Direct observation of photoinduced ultrafast melting of metallic glass nanoparticles using XFEL single-pulse imaging*

The exploration of ultrafast light-matter interactions has invigorated research on light-induced quantum control of material properties by opening new avenues for producing new phases of matter in nonequilibrium states. In this study, we investigate ultrafast melting of metallic glass materials using time-resolved single pulse imaging. The metallic glass exhibits an amorphous structure without long range atomic orders. The absence of crystalline order gives rise to distinctive melting reactions. Its melting process is expected to be different from the crystalline metal as the absence of phonons. However, the fundamental mechanisms underlying the rearrangement of the metallic glass structure remain elusive. By conducting the single-pulse time-resolved imaging experiments performed at PAL-XFEL, we obtained direct nanoscale images of single metallic glass nanoparticles (MG-NP) undergoing nonequilibrium melting excited by femtosecond IR laser (800 nm). Upon laser illumination, anisotropic shape deformation of the MG-NP occurred promptly (within 1 ps). The NP deforms, accompanied by an inhomogeneous distribution of the mass density, more severely as the melting proceeds. Inhomogeneity in the density distribution becomes more apparent. It displays highly inhomogeneous distribution with low-density regions localized to form bands, which prevents further development of shape deformation. We present the nanoscale imaging results that visualize ultrafast melting in metallic glasses, which promote in-depth understanding of structural deformation accompanied during the melting (glass) transition.