Boson Peak Shifts Research Articles

Critical point of jamming transition in two-dimensional monodisperse systems.

The existence of amorphous packings in two-dimensional monodisperse system is a classical unsolved problem. We get the energy minimum state by the energy minimization method of enthalpy under constant pressure conditions. Firstly, we find that there are two peaks in the experiment, which demonstrate the interesting features of the coexistence of crystals and amorphous crystals. And then, we confirm the critical point of jamming transition of the two-dimensional monodisperse is [Formula: see text]. Finally, we prove that the jamming scaling is still satisfied in two-dimensional monodispersed system: [Formula: see text] and vanishes as [Formula: see text], and the boson peak shifts to lower frequencies for less compressed systems.

Boson Peak in Amorphous Graphene in the Stable Random Matrix Model

The effect of disorder in the distribution of force constants on optical and acoustic phonons in the scalar model of crystalline graphene is studied for both oscillations lying in the sheet plane and for flexural modes. It was shown that in the model of stable random matrices with translational symmetry, an additional to Debye vibrational density of states arises at a sufficient degree of disorder, i.e., the boson peak. The boson peak shifts to lower frequencies with increasing relative fluctuations of force constants and decreasing Young’s modulus of the system. At a weak disorder (or with no disorder), there are two peaks in the density of states g(ω), which correspond to logarithmic van-Hove singularity for acoustic and optical phonons of crystalline graphene. These peaks broaden and merge into a single boson peak with increasing disorder. Optical phonons are first destroyed due to disorder, while acoustic phonons gradually transform to the boson peak. For flexural modes there is a slightly different situation. Van-Hove singularities still spread disorder, but lead to the appearance of phonons in the system, which form the boson peak and move with it to low frequencies with increasing disorder.

Boroxol ring modification driven by plastic deformations of compacted B2O3 glasses

Structural changes of glassy B2O3, compacted under GPa pressures at temperatures below the glass transition, have been monitored by Raman spectroscopy, disclosing a progressively decreasing intensity of both the Boson peak and the main band at 808 cm−1 with increasing densification. Both these components are determined by vibrational activity of boroxol rings (B3O6), the glassy super-structural units formed by connected BO3 planar triangles. It is suggested that growing density by glass compaction (up to 13%) causes severe deformation of boroxol rings, determining a decrease of structural units which preserve unaltered their vibrational activity. The density of compacted glasses, annealed at ambient temperature and pressure, is time-dependent and decreases to a lower constant value, but still higher than that of normal vitreous B2O3, evidencing a permanent plastic deformation. The final structural configuration is achieved over a period of few months during which the local strain is partially relaxed allowing some boroxol groups to recover their original shape and vibrational modes. Growing glass densification gives rise to a substantial hardening of the network with a well-defined linear increase of both the bulk and rigidity moduli, while leaving nearly constant the Poisson's ratio. Also the frequency νBP of the Boson peak shifts from 26 cm−1 in v-B2O3 up to 37 cm−1 in the glass having the highest density (2069 kg/m3), this increase being stronger than that expected from the simple hardening of elastic continuum. The observed effects are consistent with the gradual vibrational collapse of boroxol rings driven by densification and emphasize the basic role of these glassy units in determining the softening degree and the low-energy vibrational dynamics of v-B2O3.

High-Energy Shift of Boson Peak and Its Broadening in Crystallization Regime: A Qualitative Aspect

