Mean-square-displacement distribution in crystals and glasses: An analysis of the intrabasin dynamics

A Comparative Study of Electron Radiation Responses of Pu2Zr2O7 and La2Zr2O7: An abinitio Molecular Dynamics Study

In this study, the response of Pu₂Zr₂O₇ and La₂Zr₂O₇ to electronic radiation is simulated, employing an ab initio molecular dynamics method. It is shown that Pu₂Zr₂O₇ undergoes a crystalline-to-amorphous structural transition with 0.3% electronic excitation, while for La₂Zr₂O₇, the structural amorphization occurs with 1.2% electronic excitation. During the microstructural evolution, the anion disorder further drives cation disorder and eventually results in the structural amorphization of Pu₂Zr₂O₇ and La₂Zr₂O₇. The difference in responses to electron radiation between Pu₂Zr₂O₇ and La₂Zr₂O₇ mainly results from the strong correlation effects between Pu 5f electrons and the smaller band gap of Pu₂Zr₂O₇. These results suggest that Pu₂Zr₂O₇ is less resistant to amorphization under local ionization rates that produce a low level of electronic excitation, since the level of the concentration of excited electrons is relatively low in Pu₂Zr₂O₇. The presented results will advance the understanding of the radiation damage effects of zirconate pyrochlores.

Escape time, relaxation, and sticky states of a softened Henon-Heiles model: Low-frequency vibrational mode effects and glass relaxation

Here we study the relaxation of a chain consisting of three masses joined by nonlinear springs and periodic conditions when the stiffness is weakened. This system, when expressed in their normal coordinates, yields a softened Henon-Heiles system. By reducing the stiffness of one low-frequency vibrational mode, a faster relaxation is enabled. This is due to a reduction of the energy barrier heights along the softened normal mode as well as for a widening of the opening channels of the energy landscape in configurational space. The relaxation is for the most part exponential, and can be explained by a simple flux equation. Yet, for some initial conditions the relaxation follows as a power law, and in many cases there is a regime change from exponential to power-law decay. We pinpoint the initial conditions for the power-law decay, finding two regions of sticky states. For such states, quasiperiodic orbits are found since almost for all components of the initial momentum orientation, the system is trapped inside two pockets of configurational space. The softened Henon-Heiles model presented here is intended as the simplest model in order to understand the interplay of rigidity, nonlinear interactions and relaxation for nonequilibrium systems such as glass-forming melts or soft matter. Our softened system can be applied to model β relaxation in glasses and suggest that local reorientational jumps can have an exponential and a nonexponential contribution for relaxation, the latter due to asymmetric molecules sticking in cages for certain orientations.

Simple solvable energy-landscape model that shows a thermodynamic phase transition and a glass transition

When a liquid melt is cooled, a glass or phase transition can be obtained depending on the cooling rate. Yet, this behavior has not been clearly captured in energy-landscape models. Here, a model is provided in which two key ingredients are considered in the landscape, metastable states and their multiplicity. Metastable states are considered as in two level system models. However, their multiplicity and topology allows a phase transition in the thermodynamic limit for slow cooling, while a transition to the glass is obtained for fast cooling. By solving the corresponding master equation, the minimal speed of cooling required to produce the glass is obtained as a function of the distribution of metastable states.