Metal-insulator transition and Cu K-edge XANES studies for Pr_1.85Ce_0.15CuO_4+δ high-T_c superconductors

Metal-insulator transition near oxygen content parameter δ ≃ 0.018 was observed for electron-doped Pr_1.85 Ce_0.15 CuO_4+δ (-0.003 ≤ δ ≤ 0.03 cuprates. Cu K -edge X-ray absorption near-edge structure (XANES) studies with nearly identical threshold edge energy E_o of 8980.8 eV indicate a Cu formal valence smaller than 2 for all samples, which is consistent with the estimated Cu valence of ∼ 1.84 for 20.5 K superconductor Pr_1.85 Ce_0.15 CuO_3.997 and ∼ 1.91 for Pr_1.85 Ce_0.15 CuO_4.03 insulator. The XANES spectrum reflects the Cu 3dⁿ character where low energy peak A₁ reflects the 3d¹⁰ configuration of Cu(I) oxidation state and A₂ peak reflects the 3d¹⁰ L (L for a oxygen ligand hole) configuration for Cu(II) oxidation state. The variation of energy separation Δ E (A₂-A₁) is consistent with the observed metal-insulator transition, increases sharply from 2.42 e V for Pr_1.85 Ce_0.15 CuO_4.018 insulator to 2.74 e V for 15 K underdoped superconductor Pr_1.85 Ce_0.15 CuO_4.015.