N.V. Khotkevych, Yu.A. Kolesnichenko, and J.M. van Ruitenbeek, Electron tunneling into surface states through an inhomogeneous barrier: asymptotically exact solution of the problem and STM theory, Low Temp. Phys. 39, (2013) [Fiz. Nizk. Temp. 39, 384-389, (2013)] DOI: https://doi.org/10.1063/1.4795003.