Crossover between weak anti-localization and weak localization by Co doping and annealing in gapless PbPdO₂ and spin gapless Co-doped PbPdO₂

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 l_φ.