CAREER: Unusual phonons in bulk ferroelectric HfO2 single crystals

摘要：由于优异的性能和与硅技术的相容性，HfO2是当今下一代非易失性、高密度和高速技术中最有前途的材料。然而，HfO2性质的微观起源仍然难以捉摸，阻碍了HfO2基器件的发展。该项目旨在通过揭示HfO2独特性质的微观起源来填补认识上的重要空白。这一结果将为HfO2未来的原子尺度应用奠定科学基础。该项目的研究结果将为量子材料领域提供新的知识，并有助于合理合成和制造更好的hfo2基器件。研究活动包括在位于国家实验室的世界级中子设施中进行的大型晶体研究。PI有效地将研究活动与所有学术水平（K-12学生，本科生和研究生）的教育相结合，以培养下一代科学家。特别是，PI与自由科学中心、富兰克林矿物博物馆、罗格斯大学和当地高中合作，开发了一个“天然矿物和量子材料”教育项目，为K-12学生提供了很好的学习机会。技术摘要：HfO2表现出非凡的铁电性：它是唯一具有可切换极化的二元化合物，在原子尺度上具有鲁棒性。更重要的是，HfO2表现出优异的互补金属氧化物半导体相容性。因此，HfO2是当今下一代非易失性、高密度和高速铁电存储器最有前途的材料。然而，HfO2铁电性的微观起源仍然难以确定，这阻碍了HfO2基铁电器件的发展。最近的理论研究表明，HfO2的铁电性可能源于奇异的平坦极性声子带，这些平坦带与极性层和空间层的原子尺度分离直接相关，从而可以在原子尺度上操纵极化。本项目旨在填补这一重要空白：利用激光浮区晶体生长方法稳定大块晶体中的亚稳铁电相，利用晶体上的非弹性中子散射技术验证平极性声子带的情景。非弹性中子散射法可以探测到各种波长和能量范围内的声子带。众所周知，量子材料中的平带，如电子的平带，会引起各种新的量子现象，如超导性。然而，人们对平极声子带的功能了解甚少。该项目将揭示平极性声子带对铁电性的影响，这可以直接关系到原子尺度的铁电操纵。该研究结果也将为改善hfo2基材料的铁电性能提供新的途径，例如通过设计平极性声子带来微调铁电性能。PI有效地将研究活动与所有学术水平（K-12学生，本科生和研究生）的教育相结合，以培养下一代科学家。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。

Non-technical Abstract: Due to extraordinary properties and compatibility with silicon technology, HfO2 is today's most promising material for next-generation non-volatile, high-density, and high-speed technologies. However, the microscopic origin of HfO2's properties remains elusive, hindering the development of HfO2-based devices. This project aims to fill the important gap in understanding by revealing the microscopic origin of HfO2's unique properties. The results will build the scientific foundation of HfO2 for future atomic-scale applications. The results of this project will provide new knowledge in the field of quantum materials, in addition to helping the rational synthesis and fabrication of better HfO2-based devices. The research activities involve the study of large crystals performed at world-class neutron facilities located at national labs. The PI effectively integrates the research activities with education at all academic levels (K-12 students, undergraduates, and graduates) to train the next generation of scientists. In particular, the PI collaborates with the Liberty Science Center, Franklin Mineral Museum, Rutgers University, and local high schools and develops a “Natural Minerals & Quantum Materials” educational project that provides great learning opportunities for K-12 students.Technical Abstract: HfO2 exhibits extraordinary ferroelectricity: it is the only binary compound with switchable polarization, which is robust down to atomic scales. More importantly, HfO2 shows excellent complementary metal-oxide semiconductor compatibility. Thus, HfO2 is today's most promising material for next-generation non-volatile, high-density, and high-speed ferroelectric memories. However, the microscopic origin of HfO2's ferroelectricity remains elusive, hindering the development of HfO2-based ferroelectric devices. Recent theoretical work suggested that HfO2's ferroelectricity may originate from exotic flat polar phonon bands, and these flat bands are directly associated with the atomic-scale separation of polar and space layers, which can enable atomic-scale manipulation of polarization. This project aims to fill the important gap: utilize the laser floating zone crystal growth method to stabilize the metastable ferroelectric phase in large bulk crystals and the inelastic neutron scattering technique on the crystals to validate the scenario of flat polar phonon bands. The inelastic neutron scattering method can detect phonon bands in a wide range of wavevectors and energies. Flat bands in quantum materials, such as flat bands of electrons, have been known to cause various novel quantum phenomena, such as superconductivity. However, the function of flat polar phonon bands is poorly understood. This project would reveal the effect of flat polar phonon bands on ferroelectricity, which can be directly relevant to atomic-scale ferroelectric manipulation. The results would also provide a new approach for improving the ferroelectric properties of HfO2-based materials, e.g., fine tunning the ferroelectric properties by engineering the flat polar phonon bands. The PI effectively integrates the research activities with education at all academic levels (K-12 students, undergraduates, and graduates) to train the next generation of scientists.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.