Current-voltage characteristics were measured in the mixed state of Y 1 Ba 2 Cu 3 O 72d superconducting films in the regime where flux flow becomes unstable and the differential conductivity dj͞dE becomes negative. Under conditions where its negative slope is steep, the j͑E͒ curve develops a pronounced staircaselike pattern. We attribute the steps in j͑E͒ to the formation of a dynamical phase consisting of the successive nucleation of quantized distortions in the local vortex velocity and flux distribution within the moving flux matter. DOI: 10.1103/PhysRevLett.87.177001 PACS numbers: 74.40. +k, 74.60.Ge, 74.72.Bk In a type II superconductor, a magnetic field H above the lower critical value H c1 introduces flux vortices containing an elementary quantum of flux F o h͞2e, and interactions between the vortices tend to align them into a uniform lattice A transport current exerts a Lorentz driving force F L j 3 F o on the vortices and the motion is opposed by a viscous drag F d 2hv f , where h is the coefficient of viscosity. If we assume that pinning forces F p are negligible (because F L ¿ F p ), then the steady state motion reflects a balance between driving (F L ), drag (F d ), and elastic forces (F e ) on each vortex. For a perfectly uniform distribution, the net elastic force on each vortex vanishes, resulting in free flux flow A different scenario prevails at ultrahigh dissipation levels and electric fields sufficient to alter the electronic distribution function and/or the electronic temperature. Here j͑E͒ becomes nonlinear and can develop an unstable region with negative differential conductivity (NDC) (region "C" i
