Floppy modes in crystalline and amorphous silicates

We present the results of comparative inelastic neutron scattering measurements of both amorphous and crystalline silicates. We demonstrate the presence of floppy modes in amorphous silica and potassium disilicate over the energy scale 0-5 meV. A peak is also observed at 5 meV in amorphous silica, which is traditionally associated with the boson peak. By comparison with the spectrum of crystalline alpha-cristobalite, we show that it, in fact, arises from the transverse acoustic modes. The same peak in amorphous potassium disilicate cannot be resolved because it is obscured by a large number of floppy modes.