The microscopic details of local particle dynamics is studied in a glass-forming one component supercooled liquid modeled by a Dzugutov potential developed for simple metallic glass formers. Our main goal is to investigate particle motion in the supercooled liquid state, and to ascertain the extent to which this motion is cooperative and occurring in quasi-one-dimesional, string-like paths. To this end we investigate in detail the mechanism by which particles move along these paths. In particular, we show that the degree of coherence—that is, simultaneous motion by consecutive particles along a string—depends on the length of the string. For short strings, the motion is highly coherent. For longer strings, the motion is highly coherent only within shorter segments of the string, which we call “microstrings.” Very large strings may contain several microstrings within which particles move simultaneously, but individual microstrings within a given string are temporally uncorrelated with each other. We discuss possible underlying mechanism for this complex dynamical behavior, and examine our results in the context of recent work by Garrahan and Chandler [Phys. Rev. Lett. 89, 035704 (2002)] in which dynamic facilitation plays a central role in the glass transition.

We study the Johari–Goldstein β process of organic glass formers by one- (1D) and two-dimensional (2D) ${}^{\mathrm{2}}\mathrm{H}$ nuclear magnetic resonance (NMR). In particular, we compare systems with pronounced secondary relaxation in dielectric spectroscopy, namely toluene-${\mathrm{d}}_{\mathrm{5}}$ and polybutadiene-${\mathrm{d}}_{\mathrm{6}}$ (PB), with compounds which do not exhibit a distinct β peak, i.e., glycerol-${\mathrm{d}}_{\mathrm{5}}$ and polystyrene-${\mathrm{d}}_{\mathrm{3}}$ (PS). Choosing large interpulse delays in the applied echo pulse sequences we increase the sensitivity on small angle rotational jumps. This way, we are able to probe clearly the β process of toluene and PB in the line shape of 1D ${}^2\mathrm{H}$ NMR spectra and in the orientational correlation functions of 2D ${}^{\mathrm{2}}\mathrm{H}$ NMR in time domain which is not possible when using the conventional techniques. Below the glass transition temperature $T_g,$ the secondary relaxation of both glass formers is caused by a highly restricted reorientation of essentially all molecules. Comparing our results with simulations we estimate that the reorientation of most toluene molecules and PB monomeric units is characterized by an amplitude $\text{χ<10°.}$ This amplitude is approximately unchanged below $T_g,$ but strongly increases above the glass transition. Closer investigating the 1D ${}^{\mathrm{2}}\mathrm{H}$ NMR line shape for large interpulse delays we moreover demonstrate that the reorientation involved in the β process takes place step-by-step via many elementary rotational jumps. On the other hand, for glycerol and PS, hardly any effects are observed in 1D and 2D ${}^{\mathrm{2}}\mathrm{H}$ NMR experiments below $T_g$ when applying comparable experimental parameters. We conclude that reorientations with an amplitude $\text{χ>1°}$ do not occur on a time scale of $\mu \mathrm{s-ms}$ for the majority of molecules in glassy glycerol and PS.

We perform molecular dynamics simulations to study the structural relaxation dynamics of a model of viscous silica, the prototype of a strong glass former. We find that the melt dynamics are spatially heterogeneous regardless of whether the bulk relaxation is non-Arrhenius or Arrhenius, and cannot be understood as a statistical bondbreaking process. Further, we show that stringlike motion is suppressed by the covalent bondings, yet high particle mobility propagates continuously, supporting the concept of dynamic facilitation emphasized in recent theoretical work.

${}^2\mathrm{H}$ NMR is used to study the mechanisms for the reorientation of protein hydration water. In the past, crossovers in temperature-dependent correlation times were reported at $T_{x1}\approx 225\mathrm{K}$ ($X_1$) and $T_{x2}\approx 200\mathrm{K}$ ($X_2$). We show that neither $X_1$ nor $X_2$ are related to a fragile-to-strong transition. Our results rule out an existence of $X_1$. Also, they indicate that water performs thermally activated and distorted tetrahedral jumps at $T<T_{x2}$, implying that $X_2$ originates in an onset of this motion, which may be related to a universal defect diffusion in materials with defined hydrogen-bond networks.