Inelastic light scattering (i.e., Brillouin and Raman) can provide information about chemical bonding and coordination states, making it an efficient technique for studying amorphous matter in supercooled liquid (SCL) and melt regimes. Several studies have reported that, during crystallization (which occurs between these regimes), binary system glasses exhibit anomalous elastic behavior [i.e., simultaneous occurrence of increases in sound velocity (or elasticity), which is estimated by Brillouin spectra] and significant damping. Such behavior has also been reported in the low-energy vibration mode ( 1THz) of Raman scattering (known as the Boson peak, BP) during transition from SCL to crystals (SCL–crystal transition). Although the anomalies appear to be related to the crystallization phenomenon, in-depth analysis has not been conducted yet. Therefore, in this study, in order to obtain additional information concerning the anomaly in the crystallization regime, we prepared an appropriate sample for the in situ observation of the BP and examined the behavior of the BP under nonisothermal and isothermal heating conditions. For observing crystallization dynamics through inelastic light scattering, glass samples that show a strong tendency of homogeneous nucleation are preferable because the samples have a large interface, at which the SCL–crystal transition occurs. Since a BaSi2O5 glass possesses high nucleation ability and TiO2 is known to be an effective nucleation reagent in a silicate glass system, we prepared the glasses in which SiO2 was substituted by TiO2, i.e., BaTixSi2 xO5 (x 1⁄4 0{0:25), using a melt-quenching technique (melting condition: 1773K for 1 h). The as-quenched samples were annealed at each glass-transition temperature (Tg) [Fig. 1(b)] for 20 h to obtain nucleated samples. This was done because for glass with a homogeneous nucleation trend, the temperature at which the maximum nucleation rate is evident is close to Tg. 9) The samples were characterized by differential thermal analysis (DTA), transmission electron microscopy (TEM), and in situ inelastic light scattering observation. Detailed conditions for these analyses are described elsewhere. The spectra that were obtained in the low-energy region were reduced with respect to the Bose–Einstein factor and were fitted using the sum of the BP and the Gaussian function. This was done because the vibrational band, which results from the Ba–O bond, contributes to the spectra. The observed BP was analyzed using a log-normal function to evaluate the position of its maximum !BP and its full width at half maximum (FWHM). Figure 1 shows the results of thermal analysis in the studied samples. Here the samples with x 1⁄4 0:125 are representative of all the samples. In the as-quenched samples, their Tg increased with the value of x. The annealed samples at each Tg tended to show lower crystallization-peak temperatures (Tp) than the as-quenched samples [Figs. 1(a) and 1(b)]. In particular, the difference in Tp between the as-quenched and annealed samples (i.e., Tp) indicated the maximum value at x 1⁄4 0:125 (BaTi0:125Si1:875O5; Tp 1⁄4 73 K). This value was much larger than that of the base glass (BaSi2O5; Tp 1⁄4 40K). According to Marotta et al., oxide glass showing homogeneous nucleation and subjected to thermal treatment reveals a shift of Tp to a temperature that is lower than that of the nontreated glass because of the evolution of an excess of internal nuclei. These results suggest that the BaTi0:125Si1:875O5 glass possesses high nucleation ability. In addition, the annealed sample showed no structural development, and only the halo pattern was observed in the corresponding electron diffraction pattern [Fig. 1(c)]. We, therefore, selected the BaTi0:125Si1:875O5 glass as a test sample for the in situ observation. Figure 2 shows the result of the in situ measurements of the inelastic light scattering spectra of the test sample. We observed a BP around 1–4 THz, which is attributed to an excess of the vibrational density of states because of the nanometric heterogeneity/cohesive nanodomain. From the fitting, we estimated !BP to be 1:3THz at room temperature [Fig. 2(a)]. During heating (nonisothermal; 20K/min), the spectra showed no significant change up to 1273K. When the temperature was further increased, the BP revealed progressive broadening (or overdamping) in the isothermal stage (0–21min at 1273K; T=Tg 1:28). Eventually, a group of small peaks (closed circles) (as opposed to the BP) began to appear, indicating full crystallization of the test sample. In terms of the reduced temperature (T=Tg) dependence of !BP [Fig. 2(b)], !BP 0 0.1 0.2 970 980 99

Boson peak in alkaline borate glasses: Raman spectroscopy, neutron scattering, and specific-heat measurements

The dependence of the boson peak on the alkaline ion in modified borate glasses ${({M}_{2}\mathrm{O})}_{0.14}{({\mathrm{B}}_{2}{\mathrm{O}}_{3})}_{0.86}$ $({M}^{+}={\mathrm{Li}}^{+},{\mathrm{Na}}^{+},{\mathrm{K}}^{+},{\mathrm{Cs}}^{+})$ was analyzed by performing Raman spectroscopy, inelastic neutron scattering, and low-temperature specific-heat measurements. It is found that the distribution of vibrations merging into the boson peak shifts to higher frequency by going from cesium to lithium. A linear correlation between the boson peak frequency and the transverse sound velocity is evidenced. The dependence on the polarizing power of the metallic cation is analyzed, stemming from considerations about elastic moduli. These findings suggest a mainly transverse character of the excess vibrational modes in glasses.

