Prof. Olivier Delaire (Duke University)
Wed, November 9, 1:25pm – 2:25pm, PAN 110
Zoom: https://umn.zoom.us/j/97640459154?pwd=WjlpbmJUQUcxT2txMWM2SnNxSm8rQT09
A detailed view of atomic motions in solids is needed to refine microscopic theories of transport and thermodynamics, and to design improved materials. In particular, the behavior of atomic vibrations (phonons) is key to rationalize numerous functional properties, ranging from multiferroics for information processing and superionics for safer solid batteries, to thermoelectrics for waste-heat harvesting, or metal-insulator transitions for ultrafast transistors. Near phase transitions associated with phonon instabilities, one needs to properly account for the effect of strong anharmonicity, which disrupts the quasiharmonic phonon gas model through large phonon-phonon coupling terms. Large phonon amplitudes can also amplify the electron-phonon interaction and lead to renormalization of a material’s electronic structure. These interactions, often neglected in textbooks, remain insufficiently understood but could open the door to new and improved material functionalities. The effects of phonon anharmonicity and couplings to other degrees of freedom are revealed by mapping phonon spectral functions throughout reciprocal space. To this end, our group uses state-of-the-art neutron and x-ray scattering techniques. Our first-principles simulations enable the quantitative rationalization of measurements, for example with ab-initio molecular dynamics simulations augmented with machine learning, and anharmonic renormalization at finite-temperature. This presentation will highlight results from our investigations of atomic dynamics in several classes of materials beyond the traditional phonon quasiparticle picture, such as halide perovskite photovoltaics [1], ferroelectrics and multiferroics [2,3], superionic conductors [4,5], and VO 2 across its metal insulator transition [6,7].