Fluorine Doping Effect On H$_{c2}$ and R$_{h}$ In LaFeAsO$_{1-x}$F$_{x}$

The iron arsenide superconductors discovered earlier this year have attracted much interest, and some families showing high-$T_{c}$ have been identified. \textit{Ln}FeAsO (\textit{Ln}; lanthanide) is the first copper-free family of compounds with $T_{c}$ exceeding 50 K. Here, we present the first systematic study of $H_{c2}$ and $R_{H}$ in the wide fluorine doping region (LaFeAsO$_{1-x}$F$_{x}$; $x$ = 0, 0.25, 0.05, 0.07, 0.11 and 0.14). We found that $H_{c2}$ increases monotonically with decreasing $x$, while the superconducting phase diagram ($T_{c}$, $x)$ displays the classic dome-shaped structure. Furthermore, the shape of $H_{c2}(T)$ depends strongly on $x$. This, according to Gurevich's model, suggests a multiband electronic structure. The Hall resistivity for non-superconducting samples $x$ = 0, 0.025 show a non-linear magnetic field dependence, which also supports a multiband electronic structure interpretation. In addition, the estimated low-field limit of the $R_{H}$ for $x$ = 0, 0.025 detect a gap opening at the structural transition and magnetic transition. The evidence for multiband electronic structure links these materials to famous multiband superconductor MgB$_{2}$ rather than high-$T_{c}$ cuprates.