Abstract
For a type-II superconductor in an applied magnetic field greater than the lower critical field Hc1, the magnetic flux will penetrate into the superconductor and form quantized vortices with Andreev bound states in the vortex cores. The characteristics of the bound states are related to the pairing symmetry and band structure of the superconductor. Recently, a new family of high temperature (high-Tc) superconductors, the iron pnictides, has been discovered. Surprisingly, in electron-doped Ba(Fe1−xCox)2As2, no bound state was found in the vortices. Here, we use a low-temperature scanning tunnelling microscope to study the electronic structure of hole-doped Ba0.6K0.4Fe2As2. Two superconducting gaps (with gap values 2Δ/kBTc≈2.2 and 5.1, where kB is Boltzmann’s constant) were observed in the superconducting state. By applying magnetic fields, we observed ordered vortices with Andreev bound states in the vortex cores. The bound states and their spatial evolution can be qualitatively explained by our numerical calculations for multiband s-wave superconductivity.
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Acknowledgements
We thank D. H. Lee, K. Kuroki, T. Xiang and T. Hanaguri for comments and suggestions. This work was supported by the Natural Science Foundation of China (No. 10774170, No. 10974086, No. 10734120), the Ministry of Science and Technology of China (973 Projects No. 2011CBA00100, No. 2011CB922101, No. 2010CB923002), and the Chinese Academy of Sciences.
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The low-temperature STM was constructed by L.S., Y-L.W. and Y.H. with the help of S.H.P., A.L. and H-H.W., B.S. and B.Z. prepared the samples and measured the sample quality. L.S. designed and performed STM experiments and analysed data. Y-L.W. performed STM experiments and analysed data. D.W. and Q-H.W. contributed to numerical calculations. H-H.W. coordinated the whole work, contributed to sample preparation, STM experiments and data analysis. The paper was written jointly by L.S., H-H.W., Q-H.W. and S.H.P. All authors have discussed the results and the interpretation.
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Shan, L., Wang, YL., Shen, B. et al. Observation of ordered vortices with Andreev bound states in Ba0.6K0.4Fe2As2. Nature Phys 7, 325–331 (2011). https://doi.org/10.1038/nphys1908
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DOI: https://doi.org/10.1038/nphys1908
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