Evolution of superconductivity in K2-xFe4+ySe5: Spectroscopic studies of X-ray absorption and emission

H T Wang; Anirudha Ghosh; C H Wang; S H Hsieh; Y C Shao; J W Chiou; C L Chen; C W Pao; J F Lee; Y S Liu; Y D Chuang; J H Guo; M K Wu; W F Pong

doi:10.1073/pnas.1912610116

Evolution of superconductivity in K_2-xFe_4+ySe₅: Spectroscopic studies of X-ray absorption and emission

Proc Natl Acad Sci U S A. 2019 Nov 5;116(45):22458-22463. doi: 10.1073/pnas.1912610116. Epub 2019 Oct 22.

Authors

H T Wang¹, Anirudha Ghosh², C H Wang^{3

4}, S H Hsieh^{2

5}, Y C Shao^{2

6}, J W Chiou⁷, C L Chen⁵, C W Pao⁵, J F Lee⁵, Y S Liu⁶, Y D Chuang⁶, J H Guo⁶, M K Wu^{8

3}, W F Pong⁹

Affiliations

¹ Department of Physics, National Tsing Hua University, 30013 Hsinchu, Taiwan.
² Department of Physics, Tamkang University, 25137 Tamsui, Taiwan.
³ Institute of Physics, Academia Sinica, 11529 Taipei, Taiwan.
⁴ Department of Electronic and Computer Engineering, National University of Science and Technology, 10607 Taipei, Taiwan.
⁵ National Synchrotron Radiation Research Center, 30076 Hsinchu, Taiwan.
⁶ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.
⁷ Department of Applied Physics, National University of Kaohsiung, 81148 Kaohsiung, Taiwan.
⁸ Department of Physics, National Tsing Hua University, 30013 Hsinchu, Taiwan; [email protected] [email protected].
⁹ Department of Physics, Tamkang University, 25137 Tamsui, Taiwan; [email protected] [email protected].

Abstract

This study investigates the evolution of superconductivity in K_2-xFe_4+ySe₅ using temperature-dependent X-ray absorption and resonant inelastic X-ray scattering techniques. Magnetization measurements show that polycrystalline superconducting (SC) K_1.9Fe_4.2Se₅ has a critical temperature (T _c) of ∼31 K with a varying superconducting volume fraction, which strongly depends on its synthesis temperature. An increase in Fe-structural/vacancy disorder in SC samples with more Fe atoms occupying vacant 4d sites is found to be closely related to the decrease in the spin magnetic moment of Fe. Moreover, the nearest-neighbor Fe-Se bond length in SC samples exceeds that in the non-SC (NS) sample, K₂Fe₄Se₅, which indicates a weaker hybridization between the Fe 3d and Se 4p states in SC samples. These results clearly demonstrate the correlations among the local electronic and atomic structures and the magnetic properties of K_2-xFe_4+ySe₅ superconductors, providing deeper insight into the electron pairing mechanisms of superconductivity.

Keywords: Fe vacancy disorder; X-ray absorption spectroscopy; resonant inelastic X-ray scattering; superconductivity.