Magnetic properties of gapless semiconductors: PbPdO₂ and PbPd_0.9Co_0.1O₂

PbPdO₂ is a new class of gapless semiconductors, which is extremely sensitive to external influences such as temperature, magnetic field, and carrier doping, because of their peculiar band structure. With varying temperature, a broad transition from a high-temperature metallic behavior to a low-temperature insulating behavior was observed at T_MI=100 K in the electrical resistivity, which is related to the thermally assisted excitation near the Fermi level due to its gapless band structure. By doping 10% Co for Pd in PbPdO₂, the number of hole charge carriers was increased by ten times, and the transition temperature was increased to T_MI=150 K. When applying a magnetic field, a ferromagnetic component was found at low temperatures in the magnetization curves of both materials, in addition to diamagnetic background signals for PbPdO₂ and paramagnetic background signals for PbPd_0.9Co_0.1O₂. In the low temperature regime, the slope of magnetoresistance is negative, while it is changed into positive with a quadratic form at high temperatures. These results of magnetic properties identify a tendency of strong spin-orbit coupling in the gapless semiconducting compounds.