Abstract
In superconductors, electrons are paired and condensed into the ground state. An impurity can break the electron pairs into quasiparticles with energy states inside the superconducting gap. The characteristics of such in-gap states reflect accordingly the properties of the superconducting ground state1. A zero-energy in-gap state is particularly noteworthy, because it can be the consequence of non-trivial pairing symmetry1 or topology2,3. Here we use scanning tunnelling microscopy/spectroscopy to demonstrate that an isotropic zero-energy bound state with a decay length of ∼10 Å emerges at each interstitial iron impurity in superconducting Fe(Te,Se). More noticeably, this zero-energy bound state is robust against a magnetic field up to 8 T, as well as perturbations by neighbouring impurities. Such a spectroscopic feature has no natural explanation in terms of impurity states in superconductors with s-wave symmetry, but bears all the characteristics of the Majorana bound state proposed for topological superconductors2,3, indicating that the superconducting state and the scattering mechanism of the interstitial iron impurities in Fe(Te,Se) are highly unconventional.
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References
Balatsky, A. V., Vekhter, I. & Zhu, J-X. Impurity-induced states in conventional and unconventional superconductors. Rev. Mod. Phys. 78, 373–433 (2006).
Fu, L. & Kane, C. L. Superconducting proximity effect and Majorana fermions at the surface of a topological insulator. Phys. Rev. Lett. 100, 096407 (2008).
Linder, J., Tanaka, Y., Yokoyama, T., Sudbø, A. & Nagaosa, N. Unconventional superconductivity on a topological insulator. Phys. Rev. Lett. 104, 067001 (2010).
Pan, S. H. et al. Imaging the effects of individual zinc impurity atoms on superconductivity in Bi2Sr2Ca(Cu, Zn)2O8+δ . Nature 403, 746–750 (2000).
Hudson, E. W. et al. Interplay of magnetism and high-Tc superconductivity at individual Ni impurity atoms in Bi2Sr2Ca(Cu, Zn)2O8+δ . Nature 411, 920–924 (2001).
Wang, Z. et al. Close relationship between superconductivity and the bosonic mode in Ba0.6K0.4Fe2As2 and Na(Fe0.975Co0.025)As. Nature Phys. 9, 42–48 (2013).
Yang, H. et al. In-gap quasiparticle excitations induced by non-magnetic Cu impurities in Na(Fe0.96Co0.03Cu0.01)As revealed by scanning tunnelling spectroscopy. Nature Commun. 4, 2749 (2013).
Grothe, S. et al. Bound states of defects in superconducting LiFeAs studied by scanning tunneling spectroscopy. Phys. Rev. B 86, 174503 (2012).
Hanaguri, T., Niitaka, S., Kuroki, K. & Takagi, H. Unconventional s-wave superconductivity in Fe(Se, Te). Science 328, 474–476 (2010).
Allan, M. P. et al. Anisotropic energy gaps of iron-based superconductivity from intraband quasiparticle interference in LiFeAs. Science 336, 563–567 (2012).
Liu, T. J. et al. Charge-carrier localization induced by excess Fe in the superconductor Fe1+yTe1−xSex . Phys. Rev. B 80, 174509 (2009).
Rößler, S. et al. Disorder-driven electronic localization and phase separation in superconducting Fe1+yTe0.5Se0.5 single crystals. Phys. Rev. B 82, 144523 (2010).
Taen, T., Tsuchiya, Y., Nakajima, Y. & Tamegai, T. Superconductivity at Tc ∼ 14 K in single-crystalline FeTe0.61Se0.39 . Phys. Rev. B 80, 092502 (2009).
Miao, H. et al. Isotropic superconducting gaps with enhanced pairing on electron Fermi surfaces in FeTe0.55Se0.45 . Phys. Rev. B 85, 094506 (2012).
Massee, F. et al. Cleavage surfaces of the BaFe2−xCoxAs2 and FeySe1−xTex superconductors: A combined STM plus LEED study. Phys. Rev. B 80, 140507(R) (2009).
Li, S. et al. First-order magnetic and structural phase transitions in Fe1+ySexTe1−x . Phys. Rev. B 79, 054503 (2009).
Thampy, V. et al. Friedel-like oscillations from interstitial iron in superconducting Fe1+yTe0.62Se0.38 . Phys. Rev. Lett. 108, 107002 (2012).
Yin, Y. et al. Scanning tunneling spectroscopy and vortex imaging in the iron pnictide superconductor BaFe1.8Co0.2As2 . Phys. Rev. Lett. 102, 097002 (2009).
Shan, L. et al. Observation of ordered vortices with Andreev bound states in Ba0.6K0.4Fe2As2 . Nature Phys. 7, 325–331 (2011).
Hirschfeld, P. J., Korshunov, M. M. & Mazin, I. I. Gap symmetry and structure of Fe-based superconductors. Rep. Prog. Phys. 74, 124508 (2011).
Yazdani, A., Jones, B. A., Lutz, C. P., Crommie, M. F. & Eigler, D. M. Probing the local effects of magnetic impurities on superconductivity. Science 275, 1767–1770 (1997).
Tsai, W-F., Zhang, Y-Y., Fang, C. & Hu, J. P. Impurity-induced bound states in iron-based superconductors with s-wave cosk x ⋅ cosk y pairing symmetry. Phys. Rev. B 80, 064513 (2009).
Zhang, D. Nonmagnetic impurity resonances as a signature of sign-reversal pairing in FeAs-based superconductors. Phys. Rev. Lett. 103, 186402 (2009).
Hu, J. P. Iron-based superconductors as odd-parity superconductors. Phys. Rev. X 3, 031004 (2013).
Song, C-L. et al. Direct observation of nodes and twofold symmetry in FeSe superconductor. Science 332, 1410–1413 (2011).
Acknowledgements
The authors thank Z. Fang, X. Dai, T. Xiang, D-H. Lee, T-K. Lee, G-M. Zhang and P. Coleman for stimulating discussions. This work is supported by the State of Texas through TcSUH, the Chinese Academy of Sciences, US Air Force Office of Scientific Research (FA9550-09-1-0656), Robert A. Welch Foundation (E-1146), US DOE (DE-SC0002554, DE-FG02-99ER45747), National Science Foundation of China (11322432, 11190020), and Ministry of Science and Technology of China (2012CB933000).
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J-X.Y. carried out the STM/S experiments with contributions from Z.W., J-H.W., Z-Y.Y., J.G., X-Y.H., L.S., A.L. and X-J.L.; Z.W. synthesized and characterized the sequence of samples; J-X.Y., S.H.P. and H.D. performed the data analysis, figure development and wrote the paper with contributions from J-P.H., Z-Q.W., C-S.T., P-H.H., J.L. and X-X.W.; S.H.P. supervised the project. All authors discussed the results and the interpretation.
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Yin, JX., Wu, Z., Wang, JH. et al. Observation of a robust zero-energy bound state in iron-based superconductor Fe(Te,Se). Nature Phys 11, 543–546 (2015). https://doi.org/10.1038/nphys3371
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DOI: https://doi.org/10.1038/nphys3371
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