Xing He, Duke University, Wed, June 15, 12pm – 2pm
Phonon anharmonicity is crucial to understand thermal transport properties of materials, phase transition mechanisms, as well as atomic disorder/diffusions. Time-of-flight inelastic neutron scattering instrument covering large four-dimensional momentum and energy space, is suitable to probe phonon frequency and lifetime as a function of temperature. Furthermore, first-principles simulation including temperature effects, such as ab-initio molecular dynamics, and recently developed machine-learning accelerated molecular dynamics provide tools for large-scale analysis of strong anharmonic systems. In this presentation, I investigate phonon anharmonicity in a variety of perovskite materials, including oxide and halide, using inelastic neutron scattering, neutron/x-ray diffuse scattering, and first-principles computation. Oxide perovskites, exhibiting multiple phase transitions, lattice instabilities and complex ferroelectric polarization behaviors, have drawn interest for decades, with recent emerging quantum critical behaviors, anomalous thermal transport properties, and thermal hall effects at low temperature, all of which are related to low-energy acoustic and optic modes. In halide perovskite, despite its high performance in photovoltaics and radiation detectors, ultralow thermal conductivity, potential thermoelectric application, and optoelectronic properties, lattice dynamics and their influence on these phenomena have received little attention. Here, I focus on several perovskite compounds in this presentation: phonon eigenvector anharmonicity in SrTiO3 as approaching quantum critical point; large octahedron fluctuation, strong diffuse scattering, and their coupling to electronic bandgap in CsPbBr3; multiple phonon instabilities and complex ground state in double perovskite Cs2AgBiBr6; and atomic disorder and its selective coupling with phonon in Nb doped KTaO3. In summary, I use advanced scattering techniques and simulation tools to probe and rationalize phonon anharmonicities including frequency shifts, linewidth broadening, and intensities renormalization, as well as their corresponding real-space force-constants changes. These findings will be crucial in understanding the unusual thermal transport and optoelectronic properties in various materials.
References:
References:
[1] Xing He et al., Phys. Rev. Lett. 124 (2020) 145901
[2] T. Lanigan-Atkins, Xing He et al., Nat. Mater. 20 (2021) 977
[3] Xing He et al., arXiv preprint arXiv:2112.04717 (2021)