| 摘要: | 強耦合多體系統在液態下，邊界平面所造成的拓樸侷限抑制垂直於平面的運動並使粒子沿邊界排列，進而導致平行平面的層狀有序結構。過去研究主要探討液態中層狀有序結構的形成和動力減緩，以及深度焠冷液態中碎形狀的層-液介面之成長和其三維晶格結構，但未曾探討三維異質結構，及其結構變化與動力學源頭。
在三維焠冷液體系統，受邊界平面誘發的層狀區域可被視為由二維單層相疊組成的2+1維系統。就二維單層而言，近期研究發現，不受邊界侷限的二維Yukawa冷液體由不同晶軸方向的三角晶塊所組成。粒子受熱擾動激發，展現局部震盪或集體躍動，後者使晶塊破裂成相同旋轉方向的晶塊，並伴隨著癒合以重整結構。相較於二維，三維受邊界平面誘發的層狀區域中各層間的耦合使單層內的結構與運動更複雜，因不同層的結構與運動會互相影響，進而影響非均質(heterogeneous)三維結構與其結構重整。
此研究中，我們藉由焠冷三維微粒電漿液體探討下列重要議題: (1) 層-液介面的時空間演變與擾動 (2) 二維單層內結構與其對三維結構之影響 (3) 二維單層內與層間的運動以及其對三維結構變化之影響。我們發現焠冷液體至近乎凝固點，層狀結構從邊界平面向三維液體內部成長，形成多層堆疊之有序結構。單層內與層間之交互作用與熱擾動各自扮演著穩定與破壞結構的角色。兩者競爭之下，導致液態與層狀結構之介面成長後達到一個在穩定平均高度上下起伏震盪的狀態，此震盪使介面高度的空間和時間頻譜呈現似紊流(turbulence)多尺度之冪次律分佈。結構而言，二維各層處於hexatic phase，由具有長尺度同晶軸方向次序(long range order with the same lattice orientations)的六角晶格組成，單層內結構雖可重組，相鄰層透過層間交互作用，導致相鄰層仍具相同且有長時間穩定度的層內晶軸方向。鄰層間交錯堆疊形成不同三維結構，以典型面心立方、體心立方、六方最密晶體為主，而其分別以[111]、[110]、[001]晶格方向垂直邊界平面。運動而言，層內粒子受熱擾動誘發區域性集體運動，局部震盪與晶格斷裂-重連，層間交互作用侷限粒子運動於單層內，且影響結構重組時晶格斷裂之尺度(不同於以往冪次律尺度分佈於二維系統)。鄰層間的相異層內運動導致鄰層相對位置改變，進而改變三維結構。
;Boundary surface induced layering occurs in many strongly coupled systems, such as colloids, metallic glasses, and dusty plasmas. The topological constraint from the boundary surface tends to line up particles, form layered structure and suppress particle trans-layer motions through mutual interaction. Thermal agitations tend to deteriorate structural order. Past studies mainly focused on layering formation and the associated dynamical slowing down in confined liquids, and the fractal like behavior of the layering front and the formation of different three dimensional (3D) crystalline ordered domains (CODs) after deep quenching. Nevertheless, the heterogeneous structural rearrangements of 3D CODs and their dynamical origins are open fundamental issues.
Microscopically, for the quenched 3D liquid under confinement, the layered region can be viewed as a 2+1D system composed of a stack of coupled 2D layers. Recent studies on 2D Yukawa liquids without confinement effect showed that the 2D cold liquid around freezing can be viewed as a patchwork of CODs with triangular lattice structure and different lattice orientations, which can be rearranged through rupturing/healing of CODs by thermally induced cooperative hopping. Nevertheless, comparing with 2D systems, in the layered region of the quenched 3D liquid, the interlayer coupling under various 3D COD structures can further complicate the intralayer structures and motions, which in turn affect the 3D COD structures and motions. Moreover, under the competition of thermal agitation and mutual interaction, the interface between the layered and unlayered regions might be a rough surface with various fluctuation scales.
Therefore, it is intriguing to unravel the following unexplored important issues. A) What are the spatiotemporal evolution and fluctuations of the layering front? B) In the layered region, what are the basic intralayer 2D structure and how does the local relative intralayer structures of adjacent layers affect the local 3D COD structures? C) What are the basic thermally excited intra- and interlayer cooperative motions, and how do they affect 3D COD structural rearrangement?
Here, these issues are experimentally addressed in a quenched 3D dusty plasma liquid by correlating with intra and interlayer motions. The topological constraint from the bottom sheath surface lines up the particles and induces layering. It is found that the scale free turbulent layering front first invades upward into the liquid region, and then fluctuates around a saturated level. The layered region can be viewed as a 2+1D system of vertically coupled layers exhibiting hexatic intra-layer structure with slow decay of long range triangular lattice order. The similar intralayer lattice orientations of adjacent layers with different horizontal shifts of intralayer lattice lines allow the formation of the 3D FCC, BCC, and HCP structures with specific lattice orientations vertical to the boundary. Particles in each layer alternatively exhibit heterogeneous thermally induced intralayer cooperative cage rattling and hopping. Heterogeneous cooperative motions of adjacent layers are the key for causing the interlayer sliding and leading to heterogeneous 3D structures and structural rearrangements. |
