Ptychographical imaging of the phase vortices in the x-ray beam formed by nanofocusing lenses

D. Dzhigaev, U. Lorenz, R.P. Kurta, F. Seiboth, Tomas Stankevic, S. Mickevicius, A. Singer, A. Shabalin, O. M. Yefanov, M. N. Strikhanov, G. Falkenberg, C. G. Schroer, Robert Krarup Feidenhans'l, I. A. Vartanyants