Shape of a moving fluxon in stacked Josephson junctions

We study numerically and analytically the shape of a single fluxon moving i n double stacked Josephson junctions (SJJ's) for various junction parameter s. We show that the fluxon in a double SJJ consists of two components, whic h are characterized by different Swihart velocities and Josephson penetrati on depths. The weight coefficients of the two components depend on the para meters of the junctions and the velocity of the fluxon. It is shown that th e fluxon in SJJ's may have an unusual shape with an inverted magnetic field in the second junction when the velocity of the fluxon is approaching the lower Swihart velocity. Finally, we study the influence of fluxon shape on flux-flow current-voltage characteristics and analyze the spectrum of Chere nkov radiation for fluxon velocity above the lower Swihart velocity. An ana lytic expression for the wavelength of Cherenkov radiation is derived, [S01 63-1829(99)03632-5].