Reversal of crystallization in cryoprotected samples by laser editing

Advances in cryobiology techniques commonly target either the cooling or the warming cycle, while little thought has been given to ≪repair≫ protocols applicable during cold storage. In particular, crystallization is the dominant threat to cryopreserved samples but proceeds from small nuclei that are innocuous if further growth is forestalled. To this end, we propose a laser editing technique that locally heats individual crystals above their melting point by a focused nanosecond pulse, followed by amorphization during rapid resolidification. As a reference, we first apply the approach to ice crystals in cryoprotected solution and use Raman confocal mapping to study the deactivation of crystalline order. Then, we examine dimethyl sulfoxide trihydrate crystals that can germinate at low temperatures in maximally freeze concentrated regions, as commonly produced by equilibrium cooling protocols. We show how to uniquely identify this phase from Raman spectra and evidence retarded growth of laser-edited crystals during warming.

Raman scattering spectroscopy of MBE grown thin film topological insulator Bi2-xSbxTe3-ySey

BSTS epitaxial thin film topological insulators were grown using the MBE technique on two different types of substrates i.e., Si (111) and SiC/graphene with Bi0.7Sb1.6Te1.8Se0.9 and Bi0.9Sb1.5Te1.8Se1.1, respectively. The crystallographic properties of BSTS films were investigated via X-ray diffraction, which showed the strongest reflections from the (0 0 l) facets corresponding to the rhombohedral phase. Superior epitaxial growth, homogeneous thickness, smooth surfaces, and larger unit cell parameters were observed for the films grown on the Si substrate. Polarization dependent Raman spectroscopy showed a weak appearance of the Ag mode in cross--polarized geometry. In contrast, a strong Eg mode was observed in both parallel and cross-polarized geometries which correspond to the rhombohedral crystal symmetry of BSTS films. A redshift of Ag and Eg modes was observed in the Raman spectra of BSTS films grown on the Si substrate, compared to those on SiC/graphene, which was directly associated with the unit cell parameter and composition of the films. Raman spectra showed four fundamental modes with asymmetric line shape, and deconvolution of the peaks resulted in additional modes in both the BSTS thin films. The sum of relative ratios of linewidths of fundamental modes (Ag and Eg) of BSTS films grown on Si substrate was lower, indicating a more ordered structure with lower contribution of defects as compared to BSTS film grown on SiC/graphene substrate.

Irreversible lipid phase transition detected in a porcine oocyte at chilling

Under physiological conditions, the membranes and lipid droplets of germ cells are in a conformationally disordered phase. Typically, during cooling, lipids undergo the transition to ordered phases and, upon heating, melt into a disordered phase. In this communication, we report the lipid phase transition in lipid droplets observed in porcine oocytes. Upon cooling, a sharp lipid phase transition from conformationally disordered to ordered state was detected within the temperature range between 20 and 15 °C. Subsequent heating to 45 °C does not return lipids to their original phase state. To the best of our knowledge, this is the first observation of an irreversible phase transition in lipid droplets of biological cells with native lipid composition.

In-plane and out-of-plane gigahertz sound velocities of saturated and unsaturated phospholipid bilayers from cryogenic to room temperatures

Here, we examined the gigahertz sound velocities of hydrated multibilayers of saturated (1,2-dimyristoyl-sn-glycero-3-phosphocholine, DMPC) and unsaturated (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC) phospholipids by Brillouin spectroscopy. Out-of-plane and in-plane (lateral) phonons were studied independently of each other. Similar strong temperature dependences of the sound velocities were found for phonons of both types. The sound velocities in the low-temperature limit were two-fold higher than that at physiological temperatures; a significant part of the changes in sound velocity occurs in the solid-like gel phase. The factors that may be involved in the peculiar behavior of sound velocity include changes in the chain conformational state, relaxation susceptibility, changes in the elastic modulus at infinite frequencies, and lateral packing of molecules.

