High-frequency dynamics of vitreousGeSe2

Abstract

The vibrational dynamics of vitreous germanium diselenide ͑v-GeSe 2 ͒, an amorphous glass belonging to the family of continuous random network forming glasses, has been investigated by means of inelastic neutron scattering. We employed three different neutron spectrometers, each of them providing different energy resolutions and spanning different portions of the energy-wave-vector ͑ប , Q͒ plane, which partially overlap so that complementary information can be obtained. Two well-defined peaks are resolved in the dynamic structure factor in a broad Q region, below and above the position of the first sharp diffraction peak in the static structure factor. The low-lying excitation is nondispersive and located around the boson peak frequency. The highfrequency mode presents a pseudoperiodic behavior and it can be associated to the high-frequency counterpart of the longitudinal acoustic ͑LA͒ mode. In the low-Q region this mode shows a positive dispersion of its apparent sound velocity and an attenuation which follows a Q 2 law. The combined use of a set of appropriate instruments allows an accurate determination of the nonergodicity parameter f͑Q͒. The LA dispersion curve of v-GeSe 2 is similar to that measured in other glasses where a second excitation appears in the spectra. On the contrary other glass forming materials, where this second mode is not detected, present a sinusoidal-like dispersion curve with a deeper minimum in the second pseudo-Brillouin zone.

Neutron scattering investigation of high-frequency dynamics in glassy glucose

“…Nonetheless, the H mode dispersion curve shows a secondary minimum which seems to be related to the first diffraction peak in a position where also the damping has a visible decrease. The pseudoperiodic behavior confirms the previous suggestion that the wave vectors corresponding to the first diffraction peaks act as pseudoreciprocal lattice 6 vectors for the glass, thus defining pseudo-Brillouin zones. The evidences from this and previous works suggest the existence of a medium-range order in topologically disordered systems, which has some similarity to the local order present in the crystalline phase within the unit cell.…”

“…These findings parallel the results already obtained in strong glasses. 6 Here, in the investigated Q range, the low-frequency mode shows a rather flat dispersion curve, and it is located in the energy range of the boson peak. As seen in all other glasses, the high-frequency mode shows typical features of an acoustic mode and a rather well defined dispersion curve is obtained in a wide Q interval.…”

“…3) and GeSe 2 (Ref. 6), and it shows an almost constant frequency. Nonetheless the energy position of the Ioffe-Regel limit of the L mode is about 5 meV, a value similar to the position of the boson peak as in the case of the simulation, 27 whereas for the H mode it is about 11 meV, much higher than the position of the boson peak.…”

“…The dispersion curve shows a linear slope at small wave vectors, which results in a propagation velocity usually slightly higher than the low frequency (say 1 MHz) sound velocity. In addition, a quasiperiodic trend of dispersion curves is often observed, 5,6 which recalls the phonon dispersion beyond the first Brillouin zone in crystals. As to the behavior of the peak broadening on varying the momentum transfer, a Q 4 dependence below 10 nm −1 is observed 7 for both random-network and molecular glasses, while a Q 2 dependence of the line width is observed in the nm −1 range.…”

“…3), 3-methylpentane, 4 ethanol, 5 and GeSe 2 (Ref. 6)] enhance the presence of rather broad peaks in the total dynamic structure factor S(Q,ω) when it is measured at constant momentum transfer Q as a function of the energy transferh ω. The energy of these peaks follows a dispersion curve which is similar to that observed for the phonons in crystals when Q is smaller than the position Q p of the first sharp diffraction peak (FSDP) of the static structure factor S(Q).…”