Relations between nearest-neighbor decorrelation and other relaxation processes in supercooled liquids

The neighbor-decorrelation metric C_B(t), which varies smoothly from 1 to 0 as particles lose the neighbors that were present in their original first coordination shell, captures the extent to which individual particles have “forgotten” their original local environments. Using MD simulations, we examine how the decay of C_B(t) compares with those of other, more conventionally utilized relaxation metrics. For temperatures below the crossover temperature T_c at which the alpha relaxation time τ_α starts to increase sharply, we find that C_B(t) is well fit by C_B(t) ~= (1-A) exp[-(t/τ_fast)^s_fast] + A exp[(-t/τ_slow)^s_slow], a functional form that is often utilized to fit the self-intermediate scattering function S(q, t). The τ_fast, s_fast and s_slow for C_B(t) are close to those obtained by fitting the corresponding fast-β and α relaxation processes of S(q,t), but the A(T) are larger, and the characteristic neighbor lifetime τ_slow is substantially longer than τ_α. Instead, τ_slow is close to both τ_ov (the time over which a typical particle moves by half its diameter, a natural “hopping time”) and τ_χ, the peak time for the susceptibility χ₄(t). Thus we show that the characteristic "slow" timescale for nearest-neighbor decorrelation is close to those obtained from dynamic heterogeneity (DH) metrics associated with immobile-particle clusters, and the separation between τ_bond and timescales which have previously been associated with "fast" DH metrics such as the non-Gaussian parameter α₂(t) increases with decreasing T.