臺灣博碩士論文加值系統

本論文針對兩方面研究微小超導穿隧接合的磁通,電荷與自旋傳輸。其一是約瑟芬結的整體行為,其二是超導單電子電晶體中的自旋堆積。在第一部分我們研究約瑟芬結串聯組成的一維陣列,觀察其中由磁場所引發的的超導-絕緣體相變。此相變的臨界磁場,和電流-電壓特性曲線中所觀察到庫柏電對的庫倫阻斷,可相互對應。我們利用一維的超流-絕緣體理論來分析測量的電阻訊號。相變的度規分析提供我們該相變的臨界指數。
我們發現動力學指數$z$接近於1,而相關長度指數$\nu$在兩組樣品分別得到是0.3和0.45。這些結果可以提供我們一維約瑟芬結陣列中超導-絕緣體相變的相圖。該相圖以約瑟芬耦合強度和耗散強度來兩種參數表示。第二部分我們研究磁性/超導/磁性單電子電晶體的自旋堆積現象。
我們發現當兩磁性電極的磁化方向反平行排列時,超導的能隙會突然的減小。此能隙減少的效應會隨著外加源汲極電壓的增加而增強。能隙減少可以解釋為自旋堆積的影響,當磁性電極的磁化方向反平行排列時自旋堆積便發生。理論上可以自洽的考慮電荷效應和能隙減少的效應,給出結果與實驗比較。我們也考慮了閘極電壓對自旋堆積的影響。理論上閘極可以調控通過磁性單電子電晶體電流的偏極性。在實際應用上,我們討論了自旋翻轉散射和能量弛豫等等會削弱自旋堆積現象的物理過程。

We study vortex, charge and spin transport in small superconducting tunnel junctions in two aspects, the collective behavior of small Josephson junctions, and the spin accumulation in superconducting single electron transistors. For the first part, we experimentally study the magnetic field-induced superconductor-insulator quantum phase transition in one-dimensional arrays of small Josephson junctions. It is found that the critical magnetic field that separates the two
phases corresponds to the onset of Coulomb blockade of Cooper pairs tunneling in the current-voltage characteristics. The resistance data are analyzed in the context of the superfluid-insulator transition in one dimension, and a finite temperature scaling analysis isperformed to extract the critical exponents. The dynamical exponents $z$ are determined to be close to 1, and the correlation length exponents $\nu$ are found to be approximately 0.3 and 0.45 in the two groups of measured samples. We also construct an experimental phase diagram using Josephson coupling-to-charging energy ratio($E_J/E_{CP}$) and dissipation strength. For the second part, we both experimentally and theoretically study the spin accumulation in ferromagnet/superconductor/ferromagnet single electron transistors. The measured superconducting
gap as a function of magnetic field reveals a dramatic decrease when the magnetizations of the two leads are in nearly anti-parallel orientations. The effect of suppression increases with increasing source-drain voltage. This phenomena can be account for the spin accumulation, which takes place
when the ferromagnetic leads are in anti-parallel alignment, suppressing superconductivity in the central island. A comparison with theoretical calculations, in which the charging effect and gap suppression are self-consistently considered is presented. We also theoretically investigated the
spin accumulation under the influence of gate voltage. It is found that the gate can be used to tune polarization of current passing through the ferromagnetic single electron transistor when spin accumulates. Processes that weaken this phenomena such as spin-flipping and energy relaxation are
modeled and discussed.

Abstract ...iii
Table of Contents ...viii
1 Introduction ...1
1.1 Vortices, charges and spins ...1
1.2 JJA: from2D to 1D ...3
1.3 Interplay between spins and charges: FSETs ...8
2 1DJJA: Theoretical Overview ...15
2.1 Dynamics of a single Josephson junction ...15
2.2 Electro-statics and charge screening length...20
2.3 LC transmission line and charge soliton picture...24
2.4 Non-interacting model of 1DJJA...26
2.5 Phase transitions in 2D XY model...31
2.6 Effective two level system ...33
2.7 Gate charge effect...35
2.8 Effect of dissipation ...35
3 1DJJA: Experimental Results ...39
3.1 Introduction ...39
3.2 Fabrication ...40
3.3 Measurement setup...42
3.4 Current-voltage characteristics ...45
3.5 Temperature dependence ...54
3.6 Theoretical model and phase diagram ...55
3.7 Scaling...60
3.8 Conclusion...64
4 FSET: Results of Simulations 67
4.1 Introduction ...67
4.2 Spin dependent transport ...68
4.3 Charging effect ...69
4.4 The master equation for a SET ...73
4.5 Spin dependent charge states ...75
4.6 F/N/F SET: polarizations of current ...79
4.7 Co-tunneling ... 84
4.8 Spin polarizer ... 87
4.9 F/S/F SET: spin chemical potentials ...89
4.10 Suppression of superconducting gap ...92
4.11 Conclusion ...95
5 Suppression of Superconductivity by Spin Imbalance ...101
5.1 Introduction ...101
5.2 Sample fabrication ...102
5.3 V (H) hysteresis ... 105
5.4 Parameters fitting ...107
5.5 Voltage dependence of the superconducting gap ...110
5.6 Important time scales ...114
5.7 Stray fields ... 115
5.8 Relaxation of magnetizations...117
5.9 Conclusion ...120
6 General Conclusion ...121
Appendix ...125
Bibliography ...132

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