Characterization of conformational states of POPC and DPPCd62 in POPC/DPPCd62/cholesterol mixtures using Raman spectroscopy

Conformational states of phospholipid chains in ternary mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), deuterated 1,2-dipalmitoyl-d62-sn-glycero-3-phosphocholine (DPPCd62), and cholesterol (Chol) were studied by Raman spectroscopy. Parameters of Raman peaks sensitive to conformational order have been used to determine chain order for mixtures over a wide range of compositions. A ternary diagram of fractions of phospholipid chains in conformationally ordered and disordered states has been constructed. It was found that the addition of POPC and cholesterol increases the fraction of DPPC chains in disordered conformations. The so-called liquid-ordered phase includes DPPC molecules in both ordered and disordered states in comparable proportions. It was found that POPC chains are partially ordered in mixtures with DPPC and cholesterol, in contrast to the case of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). This maybe the underlying reason why ternary mixtures with POPC have different miscibility behavior compared to DOPC.

Gigahertz elastic modulus and OH stretching frequency correlate with Jones-Dole's B-coefficient in aqueous solutions of the Hofmeister series

The ability of salts to change the macroscopic viscosity of their aqueous solutions is described by the Jones-Dole equation with B-coefficient for the linear concentration term. The sign and value of this coefficient are often considered as a measure of the salt's structure-making/breaking ability, while the validity of this assignment is still under discussion. Here, by applying Raman and Brillouin scattering spectroscopy to various salts from the Hofmeister series, we studied a possible relation between macroscopic Jones-Dole's B-coefficient and the microscopic dynamic response. Raman spectroscopy provides information about molecular vibrations and Brillouin spectroscopy about acoustic phonons with wavelengths of hundreds of nanometers. It has been found that Jones-Dole's B-coefficient correlates linearly with the coefficients, describing the concentration dependences of the average OH stretching frequency, real and imaginary parts of gigahertz elastic modulus. These relationships have been interpreted to mean that the OH stretching frequency is a measure of the ion-induced changes in the water network that cause changes in both viscosity and gigahertz relaxation. Depolarized inelastic light scattering revealed that the addition of structure-making ions not only changes the frequency of the relaxation peak but also increases the low-frequency part of the relaxation susceptibility. It was shown that the ion-induced increase in the gigahertz elastic modulus can be described by changes in the relaxational susceptibility without a noticeable change in the instantaneous elastic modulus. The isotropic Raman contribution associated with the tetrahedral-like environment of H₂O molecule does not correlate with Jones-Dole's B-coefficient, suggesting a minor influence of these tetrahedral-like configurations on viscosity.

Cryopreservation increases accumulation of exogenous stearic acid in mouse embryos

Cryopreservation of preimplantation embryos is a widely used technique, but this procedure might impact the subsequent embryo development. The effect of slow freezing and vitrification on the lipid metabolism in preimplantation mammalian embryos is not well studied. In this work, we applied Raman spectroscopy of isotopically labeled molecules to address the effects of cryopreservation on fatty acid accumulation in mouse embryos. Embryos after slow freezing or vitrification were cultured for 20 h in a medium supplemented with bovine serum albumin saturated with deuterated stearic acid (dSA). After this period the concentration of dSA estimated from Raman spectra of frozen-thawed and vitrified-warmed embryos at the morula stage was almost twice higher compared to non-cryopreserved morulas. At the same time, frozen-thawed and vitrified-warmed 4-cell embryos did not demonstrate any difference in the level of stearic acid uptake from non-cryopreserved embryos of the same stage. After an additional 24 h culture, cryopreserved and non-cryopreserved embryos demonstrated similar dSA uptake.

