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. The Seebeck coefficient at room temperature abruptly changes around the critical doping level from a large value (∼-300 μV/K) to a small one (∼-50 μV/K) both with negative sign. In the metallic compounds with x=0.2 and 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. A gigantic positive magnetoresistance effect is observed in a wide range of temperatures, 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 multiorbital correlated electron system.