Entropy-dominated stacking fault nucleation in compressed Cu thin films

We study stacking fault (SF) formation in compressed Cu thin films using harmonic transition state theory (HTST), harmonic variational transition state theory (HVTST), and molecular dynamics (MD). Motivated by prior simulations showing extremely large transition rates, we quantify the temperature, strain, and size dependence of SF nucleation free-energy barriers. Due to a very large change in vibrational entropy along the SF nucleation pathway, HTST predicts prefactors up to ${10}^{46}/\mathrm{sec}$, around 34 orders of magnitude higher than so-called standard prefactors. HTST rates are, however, significantly higher than direct MD observations. HVTST reconciles these two approaches by predicting the occurrence of strong non-Arrhenius behavior at high temperatures due to the displacement of the optimal dividing surface for the reaction. Our results demonstrate that entropic effects can dominate defect formation in strained metallic films, leading to strongly anomalous kinetics and underscoring the necessity of a variational treatment for obtaining quantitatively reliable nucleation rates.