  The phenomena of AC oscillation generated by a DC drive, such as the famous
Josephson AC effect in superconductors and Bloch oscillation in solid physics,
are of great interest in physics. Here we report another example of such
counter-intuitive phenomenon that a spin soliton in a two-component
Bose-Einstein condensate is driven by a constant force: The initially static
spin soliton first moves in a direction opposite to the force and then changes
direction, showing an extraordinary AC oscillation in a long term. In sharp
contrast to the Josephson AC effect and Bloch oscillation, we find that the
nonlinear interactions play important roles and the spin soliton can exhibit a
periodic transition between negative and positive inertial mass even in the
absence of periodic potentials. We then develop an explicit quasiparticle model
that can account for this extraordinary oscillation satisfactorily. Important
implications and possible applications of our finding are discussed.
