AC-driven motion of fluxons in weakly damped long Josephson
junctions

Boris Malomed

Dept. of Interdisciplinary Studies, Faculty of Engineering
Tel Aviv University, Tel Aviv 69978, Israel

Alexey Ustinov

Dept. of Physics, University of Nuremberg-Erlangen
Erlangen, Germany

We report the first experimental observation of a progressive
motion of a magnetic-flux quantum supported by a pure ac drive
(no dc component) in a lossy long Josephson junction of a
circular form placed into a uniform magnetic field. In a
series of earlier theoretical works, it was shown, analytically
and numerically, that a (quasi) particle in a periodic potential
may move with a nonzero mean velocity under the action of an
ac driving force. The absolute value of the mean velocity is
determined by a resonance between the frequency of the
driving force and the spatial period of the potential, while
the sign of the velocity is arbitrary (it is selected by a
random initial perturbation).
In the present experiment, the necessary effective potential
for a fluxon is induced by the external uniform magnetic field.
The fluxon's motion with a nonzero mean velocity is directly
observed in terms of a dependence of the junction's voltage
on the ac-drive's frequency. The dependence
exactly follows the theoretical prediction. We stress that that
the configuration considered is strictly reflectionally symmetric,
which is a drastic difference from the "ratchets", where the symmetry
is deliberately broken.