Carrier injection into pnictide superconductors

1111 pnictides have a layered structure consisting of FeAs planes separated by LaO layers. Doping is achieved by partial replacement of oxygen by fluorine. We have already shown that layered high-Tc cuprates can be doped by carrier injection along c-axis direction. For our c-axis transport measurements of pnictides we selected very small single crystals (approximately 5 x 5 x 1 μm3) from powder samples of LaO1-xFxFeAs with x = 0.06 and x = 0.1. Mesa structures were prepared by electron beam lithography. Above a certain bias threshold, low temperature IV characteristics showed switching from a low resistive state to a high resistive state and back, indicating charge carrier trapping and release. By this type of switching, the resistance at Tc was changed approximately by a factor of 2. The c-axis critical current belonging to the low resistive state was higher by a similar ratio. This increase of critical current was accompanied by a Tc increase of ca. 2 K. We interpret these results by electron trapping in the LaO layers and compensation of this additional charge by a decrease of electron concentration in the FeAs layers.