Fast dynamics in a model metallic glass-forming material

We investigate the fast beta- and Johari-Goldstein (JG) beta -relaxation processes, along with the elastic scattering response of glass-forming (GF) liquids and the boson peak, in a simulated Al-Sm GF material exhibiting a fragile-strong (FS) transition. These dynamical processes are universal in ordinary GF fluids and collectively describe their fast dynamics, and we find these relaxation processes also arise in a GF liquid exhibiting a FS transition. String-like particle motion, having both an irreversible and a reversible nature (stringlets) component, occurs in the fast-dynamics regime, corresponding to a ps timescale. String-like collective motion associated with localized unstable modes facilitates irreversible and intermittent particle jumping events at long times associated with the JG beta -relaxation process, while stringlets associated with localized stable modes and corresponding perfectly reversible atomic motion give rise to the boson peak. To further clarify the origin of the boson peak, we calculate the density of states for both the stringlet particles and the normal particles and find that the stringlet particles give rise to a boson peak, while the normal atoms do not. The growth of stringlets upon heating ultimately also leads to the softening of these excitations, and the boson peak frequency and shear modulus drop in concert with this softening. The growth of string-like collective motion upon heating in the fast-dynamics regime is further shown to be responsible for the growth in the intensity of the fast relaxation process. Relaxation in cooled liquids clearly involves a hierarchy of relaxation processes acting on rather different timescales and spatial scales.

Automated summary

The study investigates the fast beta- and Johari-Goldstein relaxation processes, along with the elastic scattering response of glass-forming liquids and the boson peak. These processes are found to be universal, even in glass-forming liquids with a fragile-strong transition. Heating leads to the growth of stringlets, resulting in an increase in relaxation process intensity and excitation softening.