Conformational state diagram of DOPC/DPPCd62/cholesterol mixtures

Raman spectra of aqueous suspensions of vesicles composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), deuterated 1,2-dipalmitoyl-d62-sn-glycero-3-phosphocholine (DPPC_d62), and cholesterol (Chol) were studied at room temperature to determine the conformational states of the phospholipid hydrocarbon chains. Deuteration of DPPC_d62 allowed us to characterize the conformational states of DOPC and DPPC_d62 independently. The parameters of Raman peaks, which are sensitive to the conformational order, were studied in a wide range of compositions. It was found that the DOPC molecules are conformationally disordered for all compositions. The conformational state of the DPPC_d62 molecules changes with composition. Their conformational state is influenced by cholesterol-induced partial disordering and DOPC solvation, transforming the DPPC molecules into the disordered state. The conformational state diagram from the Raman experiment was compared with outcomes from the differential scanning calorimetry (DSC) experiment. The Raman spectra also revealed that the DPPC molecules coexist in the disordered and all-trans ordered states for the DOPC/DPPC_d62/Chol mixtures except for the pure liquid-disordered phase.

Raman scattering in protonated and deuterated 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC): Indicators of conformational and lateral orders

The use of deuterocarbons is an effective method in the Raman spectroscopy of multicomponent lipid materials and biological samples. Here, Raman spectra of hydrated multilamellar vesicles of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), its deuterated analog 1,2-dipalmitoyl-d62-sn-glycero-3-phosphocholine (DPPC_d62), and DPPC-DPPC_d62 mixtures were studied in a wide temperature range to specify the Raman indicators of conformational and lateral orders. The temperature dependence of the 985 cm^-1 line in the deuterated phospholipid unequivocally indicates that this line corresponds to the CC stretching vibrations of deuterated hydrocarbon chains in the all-trans conformation. It was also concluded that the ratio of Raman intensities at the maximum of the peak of the symmetric CD₂ stretching and at a maximum near 2168 cm^-1 reflects the conformational order of the hydrocarbon chain and can be used for an evaluation of the fraction of the all-trans sequences. The frequency of the symmetric CD₂ stretching peak is sensitive to the phase state (gel or fluid) but has a low sensitivity to the partial conformational disordering within the gel phase. The Raman study of DPPC-DPPC_d62 mixtures reveals that the lateral order contributes to the ratio of intensities of the antisymmetric and symmetric CH₂ stretching peaks as a prefactor enhancing the effect of conformational ordering.

Dynamic response on a nanometer scale of binary phospholipid-cholesterol vesicles: Low-frequency Raman scattering insight

Low-frequency Raman spectroscopy was used to study the dynamic response on a nanometer scale of aqueous suspensions of two-component lipid vesicles. Binary mixtures of saturated phospholipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) and cholesterol are interesting for possible coexistence of solidlike and liquid-ordered phases, while the phase coexistence was not reported for unsaturated phospholipid (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC) and cholesterol mixtures. The DOPC-DPPC mixtures represent the well-documented case of coexisting domains of solidlike and liquid-disordered phases. These three series of lipid mixtures are studied here. A broad peak with the maximum in the range of 30-50cm^{-1} and a narrow peak near 10cm^{-1} are observed in the Raman susceptibility of the binary mixtures and attributed to the acousticlike vibrational density of states and layer modes, respectively. Parameters of the broad and narrow peaks are sensitive to lateral and conformational hydrocarbon chain ordering. It was also demonstrated that the low-frequency Raman susceptibility of multicomponent lipid bilayers allows one to determine the phase state of lipid bilayers and distinguish the homogeneous distribution of molecular complexes from coexisting domains with sizes above several nanometers. Thus, the low-frequency Raman spectroscopy provides unique information in studying phase coexistence in lipid bilayers.

