Dynamics of quasiparticle formation in structurally tunable materials

非技术摘要：准粒子是粒子的集合，例如固体中的原子或电子，它们一起工作，就像它们是一个单一的复合粒子一样。我们对现代设备中许多重要材料的理解依赖于准粒子的概念，而且我们对准粒子一旦产生后的行为有了很好的理解。然而，我们对准粒子产生过程的理解要有限得多，因为准粒子在决定广泛材料的性质方面发挥着关键作用。极化子是由局域载流子形成的准粒子的一个例子，它对现代半导体材料的导电性、光学性质和能量传输有深远的影响。这个项目将利用各种实验探测器来研究准粒子形成的动力学以及电子动力学、振动动力学和结构动力学的影响。该项目支持两名博士生和一名本科生的工作。涉及凝聚态物理基础问题的基础研究与实验发展中的实际问题相结合，提供了一个很好的培训基础；从事电子材料超快光谱工作的学生，考虑到他们的基础和应用工作背景，已经为进入学术界、政府实验室和高科技行业的职位做好了充分的准备。技术摘要：局域准粒子态在决定多种材料的性质方面起着至关重要的作用：极化子的形成对电子材料的电荷输运性质有着深远的影响，而自陷态激子或激子-极化子的形成则极大地改变了光学性质和能量传输机制。虽然准粒子的平衡性质已经在许多系统中得到了很好的确立，但对准粒子形成过程的动力学还没有完全的理解。除了它的基本意义之外，这种知识还承诺提供一种手段来理解并从而利用材料的快速电学和光学响应。这个项目包括对准粒子形成所固有的耦合电子和振动动力学的系统研究。这些研究是在准一维材料中进行的，其中驱动局域动力学的电子-电子和电子-声子相互作用的相对强度可以通过它们的组成来系统地调节。这些研究使用了对电子、振动和结构动力学敏感的飞秒时间分辨技术的组合，允许观察随着系统演化形成准粒子状态时电子电荷分布和局部结构的变化，以及特定振动模式在创建最终稳定结构中的作用。实验研究得到了理论建模的补充。

Non-technical abstract:Quasiparticles are collections of particles, such as atoms or electrons in a solid, that act together as if they were a single composite particle. Our understanding of many of the materials that are important in modern day devices depends on the idea of quasiparticles and we have a good understanding of the behavior of quasiparticles once they have been created. However, our understanding of the process of creation of quasiparticles, which plays a critical role in determining the properties of a wide range of materials, is much more limited. The polaron is one example of a quasiparticle, formed from localized charge carriers, which has a profound impact on the conductivity, optical properties and energy transport in modern semiconductor materials. This project will utilize a variety of experimental probes to study the dynamics of quasiparticle formation and the influence of electronic, vibrational, and structural dynamics. This project supports the work of two Ph.D. students and an undergraduate student. The combination of basic research involving fundamental issues in condensed matter physics with practical issues in experiment development provides an excellent training ground; students working in ultrafast spectroscopy of electronic materials are well-prepared to enter positions in academia, government laboratories, and high-tech industry, given their background in both basic and applied work. Technical abstract:Localized quasiparticle states play a critical role in determining the properties of a wide range of materials: polaron formation has a profound impact on the charge transport properties of electronic materials, and formation of self-trapped excitons, or exciton-polarons, dramatically changes optical properties and energy transport mechanisms. While the equilibrium properties of quasiparticles are well-established in many systems, a full understanding of the dynamics of the process of quasiparticle formation has yet to be achieved. In addition to its fundamental significance, such knowledge promises a means to understand, and thereby exploit, the fast electronic and optical response of materials. This project is comprised of systematic studies of the coupled electronic and vibrational dynamics inherent to quasiparticle formation. The studies are carried out in quasi-one-dimensional materials in which the relative strengths of the electron-electron and electron-phonon interactions that drive the localization dynamics can be systematically tuned via their composition. The studies use a combination of femtosecond time-resolved techniques that are sensitive to electronic, vibrational, and structural dynamics, allowing observation of changes in the electronic charge distribution and local structure as the system evolves to form the quasiparticle state, and the role of specific vibrational modes in creating the final stabilized structure. Experimental studies are complemented by theoretical modeling.