Preparation, magnetization and electrical resistivity of electrochemically oxidized La₂CuO_4+δ single crystals

We report that oxygen can be intercalated at room temperature into single crystal La₂CuO₄ through slow electrochemical oxidation in aqueous base. Magnetic susceptibility χ(T) measurements indicate that homogeneous La₂CuO_4+δ crystals can be obtained with T_c≈40 K. With increasing δ, two distinct superconducting phases are observed with T_c≈32 and 40 K, respectively; the latter is lower than the maximum T_c≈45 K of the corresponding polycrystalline samples, whereas the former is very similar. The resistivity anisotropy ρ{variant}_c/ρ{variant}_ab increases from ∼ 100 at 290 K to ∼ 300 at 50 K. The susceptibility anisotropy, χ_c-χ_ab, is significantly larger than in the La₂CuO₄ parent crystals. The magnetically determined critical current density J_c(H) of a La₂CuO_4+δ crystal with T_c = 40 K decreases exponentially with magnetic field H, with J_c (0) ∼ 5 × 10⁴ and 9 × 10⁴ A/cm² for H{norm of matrix}c and H{norm of matrix}ab, respectively.