Electron and phonon states in an ideal continuous random network model of a-SiO2 glass

A previously constructed large continuous random network model of amorphous SiO2 (a-SiO2) glass with 1296 atoms and periodic boundary condition has been relaxed with four different sets of pair potentials under constant pressure. By removing the restriction of the cubic symmetry in the original model, the bond-length and bond-angle distortions can be further reduced, resulting in an ideal fully coordinated random network model for a-SiO2. On the basis of the calculated vibrational states, it is concluded that the pair potential of Tsuneyuki et al. [Phys. Rev. Lett. 61, 869, 1988] gives the most realistic results for low-energy excitations. Specific-heat calculations using normal modes of vibration show good agreement with experimental data down to 8 K. Analysis of eigenvector components of the lowest nonzero vibrational mode shows that the low-energy atomic vibration involves the floppy movements of more than ten atoms in a chainlike structure. Thus the low-energy vibration in a-SiO2 is not associated with localized movement of a few atoms. Implications of this finding for the two-level tunneling model in glasses are then discussed. Using this ideal model structure, a first-principles calculation of electronic and optical properties of a-SiO2 glass is carried out. The reduction in the bond-length and bond-angle distortions leads to a more uniform distribution of effective charges and a slight reduction in the band gap. It is shown that such a topologically disordered network has sharp valence-band edges in the electronic density of states. The mobility edge at the top of the valence band is estimated to be only 0.06 eV, and there is no evidence for any localization for states at the conduction-band edge. The calculated dielectric function for a-SiO2 is in reasonable agreement with experimental measurements. copyright 1999 The American Physical Society