Atomistic simulations of out-of-equilibrium quantum nuclear dynamics

TL;DR

This paper introduces a first-principles simulation method for out-of-equilibrium quantum nuclear dynamics in crystals, enabling detailed interpretation of ultrafast laser experiments involving lattice excitations.

Contribution

The work develops a general, energy-conserving stochastic simulation approach based on the nonequilibrium TD-SCHA, applicable to any crystal with potential integration of electrons or machine learning potentials.

Findings

Validated with a 1D model for accuracy.

Applied to a 40-atom SrTiO3 crystal under THz laser pump.

Enables simulation of ultrafast pump-probe spectroscopy experiments.

Abstract

The rapid advancements in ultrafast laser technology have paved the way for pumping and probing the out-of-equilibrium dynamics of nuclei in crystals. However, interpreting these experiments is extremely challenging due to the complex nonlinear responses in systems where lattice excitations interact, particularly in crystals composed of light atoms or at low temperatures where the quantum nature of ions becomes significant. In this work, we address the nonequilibrium quantum ionic dynamics from first principles. Our approach is general and can be applied to simulate any crystal, in combination with a first-principles treatment of electrons or external machine-learning potentials. It is implemented by leveraging the nonequilibrium time-dependent self-consistent harmonic approximation (TD-SCHA), with a stable, energy-conserving, correlated stochastic integration scheme that achieves an accuracy of $\mathcal{O}(dt^3)$. We benchmark the method with both a simple one-dimensional model to test its accuracy and a realistic 40-atom cell of SrTiO3 under THz laser pump, paving the way for simulations of ultrafast THz-Xray pump-probe spectroscopy like those performed in synchrotron facilities.