Jump-precursor state emerges below the crossover temperature in supercooled o-terphenyl

In a supercooled liquid, the crossover temperature ${T}_{c}$ separates a high-temperature region of diffusive dynamics from a low-temperature region of activated dynamics. A molecular-dynamics simulation of all-atom, flexible $o$-terphenyl [Eastwood et al., J. Phys. Chem. B 117, 12898 (2013)] is analyzed with advanced statistical methods to reveal the molecular features associated with this crossover. The simulations extend to an \ensuremath{\alpha}-relaxation time of 14 \ensuremath{\mu}s (272.5 K), two orders of magnitude slower than at ${T}_{c}$ (290 K). At ${T}_{c}$ and below, a distinct state emerges that immediately precedes an orientational jump. Compared to the initial, tightly caged state, this jump-precursor state has a looser cage, with solid-angular excursions of 0.054--0.0125 \ifmmode\times\else\texttimes\fi{} 4\ensuremath{\pi} sr. At ${T}_{c}$ (290 K), rate heterogeneity is already the dominant cause of stretched relaxation. Exchange within the distribution of rates is faster than \ensuremath{\alpha} relaxation at ${T}_{c}$, but becomes equal to it at the lowest temperature simulated (272.5 K). The results trend toward a recent experimental observation near the glass transition (243 K) [Kaur et al., Phys. Rev. E 98, 040603(R) (2018)], which saw exchange substantially slower than \ensuremath{\alpha} relaxation. Overall, the dynamic crossover comprises multiple phenomena: the development of heterogeneity, an increasing jump size, an emerging jump-precursor state, and a lengthening exchange time. The crossover is neither sharp, nor a simple superposition of the high- and low-temperature regimes; it is a broad region that contains unique and complex phenomena.