Metal–insulator transition, Cu K-edge x-ray absorption near-edge structures and pairing symmetry for electron-doped Pr[sub 1.85]Ce[sub 0.15]CuO[sub 4+δ] superconductors.

A metal-insulator transition near oxygen content parameter δ ∼0.018(5) was observed for the Ce-optimal-doped (x50.15) Pr[SUB1.85]Ce[SUB0.15]CuO[SUB4=δ] (-0.003⩽δ⩽0.03) electron superconductors. Cu K-edge x-ray absorption near-edge structure reflects the 3d[SUPn] character where low-energy peak A[SUB1] reflects the 3d[SUP10] configuration of Cu(I) oxidation state and A[SUB2] peak reflects the 3d[SUB10]L (L for oxygen ligand hole) for Cu(II) state. The abrupt increase of A[SUB1] peak intensity and energy separation ΔE(A[SUB2] - A[SUB1]) 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 λ[SUBab] (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 [d50.011(5)], while a quadratic-T[SUP2] temperature dependence was observed for the 21 K optimal-doped superconductor (δ = 20.002-0.003). The superfluid density n[SUBs] ∼ λ[SUP-2,SUBab] increases from the underdoped to the optimal-doped region. [ABSTRACT FROM AUTHOR]