The structural relaxation dynamics in liquid and 20 K undercooled gallium has been studied by neutron spectroscopy. The intermediate scattering function at the structure factor maximum was measured over a wide temperature range up to twice the melting temperature. A stretched exponential decay was observed near the melting point which transforms to an exponential decay above $1.2{\text{T}}_{\text{melting}}$. The structural relaxation process beyond 3 ps can be described as a further slow exponential decay. Mode coupling theory is able to calculate the decay rate of this slow structural relaxation, which agrees well with the measurement. This additional structural relaxation, observed in the collective particle motions on next-neighbor distances, can be understood as a slow cage relaxation process, setting in around 370 K and indicating the slowing down of dynamics towards solidification. This process might be the microscopic origin for observed changes in the collective particle dynamics of liquid gallium with decreasing temperature. Liquid aluminium, lead, and rubidium demonstrate a similar behavior, which suggests a universal crossover in dynamics of the equilibrium liquid metal state.

• Received 17 October 2019
• Revised 13 January 2020

DOI:https://doi.org/10.1103/PhysRevB.101.014207