LAUSR

The majority of thin film studies that were devoted to Fe-oxyarsenides has focused so far on F-substituted (i. e. indirectly electron doped) \textit{Ln}OFeAs (\textit{Ln} = La, Nd, Sm). Here we turn our attention towards Co-substituted (i. e. directly electron doped) LaOFeAs and SmOFeAs in order to investigate its growth on different substrate materials by using pulsed laser deposition (PLD). We detected dominant \textit{Ln}OFeAs phase formation and discuss the occurrence of minor impurity phases in the different films on different substrates. The lack of a superconducting transition in \textit{Ln}OFe$_{0.85}$Co$_{0.15}$As films on MgO(100) could be due to strain, since we observe an onset of superconductivity in SmOFe$_{1-x}$Co$_x$As ($x$ = 0.07, 0.15) films on other oxide substrates. In addition, Co-substitution (i.e. within the Fe$_2$As$_2$ layers) and F-substitution (i.e. within the \textit{Ln}$_2$O$_2$ layers) leading to direct and indirect electron doping respectively, appears for films deposited on CaF$_2$ substrates. In contrast to the F-substituted but Co-free Fe-oxyarsenides, the co-doped SmO$_{1-x}$F$_x$Fe$_{0.85}$Co$_{0.15}$As film has experimentally accessible upper critical fields down to the lowest temperatures and may serve as an ideal test bed for further theoretical modeling of Fe-oxyarsenides.