Dynamic heterogeneities, boson peak, and activation volume in glass-forming liquids

The Cooperativity of Atomic Fluctuations in Highly Supercooled Glass-Forming Metallic Melts

The behavior of supercooled glass-forming metals depends on the cooperative atomic fluctuations caused by dynamic heterogeneities in the melt. These spatial and temporal heterogeneities form dynamic clusters, which are regions of cooperative rearrangement (CRR). In this study, the macroscopic kinetics and the correlation length ξ, of the CRR, are derived for Pt57.4Cu14.7Ni5.3P22.6 and Pd43Cu27Ni10P20 metallic glass-formers by fast differential scanning calorimetry near the glass transition. While the alloy composition influences the α-relaxation and vitrification kinetics, typically defined by the glass transition, as well as the limiting temperature of the Vogel-Fulcher-Tammann-Hesse equation and the fragility index, it has no significant influence on the correlation length of the cooperative atomic motions. In agreement with many other materials, ξ is about 3 nm at the glass transition for both metallic glasses. The temperature dependence of ξ correlates with the apparent activation energy of the α-relaxation and is the reason for its non-Arrhenius behavior.

Effect of Alkyl Chain Length on the Magnitude of Dynamically Correlated Molecules and Dynamical Crossover in Alkyltriethylammonium-Based Ionic Liquids

This study explores the impact of alkyl chain length on dynamic heterogeneity and dynamic crossover in alkyltriethylammonium-based ionic liquids ([TEA-R][TFSI]) with varying alkyl chain lengths (R = C6, C8, and C10). Using differential scanning calorimetry and broadband dielectric spectroscopy, we observed that these ionic liquids are excellent glassformers with notable ionic conductivity at room temperature. Furthermore, the number of dynamically correlated molecules at the glass transition temperature, reflecting the dynamic heterogeneity, is exceptionally small for TEA-R ILs and becomes more reduced with longer alkyl chains. Moreover, the temperature dependence of conductivity requires two Vogel-Fulcher-Tammann equations with distinct sets of fitting parameters for an accurate description. The crossover temperature Tb, indicating the transition to complex dynamics, increases with the alkyl chain length.

Dispersionless Flat Mode and Vibrational Anomaly in Active Brownian Vibrators Induced by Stringlike Dynamical Defects

In recent years, active Brownian particles have emerged as a prominent model system for comprehending the behaviors of active matter, wherein particles demonstrate self-propelled motion by harnessing energy from the surrounding environment. A fundamental objective of studying active matter is to elucidate the physical mechanisms underlying its collective behaviors. Drawing inspiration from advancements in molecular glasses, our study unveils a low-energy "flat mode" within the transverse spectrum of active Brownian vibrators-a nearly two-dimensional, bidisperse granular assembly. We demonstrate that this collective excitation induces an anomalous excess in the vibrational density of states (VDOS) beyond the phononic Debye contribution. We characterize the properties of this flat mode by exploring the parameter space of our experimental system and tuning the packing fraction, the vibrational frequency, the particle size ratio, and the mixture ratio. Additionally, we establish through empirical evidence that stringlike dynamical defects, discerned via the spatial distribution of each particle's contribution to the reduced transverse VDOS, serve as the microscopic origin of the flat mode and its associated anomalies.

Temperature dependence of spatial nanoheterogeneities of shear modulus in supercooled glycerol

The boson peak in the terahertz vibrational spectrum carries information about nano-heterogeneities in the shear modulus in glass formers. Its evolution upon heating or cooling in a supercooled liquid state may shed light on the temperature dependence of heterogeneities. For this purpose, an analysis of the light scattering spectra of supercooled glycerol in the spectral range of the boson peak and fast relaxation was carried out and the parameters of the boson peak in the temperature range 180-330 K were determined. The temperature dependent frequency of the boson peak was then expressed in terms of the mean-square amplitude of the shear modulus fluctuations. This was done using the heterogeneous elasticity theory in combination with the perturbation theory on small fluctuations and Ioffe-Regel criterion for transverse vibrations in glass formers. The contribution of structural relaxation effects to phonon damping becomes significant with increasing temperature. It is shown here that structural relaxation largely determines the temperature dependence of the mean-square fluctuations of the shear modulus at high temperatures. By solving the inverse problem, the temperature dependence of shear modulus fluctuations was obtained. It shows a rapid decrease above ∼250 K with a linear extrapolation going to zero at the so-called Arrhenius temperature TA = 350 K. Comparison with literature data on the Landau-Placzek ratio shows that they have a similar temperature dependence at T < TA, which is explained by the appearance of nanometer scale spatial heterogeneities below TA. This is confirmed by the temperature dependence of the amplitude of the boson peak.

