Non-perturbative calculation of the two-loop Lamb shift in Li-like ions
  A calculation valid to all orders in the nuclear-strength parameter is presented for the two-loop Lamb shift, notably for the two-loop self-energy correction, to the 2p-2s transition energies in heavy Li-like ions. The calculation removes the largest theoretical uncertainty for these transitions and yields the first experimental identification of two-loop QED effects in the region of the strong binding field. 
V. A. Yerokhin, P. Indelicato, V. M. Shabaev