高溫超導體的磁滯、磁通運動及表面超導問題的探討

Abstract

本論文分成四個主要的部分。在第一部分中，我們利用低場近似將廣義臨 界態模型推廣到低場的情況，同時計算了Hmax<H*和Hmax>H*時的磁滯曲線 ，我們得到了新的且不同於由Bean和Kim模型所推導出的M(H)的行為，我 們的理論在定性上符合高溫超導體的實驗結果，同時經由理論與實驗的符 合中可獲得一些重要的參數．在第二部分中，我們將Bardeen-Stephen的 磁通流動理論修改和推廣到高場的情形，藉由Clem模型和Wigner-Seitz圓 型晶胞近似，在BS理論中單一磁通線的正常蕊心的邊界將可自動的移去而 且很自然的處理掉。我們推導出了一連串有趣的結果：Lorentz力完全由 正常電流所貢獻而不是超導電流，但超導電流卻是維持著磁通量子的磁場 分佈。從能量損耗的觀點考慮，我們得到Lorentz力自動等於黏滯力而不 用如 BS理論中把它視成一個假設，同時我們也得到了黏滯係數的公式。 This dissertation contains four main parts. In the first part, the generalized critical-state model is extended to the low field case by the new low-field approximation. We calculate the hysteresis loops of magnetization for Hmax<H* and Hmax>H*. We obtain new behaviors of M(H) which are different from the results derived from Bean and Kim models. Our theory quali- tatively agrees with the experimental results for high-Tc superconductors, and some important macroscopic parameters may be obtained by fitting our theory with the experimental data. In the second part, the Bardeen-Stephen(BS) flux-flow theory is modified and extended to the high field case. With the help of the Clem model and the Wigner-Seitz circle cell approximation for vortices, the distinct boundary of the normal core of a vortex in the BS theory is removed and treated naturally. Several interesting results come out as a consequence. The Lorentz force is determined by the normal current rather than the supercurrent. But the supercurrent can sustain the magnetic field distribution of flux quanta. From energy dissipation considerations, the Lorentz force is equal to the viscosity force automatically without assumption as made in the BS theory. A new expression of the viscosity is also obtained.