Stringlet excitation model of the boson peak

The boson peak (BP), a low-energy excess in the vibrational density of states over the Debye contribution, is often identified as a characteristic of amorphous solid materials. Despite decades of efforts, its microscopic origin still remains a mystery. Recently, it has been proposed, and corroborated with simulations, that the BP might stem from intrinsic localized modes involving one-dimensional (1D) string-like excitations ("stringlets"). We build on a theory originally proposed by Lund that describes the localized modes as 1D vibrating strings, but we specify the stringlet size distribution to be exponential, as observed in simulations. We provide an analytical prediction for the BP frequency ωBP in the temperature regime well below the observed glass transition temperature Tg. The prediction involves no free parameters and accords quantitatively with prior simulation observations in 2D and 3D model glasses based on inverse power law potentials. The comparison of the string model to observations is more uncertain when compared to simulations of an Al-Sm metallic glass material at temperatures well above Tg. Nonetheless, our stringlet model of the BP naturally reproduces the softening of the BP frequency upon heating and offers an analytical explanation for the experimentally observed scaling with the shear modulus in the glass state and changes in this scaling in simulations of glass-forming liquids. Finally, the theoretical analysis highlights the existence of a strong damping for the stringlet modes above Tg, which leads to a large low-frequency contribution to the 3D vibrational density of states, observed in both experiments and simulations.

The influence of molecular shape on reorientation dynamics of sizable glass-forming isomers at ambient and elevated pressure

We used dielectric spectroscopy to access the molecular dynamics of three isomers with a structure based on a sizable, partially rigid, and non-polar core connected to a polar phenylene unit differing in the position of the polar group, and, consequently, the direction and magnitude of the dipole moment to address the question how unique molecular properties, in particular large size and elongated shape, affect the dynamics. The position of the polar group differentiates the molecular shape and isomer's anisotropy and leads to different thermal and dynamic properties of the isomers. The shape of permittivity loss spectra was governed by magnitudes of the longitudinal and transverse components of dipole moment to a large extent. For para isomer with negligible traverse component of dipole moment, the narrowest loss peak was found while for meta isomer, the bimodal loss peak was observed at high temperatures. Its shape evolved on cooling limiting the possibility of individual mode separation near glass transition where the dynamics were more cooperative. High-pressure dielectric studies showed that sizable isomers were characterized by the pronounced sensitivity of glass transition temperature, Tg, to compression. Observed high activation volumes, such as 735 cm3/mol at Tg for para isomer, were found to correlate with the length scale of dynamic cooperativity. The number of dynamically correlated molecules depended on molecular shape and varied among isomers while the determined values were much smaller than that reported for other glass-forming liquids. We discussed here the obtained results in the context of the specific properties of the systems studied showing the overriding role of anisotropy.

Boson-peak vibrational modes in glasses feature hybridized phononic and quasilocalized excitations

A hallmark of structural glasses and other disordered solids is the emergence of excess low-frequency vibrations on top of the Debye spectrum DDebye(ω) of phonons (ω denotes the vibrational frequency), which exist in any solid whose Hamiltonian is translationally invariant. These excess vibrations-a signature of which is a THz peak in the reduced density of states D(ω)/DDebye(ω), known as the boson peak-have resisted a complete theoretical understanding for decades. Here, we provide direct numerical evidence that vibrations near the boson peak consist of hybridizations of phonons with many quasilocalized excitations; the latter have recently been shown to generically populate the low-frequency tail of the vibrational spectra of structural glasses quenched from a melt and of disordered crystals. Our results suggest that quasilocalized excitations exist up to and in the vicinity of the boson-peak frequency and, hence, constitute the fundamental building blocks of the excess vibrational modes in glasses.

