Modeling of rigidity percolation and incipient plasticity in germanium-selenium glasses

We compute the bulk and surface structures of glasses in the germanium-selenium (Ge-Se) system using Monte Carlo simulations and our previously derived set of ab initio potentials. We investigate the elastic response of the Ge-Se glasses under a flat "micro"-indentation and incipient plasticity under a spherical nanoindentation. The glasses with a high average coordination number (< m >> 2.4) display structural frustration owing to an excess of bond constraints, leading to permanent densification from both types of indentations. The glasses with a low average coordination number (< m >< 2.4) exhibit a large number of floppy modes, enabling continuous shear flow. According to the Phillips theory of topological constraints, the ideal glass former is one in which the number of constraints exactly equals the number of degrees of freedom (GeSe4, where < m >=2.4). In both types of indentation simulations, we find that the GeSe4 glass structure is most resistant to distortions of its basic structural unit.