Observable-dependence of the effective temperature in off-equilibrium diatomic molecular liquids

TL;DR

This study investigates how the effective temperature, derived from fluctuation-dissipation relations, varies depending on the observable in off-equilibrium diatomic molecular liquids, revealing dependence on molecular elongation and quench depth.

Contribution

It demonstrates the observable-dependence of the effective temperature in out-of-equilibrium molecular liquids and highlights the role of molecular elongation and coupling between degrees of freedom.

Findings

At high elongations, $T_{eff}$ is the same for translational and rotational observables.

At low elongations, $T_{eff}$ coincides only for very deep quenches.

Observable-dependence emphasizes the importance of coupling between orientational and translational variables.

Abstract

We discuss the observable-dependence of the effective temperature $T_{eff}$, defined via the fluctuation-dissipation relation, of an out-of-equilibrium system composed by homonuclear dumbbell molecules. $T_{eff}$ is calculated by evaluating the fluctuation and the response for two observables associated respectively to translational and to rotational degrees of freedom, following a sudden temperature quench. We repeat our calculation for different dumbbell elongations $\zeta$. At high elongations ($\zeta > 0.4 $), we find the same $T_{eff}$ for the two observables. At low elongations ($\zeta \leq 0.4$), only for very deep quenches $T_{eff}$ coincides. The observable-dependence of $T_{eff}$ for low elongations and shallow quenches stresses the importance of a strong coupling between orientational and translational variables for a consistent definition of the effective temperature in glassy systems.