Glassy dynamics in polyalcohols: intermolecular simplicity vs. intramolecular complexity

Using depolarized light scattering, we have recently shown that structural relaxation in a broad range of supercooled liquids follows, to good approximation, a generic line shape with high-frequency power law ω^-1/2. We now continue this study by investigating a systematic series of polyalcohols (PAs), frequently used as model-systems in glass-science, i.a., because the width of their respective dielectric loss spectra varies strongly along the series. Our results reveal that the microscopic origin of the observed relaxation behavior varies significantly between different PAs: while short-chained PAs like glycerol rotate as more or less rigid entities and their light scattering spectra follow the generic shape, long-chained PAs like sorbitol display pronounced intramolecular dynamic contributions on the time scale of structural relaxation, leading to systematic deviations from the generic shape. Based on these findings we discuss an important limitation for observing the generic shape in a supercooled liquid: the dynamics that is probed needs to reflect the intermolecular dynamic heterogeneity, and must not be superimposed by effects of intramolecular dynamic heterogeneity.

Mechanical disorder of sticky-sphere glasses. I. Effect of attractive interactions

Recent literature indicates that attractive interactions between particles of a dense liquid play a secondary role in determining its bulk mechanical properties. Here we show that, in contrast with their apparent unimportance to the bulk mechanics of dense liquids, attractive interactions can have a major effect on macro- and microscopic elastic properties of glassy solids. We study several broadly applicable dimensionless measures of stability and mechanical disorder in simple computer glasses, in which the relative strength of attractive interactions-referred to as "glass stickiness"-can be readily tuned. We show that increasing glass stickiness can result in the decrease of various quantifiers of mechanical disorder, on both macro- and microscopic scales, with a pair of intriguing exceptions to this rule. Interestingly, in some cases strong attractions can lead to a reduction of the number density of soft, quasilocalized modes, by up to an order of magnitude, and to a substantial decrease in their core size, similar to the effects of thermal annealing on elasticity observed in recent works. Contrary to the behavior of canonical glass models, we provide compelling evidence indicating that the stabilization mechanism in our sticky-sphere glasses stems predominantly from the self-organized depletion of interactions featuring large, negative stiffnesses. Finally, we establish a fundamental link between macroscopic and microscopic quantifiers of mechanical disorder, which we motivate via scaling arguments. Future research directions are discussed.

Key role of retardation and non-locality in sound propagation in amorphous solids as evidenced by a projection formalism

We investigate acoustic propagation in amorphous solids by constructing a projection formalism based on separating atomic vibrations into two, "phonon" (P) and "non-phonon" (NP), subspaces corresponding to large and small wavelengths. For a pairwise interaction model, we show the existence of a "natural" separation lengthscale, determined by structural disorder, for which the isolated P subspace presents the acoustic properties of a nearly homogenous (Debye-like) elastic continuum, while the NP one encapsulates all small scale non-affinity effects. The NP eigenstates then play the role of dynamical scatterers for the phonons. However, at variance with a conjecture of defect theories, their spectra present a finite low frequency gap, which turns out to lie around the Boson peak frequency, and only a small fraction of them are highly localized. We then show that small scale disorder effects can be rigorously reduced to the existence, in the Navier-like wave equation of the continuum, of a generalized elasticity tensor, which is not only retarded, since scatterers are dynamical, but also non-local. The full neglect of both retardation and non-locality suffices to account for most of the corrections to Born macroscopic moduli. However, these two features are responsible for sound speed dispersion and have quite a significant effect on the magnitude of sound attenuation. Although it remains open how they impact the asymptotic, large wavelength scaling of sound damping, our findings rule out the possibility of representing an amorphous solid by an inhomogeneous elastic continuum with the standard (i.e., local and static) elastic moduli.

Low frequency vibrational density of state of highly permeable super glassy polynorbornenes - the Boson peak

Inelastic incoherent neutron time-of-flight scattering was employed to measure the low frequency density of states for a series of addition polynorbornenes with bulky side groups. The rigid main chain in combination with the bulky side groups give rise to a microporosity of these polymers in the solid state. The microporosity characterized by the BET surfaces area varies systematically in the considered series. Such materials have some possible application as active separation layer in gas separation membranes. All investigated materials show excess contributions to the Debye type density of states characteristic for glasses known as Boson peak. The maximum position of the Boson peak shifts to lower frequency values with increasing microporosity. Data for PIM-1 and Matrimid included for comparison are in good agreement to this dependency. This result supports the sound wave interpretation of the Boson peak.