Hydration Affects Both Harmonic and Anharmonic Nature of Protein Dynamics

To understand the effect of hydration on protein dynamics, inelastic neutron-scattering experiments were performed on staphylococcal nuclease samples at differing hydration levels: dehydrated, partially hydrated, and hydrated. At cryogenic temperatures, hydration affected the collective motions with energies lower than 5meV, whereas the high-energy localized motions were independent of hydration. The prominent change was a shift of boson peak toward higher energy by hydration, suggesting a hardening of harmonic potential at local minima on the energy landscape. The 240K transition was observed only for the hydrated protein. Significant quasielastic scattering at 300K was observed only for the hydrated sample, indicating that the origin of the transition is the motion activated by hydration water. The neutron-scattering profile of the partially hydrated sample was quite similar to that of the hydrated sample at 100K and 200K, whereas it was close to the dehydrated sample at 300K, indicating that partial hydration is sufficient to affect the harmonic nature of protein dynamics, and that there is a threshold hydration level to activate anharmonic motions. Thus, hydration water controls both harmonic and anharmonic protein dynamics by differing means.

Inelastic light scattering in B2O3 glasses with different thermal histories

This contribution presents a systematic light scattering study of a series of boron oxide glasses which are characterized by different thermal histories. The thermal treatment was obtained by annealing the samples close to the glass transition temperature for times of several hours. Both low-frequency (0.1–30 cm−1) and high-frequency (5–1600 cm−1) spectra were monitored by using a tandem Fabry–Perot interferometer and a Raman spectrometer, respectively. The low-frequency spectra include quasielastic contributions and the boson peak. It was found that different thermal histories lead to pronounced changes in the low-frequency spectrum. The position of the boson peak shifts to higher frequencies and the magnitude of the quasielastic contribution decreases as a function of annealing time. Both quantities correlate linearly with the density of the samples (ρ=1.804–1.866 g/cm3). On the other hand, the high-frequency modes do not show discernible changes. In particular, no alteration of the modes which correspond to the boroxol ring is found, indicating that the fraction of boroxol rings is constant within 2% accuracy. Taking the boson peak as a manifestation of medium-range order, we conclude that annealing the glass influences the intermediate-range order rather than the short-range order.

Densification effects on the Boson peak in vitreous silica: A molecular-dynamics study

We perform classical molecular-dynamics simulation to study the effect of densification on the vibrational spectrum of a model silica glass. We concentrate this study on the so-called Boson peak and compare our results, obtained from a direct diagonalization of the dynamical matrix, with experimental Raman data. We show that, upon densification, the position of the Boson peak shifts towards higher frequencies while its magnitude decreases which is in agreement with a recent experimental study.

A Molecular Dynamics Study of the Vibrational Spectra of Silica Polyamorphs

The vibrational spectra (inelastic neutron scattering, infrared absorption and Raman scattering) of fused silica in the amorphous and in the liquid state are investigated by molecular dynamics simulation in using the TTAM interionic potential (Tsuneyuki et al., Phys. Rev. Lett., 61, 869 (1988)). The simulation predicts in the low frequency region a boson peak, quite similar to the one observed experimentally in different spectroscopies. This mode is attributed to concerted motions between SiO4 tetrahedra and involves the medium-range order in the glass. Upon densification the boson peak shifts to higher frequencies and is less pronounced. Correlatively, the evolution of the density of states with the temperature is found to be strongly dependent on the glass coordination, a finding which suggests an increasing fragility of silica glass with the pressure as recently proposed in the literature. Finally the inability of the TTAM potential to reproduce in details the density of states of silica glass is pointed out.