CAREER: Unusual phonons in bulk ferroelectric HfO2 single crystals

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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.