Universality of the Nonphononic Vibrational Spectrum across Different Classes of Computer Glasses

It has been recently established that the low-frequency spectrum of simple computer glass models is populated by soft, quasilocalized nonphononic vibrational modes whose frequencies ω follow a gapless, universal distribution D(ω)∼ω^{4}. While this universal nonphononic spectrum has been shown to be robust to varying the glass history and spatial dimension, it has so far only been observed in simple computer glasses featuring radially symmetric, pairwise interaction potentials. Consequently, the relevance of the universality of nonphononic spectra seen in simple computer glasses to realistic laboratory glasses remains unclear. Here, we demonstrate the emergence of the universal ω^{4} nonphononic spectrum in a broad variety of realistic computer glass models, ranging from tetrahedral network glasses with three-body interactions, through molecular glasses and glassy polymers, to bulk metallic glasses. Taken together with previous observations, our results indicate that the low-frequency nonphononic vibrational spectrum of any glassy solid quenched from a melt features the universal ω^{4} law, independently of the nature of its microscopic interactions.

An inelastic neutron scattering, Raman, far-infrared, and molecular dynamics study of the intermolecular dynamics of two ionic liquids

The intermolecular dynamics in the THz frequency range of the ionic liquids n-butyl-trimethylammonium bis(trifluoromethanesulfonyl)imide, [N1114][NTf2], and methyl-tributylammonium bis(trifluoromethanesulfonyl)imide, [N1444][NTf2], were investigated by a combined usage of inelastic neutron scattering (INS), Raman, and far-infrared (FIR) spectroscopies and the power spectrum calculated by molecular dynamics (MD) simulations. The collective dynamics of the simulated systems is also discussed by the calculation of time correlation functions of charge and mass currents that are projected onto acoustic- and optic-like motions. The INS and Raman measurements have been performed as a function of temperature in the glassy, crystalline, and liquid phases. The excess in the vibrational density of states over the expectation of the Debye theory, the so-called boson peak, is found in the INS and Raman spectra as a peak at ∼2 meV (∼16 cm-1) and also in the direct measurement of heat capacity at very low temperatures (4-20 K). This low-frequency vibration is incorporated into the curve fits of Raman, FIR, and MD data at room temperature. Fits of spectra from these different sources in the range below 100 cm-1 are consistently achieved with three components at ca. 25, 50, and 80 cm-1, but with distinct relative intensities among the different techniques. It is proposed as the collective nature of the lowest-frequency component and the anion-cation intermolecular vibration nature of the highest-frequency component. The MD results indicate that there is no clear distinction between acoustic and optic vibrations in the spectral range investigated in this work for the ionic liquids [N1114][NTf2] and [N1444][NTf2]. The analysis carried out here agrees in part, but not entirely, with other propositions in the literature, mainly from optical Kerr effect (OKE) and FIR spectroscopies, concerning the intermolecular dynamics of ionic liquids.

Pinching a glass reveals key properties of its soft spots

It is now well established that glasses feature quasilocalized nonphononic excitations-coined "soft spots"-, which follow a universal [Formula: see text] density of states in the limit of low frequencies ω. All glass-specific properties, such as the dependence on the preparation protocol or composition, are encapsulated in the nonuniversal prefactor of the universal [Formula: see text] law. The prefactor, however, is a composite quantity that incorporates information both about the number of quasilocalized nonphononic excitations and their characteristic stiffness, in an apparently inseparable manner. We show that by pinching a glass-i.e., by probing its response to force dipoles-one can disentangle and independently extract these two fundamental pieces of physical information. This analysis reveals that the number of quasilocalized nonphononic excitations follows a Boltzmann-like law in terms of the parent temperature from which the glass is quenched. The latter, sometimes termed the fictive (or effective) temperature, plays important roles in nonequilibrium thermodynamic approaches to the relaxation, flow, and deformation of glasses. The analysis also shows that the characteristic stiffness of quasilocalized nonphononic excitations can be related to their characteristic size, a long sought-for length scale. These results show that important physical information, which is relevant for various key questions in glass physics, can be obtained through pinching a glass.

