Secondary relaxation in the glass-transition regime of ortho -terphenyl observed by incoherent neutron scattering

Phys. Rev. E (23feb00), resubmitted (25apr00), cond-mat/0002363) Elastic and inelastic incoherent neutron scattering experiments are simulated for simple models: a rigid solid (as used for normalisation), a glass (with a smooth distribution of harmonic vibrations), and a viscous liquid (described by schematic mode-coupling equations). As long as the spectral distribution of the input scattering law does not vary with wavenumber, it is only weakly distorted by multiple scattering. The wavenumber dependence of the scattering intensity suffers much more.

Section: Discussionmentioning

confidence: 83%

Multiple-scattering effects on smooth neutron-scattering spectra

Wuttke

2000

“…Extensive experiments on OTP have been performed to monitor the onset of glassy structural relaxation on microscopic time and length scales. Using in particular quasielastic neutron scattering, the decay of * Present address: Laboratoire de Physique Mathématique et Théorique, Université Montpellier II, 34095 Montpellier, France collective and self density fluctuations has been measured as a function of temperature, pressure, and wavenumber [4,[7][8][9][10][11][12]. Based on these studies, the validity of the universal predictions of MCT, such as the factorization theorem and the time-temperature superposition principle, has been established.…”

Section: Introductionmentioning

confidence: 99%

Structural relaxation in supercooled orthoterphenyl

Chong

Sciortino

2004

We report molecular-dynamics simulation results performed for a model of molecular liquid orthoterphenyl in supercooled states, which we then compare with both experimental data and modecoupling-theory (MCT) predictions, aiming at a better understanding of structural relaxation in orthoterphenyl. We pay special attention to the wavenumber dependence of the collective dynamics. It is shown that the simulation results for the model share many features with experimental data for real system, and that MCT captures the simulation results at the semiquantitative level except for intermediate wavenumbers connected to the overall size of the molecule. Theoretical results at the intermediate wavenumber region are found to be improved by taking into account the spatial correlation of the molecule's geometrical center. This supports the idea that unusual dynamical properties at the intermediate wavenumbers, reported previously in simulation studies for the model and discernible in coherent neutron-scattering experimental data, are basically due to the coupling of the rotational motion to the geometrical-center dynamics. However, there still remain qualitative as well as quantitative discrepancies between theoretical prediction and corresponding simulation results at the intermediate wavenumbers, which call for further theoretical investigation.

“…The data points are taken directly from the graphs shown in Refs. [22,23]. The solid curves are the theoretical results described in the text.…”

Section: Excitation-chain Theorymentioning

confidence: 99%

“…A selection from the data of [22,23] is shown in Fig.1 along with theoretical curves computed using Eqs. (2.19 -2.22).…”

Section: A Scattering Datamentioning

confidence: 99%

“…The most extensively studied laboratory glass is orthoterphenyl (OTP), for which the viscosity [19,20] and the diffusion constant [21] are known down to about the glass temperature T g ∼ = 240 K. The self intermediate scattering functionF s (k, t) for this material is known from neutron scattering measurements down to T = 293 K [22,23,24], i.e. down to about its mode-coupling temperature T MC [25,26].…”

Section: Lessons From Ortho-terphenylmentioning

confidence: 99%

See 1 more Smart Citation

Anomalous diffusion in heterogeneous glass-forming liquids: Temperature-dependent behavior

Langer

2008

In a preceding paper [1], Mukhopadhyay and I studied the diffusive motion of a tagged molecule in an heterogeneous glass-forming liquid at temperatures just above a glass transition. Among other features of this system, we postulated a relation between heterogeneity and stretched-exponential decay of correlations, and we also confirmed that systems of this kind generally exhibit non-Gaussian diffusion on intermediate length and time scales. Here I extend this analysis to higher temperatures approaching the point where the heterogeneities disappear and thermal activation barriers become small. I start by modifying the continuous-time random-walk theory proposed in [1], and supplement this analysis with an extension of the excitation-chain theory of glass dynamics. I also use a key result from the shear-transformation-zone theory of viscous deformation of amorphous materials. Elements of each of these theories are then used to interpret experimental data for ortho-terphenyl, specifially, the diffusion and viscosity coefficients and neutron scattering measurements of the self intermediate scattering function. Reconciling the theory with these data sets provides insights about the crossover between super-Arrhenius and Arrhenius dynamics, length scales of spatial heterogeneities, violation of the Stokes-Einstein relation in glass-forming liquids, and the origin of stretched exponential decay of correlations.