Metal–insulator transition, Cu K-edge x-ray absorption near-edge structures and pairing symmetry for electron-doped Pr1.85Ce0.15CuO4+δ superconductors

A metal–insulator transition near oxygen content parameter δ∼0.018(5) was observed for the Ce-optimal-doped (x=0.15) Pr1.85Ce0.15CuO4+δ (−0.003⩽δ⩽0.03) electron superconductors. Cu K-edge x-ray absorption near-edge structure reflects the 3dn character where low-energy peak A1 reflects the 3d10 configuration of Cu(I) oxidation state and A2 peak reflects the 3d10L (L for oxygen ligand hole) for Cu(II) state. The abrupt increase of A1 peak intensity and energy separation ΔE(A2−A1) near δ=0.018 coincide with the metal–insulator transition due to the formation of a broader 3d conduction band. The basal-plane penetration depth λab(T) for c-axis aligned powders show a d-wave power-law behavior with a linear-T temperature dependence for the 17.5 K underdoped superconductor [δ=0.011(5)], while a quadratic-T2 temperature dependence was observed for the 21 K optimal-doped superconductor (δ=−0.002–0.003). The superfluid density ns∼λab−2 increases from the underdoped to the optimal-doped region.