Variation of Metal-Insulator Transition and Cu K-edge XANES with Oxygen Content for Electron-doped Pr2-xCexCuO4+d Superconductors

• Main Authors: Yung-yuan Hsu, 徐永源
• Other Authors: Huan-chiu Ku
• Format: Others
• Language: en_US
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/96702907408600739751

博士 === 國立清華大學 === 物理學系 === 90 === Metal-insulator transition near oxygen content parameter d ~ 0.018 was observed for electron-doped Pr1.85Ce0.15CuO4+d (-0.003 £ d £ 0.03) high-Tc superconductors. Cu K-edge X-ray absorption near-edge structure (XANES) studies with almost identical threshold edge energy E0 around 8980.8 eV indicates a Cu formal valence lower than 2, which is consistent the estimated Cu formal valence of ~1.84 for the 20.5 K superconductor Pr1.85Ce0.15CuO3.997 and ~1.91 for insulating Pr1.85Ce0.15CuO4.03. The XANES reflects the 3dn character where low energy peak A1 reflects the 3d10 occupation of Cu(I) oxidation state and A2 peak reflects the 3d10L (L for oxygen ligand hole) configuration for Cu(II) oxidation state. The variation of energy separation DE(A2-A1) is consistent with the metal-insulator transition and increases sharply from 2.42 eV for insulating Pr1.85Ce0.15CuO4.018 to 2.74 eV for 15 K underdoped superconductor Pr1.85Ce0.15CuO4.015. The superconducting paring symmetry of the Pr1.85Ce0.15CuO4+d system and Pr2-xCexCuO4+d (x = 0.14, 0.15, 0.16) system were studied though the temperature dependence of the ab-plane electromagnetic penetration depth lab(T). A systematic change of the normalized penetration length difference lab(T)/ lab(0) — 1 from linear to quadratic then to a T2.39 power law for the oxygen content of Pr1.85Ce0.15CuO4+d varies from underdoped to overdoped. Similar systematic change from quadratic for under- and optimal-doped to T2.67 power law for overdoped sample was observed for the Ce varying system with d ~ 0. Systems basically can be described by the d-wave pairing symmetry, since the quadratic temperature dependence provides good fittings. The observed changes of power law exponent larger than 2 indicates a topological change of the gap function in the overdoped region. No direct evidence of s-wave paring symmetry was observed for the systems studied.