Communication: Observation of ultra-slow relaxation in supercooled selenium and related glass-forming liquids

The rheological behavior of supercooled Se, As₁₀Se₉₀, and As₂₀Se₈₀ liquids is studied in the linear regime as a function of frequency, extending over nearly 11 orders of magnitude, using oscillatory parallel plate rheometry. While the viscoelastic response of the As₂₀Se₈₀ liquid is characterized by a single relaxation time scale, the Se and the As₁₀Se₉₀ liquids display two distinct relaxation processes, both of which are coupled to viscosity, although their time scales differ by nearly 3-5 orders of magnitude. The ultra-slow relaxation process appears to be related to the dynamics of -Se-Se-Se- chain segments in the structure of these liquids, with characteristic time scale and shear modulus that are dependent on the average chain length. The fast mode, on the other hand, is associated with the glassy modulus and is tentatively assigned to a Johari-Goldstein β-process. These results, when taken together, are consistent with the presence of a hierarchical free energy landscape that characterizes the dynamics of the fragile Se and As₁₀Se₉₀ liquids.