Figure 4. Theoretical versus experimental band diagrams.

(a) Reciprocal fcc lattice showing a [112] plane (orange hexagon) and the ΓM direction probed in the experiment. (b) Schematic presentation of PBC and IBC models applied at the SiO₂-PS interface. In the IBC model, the springs of respective stiffness k_L,T represent the discontinuity of the displacement field across this interface. The theoretical band diagram of the DP400 is shown for PBC along [111] with the bulk PS sound velocities in c and for IBC with PS velocities (c_L=2,560 m s⁻¹, c_T=1,320 m s⁻¹) about 9% higher than bulk PS along [111] (d) and [112] (e). Along ΓL dark/light solid and dotted blue lines denote non-degenerate (longitudinal), double degenerate (transverse) and deaf computed bands, respectively. In e, all bands are non-degenerate of mixed character; the flat band is highlighted (dotted line). Solid and open circles indicate the experimental points (see Fig. 3). Hatched regions denote hybridization gaps (LHG for longitudinal modes; HG for all modes).