Temperature dependence of elastic properties of the phospholipid vesicles in aqueous suspension probed by Brillouin spectroscopy

The aqueous suspension of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) vesicles with different hydration levelsα(water-to-lipid mass ratio) have been studied by Brillouin spectroscopy in the temperature range from -190 °C to 70 °C. The samples with different hydration levels demonstrate similar temperature behavior of their sound velocity in the temperature range from -190 °C to -25 °C. There is a strong correlation between the hydration level of the sample and the character of the sound velocity temperature dependence at higher temperatures. Nevertheless, all hydrated samples demonstrate a jump in the sound velocity at the gel-fluid phase transition temperature. The amplitude of this jump depends on the hydration levelαof the sample. It has a maximum value in the sample with minimalαnecessary for the phospholipid membrane's full hydration. To evaluate the sound velocity in the phospholipid membrane, we applied the two-component model to analyze the experimental data obtained in the sample withα= 0.25 (close to the minimal necessary value for the full DPPC membrane hydration). It was found that for temperatures higher than 0 °C, the two-component model works well if we consider that sound velocity in water between vesicle layers is approximately a factor of two higher than in bulk water.

Simulated Raman spectra of bulk and low-dimensional phosphorus allotropes

We present a comprehensive theoretical and experimental Raman spectroscopic comparative study of bulk Phosphorus allotropes (white, black, Hittorf's, fibrous) and their monolayer equivalents, demonstrating that the application of the Placzek approximation to density functional theory calculated frequencies allows reliable and accurate reproduction of the bulk spectra at a relatively low computational cost. As well as accurate frequencies, peak intensities are also reproduced with reasonable accuracy. Having established the viability of the method we apply it to other less well characterised phosphorus forms such as isolated P4 cages and the planar blue-phosphorus phase. There are several speculative structural models in the literature for amorphous red phosphorus, and we predict Raman spectra for several of these. Via comparison with experiment this allows us to eliminate many of them such as the P2P2-zigzag chain and connected P4 models. The combination of Density functional theory (DFT) modelling, Placzek approximation for intensities with experimental Raman spectroscopy is demonstrated as a powerful combination for accurate characterisation of phosphorus species.

Cu2ZnSnS4 crystal growth using an SnCl2 based flux

Single crystal growth of Cu₂ZnSnS₄ in an SnCl₂-based flux.

Effect of the Hydrocarbon Chain Disorder in Phosphatidylcholine Bilayers on Gigahertz Sound Velocity

Suspensions of multilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and hydrated and dehydrated aligned multilamellar samples of DMPC were studied by Brillouin spectroscopy in the temperature range from 90 to 333 K. The sound velocity of the longitudinal acoustic wave was evaluated from the Brillouin spectra. It was found that phase transition, hydration state, and planar or vesicular form of bilayers affect the gigahertz sound velocity. Usually, the temperature dependence of the sound velocity is weak in solid substances. Amazingly, the sound velocity of hydrated DMPC samples showed significant temperature-induced changes of up to 1.8 times, even within the solid-like gel phase. We explained this effect by temperature-induced excitations of the disordered conformational states of the hydrocarbon chains as well as anharmonic effects. In addition, the relevance of the gigahertz sound velocity to the description of subterahertz Raman features was demonstrated.

Second-order-derivative analysis of structural relaxation time in the elastic model of glass-forming liquids

Recently it was shown [V. N. Novikov and A. P. Sokolov, Phys. Rev. E 92, 062304 (2015)10.1103/PhysRevE.92.062304] that the second derivative with respect to inverse temperature of the structural relaxation time in some supercooled molecular liquids has a sharp maximum. It marks the point at which the apparent activation energy begins to saturate with decreasing temperature. The elastic model of glass-forming liquids expresses the temperature dependence of the structural relaxation time through that of the shear modulus. In this paper, we test whether this model is able to predict the maximum of the second derivative. We confirm its presence in the elastic model by analyzing the temperature dependence of the Brillouin light scattering in salol. This is a very subtle feature of the temperature dependence, which is greatly enhanced when taking derivatives. Its presence in the Brillouin data provides strong support to the elastic model of glass-forming liquids.

