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研究生： 高潤婷
Jun-Ting Kao
論文名稱： Upward Curvature of Upper Critical Field in NaxCoO2.yH2O
指導教授： 牟中瑜
Chung-Yu Mou
口試委員:
學位類別： 碩士
Master
系所名稱： 理學院 - 物理學系
Department of Physics
論文出版年： 2006
畢業學年度： 94
語文別： 英文
論文頁數： 53
中文關鍵詞： Upper critical field Hc2
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    • In the experiment of upper critical field H_c2 in Na_xCoO_2. yH_2O, it showed that there existed a change of slope near T_c. In the thesis we study how to solve the anomalous phenomenon under macroscopic and microscopic descriptions.Since it is a second order phase transition, we apply the Ginzburg-Landau approach and propose two allowed pairing symmetries, f-p mixing, to treat the problem. Under the macroscopic description, we obtain the result which explains the change of slope well. Secondly, we adopt the method of microscopic calculations in one-band consideration and, however, it does not support the result in the Ginzburg-Landau approach. We find the reason why the two approaches conflict with each other. Moreover we consider the influence of two-band system on the upper critical field Hc2(T). In the result we discover that two-band system containing some microscopic characteristics which dominate the slope of each pairing symmetry may cause the upward curvature in Na_xCoO_2 yH_2O.

    1 Introduction 2 2 The Ginzburg Landau Approach 4 2.1 The Ginzburg Landau Free Energy in Nax CoO2 • yH2 O System : f-p wave mixing 4 2.2 Hc1 : Single Vortex Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.3 The Upper Critical Field of A Type II Superconductor . . . . . . . . . . . . 12 2.4 Hc2 -T Phase Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3 The Microscopic Calculations of Hc2 (T ) Curve 17 3.1 Linearized Gap Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.2 The Method of Hc2 in s-Wave Pairing Symmetry . . . . . . . . . . . . . . . . . 19 3.3 Two Pairing Symmetries of f and p Wave . . . . . . . . . . . . . . . . . . . . . 25 4 Microscopic Calculation of Hc2 (T ) Curve : Two Band Consideration 36 4.1 Different Pairings in Different Band . . . . . . . . . . . . . . . . . . . . . . . 37 4.2 Singlet Pairing s-dx^2 −y^2 + 2idxy Mixing . . . . . . . . . . . . . . . . . . . . . . 42 4.3 The Effect of Impurity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 4.3.1 Strong Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 4.3.2 Weak Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 5 Conclusion 50

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