PbPdO 2 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 TMI=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 TMI=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.
We report the dramatic change of gapless semiconductor properties by different chemical doping elements of Co and Mn into PbPdO2. The metal-insulatorlike transition temperature TMI = 100 K for PbPdO2 shifts to a higher temperature of 150 K by the Co doping and to a lower temperature of 70 K by the Mn doping. Because of the anisotropic band structure with the majority of heavy holes and the minority of light electrons, the transport and magnetic properties are significantly changed by the chemical doping elements. At low temperatures, the Co doping enhances ferromagnetic interactions, whereas the Mn doping favors antiferromagnetic interactions. These results are of great interests because you can control the magnetic ordering as well as manipulate the carrier density by changing the doping elements. These materials could be a good candidate for spintronics applications.
PbPdO2 is a gapless semiconductor, of which the physical properties are easily tuned by external parameters such as temperature, magnetic field, and chemical doping. We have studied the physical properties tuned by magnetic and nonmagnetic ion substitutions. When Pd in PbPdO2 is substituted by Zn, that is, divalent and nonmagnetic with 3d10 (S = 0), the electrical resistivity decreases and the magnetic properties are not changed to remain diamagnetic. However, by substituting Cu2+ (3d9) with S = 1/2, the electrical resistivity increases and the magnetization shows paramagnetic behavior. Another noticeable feature in the magnetic versus nonmagnetic ion substitution is found in the magneto-transport data. The magnetoresistance for PbPd0.9Cu0.1O2 is positive, compared with the negative behavior for PbPd0.9Zn0.1O2. These results propose that chemical dopants play an important role in optimizing the tunability of the physical properties of gapless semiconductors.
The magnetotransport properties of Pb(Pd,Co)O2 and PbPdO2 thin films were investigated. In magnetoconductance curves, we observed a crossover between weak anti-localization (WAL) and weak localization (WL) depending on the annealing and Co doping in PbPdO2 thin films. For the Pb(Pd,Co)O2 case showing WAL signals, the ex-situ annealing weakens the Pd-O hybridization by stabilizing Co3+ states and generating Pd1+ states, instead of Pd2+, so that the spin-orbit coupling (SOC) strength is significantly reduced. It causes the dominant magnetotransport mechanism change from WAL to WL. This annealing effect is compared with the PbPdO2 case, which possesses WL signals. The annealing process stabilizes the oxygen states and enhances the Pd-O hybridization, and consequently the SOC strength is enhanced. Our experimental results are well explained by the Hikami-Larkin-Nagaoka theory in terms of two important physical parameters; SOC strength-related α and inelastic scattering length lϕ.
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