Session H25: JCP Editor's Choice Session

Universal Scaling of Dynamic Heterogeneities in Aging SiO2

We investigate the aging dynamics of a strong glass former and find a strikingly simple scaling behavior. Using molecular dynamics simulations, we quench the system from high temperature to 2500 K, below the glass transition and investigate dynamic heterogeneities as function of waiting time, the time elapsed since the quench. We find that both the dynamic susceptibility and the probability distribution of the local incoherent intermediate scattering function can be described by simple scaling forms in terms of the global incoherent intermediate scattering function. The scaling forms are the same that have been found to describe the aging of several fragile glass formers. A similarity of fragile and strong glass formers had also been found on a microscopic level via analysis of single particle jumps. Furthermore we find for the scaling of dynamic heterogeneities that the aging dynamics is controlled by a unique aging clock which is the same for Si and O atoms.[JCP 144, 234510 (2016)]   

Phase-resolved two-dimensional terahertz spectroscopy - a probe of highly nonlinear light-matter interactions

Terahertz (THz) spectroscopy gives insight into low-frequency excitations and charge dynamics in condensed matter. So far, most experiments in a frequency range from 0.5 to 30 THz have focused on the linear THz response to determine linear absorption and disperion spectra, and/or electric conductivities. The generation of ultrashort THz transients with peak electric fields up to megavolts/cm has allowed for addressing nonlinear light-matter interactions and inducing excitations far from equilibrium. The novel method of two-dimensional THz (2D-THz) spectroscopy allows for mapping ultrafast dynamics and couplings of elementary excitations up to arbitrary nonlinear order in the electric field, both under resonant and nonresonant excitation conditions. In particular, different contributions to the overall nonlinear response are separated by dissecting it as a function of excitation and detection frequencies and for different waiting times after excitation. This talk gives an introduction in 2D-THz spectroscopy, including its recent extension to 3-pulse sequences and interaction schemes. To illustrate the potential of the method, recent results on two-phonon coherences and high-order interband excitations in the semiconductor InSb will be presented. Nonlinear THz excitation of two-phonon coherences exploits a resonance enhancement by the large electronic interband dipole of InSb and is, thus, far more efficient than linear excitation via resonant two-phonon absorption. As a second application, the nonlinear softmode response in a crystal consisting of aspirin molecules will be discussed. At moderate THz driving fields, the pronounced correlation of rotational modes of CH$_3$ groups with collective oscillations of $\pi$-electrons drives the system into the regime of nonperturbative light-matter interaction. Nonlinear absorption around 1.1 THz leads to a blue-shifted coherent emission at 1.5 THz, revealing a dynamic breakup of the strong electron-phonon correlations.   

Insights into the nonadiabatic dynamics of radical cations

The fate of molecular systems when they interact with photons is almost always affected by nonadiabatic processes. Conical intersections between two or more electronic states are often present playing a crucial role in the dynamics. In this talk we focus on the importance of nonadiabatic effects in radical cations formed during photoionization. Motivation for this work has been the importance of the dynamics of radical cations in interpreting pump-probe experiments. Using surface hopping molecular dynamics and the Multi-Configurational Time-Dependent Hartree (MCTDH) approach we have studied the relaxation of several radical cations initially prepared in excited ionic states. We have found that radiationless decay to the lowest ionic state occurs very fast, and the direction of the derivative coupling plays an important role in the efficiency of the decay.   

Labyrinthine flows across multilayer graphene-based membranes

Graphene-based materials have recently found extremely wide applications for fluidic purposes thanks to remarkable developments in micro-/nano-fabrication techniques. In particular, high permeability and specific selectivity have been reported for these graphene-based membranes, such as the graphene-oxide membranes, with however controversial experimental results. There is therefore an urgent need to propose a theoretical framework of fluid transport in these architectures in order to rationalize the experimental results.\par In this presentation, we report a theoretical study of mass transport across multilayer graphene based membranes, which we benchmark by atomic-scale molecular dynamics. Specifically, we consider the water flow across multiple graphene layers with an inter-layer distance ranging from sub-nanometer to a few nanometers. The graphene layers have nanoslits aligned in a staggered fashion, and thus the water flows involve multiple twists and turns. We compare the continuum model predictions for the permeability with the lattice Boltzmann calculations and molecular dynamics simulations. The highlight is that, in spite of extreme confinement, the permeability across the graphene-based membrane is quantitatively predicted on the basis of a properly designed continuum model [1]. The framework of this study constitutes a benchmark to which we compare favourably published experimental data.\par In addition, flow properties of a water-ethanol mixture are presented, demonstrating the possibility of a novel separation technique. While the membrane is permeable to both pure liquids, it exhibits a counter-intuitive ``self-semi-permeability'' to water in the presence of the mixture. This suggests a robust and versatile membrane-based separation method built on a pressure-driven reverse-osmosis process, which is considerably less energy consuming than distillation processes [2].\par \noindent References:\par \noindent [1] ``Labyrinthine water flows across multilayer graphene-based membranes: molecular dynamics versus continuum predictions,'' H. Yoshida and L. Bocquet, J. Chem. Phys. 144, 234701 (2016).\par \noindent [2] ``Carbon membranes for efficient water-ethanol separation,'' S. Gravelle, H. Yoshida, L. Joly, C. Ybert, L. Bocquet, J. Chem. Phys. 145, 124708 (2016).   

Quantum Dynamics of Incoherently Driven Systems

Understanding the molecular response to natural incoherent light, such as sunlight, is crucial to efforts to model natural photo-induced molecular and biomolecular processes, such as photosynthesis and vision. Here we introduce and discuss the significance of analytic solutions to the non-secular master equation for excitation by natural light of a model V-level system. Emphasis will be on the nature and character of the light-induced Fano coherences.