Effects of charge density waves on flux dynamics in weak-pinning single crystals of NbSe$_{2}$ : free flux flow, flux-core size effects, and unexpected doubling of$ J_{c}(H)$ `peak effect'

The interaction of two different ordering schemes -- charge density waves (CDWs) and superconductivity -- is studied in high-quality samples of NbSe$_{2}$, particularly in the motion of magnetic flux quanta. More specifically, the study is on the effect of ``switching off'' the CDW phase -- effected by doping with Ta -- on the magnetic-field $H $dependence of: (i) the Lorentz-force-driven free flux flow (FFF) resistivity \textit{$\rho $}$_{f}$ associated with the ordered motion of vortices, and (ii) critical current density $J_{c}$. FFF is achieved for the first time in this material. The field dependence of \textit{$\rho $}$_{f}$ deviates from traditional Bardeen-Stephen flux flow and is more consistent with effects of flux \textit{core size }as predicted by Kogan and Zelezhina. However, the suppression of CDW's seems to have no significant effect on these properties. On the other hand, $J_{c}(H)$ shows a surprising \textit{double} peak for the CDW-suppressed sample --contrary to previous studies in which the $J_{c}$ peak was shown to \textit{disappear. }Possible mechanisms are discussed.