Probing Strong Electronic Correlations in Ferroelectrics and Multiferroics Through High-Order Harmonic Spectroscopy and First-Principles Calculations

摘要：现代凝聚态物理学的目标之一是控制材料的性质，使它们可以“按需”修改。在相关材料中，强烈的激光脉冲可以迫使材料远离平衡状态，极大地改变其电子、磁性和光学性质。这项工作的重点是这些材料的非平衡特性的实验和计算研究，这些材料在强红外激光场的诱导下，对外部刺激表现出独特的电子和磁响应。利用先进的激光技术，通过测量高次谐波产生的过程发射的光来重建这些材料中的相互作用动力学。虽然之前对固体中超快动力学的大多数研究都是在飞秒和更长的时间尺度上进行的（1飞秒= 0.000000000000001秒），但本项目旨在揭示亚飞秒动力学的细节，并建立非平衡材料中超快相互作用与高次谐波发射的光谱和时域结构之间的联系。通过该项目开发的实验和理论工具也可用于描述与太阳能和自旋电子学等技术应用相关的过程。该奖项培训的学生将获得先进的激光和计算技术以及最先进的材料合成方面的经验，为他们从事高科技职业做好准备。技术摘要：理解和控制强驱动、强相关材料的量子多体动力学是现代科学的重大挑战之一。虽然强相关材料的非平衡动力学已经研究了几十年，但强电子相关的几至亚飞秒动力学一直隐藏在实验中。通过测量和第一性原理计算中红外强激光场在块状和薄膜固体中产生的高次谐波，本项目旨在解决铁电钛酸钡和多铁铋铁氧体晶体中的电子相关性，以及这些相关性在强场激光响应中的作用。通过对发射高次谐波辐射的光谱、极化和时域结构的测量，揭示了晶体结构、铁电极化、Berry曲率和相关电子动力学对高次谐波产生过程的影响。计算，使用时间依赖的密度泛函理论和通过动态平均场理论获得的交换相关函数，准确地捕获了强电子相关核心的时域库仑相互作用。通过该项目开发的实验和理论工具也可用于描述与太阳能和自旋电子学等技术应用相关的过程。该奖项培训的学生将获得先进的激光和计算技术以及最先进的材料合成方面的经验，为他们从事高科技职业做好准备。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Nontechnical Abstract: One of the goals of modern condensed matter physics is to control the properties of materials so that they can be modified 'on demand'. In relevant materials intense pulses of laser light can force the materials far from equilibrium, dramatically changing their electronic, magnetic, and optical properties. This work focuses on experimental and computational studies of the non-equilibrium properties of such materials which exhibit unique electronic and magnetic responses to external stimuli, induced by intense infrared laser fields. Using advanced laser techniques, the dynamics of interactions in such materials are reconstructed from measurements of light emitted through a process known as high-order harmonic generation. While most previous studies of ultrafast dynamics in solids are performed at the femtosecond and longer timescales (1 femtosecond = 0.000000000000001 seconds), this project seeks to uncover details of the sub-femtosecond dynamics and to establish the link between ultrafast interactions in non-equilibrium materials and the spectral- and time-domain structure of high-order harmonic emission. The experimental and theoretical tools developed through this project can also be used to describe processes relevant to technological applications such as solar energy and spintronics. Students trained as a result of the award will gain experience in advanced laser and computational techniques, as well as state-of-the-art materials synthesis, preparing them for high-tech careers.Technical Abstract: Understanding and controlling the quantum many-body dynamics of strongly-driven, strongly-correlated materials is one of the grand challenges of modern science. Although the nonequilibrium dynamics of strongly-correlated materials have been studied for decades, the few- to sub-femtosecond dynamics of strong electronic correlation have been hidden from experiments. Through measurements and first-principles calculations of the high-order harmonics generated from intense mid-infrared laser fields in bulk and thin-film solids, this project aims to resolve electronic correlations in ferroelectric barium titanate and multiferroic bismuth ferrite crystals, and the role of these correlations in the strong-field laser response. Through measurements of the spectrum, polarization, and time-domain structure of the emitted high-order harmonic radiation, the project reveals the impacts of crystal structure and ferroelectric polarization, Berry curvature, and correlated electron dynamics on the high-order harmonic generation process. Calculations, using time-dependent density functional theory with exchange-correlation functions obtained via dynamical mean-field theory, accurately capture the time-domain Coulomb interactions at the heart of strong electronic correlations. The experimental and theoretical tools developed through this project can also be used to describe processes relevant to technological applications such as solar energy and spintronics. Students trained as a result of the award will gain experience in advanced laser and computational techniques, as well as state-of-the-art materials synthesis, preparing them for high-tech careers.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.