Magnetic properties of gapless semiconductors: PbPdO2 and PbPd0.9Co0.1O2

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 PbPd0.9Co0.1O2. 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. (C) 2010 American Institute of Physics. [doi:10.1063/1.3352984]