  Thin stacks consisting of a single intrinsic Josephson junction on
(Bi,Pb)-Sr-Ca-Cu-O thin films are investigated under the influence of external
microwave fields. The $I$-$V$-characteristic shows a single resistive branch, a
clear superconducting gap edge structure and a pronounced current step in
external microwave fields. With increasing irradiation power it shifts to
higher voltages, while the height of the step remains practically unchanged. In
a numerical simulation including an ac-magnetic field parallel to the
superconducting layers the experimental features of the structure can be
explained by a collective motion of Josephson fluxons.
