Atomic Dynamics in Energy and Functional Materials: Scattering Experiments and First-Principles Simulations
Abstract: A detailed understanding of atomic dynamics is needed to refine microscopic theories of transport and thermodynamics, and it is of vital interest for the design of new functional and energy materials. In particular, the behavior of atomic vibrations (phonons) near lattice instabilities remains sparsely investigated, yet it impacts numerous functional properties, such as ferroelectricity/multiferroicity, superionic transitions in solid-electrolytes, thermoelectricity, and metal-insulator transitions. Near phase transitions driven by phonon instabilities, one needs to properly account for the effect of strong anharmonicity, which disrupts the quasi-harmonic phonon gas model through large phonon-phonon coupling terms. Large phonon amplitudes can also amplify the electron-phonon interaction and lead to renormalizations of a material's electronic structure. These interactions, while often neglected in textbooks and traditional studies, could open the door to further tuning of materials properties for improved functionality.
This presentation will highlight results from our investigations of atomic dynamics in several classes of materials impacted by lattice instabilities, such as ferroelectrics and multiferroics (EuTiO3, YMnO3), thermoelectrics (PbTe, SnSe), superionic conductors, and VO2 across its metal insulator transition. Our group takes advantage of advances in modern neutron and X-ray spectrometers, which have revolutionized our ability to probe atomic dynamics. By mapping phonon spectral functions throughout reciprocal space, via the dynamical structure factor of single crystals, S(Q,E), phonon anharmonicity and couplings to other degrees of freedom can now be revealed in great detail. Such mode-resolved investigations bring direct insights into phonon scattering mechanisms, including anharmonicity, electron-phonon coupling, spin-phonon coupling, or scattering by defects and nanostructures. Increasingly, first-principles simulations of atomic dynamics enable the quantitative rationalization of these effects, for example with ab initio molecular dynamics simulations or anharmonic renormalization techniques at finite-temperature, and our group systematically integrates such modeling with our scattering experiments. The presentation will conclude with some possible scientific opportunities.