Disentangling structural and kinetic components of the $α$-relaxation in supercooled metallic liquids

The particle motion associated to the $α$-relaxation in supercooled liquids is still challenging scientists due to its difficulty to be probed experimentally. By combining synchrotron techniques, we report the existence of microscopic structure-dynamics relationships in Pt$_{42.5}$Cu$_{27}$Ni$_{9.5}$P$_{21}$ and Pd$_{42.5}$Cu$_{27}$Ni$_{9.5}$P$_{21}$ liquids which allows us to disentangle structural and kinetic contributions to the $α$-process. While the two alloys show similar kinetic fragilities, their structural fragilities differ and correlate with the temperature dependence of the stretching parameter describing the decay of the density fluctuations. This implies that the evolution of dynamical heterogeneities in supercooled alloys is determined by the rigidity of the melt structure. We find also that the atomic motion not only reflects the topological order but also the chemical short-range order, which can lead to a surprising slowdown of the $α$-process at the mesoscopic length scale. These results will contribute to the comprehension of the glass transition, which is still missing.