強耦合微粒電漿中的結構與動力行為研究

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题名:	強耦合微粒電漿中的結構與動力行為研究
作者:	阮文滔;Wen-Tau Juan
贡献者:	物理研究所
关键词:	微粒電漿;庫倫團;異常擴散;拓樸缺陷;電漿晶格;dusty plasma;Coulomb cluster;anomalous diffusion;topological defect;dusty plasma crystal
日期:	2000-12-20
上传时间:	2009-09-22 10:51:44 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	微粒電漿為一含微米微粒之弱離化氣體, 微粒上帶大量電荷所產生的巨大庫倫作用力使其成為一強耦合多體庫倫系統. 沿垂直方向的離子流可於微粒下方聚集成離子雲,進而形成垂直的電耦極矩,使得微粒於鉛直方成長鍊狀排列,並於水平面上形成六角晶格的似二維結構. 此一論文主要藉由光學顯微鏡觀測, 探討此一似二維系統在溫度, 有限邊界, 與外加應力的對稱破壞下, 所產生的各種結構改變與相應的動力行為. 於溶化狀態下, 系統展現出大尺度的規則區塊轉動並伴隨著單一拓樸耦極矩沿晶格軸線的運動, 在此一背景下, 微粒隨時間展現出由反相關, 正相關, 至無關的擴散行為. 液態時的不可壓縮性造成微粒產生如渦流般的隨機運動模式且伴隨著拓樸電荷團的快速產生與消滅, 即使結構於空間上的相關性已消失, 但在動力行為上, 由於粒子間的彼此牽引, 仍展現出空間相關性. 在有限邊界的情形下, 微粒由無限邊界的庫倫晶格轉變為圓形微粒庫倫團. 觀測有限微粒數於環形侷限井中的排列, 可架構一相應的古典原子週期表. 溫度所造成的拓樸狀態改變亦可視為同一原子序下的不同簡併態. 大型庫倫團的三角晶格核心與外圍殼狀結構, 同時具備了無限大庫倫晶格與小型庫倫團殼層狀結構的特性, 並於溫度擾動下分別展現出各向同性的渦流運動與殼層間的相對運動模式. 藉由微粒間相互運動的牽引, 外殼層的非均向運動隨時間向核心傳遞. 雷射光壓所產生的力場可推動懸浮液體庫倫團中位於中心帶狀上的微粒. 於光壓帶上,粒子的前進速度會隨整體微粒的排列隨時間上的演變而調制,並隨機產生出垂直於雷射方向傳遞的渦流狀隨機微粒運動,此一渦流強度隨與雷射區間距離的增加而遞減. 長時間的平均速度場展現出位於雷射光束兩邊相反方向的大渦流運動, 且在雷賒區域周圍強大剪流場的邊界層僅有一至兩個粒子平均間距. 溫度輔助下的多體粒子交互作用使此一系統中的多項物理量, 如驅動微粒的平均速度, 微粒的擴散係數, 與等效黏滯力等, 均展現出非線性的雷射功率相關性. 此一結果也由我們於無溫度下的分子動力模擬結果得到應證 The structural and dynamical behaviors of the strongly coupled many body systems at the microscopic level are interesting issues in physics. In a dusty plasma, μm sized particles are charged (about 104 e - / particle) and suspended in weakly ionized glow discharges. It forms a strongly coupled Coulomb system with sub-mm interparticle spacing in the low viscosity gaseous background and provides us an environment to investigate important issues through the optical microscopy. In this thesis, starting from the highly ordered triangular lattice, the structures and the cooperative particle motions caused by symmetry breaking processes, such as the thermal fluctuations, the finite system boundary under a circular confining field, and the directional driving forces, are widely investigated. In the strongly coupled system, thermal fluctuations destroy the spatial ordering, lower down the effective caging barrier of the particle from the neighbors, promote particle diffusions, and randomize the collective excitations. It governs the microscopic behaviors in different states. The stick-slip type domain rotations and the continuously excited random vortex type motions associated with different anomalous diffusions and the topological evolutions in the melting and the liquid states are observed respectively. Introducing finite numbers of dust particles in the small plasma trap, the strongly coupled quasi-2d dust clusters are first observed. The finite boundary of the Coulomb cluster not only contributes the lattice bending with six intrinsic defects to form the circular shells around the boundary but also cages the particle motion in the shell region. The generic packing rule of the small cluster with shell structures for the small clusters, and the triangular lattice core surrounded by the outer circular shells for the large cluster are observed. Under thermal fluctuations, particles exhibit the isotropic vortex type excitations in the triangular lattice region and anisotropic rotations along the azimuthal direction in the shell region respectively. Driving rows of dust particle through the cluster center by the optical pressure from a dc laser beam, the microscopic responses of the liquid cluster under the directional driving forces are investigated. The interplay between the tilted effective caging barriers for the driven particle and the vortex type particle excitations dominates the microscopic behaviors. Under the assistance of the thermal fluctuation, the collective forward hopping and the induced cascaded generation of chaotic vortices are enhanced. It further promotes the transverse diffusion with decaying strength from the line source. The mean velocity of the driven particle, viscosity, and the diffusion coefficient show nonlinear dependence on the laser power due to the complicated vortex excitation and relaxation processes. Our MD simulations at zero temperature and above the melting temperature also demonstrate that thermal fluctuations play the key roles for the nonlinear responses.
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