An excess electron at polyethylene/vacuum interfaces using a reaction-field technique

We study the surface states of an excess electron at polyethylene/vacuum interfaces using an accurate reaction-field method, specifically designed to take into account the long range interaction of the excess electron and the dielectric surface. The method is shown to validate the energy levels recently reported with a simple perturbation theory scheme, while providing a better description of the wave function at the vacuum. The use of a single particle pseudopotential allows the simulation of large interface samples, showing distinct differences between the electron surface states at amorphous and crystalline interfaces due to their different atomic density.Ministerio de Economía y Competitividad of Spain, Grant No. FIS2016-80244

Comment on “Markovian approximation in a coarse-grained description of atomic systems” [J. Chem. Phys.125, 204101 (2006)]

The authors concluded that the coarse-graining dynamics of a one-dimensional chain of oscillators interacting through Lennard–Jones potentials is Markovian, in contrast with the situation observed for harmonic lattices. However, with the help of a novel equation that relates the correlation of forces and momenta, I show that this conclusion is drawn from an incorrect analysis of the simulation data.Dirección General de Enseñanza Superior of Spain Grant No. FIS2005-0288

Control of transport in two-dimensional systems via dynamical decoupling of degrees of freedom with quasiperiodic driving fields

We consider the problem of the control of transport in higher-dimensional periodic structures by applied ac fields. In a generic crystal, transverse degrees of freedom are coupled, and this makes the control of motion difficult to implement. We show, both with simulations and with an analytical functional expansion on the driving amplitudes, that the use of quasiperiodic driving significantly suppresses the coupling between transverse degrees of freedom. This allows a precise control of the transport, and does not require a detailed knowledge of the crystal geometry.The Leverhulme Trust, and the Ministerio de Ciencia e Innovación of Spain (Grant No. FIS2008-02873

Vibrational mechanics in higher dimension: Tuning potential landscapes

This work extends the domain of vibrational mechanics to higher dimensions, with fast vibrations applied to different directions. In particular, the presented analysis considers the case of a split biharmonic drive, where harmonics of frequency ω and 2 ω are applied to orthogonal directions in a two-dimensional setting. It is shown, both numerically and with analytic calculations, that this determines a highly tunable effective potential with the same symmetry as the original one. The driving allows one not only to tune the amplitude of the potential, but also to introduce an arbitrary spatial translation in the direction corresponding to the 2 ω driving. The setup allows for generalization to implement translations in an arbitrary direction within the two-dimensional landscapes. The same principles also apply to three-dimensional periodic potentials.Ministerio de Ciencia e Innovación of Spain PID2019- 105316GB-I0

Formal derivation of dissipative particle dynamics from first principles

We show that the Markovian approximation assumed in current particle-based coarse-grained techniques, like dissipative particle dynamics, is unreliable in situations in which sound plays an important role. As an example we solve analytically and numerically the dynamics of coarse-grained harmonic systems by using first principle methods, showing the presence of long-lived memory kernels. This effect raises questions about the connection of these approaches at their current form to molecular dynamics.The European Union and the EPSR

Inhomogeneous multiscale dynamics in harmonic lattices

We use projection operators to address the coarse-grained multiscale problem in harmonic systems. Stochastic equations of motion for the coarse-grained variables with an inhomogeneous level of coarse graining in both time and space are presented. In contrast to previous approaches that typically start with thermodynamic averages the key element of our approach is the use of a projection matrix chosen both for its physical appeal in analogy to mechanical stability theory and for its algebraic properties. We show that thermodynamic equilibrium can be recovered and obtain the fluctuation dissipation theorema posteriori. All system-specific information can be computed from a series of feasible molecular dynamics simulations. We recover previous results in the literature and show how this approach can be used to extend the quasicontinuum approach and comment on implications for dissipative particle dynamics type of methods. Contrary to what is assumed in the latter models the stochastic process of all coarse-grained variables is not necessarily Markovian even though the variables are slow. Our approach is applicable to any system in which the coarse-grained regions are linear. As an example we apply it to the dynamics of a single mesoscopic particle in the infinite one-dimensional harmonic chain.EU growth project SENTIMATS under Contract No. G5RD-CT-200

Computer simulations of localized small polarons in amorphous polyethylene

We use a simple mean field scheme to compute the polarization energy of an excess electron in amorphous polyethylene that allows us to study dynamical properties. Nonadiabatic simulations of an excess electron in amorphous polyethylene at room temperature show the spontaneous formation of localized small polaron states in which the electron is confined in a spherically shaped region with a typical dimension of 5 Å. We compute the self-trapping energy to be −0.06±0.03 eV, with a lifetime on the time scale of a few tens of picoseconds

Electronic states for excess electrons in polyethylene compared to long-chain alkanes

Versión Post-Print del autorWe use a pseudopotential model to calculate the electronic states available to an excess electron in crystalline and amorphous regions of model polyethlyene as well as the molecular crystal of the linear alkane C27H56. It is shown that alkane crystals of whatever chain length are not representative of crystalline polyethylene (PE) although they are often considered to be so. We discuss the implications for electron transport in PE.This work was supported by EPSRC through Grants GR/R18222 and GR/M9442

Steady state of a fluidized granular medium between two walls at the same temperature

The steady state of a low-density gas of inelastic hard spheres confined between two parallel walls at the same temperature is studied. Because of the dissipation in collisions, the state is not uniform but highly inhomogeneous with a nonlinear temperature profile. Direct Monte Carlo simulations show that in the nearly elastic limit the pressure is uniform, but the state exhibits anisotropy of the diagonal terms of the pressure tensor, contrary to the predictions of the Navier-Stokes equations. For larger inelasticity, the pressure becomes nonuniform. These rheological effects, peculiar to granular systems, are explained by means of a model kinetic equation based on the Boltzmann equation. The equation is solved by constructing a systematic perturbative expansion in the square root of the degree of inelasticity. The theoretical predictions compare well with the simulation results for small inelasticity, but they are in conflict for larger values of the degree of inelasticity. The analysis provides strong evidence that this is due to the asymptotic but divergent character of the expansion, similarly to what happens when the usual Chapman-Enskog expansion is applied to molecular fluids.This research was partially supported by Grant No. PB96- 0534 from the Dirección General de Investigación Científica yTécnica (Spain

Exploring the limits of magnetic field focusing: Simple planar geometries

This work explores the possibility to arbitrarily shape in space low-frequency magnetic fields using a recently introduced synthesization technique (Choi et al., 2016). We investigate the ability to focus a magnetic field on a two-dimensional region using magnetic field sources distributed on a parallel plane. In agreement with the recent work, arbitrarily tight focusing is demonstrated possible. However, our results indicate that this comes at the cost of exponentially large power requirements and also leads to exponentially large fields in the region between the source and target planes. This imposes strict limitations on the application of the technique. In addition, we also demonstrate that arbitrary steering of the magnetic field focus within the target region is possible, without any additional cost in terms of power requirement. In exploring the potential for magnetic field synthesis, our findings highlight limits to be considered for practical applications, as well as promising capabilities not identified before.Ministerio de Ciencia e Innovación of Spain PID2019-105316GB-I0