A.; Takeya, H.; Mizuguchi, Y.; Takano, Y.; Noji, T. et al. Spectromicroscopy of electronic phase separation in KxFe2−y Se2 superconductor. Sci. Rep. 2014, 4, 5592.
Dudin, P.; Herriott, D.; Davies, T.; Krzton-Maziopa, A.; Pomjakushina, E.; Conder, K.; Cacho, C.; Yates, J. R.; Speller, S. C. Imaging the local electronic and magnetic properties of intrinsically phase separated RbxFe2−ySe2 superconductor using scanning microscopy techniques. Supercon. Sci. Technol. 2019, 32, 044005.
Cao, C.; Dai, J. H. Block spin ground state and three-dimensionality of (K,Tl)yFe1.6Se2. Phys. Rev. Lett. 2011, 107, 056401.
Yan, X. W.; Gao, M.; Lu, Z. Y.; Xiang, T. Ternary iron selenide K0.8Fe1.6Se2 is an antiferromagnetic semiconductor. Phys. Rev. B 2011, 83, 233205.
Chen, Z. G.; Yuan, R. H.; Dong, T.; Xu, G.; Shi, Y. G.; Zheng, P.; Luo, J. L.; Guo, J. G.; Chen, X. L.; Wang, N. L. Infrared spectrum and its implications for the electronic structure of the semiconducting iron selenide K0.83Fe1.53Se2. Phys. Rev. B 2011, 83, 220507(R).
Acknowledgments
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.
Author information
Authors and Affiliations
Corresponding authors
Rights and permissions
This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.
The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Chen, Y., Jiang, J., Yang, H. et al. Visualization of the electronic phase separation in superconducting KxFe2−ySe2. Nano Res. 14, 823–828 (2021). https://doi.org/10.1007/s12274-020-3119-8
Received:
Revised:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s12274-020-3119-8