A robust zero-energy bound state (ZBS) in a superconductor, such as a Majorana or Andreev bound state, is often a consequence of non-trivial topological or symmetry related properties, and can provide indispensable information about the superconducting state. Here we use scanning tunneling microscopy/spectroscopy to demonstrate, on the atomic scale, that an isotropic ZBS emerges at the randomly distributed interstitial excess Fe sites in the superconducting Fe(Te,Se). This ZBS is localized with a short decay length of ~ 10 {\AA}, and surprisingly robust against a magnetic field up to 8 Tesla, as well as perturbations by neighboring impurities. We find no natural explanation for the observation of such a robust zero-energy bound state, indicating a novel mechanism of impurities or an exotic pairing symmetry of the iron-based superconductivity.
@article{arxiv.1403.1027,
title = {Observation of a Robust Zero-energy Bound State in Iron-based Superconductor Fe(Te,Se)},
author = {J. -X. Yin and Zheng Wu and J. -H. Wang and Z. -Y. Ye and Jing Gong and X. -Y. Hou and Lei Shan and Ang Li and X. -J. Liang and X. -X. Wu and Jian Li and C. -S. Ting and Z. Wang and J. -P. Hu and P. -H. Hor and H. Ding and S. H. Pan},
journal= {arXiv preprint arXiv:1403.1027},
year = {2015}
}