Phase shift effects of the I-V characteristics of zero-field current spikes in long Josephson junctions

In this paper we analyse the length dependence of the zero-field current spikes (Z.F.S.) which appear in the I-V current voltage characteristics of long Josephson tunnel junctions. ZFS are described in terms of travelling fluxons. The presence of the bias current, losses and in particular of the boundary conditions is discussed: in the absence of magnetic fields fluxons are reflected either at the junction edges or for antifluxon collisions. The generated phase shift effects are dominant with respect to the variations of the power balance condition. Taking into account the phase shift and the limit of the Lorentz fluxon contraction, we derive an analytical approximate expression of the mean velocity in one-dimensional, intermediate length, overlap junctions. A rather good agreement is obtained comparing our theoretical analysis with several experimental ZFS I-V characteristics.