Shape of a moving fluxon in stacked Josephson junctions

We study numerically and analytically the shape of a single fluxon moving in double stacked Josephson junctions (SJJ's) for various junction parameters. We show that the fluxon in a double SJJ consists of two components, which are characterized by different Swihart velocities and Josephson penetration depths. The weight coefficients of the two components depend on the parameters of the junctions and the velocity of the fluxon. It is shown that the 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 Cherenkov radiation for fluxon velocity above the lower Swihart velocity. An analytic expression for the wavelength of Cherenkov radiation is derived, [S0163-1829(99)03632-5].