Bonding origin of optical contrast in phase-change memory materials

The large optical contrast between crystalline and amorphous phases of phase change memory materials is shown to arise from a large difference in the optical matrix elements. These are enhanced in the crystal by aligned rows of resonantly bonded $p$ orbitals. Amorphous phases have normal-sized matrix elements due to an absence of this order, irrespective of coordination number. This is a more general description of local order differences between the crystalline and amorphous phases, which applies even when coordinations in the amorphous phases exceed the $8\ensuremath{-}\text{N}$ value.