Activation volume in superpressed glass-formers

In pressurized glass-forming systems, the apparent (changeable) activation volume Va(P) is the key property governing the previtreous behavior of the structural relaxation time (τ) or viscosity (η), following the Super-Barus behavior: [Formula: see text], T = const. It is usually assumed that Va(P) = V#(P), where [Formula: see text] or [Formula: see text]. This report shows that Va(P) ≪ V#(P) for P → Pg, where Pg denotes the glass pressure, and the magnitude V#(P) is coupled to the pressure steepness index (the apparent fragility). V#(P) and Va(P) coincides only for the basic Barus dynamics, where Va(P) = Va = const in the given pressure domain, or for P → 0. The simple and non-biased way of determining Va(P) and the relation for its parameterization are proposed. The derived relation resembles Murnaghan - O'Connel equation, applied in deep Earth studies. It also offers a possibility of estimating the pressure and volume at the absolute stability limit. The application of the methodology is shown for diisobutyl phthalate (DIIP, low-molecular-weight liquid), isooctyloxycyanobiphenyl (8*OCB, liquid crystal) and bisphenol A/epichlorohydrin (EPON 828, epoxy resin), respectively.

Isochronal superpositioning in the equilibrium regime of superpressed propylene carbonate to ∼ 1.8 GPa: A study by diffusivity measurement of the fluorescent probe Coumarin 1

We address the problem of glass-forming of liquids by superpressing. We study the pressure-induced dynamic change of the fragile van der Waals liquid propylene carbonate towards the glassy state in the equilibrium regime by measuring the diffusivity of the fluorescent probe Coumarin 1 embedded in the host liquid. The probe diffusivity is measured by the fluorescence recovery after photobleaching (FRAP) technique across a bleached volume generated by the near-field diffracted pattern of a laser beam. The recovered fluorescence intensity fits to a stretched exponential with the diffusive time [Formula: see text] and the stretched exponent [Formula: see text] as free parameters. In the pressure range [0.3-1.0]GPa the diffusivity decouples from the Stokes-Einstein relation. The decoupling correlates well to a decrease of [Formula: see text]. The variation of [Formula: see text] is non-monotonous with [Formula: see text] showing a minimum at [Formula: see text] s. We evidence an isochronal superpositioning over about 3 decades of [Formula: see text] between ∼ 10 s and [Formula: see text] s and a density scaling in the whole investigated pressure range. The pressure at which [Formula: see text] is minimum coincides to the dynamical crossover pressure measured by other authors. This crossover pressure is compatible with the critical point of MCT theory. As our studied pressure range encompasses the critical pressure, the non-monotonous variation of [Formula: see text] opens new insight in the approach to the critical point.

Glassy dynamics predicted by mutual role of free and activation volumes

Broadband Dielectric Spectroscopy (BDS) at elevated pressures and Positron Annihilation Lifetime Spectroscopy (PALS) are employed to elucidate the importance of the ratio of activation and free volumes during vitrification. We show that this ratio has a linear correlation with the structural relaxation of glass forming liquids in a wide temperature range hence engendering it as a vital input in the description of the dynamic glass transition.

Low-frequency Raman spectra of a glass-forming ionic liquid at low temperature and high pressure

The frequency range below ∼100 cm^-1 of the Raman spectrum of a glass-forming liquid exhibits two features that characterize the short-time (THz) dynamics: the quasi-elastic scattering (QES) tail and the boson peak (BP). In this work, we follow temperature and pressure effects on the intermolecular dynamics of a typical ionic liquid, 1-butyl-1-methylpiperidinium bis(trifluoromethanesulfonyl)imide, [Pip₁₄][[NTf₂]. The glass transition temperature of [Pip₁₄][[NTf₂] at atmospheric pressure is T_g = 198 K, and the pressure of glass transition at room temperature is P_g = 1.1 GPa. Raman spectra obtained while cooling the liquid or heating the glass exhibit hysteresis in QES and BP intensities, I_QES and I_BP. The dependence of I_QES, I_BP, and the BP frequency, ω_BP, with pressure up to the glass transition is steeper than the temperature dependence due to the stronger pressure effect on density within the GPa range. The temperature and pressure behaviors of the parameters I_QES, I_BP, and ω_BP obtained here for [Pip₁₄][[NTf₂] are discussed in light of known results for other glass-formers.

