Mott-insulator to metal transition in filling-controlled SmMnAsO_(1-x)

Transport and magnetic properties have been systematically investigated for SmMnAsO_{1-x} with controlled electron-doping. As the electron band-filling is increased with the increase of oxygen deficiency (x), the resistivity monotonically decreases and the transition from Mott-insulator to metal occurs between x=0.17 and 0.2. Seebeck coefficient at room temperature abruptly changes around the critical doping level from a large value (\sim -300 \mu V/K) to a small one (\sim -50 \mu V/K) both with negative sign. In the metallic compounds with x=0.2 and x=0.3, Mn spins order antiferromagnetically around 30 K, and the Hall coefficient with the negative sign shows a reduction in magnitude upon the magnetic transition, indicating the change in the multiple Fermi surfaces. Gigantic positive magnetoresistance effect is observed in a wide range of temperature, reaching up to 60 % at 2 K for the x=0.3 compound. The effect is attributed to the field-induced change of the complex Fermi surfaces in this multi-orbital correlated electron system.