Clarifying the nature of the Johari-Goldstein β-relaxation and emphasising its fundamental importance

ABSTRACT

Since 1998 the primitive relaxation time τ₀(T,P) of the Coupling Model (CM) and the Johari-Goldstein (JG) β-relaxation time τ_JG(T,P) are shown approximately equal in many glass-formers. The CM relation between τ₀(T,P) and τ_α(T,P) at any T and P is exact. Additionally from the CM relation τ_α(T,P)/τ₀(T,P) is exactly invariant to variations of T and P while τ_α(T,P) is kept constant, and τ₀ is exactly a function of ρ^γ/T like τ_α. However, since τ_JG(T,P) ≈ τ₀(T,P), the exact invariance of τ_α(T,P)/τ₀(T,P) leads to approximate invariance of τ_α(T,P)/τ_JG(T,P), and τ_JG is approximately a function of ρ^γ/T. Notwithstanding, the CM prediction of the approximate relations between τ_β and τ_α were mistaken as exact relations by some researchers. In this paper, we remove this misunderstanding by demonstrating via simulations and experiments that the JG β-relaxation is comprised of processes with different length-scales and degrees of cooperativity, and the process is heterogeneous. The distribution of processes makes τ_JG(T,P) equivocal, because it is just a single relaxation time used to represent the different processes within the distribution, which may change on varying T and P, at constant τ_α(T,P). The problem is compounded if the β-relaxation is not resolved, and fitting procedure used to extract τ_JG(T,P) and τ_α(T,P). Despite the relations of τ_JG(T,P) to τ_α(T,P) are approximate, we show these properties of τ_JG(T,P) are truly remarkable, fundamental, general, and important.

KEYWORDS:

• Johari-Goldstein β-relaxation
• dynamic heterogeneity
• pressure dependence
• relation to α relaxation

Acknowledgements

We thank Prof. Dmitry Bedrov, Dr. Federico Caporaletti and Prof. Giulio Monaco for sharing their data with us.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the National Basic Research Program of China (973 Program No. 2015CB856805); National Natural Science Foundation of China (NSFC) (grant numbers 11474247, 51871193); and Key Research and Development Program of Hebei Province (grant number 18391502D).