Raman spectroscopy evidence of lipid separation in domestic cat oocytes during freezing

Although lipid droplets are believed to play an important role in cryopreservation of mammalian embryos and oocytes, the effect of low temperatures on lipid droplets and related mechanisms of cryodamage are still obscure. Here, we provide Raman spectroscopy evidence of lipid separation inside the lipid droplets in domestic cat oocytes during slow freezing. It was shown that at -25 °C lipids coexist in two separated phase states inside lipid droplets. The scale of detected domains was a few micrometers size. We also found that under certain conditions these areas have a specific spatial distribution. Lipids with high melting temperatures are distributed near the surface of lipid droplets while fusible lipids are located deep inside. Raman spectroscopy was found to be a prospective approach to study inhomogeneity of lipid phase transition in cells and to reveal effects of this inhomogeneity on cryopreservation of biological cells.

Brillouin spectroscopy of biorelevant fluids in relation to viscosity and solute concentration

The measurement of intracellular viscoelastic properties by Brillouin scattering is a rapidly developing field in biophysics and medicine. Here, the Brillouin spectroscopy is applied for a number of aqueous solutions of biorelevant molecules to reveal relations between the Brillouin line parameters (frequency and width) and viscosity or solute concentration. It is found that for the majority of the studied biorelevant molecules the solute concentration governs the Brillouin frequency in a universal manner. On the other hand, the relations between the macroscopic viscosity and Brillouin peak parameters are different for different solutes. We conclude that for biological fluids the viscosity evaluation from Brillouin data needs prior knowledge about the chemical composition. This result challenges the fidelity of the indirect experimental determinations of the cellular viscosity, when small molecule solutions are used for the calibration.

Vibrational layer eigenmodes of binary phospholipid-cholesterol bilayers at low temperatures

Raman spectra in the low-frequency spectral range-between 5 and 90cm^{-1}-were studied for multilamellar bilayers prepared with cholesterol (Chol) and phospholipids of three different types: doubly unsaturated lipids 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), monounsaturated lipids 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and fully saturated lipids 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The narrow peak seen below 250 K and positioned between 9 and 18cm^{-1}-depending on the system and temperature-was attributed to the vibrational eigenmode of a lipid monolayer. For the DOPC-Chol bilayer, the peak position and the peak width were found to monotonically increase and decrease, respectively, with the Chol concentration. For POPC-Chol and DMPC-Chol bilayers, these parameters revealed nonmonotonic concentration dependences, with an apparent minimum at the intermediate Chol content. The peak intensity was ascribed to interleaflet coupling. As in the literature, a coexistence of liquid-ordered and solid-ordered domains was suggested for the DMPC-Chol and POPC-Chol bilayers; the Chol concentration dependences of Raman peak parameters were discussed in line with this suggestion, under the assumption that the different composition of coexisting domains conserves upon cooling. We demonstrated that the obtained Raman data disagree with the suggested domain coexistence if the domain sizes are substantially larger than the lipid layer thickness.

Temperature dependence of the Landau-Placzek ratio in liquid water

Rayleigh-Brillouin light scattering is studied in liquid water over the range from 249 to 365 K. Experiments are carried out with a high spectral resolution (0.1 GHz), eliminating any contribution of the structural relaxation to the elastic line. The Landau-Placzek ratio is found as the ratio of the Rayleigh and Brillouin intensities. In the whole temperature range, the Landau-Placzek ratio is found to be in good agreement with a prediction of the theory with a pair of independent thermodynamic variables, pressure and entropy. This description is usually used for single-component homogeneous liquids. An excess of the Landau-Placzek ratio above the prediction is expected for inhomogeneous liquids and is observed, for example, in glass-forming liquids below a certain temperature. In contrast to glass-forming liquids, no excess of elastically scattered light increasing at low temperatures is observed for the Landau-Placzek ratio of water. This suggests that the Landau-Placzek ratio of liquid water can be described by a homogeneous structure, and the idea of the water structure consisting of two structural motifs may not be necessary to explain the experimental ratio.

