Anisotropic electronic and thermal transport properties of lightly oxygen-doped La₂CuO_4+δ

We track the systematic variation in magnetic susceptibility (χ), thermal conductivity (κ), anisotropic resistivity (ρ{variant}), and thermoelectric power (S) of crystalline La₂CuO_4+δ as a function of light hole doping. The level of hole doping (δ) was carefully varied by anneals under controlled oxygen-partial-pressure performed in such a way that the antiferromagnetic ordering temperature T_N ranged from 250 to 310 K. χ data indicate that increased oxygen hole doping acts to reduce T_N below the δ = 0 value of 320 K. Anomalies are present in κ at T_n that result from Cu spin fluctuations about the magnetically ordered ground state. Anomalies are also present in ρ{variant} and S that occur at T_N for air-annealed samples wherein T_N = 250 K; when more lightly hole-doped so that T_N >250 K, the anomalies in ρ{variant} and S do not occur at T_N, but instead occur at T<250 K. In analogy with heavily oxygen-loaded La₂CuO_4+δ, these transport anomalies may result from phase separation involving the diffusion of non-stoichiometric, excess oxygen.