臺灣博碩士論文加值系統

本論文將討論高溫電子超導體R1.85Ce0.15CuO4-d 系統之金屬絕緣體轉變、弱鐵磁性及其中超導相的d波超導性質。
在常壓下相穩定的R1.85Ce0.15CuO4-d (R=Pr,Nd,Sm,Eu,Gd) 系統中，唯有Gd1.85Ce0.15CuO4-d為非超導體，取而代之的是在絕對溫度162度時Cu2+弱鐵磁序的產生。為了更進一步瞭解晶體結構、弱鐵磁性和超導性之間的關係，我們選擇 (R1-yGdy)1.85Ce0.15CuO4-d (R=Nd, Eu) 兩個系統作更進一步的研究。在這兩個系統中，金屬-絕緣體轉變分別發生在y~0.65和y~0.1 的時候，在此同時結構也由T￠相轉變為O￠相。結構上的轉變是由於較小的(R,Gd,Ce)2O2 層和CuO2層不吻合，使得CuO2 平面上的氧原子偏離了原本的理想位置所造成的結果。從這兩個系統的相圖上我們可以發現超導只會在金屬性的T￠相出現，而在絕緣的O￠相中則可以觀察到由於非180° 的Cu-O-Cu夾角造成的 Cu2+ 弱鐵磁序。
為了確認高溫電子超導體的載子對稱性，我們選擇在磁場下排序過的Pr1.85Ce0.15CuO4-d粉末樣本做更進一步的測量。經由測量磁場垂直ab平面的磁化率對溫度的關係，我們可以推測出在ab平面上的穿透深度lab跟溫度的關係。測量結果發現，樣本的lab與溫度並不完全呈現指數關係，而有些微的次方關係摻雜在其中，顯示出高溫電子超導體的超導序參數在費米面上有節點的出現，可能是由於d波對稱性所造成。

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.

Abstract (Chinese) I
Abstract II
Contents III
List of Figures and Tables IV
Acknowledgments VII
Chapter 1Introduction 1
Chapter 2Experimental Details 6
2.1Sample preparation 6
2.2Powder X-ray Diffraction Measurement 6
2.3Powder Alignment 9
2.4Electrical resistivity Measurement 12
2.5Magnetization and Magnetic Susceptibility
Measurement 14
Chapter 3Results and Discussions 17
3.1Weak Ferromagnetism in Gd1.85Ce0.15CuO4-d17
3.2Metal-insultor Transition in
(R1-yGdy)1.85Ce0.15CuO4-d(R=Eu&Nd) 27
3.3d-wave superconductivity in
Pr1.85Ce0.15CuO4-d 44
Chapter 4Conclusions 55
References 56

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