Dynamics of quasiparticle formation in structurally tunable materials

准粒子是粒子的集合，例如固体中的原子或电子，它们一起作用，就好像它们是一个单一的复合粒子。 我们对许多在现代设备中很重要的材料的理解取决于准粒子的概念，并且我们对准粒子一旦被创建后的行为有很好的理解。 然而，我们对准粒子产生过程的理解要有限得多，而准粒子在确定各种材料的性质方面起着关键作用。 极化子是准粒子的一个例子，由局域电荷载流子形成，对现代半导体材料的导电性、光学性质和能量输运具有深远的影响。 该项目将利用各种实验探针来研究准粒子形成的动力学以及电子，振动和结构动力学的影响。 该项目支持两个博士的工作。学生和本科生。 涉及凝聚态物理学基本问题的基础研究与实验开发中的实际问题相结合，提供了一个很好的培训场所;在电子材料超快光谱学领域工作的学生为进入学术界，政府实验室和高科技行业做好了充分的准备，因为他们在基础和应用工作方面都有背景。 局域准粒子态在确定各种材料的性质方面起着关键作用：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，自陷激子或激子-极化子的形成极大地改变了光学性质和能量输运机制。 虽然准粒子的平衡性质在许多系统中得到了很好的建立，但对准粒子形成过程的动力学还没有完全理解。 除了其基本意义外，这些知识还提供了一种理解并从而利用材料的快速电子和光学响应的方法。 该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，其中驱动局域化动力学的电子-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进行系统地调整。 这些研究使用了飞秒时间分辨技术的组合，这些技术对电子，振动和结构动力学敏感，允许观察电子电荷分布和局部结构的变化，因为系统演化形成准粒子状态，以及特定振动模式在创建最终稳定结构中的作用。 实验研究辅以理论建模。

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.