Rashba 效應對於二維系統超導性質的影響與新反鐵磁半金屬
材料

這篇論文由兩部份所組成。第一部份是Rashba 效應對二維電子超導性的影響。對於垂直二維電子平面方向的電場，會對運動中電子的自旋產生偶合效應，這稱為的Rashba 效應。ＢＣＳ一般而言也適用二維電子氣（不含Rashba 效應），因此可得s-wave 的超導體。Rashba 效應會分裂二維電子氣中電子自旋的簡併度，因此會產生一個跨分裂自旋帶的能隙方程式。我們發現能隙方程式跟動量p 的關係，除了之前預期的一個相位，還會有一個cos 因子，這個cos 因子將會影響能隙的大小，並且此能隙成為類似p-wave 的態勢。對於似s-wave 的部份，因為跨自旋能帶電子對和同自旋能帶電子對對於能隙的貢獻是相消的，因此似p-wave 部份主導了能隙方程式。同時我們也詳細的分析了超導體的電子聲子的交互作用。如果我們只考慮了一般散射過程 (Normal process)，理論估計的結果是無法和實驗相符的。當我們也考慮了Umklapp process 的貢獻，估計的結果和實驗是一致的。我們發現 Umklapp process 是重要的甚至是主要的貢獻。第二部份是尋找新的反鐵磁半金屬材料。我們利用第一原理的計算研究雙鈦鈣-鉍鉛過渡金屬氧化物BiPbBB'O6，其中交換關聯能量是用推廣的密度梯度近似(GGA) 的方法，另外我們也計算了密度梯度近似加上了在位交互作用(GGA+U)。其中BB' 是可能的過渡金屬組合。我們發現雙鈦鈣-鉍鉛鉻銅氧與鉍鉛釩釕氧是反鐵磁半斤屬材料，而鉍鉛釩鋨氧則是接近反鐵磁半金屬氧化物。我們提出其中反鐵磁與半金屬特性應該是源自鄰近的過渡金屬藉由中間的氧離子所產生的雙倍交換機制(double-exchange mechanism)。

關鍵字

超導；二維；反鐵磁；半金屬

並列摘要

There are two parts in this thesis. The first part is the effect of Rashba interaction in superconductivity of the two-dimensional electron gas (2DEG). The interface electric field normal to the 2DEG plane coupled to the spin of the moving electron is the so called Rashba interaction. In the ordinary 2DEG (in the absence of Rashba interaction), BCS theory is applied and the s-wave superconductivity is obtained. The precence of Rashba interaction would lift the spin degeneracy and result in coupled gap equations of two spin bands. We find that the gap function △(p) depends on p not only through it it phase △0ei φp as was predicted before, but also depends on an additional cos(φp) factor which modulates the magnitude of the gap energy and the magnitude is of p-wave like form. The p-wave like gap energy dominates the gap equations because the s-wave like part was much suppressed due to the destructive contribution from inter-band and intra-band pairings. We perform a more detail analysis of electron-phonon interaction in superconductivity. While only considering the normal process, the calculated transition temperature can not agree with the experimental results. While the Umklapp process is included, the the calculated result is consistent with experiments. We find that the Umklapp process is important and can be the dominant contribution in superconductivity. The second part is to find new half-metallic antiferromagnetic (HM-AFM) materials. We theoretically investigated the electronic structures of doubleperovskite BiPbBB′O6 based on first-principles density functional calculation with generalized gradient approximation (GGA) and GGA incorporated with Coulomb correlation interaction U (GGA + U). BB′ are possible transition metal atomic combinations. We find that BiPbVRuO6 and BiPbCrCuO6 double perovskites are HM-AFM materials and BiPbVOsO6 is a nearly HMAFM material. We suggest that the HM and AFM properties of these materials is caused by the double-exchange mechanism between neighboring B and B′ ion via the intermediated oxygen ion.

並列關鍵字

Rashba ； superconductivity ； two-dimension ； antiferromagnetic ； half-metal

參考文獻

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[47] B. S. Shivaram, Y. H. Jeong, T. F. Rosenbaum, and D. G. Hinks, Phys.

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Rashba 效應對於二維系統超導性質的影響與新反鐵磁半金屬材料

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