Metal-insulator Transition, Weak Ferromagnetism and d-wave Superconductivity in High-Tc Electron Superconductors.

• Main Authors: Hsin0Chun Chiang, 江欣峻
• Other Authors: Huan-Chiu Ku
• Format: Others
• Language: en_US
• Published: 2000
• Online Access: http://ndltd.ncl.edu.tw/handle/65088096122497301180

碩士 === 國立清華大學 === 物理學系 === 88 === Metal-insulator transition in the Gd-doped (R1-yGdy)1.85Ce0.15CuO4-d systems occurred around y ~ 0.65 for R = Nd and y ~ 0.1 for R = Eu with a simultaneous structural transition from the tetragonal T￠-phase (space group I4/mmm) to an orthorhombic O￠-phase (Acam, ao ￡ bo ~O2 at). The structural change is due to the lattice mismatch between (R,Gd,Ce)2O2 layer and CuO2 layer which results with an oxygen distortion in the perfect square CuO2 plane. The phase diagram indicates that the superconductivity can only occur in the metallic T￠-phase cuprates where the metallic state is controlled by a delicate balance between oxygen reduction and structural stability. The oxygen distortion lowers the crystal symmetry from T￠-phase to O￠-phase (space group: Acam) at room temperature. The Cu2+ weak ferromagnetic weak-ferromagnetic/canted-antiferromagnetic (WF/CAF) order observed in the insulating region is the direct result of non-180° a(Cu-O-Cu) canting angle of the O￠-phase. In order to confirm the pairing state of the high temperature electron superconductor Pr1.85Ce0.15CuO4-d was chosen for further study. The temperature dependence of the in-plane magnetic penetration depth lab(T) of magnetically aligned powders of crystalline Pr1.85Ce0.15CuO4-d is reported. The temperature dependence of lab(T)- lab(0) shows a power law component but not all exponential decay indicative there are nodes on the Fermi surface. The result implies that the order parameter of high Tc electron superconductors should not be totally s-wave but a little d-wave symmetry.