Normal vibrational modes of phospholipid bilayers observed by low-frequency Raman scattering

Low-frequency Raman spectra of multilamellar vesicles made either of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) have been studied in a wide temperature range. Below 0^{∘}C two peaks are found at frequencies around 8-9 and 14-17cm^{-1} and attributed to the normal vibrational modes of the phospholipid bilayer, which are determined by the bilayer thickness and stiffness (elastic modulus). The spectral positions of the peaks depend on the temperature and the bilayer composition. It is suggested that the ratio of the intensities of the first and second peaks can serve as a measure of the interleaflet elastic coupling. The addition of cholesterol to the phospholipid bilayer leads to peak shift and broadening, which may be assigned to the composition heterogeneities commonly attributed to the lipid raft formation.

Cryoprotectant redistribution along the frozen straw probed by Raman spectroscopy

The distribution of cryoprotectant (10% glycerol) and ice along the frozen plastic straw (the most useful container for freezing mammalian semen, oocytes and embryos) was studied by Raman scattering technique. Raman spectroscopy being a contactless, non-invasive tool was applied for the straws filled with the cryoprotectant solution and frozen by controlled rate programs commonly used for mammalian embryos freezing. Analysis of Raman spectra measured at different points along the straw reveals a non-uniform distribution of the cryoprotectant. The ratio between non-crystalline solution and ice was found to be increased by several times at the bottom side of the solution column frozen by the standard freezing program. The increase of the cryoprotectant fraction occurs in the area where embryos or oocytes are normally placed during their freezing. Possible effects of the cooling rate and the ice nucleation temperature on the cryoprotectant fraction at the bottom side of the solution column were considered. Our findings highlight that the ice fraction around cryopreserved embryos or oocytes can differ significantly from the averaged one in the frozen plastic straws.

Transition from Arrhenius to non-Arrhenius temperature dependence of structural relaxation time in glass-forming liquids: continuous versus discontinuous scenario

The temperature dependences of α relaxation time τ(α)(T) of three glass-forming liquids (salol, o-terphenyl, and α-picoline) were investigated by a depolarized light scattering technique. A detailed description of τ(α)(T) near T(A), the temperature of the transition from the Arrhenius law at high temperatures to a non-Arrhenius behavior of τ(α)(T) at lower temperatures, was done. It was found that this transition is quite sharp. If the transition is described as switching from the Arrhenius law to the Vogel-Fulcher-Tammann law, it occurs within the temperature range of about 15 K or less. Most of the known expressions for τ(α)(T) cannot describe this sharp transition. Our analysis revealed that this transition can be described either as a discontinuous transition in the spirit of the frustration-limited domain theory [D. Kivelson, G. Tarjus, X. Zhao, and S. A. Kivelson, Phys. Rev. E 53, 751 (1996)], implying a phase transition, or by a phenomenological expression recently suggested [B. Schmidtke, N. Petzold, R. Kahlau, M. Hofmann, and E. A. Rössler, Phys. Rev. E 86, 041507 (2012)], where the activation energy includes the term depending exponentially on temperature.

Low-frequency inelastic light scattering in a ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) glass

Low-frequency (down to 30 GHz) inelastic light scattering is studied in a multicomponent glass ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) in a wide temperature range. The contributions of the THz vibrational spectrum (boson peak) and of the fast relaxation are extracted and analyzed. It is shown that the fast relaxation spectrum is described by a distribution of relaxation times leading to a power-law ν(α) dependence in the frequency range 30-300 GHz. Temperature dependence of α(T) is well described by the Gilroy-Phillips model, while the integrated intensity of the fast relaxation increases significantly with the temperature. This feature distinguishes the fast relaxation in ZBLAN from the case of most single-component glasses. Thermodynamic and kinetic fragility indexes are significantly different for the ZBLAN glass. The correlations between the boson peak intensity, elastic moduli, and fragility index, found earlier for single-component glasses, are fulfilled for the thermodynamic fragility index of ZBLAN. In contrast, the correlation between the fast relaxation intensity at Tg and the fragility holds better for the kinetic fragility index of ZBLAN. We propose that thermodynamic and kinetic fragilities reflect different aspects of glassy dynamics in the case of glass formers with the complex chemical composition and structure topology: the former correlates with the elastic properties and the boson peak, the latter with the relaxation.

