Investigation of Quantum Materials Using Pump-Probe Raman Scattering

在超导体和拓扑绝缘体等量子材料中，量子物理是最重要的性质的基础。解开这些材料带来的谜团是21世纪的主要科学挑战之一，与新一代电子、能源和医疗技术直接相关。该项目致力于观察这些材料在受到短激光脉冲撞击后的时间演化过程。微弱的激光脉冲可能会把一些电子踢出轨道。观察这些被击中的原子和其他电子的反应，可以发现电子之间以及电子和原子晶格之间的相互作用。如果激光脉冲很强，材料可以短暂地进入不同的状态，例如超导体将变成非超导。观察它是如何回到原始状态的，阐明了原本隐藏的机制。这些知识被用来测试基本理论，并有助于将量子材料用于实际应用。进行这些观测的实验技术被称为泵浦探测拉曼光谱，可以拍摄量子材料如何随时间演变的电影，每隔0.00000000000001秒拍摄一帧。在这些时间间隔内，光的传播速度只有一毫米的几分之一，因此可以在慢动作中检测到非常快的过程。这项工作是在科罗拉多大学博尔德分校新的超快激光光谱学实验室进行的，该实验室以前是由NSF提供的资金建造的。它是两名研究生博士论文的一部分，对本科教育做出了贡献。该项目还将通过现有的社区正式科学教育伙伴关系计划，为推广工作做出贡献。技术摘要这个项目专注于材料的突破性测量，其中量子物理是最重要的物理性质的基础。总的目标是了解这些材料如何脱离热平衡：被研究的样品被一个短的激光脉冲驱离平衡，另一个延时的激光脉冲将作为拉曼光谱的激发激光来探测它的弛豫。这项工作是在科罗拉多大学博尔德分校新的超快激光光谱学实验室进行的，该实验室以前是由NSF提供的资金建造的。这个实验室的核心激光系统是世界上最先进的激光系统之一。它被用于泵浦-探测角度分辨光电子能谱和泵浦-探测拉曼光谱。该项目代表了这项工作的拉曼部分。在弛豫过程中跟踪了所有的拉曼活性激发。其中包括声子、磁子和一些高达5 eV能隙的电子激发。拉曼散射直接探测激发态的能量、寿命和布居(如声子占据数)。时间分辨拉曼散射测量是在电荷密度波(CDW)系统、常规超导体和铜酸盐超导体以及Mott绝缘体上进行的。这些材料展示了各种令人感兴趣的神秘物理现象。该项目是两名研究生博士论文的一部分，有助于本科教育，也将通过现有的社区正式科学教育伙伴计划促进外联工作。

Non-Technical Abstract In quantum materials such as superconductors and topological insulators quantum physics underlies most important properties. Unraveling the puzzles posed by these materials is one of the main scientific challenges of the 21st century, with direct relevance to new generations of electronic, energy, and medical technologies. This project is devoted to observing how these materials evolve in time after being hit by a short laser pulse. A weak laser pulse may kick some electrons out of their orbits. Observing how these hit atoms and other electrons react reveals interactions between electrons as well as between electrons and the atomic lattice. If the laser pulse is strong, the material can briefly go into a different state, e.g. a superconductor will become nonsuperconducting. Observing how it comes back to the original state elucidates otherwise hidden mechanisms. This knowledge is used to test fundamental theories and helps to harness quantum materials for practical applications. The experimental technique for making these observations is known as pump-probe Raman spectroscopy and allows making movies of how quantum materials evolve in time taking frames every 0.00000000000001 sec. Light travels a fraction of a millimeter during these time intervals, so very fast processes can be detected in slow motion. The work is carried out at the new ultrafast laser spectroscopy laboratory at the University of Colorado (CU)-Boulder built with the funds previously provided by NSF. It is a part of a Ph.D. thesis of two graduate students and contributes to undergraduate education. The project will also contribute to outreach efforts through the existing Partnership in Formal Science Education in the Community program. Technical Abstract This project focuses on groundbreaking measurements of materials where quantum physics underlies most important physical properties. The general goal is to understand how these materials behave away from thermal equilibrium: The investigated sample is driven out of equilibrium by a short laser pulse, and another time-delayed laser pulse will probe its relaxation by serving as the excitation laser for Raman spectroscopy. The work is carried out at the new ultrafast laser spectroscopy laboratory at the University of Colorado (CU)-Boulder built with the funds previously provided by NSF. The laser system at the heart of this laboratory is one of the most advanced of its kind in the world. It is utilized for pump-probe angle-resolved photoemission spectroscopy and pump-probe Raman spectroscopy. This project represents the Raman component of this effort. All Raman active excitations are tracked during relaxation. These include phonons, magnons and some electronic excitations across gaps of up to 5eV. Raman scattering directly probes the energies, lifetimes, and population of excited states (e.g. phonon occupation numbers). The time-resolved Raman scattering measurements are on charge density wave (CDW) systems, conventional and cuprate superconductors, and Mott insulators. These materials display diverse and often still enigmatic physical phenomena of great interest. The project is a part of a Ph.D. thesis of two graduate students and contributes to undergraduate education and will also contribute to outreach efforts through the existing Partnership in Formal Science Education in the Community program.