Microscopic understanding of the Johari-Goldstein β relaxation gained from nuclear γ-resonance time-domain-interferometry experiments

Traditionally the study of dynamics of glass-forming materials has been focused on the structural alpha relaxation. However, in recent years experimental evidence has revealed that a secondary beta relaxation belonging to alpha special class, called the Johari-Goldstein (JG) beta relaxation, has properties strongly linked to the primary a relaxation. By invoking the principle of causality, the relation implies the JG beta relaxation is fundamental and indispensable for generating the a relaxation, and the properties of the latter are inherited from the former. The JG beta relaxation is observed together with the alpha relaxation mostly by dielectric spectroscopy. The macroscopic nature of the data allows the use of arbitrary or unproven procedures to analyze the data. Thus the results characterizing the JG beta relaxation and the relation of its relaxation time tau(beta) to the alpha-relaxation time tau(alpha) obtained can be equivocal and controversial. Coming to the rescue is the nuclear resonance time-domain-interferometry (TDI) technique covering a wide time range (10(-9) - 10(-5) s) and a scattering vector q range (9.6-40 nm(-1)). TDI experiments have been carried out on four glass formers, ortho-terphenyl [M. Saito et al., Phys. Rev. Lett. 109, 115705 (2012)], polybutadiene [T. Kanaya et al., J. Chem. Phys. 140, 144906 (2014)], 5-methyl-2-hexanol [F. Caporaletti et al., Sci. Rep. 9, 14319 (2019)], and 1-propanol [F. Caporaletti et al., Nat. Commun. 12, 1867 (2021)]. In this paper the TDI data are reexamined in conjunction with dielectric and neutron scattering data. The results show the JG beta relaxation observed by dielectric spectroscopy is heterogeneous and comprises processes with different length scales. A process with a longer length scale has a longer relaxation time. TDI data also prove the primitive relaxation time tau(0) of the coupling model falls within the distribution of the TDI q-dependent JG beta-relaxation times. This important finding explains why the experimental dielectric JG beta-relaxation times tau(beta)(T,P) is approximately equal to tau(0)(T,P) as found in many glass formers at various temperature T and pressure P. The result, tau(beta)(T,P) approximate to tau(0)(T,P), in turn explains why the ratio tau(alpha)(T,P)/tau(beta)(T,P) is invariant to changes of T and pressure P at constant tau(alpha)(T,P), the alpha-relaxation time.

Automated summary

This study focuses on the research status and techniques related to JG beta relaxation in glass-forming materials. By combining TDI technique with dielectric and neutron scattering data analysis, it reveals that the JG beta relaxation is heterogeneous and comprises processes with different length scales, with processes of longer length scale having longer relaxation time. The study also finds that the primitive relaxation time of the coupling model falls within the distribution of the TDI q-dependent JG beta-relaxation times, which explains the experimental observations of the relationship between tau(beta) and tau(0) in various glass formers.