Structural origin of the anisotropic local mechanical response of amorphous silicon

Deciphering atomic packing around ``defects'' in glasses remains a grand challenge in the glass community. Amorphous silicon (a-Si), as a prototypical network-forming glass, often exhibits opposite structure-property relations compared to metallic glasses (MGs), and the classic concept of free volume widely used for MGs is also invalid to a-Si. In this work, we surprisingly discovered that a-Si and MGs share the same structural origin responsible for their orientation-dependent local mechanical response. In addition, the favorite motif in a-Si, i.e., tetrahedra, centered on the atoms with the lowest plastic susceptibility have two dominant preferred orientations. This is in contrast to the case in MGs. These results will enhance our understanding of structure-property relationships in glasses and also provide some optimism for establishing a universal picture regarding ``defects'' for the two classes of glasses.