Broken local symmetry in paraelectric BaTiO3 proved by second harmonic generation

Precursor dynamics of a cubic to tetragonal ferroelectric phase transition in BaTiO3 is studied by the accurate measurement of the second harmonic generation (SHG) integral intensities. A finite signal holds for the SHG integrated intensity above the ferroelectric Curie temperature T(c)=403  K. Above the Burn's temperature T(d)≈580  K, the power law with the exponent γ=1 shows normal SHG nature originating from the hyper-Raman scattering by dynamical polar excitations, while, below T(d), a SHG signal from polar nanoregions becomes dominant with the larger exponent γ=2. Such a crossover of the power law exponent near T(d) is discussed on the basis of the effective Hamiltonian method and Monte Carlo simulation.

Rayleigh-Brillouin light-scattering study of a simple glass former: evidence of locally favored structures

The Rayleigh-Brillouin light scattering is studied in the glass-forming picoline with emphasis to the temperature dependence of the Landau-Placzek ratio. It was found that the Landau-Placzek ratio is in good agreement with the theoretical prediction at T>T(A), where TA is the temperature of transition from an Arrhenius-like to a non-Arrhenius behavior for the α-relaxation time dependence on temperature. Below T(A) the Landau-Placzek ratio exceeds significantly the theoretical prediction. The finding is interpreted as the evidence of the locally favored structures, which presence in an equilibrium glass-forming liquid below T(A).

Non-Debye normalization of the glass vibrational density of states in mildly densified silicate glasses

The evolution of the boson peak with densification at medium densification rates (up to 2.3%) in silicate glasses was followed through heat capacity measurements and low frequency Raman scattering. It is shown that the decrease of the boson peak induced by densification does not conform to that expected from a continuous medium; rather it follows a two step behaviour. The comparison of the heat capacity data with the Raman data shows that the light-vibration coupling coefficient is almost unaffected in this densification regime. These results are discussed in relation to the inhomogeneity of the glass elastic network at the nanometre scale.

Transition from single-molecule to cooperative dynamics in a simple glass former: Raman line-shape analysis

Parameters for orientational and inhomogeneous broadening are found from the line shape analysis of the Raman spectrum of the glass former alpha-picoline during cooling from low-viscous to glassy state. The orientational phase loss time tau(OPL), extracted from the analysis, coincides with the alpha relaxation time at T > T(A), where T(A) is the temperature of transition from an Arrhenius-like to a non-Arrhenius behavior for the alpha-relaxation time dependence on temperature. At lower temperatures tau(OPL) (T) continues the Arrhenius behavior, in contrast to the alpha-relaxation time. The width of inhomogeneous broadening of the Raman line decreases noticeably as temperature increases in the temperature range T(g) < T < T(A), approaching to zero at T approximately T(A). The findings evidence the transition of molecular dynamics from individual to cooperative at T = T(A).

Evaluation of terahertz density of vibrational states from specific-heat data: application to silica glass

The efficiency of the Tikhonov regulation method for the specific-heat phonon spectrum inversion problem is demonstrated. The regulation method is applied to the evaluation of the density of vibrational states from the low-temperature specific heat of a silica glass. The density of states obtained is found to be in good agreement with the results of inelastic neutron scattering. The extracted density of states is used for the analysis of the Raman light-vibration coupling coefficient. It is established that the coupling coefficient increases with frequency faster than approximately nu at nu<10 cm(-1). In the frequency range above 10 cm(-1) the coupling coefficient is found to be in agreement with the results of previous investigations.