CAREER: Probing Quantum Materials Modified by Terahertz Quantum Fluctuations

非技术描述真空并不是真的空的，而是包含根据量子力学不断波动的光波。同样，冷却到接近绝对零度的物质并不是真正冻结的，而是包含不断波动的原子运动。虽然这些波动在日常生活中通常可以忽略不计，但从理论上讲，当驻波被压缩成一个非常小的体积时，它们可能会变大，这种体积被称为空腔，其尺寸与波的波长相比。在这种情况下，根据最近的一些理论和实验研究，波动波可能足够强，足以改变浸入波中的材料的性质，包括原子结构、电导率和磁性。这项研究试图回答这一基于特定系统的空腔修饰材料新范式中的几个主要悬而未决的问题：电磁波和原子振动的混合，这是许多离子晶体中自然发生的。在太赫兹频率的振动共振附近，混合波的波长收缩，因此量子涨落有望增强。研究小组计划直接测量小腔内光和物质的量子涨落，并跟踪修改后的能量演变和材料性质。这些结果有望通过免费获取量子力来优化材料。该项目还支持向不同教育水平的受众传播量子材料的基本概念。首席研究人员计划制定一门模块研究生课程，以弥合现有材料教育与建设量子基础设施的社会需求之间的知识差距，提供针对社区大学中代表性不足群体的本科生研究机会，并与休斯顿独立学区的高中教师合作，设计一门关于量子技术材料的课程。这些努力是NSF量子飞跃大构想和国家量子倡议的一部分，目的是为未来建立一支量子劳动力队伍。技术描述不断涌现的实验证据和理论模型为腔增强玻色子模式(包括光子和声子)的量子涨落打开了可能性，从而显著改变材料在激发态和基态下的结构、输运和磁性行为。因此，量子涨落可以作为一个控制钮，在不需要主动输入的情况下改变量子材料。然而，对于太赫兹频率的放大起伏的可能性，或者腔耦合的明亮极化子与更多的腔去耦合的局域态之间的性质转移的可能性，实验验证很少。这一研究为超强耦合声子-极化子在亚波长腔中的量子涨落的大小和后果提供了急需的实验知识，可能指导未来通过腔工程实现各种功能量子材料的努力，可能包括超导体、铁磁体、铁电体和拓扑电子材料。首席研究员和研究团队计划开发太赫兹频率的灵敏电光显微镜，以量化波动的电场和原子位移。这些测量可以为现实中有耗量子材料中的腔量子电动力学的理论分析提供更坚实的基础。量子显微镜结合时间分辨光谱揭示了声子-极化激子激发的相干性和布居动力学，这为可能的腔诱导结构跃迁提供了线索。最后，所获得的知识有助于设计和实现混合太赫兹腔-材料系统，以修改二维层状材料和异质结构中的自旋和电荷状态，其中电子相互作用很强，并且外部可调。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Nontechnical descriptionVacuum is not truly empty but contains light waves that are constantly fluctuating according to quantum mechanics. Similarly, materials cooled to near-absolute zero temperature are not truly frozen but contain constantly fluctuating atomic motions. Although these fluctuations are usually negligible in daily life, theoretically they can grow large when a standing wave is compressed into a very small volume, called a cavity, with dimensions compared with the wave’s wavelength. Under such circumstances, according to some recent theoretical and experimental research, the fluctuating wave may be strong enough to change the properties of materials immersed in the wave, including the atomic structure, electrical conductivity, and magnetic properties. This research attempts to answer a few major open questions in this new paradigm of cavity modified materials based on a specific system: the mixing of electromagnetic waves and atomic vibrations that naturally happens in many ionic crystals. Near the vibrational resonance, which is in the terahertz frequencies, the wavelength of the mixed wave shrinks, so the quantum fluctuation is expected to enhance. The research team plans to directly measure the quantum fluctuation of the light and matter inside small cavities, and track the modified energy evolution and materials properties. The results promise insights into optimizing materials by harvesting quantum forces for free. The project also supports the dissemination of the basic concepts of quantum materials to audiences at different levels of education. The principal investigator plans to formulate a modular graduate course to bridge the knowledge gap between existing materials education and the societal need for building quantum infrastructure, to offer an undergraduate research opportunity targeting underrepresented groups in community colleges, and to work with high school teachers in the Houston Independent School District to design a lesson about materials in quantum technology. These efforts are part of NSF’s Big Idea of Quantum Leap and the National Quantum Initiative to build a quantum workforce for the future.Technical descriptionEmerging experimental evidence and theoretical models open a possibility for cavity-enhanced quantum fluctuations of bosonic modes, including photons and phonons, to significantly alter materials’ structural, transport, and magnetic behaviors in both the excited states and the ground state. The quantum fluctuations thus may serve as a control knob to transform quantum materials without the need for active input. However, experimental validation is scarce about a possibility of the amplified fluctuations in terahertz frequencies or the transfer of properties between cavity-coupled bright polaritons and much more numerous cavity-decoupled localized states. This research provides the much-needed experimental knowledge on the magnitude and consequence of the quantum fluctuations of ultra-strongly coupled phonon-polaritons in sub-wavelength cavities, which may guide future efforts to realize various functional quantum materials through cavity engineering, possibly including superconductors, ferromagnets, ferroelectrics, and topological electronic materials. The principal investigator and research team plans to develop sensitive electro-optic microscopy in the terahertz frequencies to quantify the fluctuating electric fields and atomic displacements. The measurements can provide a firmer ground to the theoretical analysis of cavity quantum electrodynamics in realistic, lossy quantum materials. The quantum microscopy combined with time-resolved spectroscopy reveal the coherence and population dynamics of phonon-polariton excitations, which shed light on possible cavity-induced structural transitions. Finally, the acquired knowledge facilitates the design and realization of hybrid terahertz cavity-materials systems to modify the spin and charge states in two-dimensional layered materials and heterostructures, where the electronic interactions are strong and externally tunable.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.