Universal Nonphononic Density of States in 2D, 3D, and 4D Glasses

It is now well established that structural glasses possess disorder- and frustration-induced soft quasilocalized excitations, which play key roles in various glassy phenomena. Recent work has established that in model glass formers in three dimensions, these nonphononic soft excitations may assume the form of quasilocalized, harmonic vibrational modes whose frequency follows a universal density of states D(ω)∼ω^{4}, independently of microscopic details, and for a broad range of glass preparation protocols. Here, we further establish the universality of the nonphononic density of vibrational modes by direct measurements in model structural glasses in two dimensions and four dimensions. We also investigate their degree of localization, which is generally weaker in lower spatial dimensions, giving rise to a pronounced system-size dependence of the nonphononic density of states in two dimensions, but not in higher dimensions. Finally, we identify a fundamental glassy frequency scale ω_{c} above which the universal ω^{4} law breaks down.

Frustration-induced internal stresses are responsible for quasilocalized modes in structural glasses

It has been recently shown [E. Lerner, G. Düring, and E. Bouchbinder, Phys. Rev. Lett. 117, 035501 (2016)PRLTAO0031-900710.1103/PhysRevLett.117.035501] that the nonphononic vibrational modes of structural glasses at low frequencies ω are quasilocalized and follow a universal density of states D(ω)∼ω^{4}. Here we show that the gapless nature of the observed density of states depends on the existence of internal stresses that generically emerge in glasses due to frustration, thus elucidating a basic element underlying this universal behavior. Similarly to jammed particulate packings, low-frequency modes in structural glasses emerge from a balance between a local elasticity term and an internal stress term in the dynamical matrix, where the difference between them is orders of magnitude smaller than their typical magnitude. By artificially reducing the magnitude of internal stresses in a computer glass former in three dimensions, we show that a gap is formed in the density of states below which no vibrational modes exist, thus demonstrating the crucial importance of internal stresses. Finally, we show that while better annealing the glass upon cooling from the liquid state significantly reduces its internal stresses, the self-organizational processes during cooling render the gapless D(ω)∼ω^{4} density of state unaffected.

Anomalies in the low frequency vibrational density of states for a polymer with intrinsic microporosity – the Boson peak of PIM-1

For the first time the low frequency vibrational density of states is reported for a polymer with intrinsic microporosity.

Statistics and Properties of Low-Frequency Vibrational Modes in Structural Glasses

Low-frequency vibrational modes play a central role in determining various basic properties of glasses, yet their statistical and mechanical properties are not fully understood. Using extensive numerical simulations of several model glasses in three dimensions, we show that in systems of linear size L sufficiently smaller than a crossover size L_{D}, the low-frequency tail of the density of states follows D(ω)∼ω^{4} up to the vicinity of the lowest Goldstone mode frequency. We find that the sample-to-sample statistics of the minimal vibrational frequency in systems of size L<L_{D} is Weibullian, with scaling exponents in excellent agreement with the ω^{4} law. We further show that the lowest-frequency modes are spatially quasilocalized and that their localization and associated quartic anharmonicity are largely frequency independent. The effect of preparation protocols on the low-frequency modes is elucidated, and a number of glassy length scales are briefly discussed.

