  We investigate the behavior of amorphous silicon under hydrostatic
compression using molecular simulations. During compression, amorphous silicon
undergoes a discontinuous nonequilibrium transition from a low-density to a
high-density structure at a pressure of around $13$-$16$~GPa. Ensemble-averaged
density and elastic constants change discontinuously across the transition.
Densification of individual glassy samples occurs through a series of discrete
plastic events, each of which is accompanied by a vanishing shear modulus. This
is the signature of a series of elastic instabilities, similar to shear
transformation zones observed during shear yielding of glasses. We compare the
structure obtained during compression with a near-equilibrium form of amorphous
silicon obtained by quenching a melt at constant pressure. This gives
structures identical to nonequilibrium compression at low and high pressure,
but the transition between them occurs gradually rather than discontinuously.
Our observations indicate that the polyamorphic transition is of a
nonequilibrium nature, and it has the characteristics of a yield transition
that occurs under compression instead of shear.
