第二類超導體的三維臨界電流密度的計算與討論

Abstract

基於超導實用性的目地, 臨界電流密度 Jc, 一直以來都是超導理論與實驗, 最關注的議題之一, 此篇博士論文題目為”第二類超導體的三維臨界電流密度的計算與討論The Calculation and Discussions in Three-Dimensional Critical Current Density in Type-II Superconductors”, 將採用量子統計方法, 針對第二類超導體的三維集體釘扎量子與熱擾動(quantum and thermal fluctuations on collective pinning), 和臨界電流密度(critical current density)Jc 作理論計算.

經過我們的詳細理論計算顯示, 當考慮外加磁場為常數, 臨界電流密度(critical current density) Jc , 在量子極限下, 是幾乎與溫度無關的; 然而在古典極限下(in classical limit),當溫度區間在 T<T_dp, 去釘扎溫度 (depinning temperature), 臨界電流密度 Jc 是微弱的隨溫度增加而減少, 當溫度區間在 T_dp< T< T_f,邊界漲落溫度(boundary fluctuation temperature), 該溫度相依的(temperature-dependence) 臨界電流密度 Jc , 則為乘冪型式遞減 (form of power law); 然而在溫度區間為 T>T_f 時, 臨界電流密度 Jc, 則隨溫度增加呈現指數型式快速衰減 (decays exponentially). 但是在考慮溫度為一常數時, 當外加磁場增加時,臨界電流密度 Jc 則呈現出, 最初減少隨後增加, 在達到一極大值後,最後再減少的行為.

我們的詳細理論計算結果,經與實驗作比較,計算結果確與實驗一致.The title of this Ph. D. thesis is “The Calculation and Discussions in Three-Dimensional Critical Current Density in Type-II Superconductors”. The critical current density Jc of superconductors has been one of the most important topics both theoretically and experimentally due to its important in applications. We shall study the quantum, thermal fluctuations on collective pinning and critical current density Jc for three-dimensional superconducting bulk materials by utilizing quantum statistics.

After detailed theoretical calculations and discussions, it is shown that Jc is nearly independent of temperature in the quantum limit for a constant magnetic field; however, in the classical limit,Jc decreases weakly with increasing temperature when T<T_dp the depinning temperature, when T_dp<T<T_f boundary fluctuation temperature,Jc is power law decaying, while T>T_f , Jc decays exponentially. For a constant temperature,Jc decreases first then increases after reaching a maximum, finally decreases again as the applied magnetic field increases.

These results are in agreement with the experiments.