Glass susceptibility: Growth kinetics and saturation under shear

We study the growth kinetics of glassy correlations in a structural glass by monitoring the evolution, within mode-coupling theory, of a suitably defined three-point function χ_{C}(t,t_{w}) with time t and waiting time t_{w}. From the complete wave-vector-dependent equations of motion for domain growth, we pass to a schematic limit to obtain a numerically tractable form. We find that the peak value χ_{C}^{P} of χ_{C}(t,t_{w}), which can be viewed as a correlation volume, grows as t_{w}^{0.5}, and the relaxation time as t_{w}^{0.8}, following a quench to a point deep in the glassy state. These results constitute a theoretical explanation of the simulation findings of Parisi [J. Phys. Chem. B 103, 4128 (1999)JPCBFK1520-610610.1021/jp983967m] and Kob and Barrat [Phys. Rev. Lett. 78, 4581 (1997)PRLTAO0031-900710.1103/PhysRevLett.78.4581], and they are also in qualitative agreement with Parsaeian and Castillo [Phys. Rev. E 78, 060105(R) (2008)PLEEE81539-375510.1103/PhysRevE.78.060105]. On the other hand, if the quench is to a point on the liquid side, the correlation volume grows to saturation. We present a similar calculation for the growth kinetics in a p-spin spin glass mean-field model where we find a slower growth, χ_{C}^{P}∼t_{w}^{0.13}. Further, we show that a shear rate γ[over ̇] cuts off the growth of glassy correlations when t_{w}∼1/γ[over ̇] for quench in the glassy regime and t_{w}=min(t_{r},1/γ[over ̇]) in the liquid, where t_{r} is the relaxation time of the unsheared liquid. The relaxation time of the steady-state fluid in this case is ∝γ[over ̇]^{-0.8}.

Ionic liquids and their bases: Striking differences in the dynamic heterogeneity near the glass transition

Ionic liquids (ILs) constitute an active field of research due to their important applications. A challenge for these investigations is to explore properties of ILs near the glass transition temperature Tg, which still require our better understanding. To shed a new light on the issues, we measured ILs and their base counterparts using the temperature modulated calorimetry. We performed a comparative analysis of the dynamic heterogeneity at Tg for bases and their salts with a simple monoatomic anion (Cl(-)). Each pair of ionic and non-ionic liquids is characterized by nearly the same chemical structure but their intermolecular interactions are completely different. We found that the size of the dynamic heterogeneity of ILs near Tg is considerably smaller than that established for their dipolar counterparts. Further results obtained for several other ILs near Tg additionally strengthen the conclusion about the relatively small size of the dynamic heterogeneity of molecular systems dominated by electrostatic interactions. Our finding opens up new perspectives on designing different material properties depending on intermolecular interaction types.

Protein dynamics: from rattling in a cage to structural relaxation

We present an overview of protein dynamics based mostly on results of neutron scattering, dielectric relaxation spectroscopy and molecular dynamics simulations. We identify several major classes of protein motions on the time scale from faster than picoseconds to several microseconds, and discuss the coupling of these processes to solvent dynamics. Our analysis suggests that the microsecond backbone relaxation process might be the main structural relaxation of the protein that defines its glass transition temperature, while faster processes present some localized secondary relaxations. Based on the overview, we formulate a general picture of protein dynamics and discuss the challenges in this field.

On the mechanism of activated transport in glassy liquids

We explore several potential issues that have been raised over the years regarding the "entropic droplet" scenario of activated transport in liquids, due to Wolynes and co-workers, with the aim of clarifying the status of various approximations of the random first-order transition theory (RFOT) of the structural glass transition. In doing so, we estimate the mismatch penalty between alternative aperiodic structures, above the glass transition; the penalty is equal to the typical magnitude of free energy fluctuations in the liquid. The resulting expressions for the activation barrier and the cooperativity length contain exclusively bulk, static properties; in their simplest form they contain only the bulk modulus and the configurational entropy per unit volume. The expressions are universal in that they do not depend explicitly on the molecular detail. The predicted values for the barrier and cooperativity length and, in particular, the temperature dependence of the barrier are in satisfactory agreement with observation. We thus confirm that the entropic droplet picture is indeed not only internally consistent but is also fully constructive, consistent with the apparent success of its many quantitative predictions. A simple view of a glassy liquid as a locally metastable, degenerate pattern of frozen-in stress emerges in the present description. Finally, we derive testable relationships between the bulk modulus and several characteristics of glassy liquids and peculiarities in low-temperature glasses.

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.

