Flux quanta driven by high-density currents in low-impurity V3Si and LuNi2B2C: Free flux flow and fluxon-core size effect

High-density direct currents are used to drive flux quanta via the Lorentz force toward a highly ordered ``free flux flow'' (FFF) dynamic state, made possible by the weak-pinning environment of high-quality, single-crystal samples of two low-${T}_{c}$ superconducting compounds, ${\text{V}}_{3}\text{Si}$ and ${\text{LuNi}}_{2}{\text{B}}_{2}\text{C}$. We report the effect of the magnetic field-dependent fluxon-core size on flux flow resistivity ${\ensuremath{\rho}}_{f}$. Much progress has been made in minimizing the technical challenges associated with the use of high currents. Attainment of a FFF phase is indicated by the saturation at highest currents of flux flow dissipation levels that are well below the normal-state resistance and have field-dependent values. The field dependence of the corresponding ${\ensuremath{\rho}}_{f}$ is shown to be consistent with a prediction based on a model for the decrease of fluxon-core size at higher fields in weak-coupling BCS $s$-wave materials.