Intrinsic Flux-Flow Resistance Steps in the Cuprate SuperconductorNd2−xCexCuOy

We have observed a novel intrinsic step structure in the flux-flow resistance of epitaxial $c$-axis-oriented films of ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_y$ as a function of current at intermediate magnetic fields $B_{c1\mathrm{}}\ll B<\mathrm{}{B}_{c2\mathrm{}}$. The effect is observed only if the sample is cooled with superfluid helium. For explaining the underlying instability, we propose a model based on the strongly energy-dependent density of states available near the Fermi energy for quasiparticle scattering in the superconducting mixed state for the clean limit.