Cooperativity length scale in nanocomposites: interfacial and confinement effects

Interfacial and confinement effects on the evolution of cooperativity on approaching the glass transition have been studied in poly(propylenecoethylene) functionalized with diethylmaleate, polyethylene 1,4-cyclohexylenedimethylene terephthalate glycol and their nanocomposites with montmorillonite. A small increase of the structural dynamic cooperativity, a weak alteration of the temperature dependence of the characteristic relaxation frequency, and no changes in the glass transition temperature observed in poly(propylenecoethylene)-based samples can be rationalized in terms of interfacial interactions between polymer and exfoliated clay. On the other hand, confinement of polymer chains in the galleries of clay (intercalated nanocomposite) produces a strong reduction of cooperativity, of the temperature dependence of the characteristic relaxation frequency, and of the glass transition temperature in polyethylene 1,4-cyclohexylenedimethylene terephthalate glycol samples. Finally, by investigating the temperature dependence of a generalized fragility and of cooperativity, we evidenced that fragility of glass formers is determined not only by cooperativity.

Effect of temperature and density fluctuations on the spatially heterogeneous dynamics of glass-forming Van der Waals liquids under high pressure

In this Letter, we show how temperature and density fluctuations affect the spatially heterogeneous dynamics at ambient and elevated pressures. By using high-pressure experimental data for van der Waals liquids, we examine contributions of the temperature and density fluctuations to the dynamics heterogeneity. We show that the dynamic heterogeneity decreases significantly with increasing pressure at a constant structural relaxation time (isochronal condition), while the broadening of the relaxation spectrum remains constant. This observation questions the relationship between spectral broadening and dynamic heterogeneity.

The importance of the activation volume for the description of the molecular dynamics of glass-forming liquids

High pressure dielectric measurements were carried out on hydrogen bonded d-glucose and two van der Waals peracetyl saccharides, i.e. α pentaacetyl glucose and α octaacetyl maltose. In this study we found that after removing H bonds, the molecular dynamics of both modified saccharides is very sensitive to pressure, as reflected by a large value of the pressure coefficient of the glass transition temperature, equal to 270 K GPa(-1) and 280 K GPa(-1) for α pentaacetyl glucose and α octaacetyl maltose, respectively. On the other hand, dT(g)/dP for d-glucose is much lower, equal to 67 K GPa(-1). Our result confirms the general rule that the hydrogen bonding glass-forming liquids exhibit much lower values of dT(g)/dP compared to the van der Waals systems. Additionally, on the basis of results reported herein and also recent literature data for polyalcohols, we point out that the activation volume correlates fairly well with the molecular volume in the case of hydrogen bonding liquids. On the other hand, much larger values of the activation volumes at T(g) with respect to the molecular volumes were found for both peracetyl saccharides.

Fragility versus activation volume: insight into molecular dynamics of glass-forming hydrogen-bonded liquids

In this Brief Report we show that key parameters, the fragility m(p) and activation volume ΔV, which characterize the sensitivity of molecular dynamics near the glass transition to temperature and pressure changes, respectively, reflect molecular properties in a nonequivalent way. Our comparative study of fragilities and activation volumes of isomeric pentiols provides evidence that the parameter m(p) can distinguish different H-bonded structures, even if molecular volumes are similar; however, the value of ΔV recognizes mainly the size of a relaxing molecular unit and correlates to the length scale of molecular cooperativity. Thus there is a striking difference in information given by m(p) and ΔV.

Comparing dynamic correlation lengths from an approximation to the four-point dynamic susceptibility and from the picosecond vibrational dynamics

Recently an alternative approach to the determination of dynamic correlation lengths ξ for supercooled liquids, based on the properties of the slow (picosecond) vibrational dynamics, was carried out [Hong, Novikov, and Sokolov, Phys. Rev. E 83, 061508 (2011)]. Although these vibrational measurements are typically conducted well below the glass transition temperature, the liquid is frozen at T(g), whereby structural correlations, density variations, etc., manifested at low temperatures as spatial fluctuations of local elastic constants, can be related to a dynamic heterogeneity length scale for the liquid state. We compare ξ from this method to values calculated using an approximation to the four-point dynamic susceptibility. For 26 different materials we find good correlation between the two measures; moreover, the pressure dependences are consistent within the large experimental error. However, ξ from Boson peak measurements above T(g) have a different, and unrealistic, temperature dependence.