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DY: Fachverband Dynamik und Statistische Physik

DY 15: Condensed-matter simulations augmented by advanced statistical methodologies (joint session DY/CPP)

DY 15.5: Vortrag

Montag, 16. März 2020, 16:30–16:45, HÜL 186

Anharmonic phonons sampled from large scale molecular dynamics based on on-the-fly machine- learning force fields — •Jonathan Lahnsteiner and Menno Bokdam — Computational materials physics University of Vienna, Sensengasse 8/12 1090 Wien

The phonon spectrum for complex dynamic solids at elevated temperatures is often ill-described by the harmonic approximation. In this talk, I will present a molecular dynamics (MD) study of the Caesium Lead Bromide perovskite (CsPbBr3) in its orthorhombic and cubic phase. In the cubic phase, the Cs cation displays 'rattling' motion and (surrogate) structures are dynamically unstable, as indicated by imaginary phonon modes obtained in the harmonic approximation. For high accuracy and efficiency, the on-the-fly machine-learning force field (MLFF) method [1] is applied. This method generates a force-field with near first-principles accuracy and is trained to mimic the potential energy surface described by the SCAN density functional. Large scale microcanonical ensembles are generated with the MLFF and the finite-temperature dynamic structure factors are computed from the Fourier transform of the ionic density-density correlation functions. The intensities of the dynamic structure factor directly give insight in the phonon properties by taking into account phonon scattering events up to infinite order. These fully dynamic phonon spectra are compared to the phonons in the harmonic approximation. With this we are able to discuss the importance of anharmonic lattice vibrations to the stabilization of CsPbBr3 perovskite phases at finite temperature.

[1] Jinnouchi et.al. Phys.Rev.Let.10.1103/PhysRevLett.122.225701