“Observation of Fermi-energy dependent unitary impurity resonances in a strong topological insulator Bi2Se3 with scanning tunneling spectroscopy” M. L. Teague, F.-X. Xiu, L. He, K.-L. Wang and N.-C. Yeh, fast-track communication in Solid State Communications 152, 747 – 751 (2012). [arXiv:1201.5618]
http://www.sciencedirect.com/science/article/pii/S0038109812000749