Free flux flow: a probe into the field dependence of vortex core size in clean single crystals

The free-flux-flow (FFF) phase has been attained successfully in a number of clean, weak-pinning, low-anisotropy, low-$T_{c}$, single-crystal samples as a unique probe into type II superconductivity that is independent of composition. The ``clean'' quality of the samples have been confirmed by reversible magnetization, high residual resistivity ratio, and low critical current densities $J_{c}$ with a re-entrant ``peak'' effect in $J_{c}(H)$ just below the critical field $H_{c2}$. The necessity of high current densities presented technical challenges that had been successfully addressed, and FFF is confirmed by a field-dependent ohmic state that is also well below the normal state. In these studies, the FFF resistivity \textit{$\rho $}$_{f}(H)$ has been measured in order to observe the field-dependent core size of the quantized magnetic flux vortices as modeled recently by Kogan and Zelezhina (KZ) who predicted a specific deviation from Bardeen-Stephen flux flow, dependent on normalized temperature and scattering parameter $\lambda $. The compounds studied are: V$_{3}$Si, LuNi$_{2}$B$_{2}$C, and NbSe$_{2}$, and results have shown consistency with the KZ model. Other applications of this method could also be used to probe normal-state properties, especially for the new iron arsenides, as will be discussed.