David Cubero

Electronic transport in disordered n-alkanes: From fluid methane to amorphous polyethylene

We use a fast Fourier transform block Lanczos diagonalization algorithm to study the electronic states of excess electrons in fluid alkanes (methane, ethane, and propane) and in a molecular model of amorphous polyethylene (PE) relevant to studies of space charge in insulating polymers. We obtain a new pseudopotential for electron–PE interactions by fitting to the electronic properties of fluid alkanes and use this to obtain new results for electron transport in amorphous PE. From our simulations, while the electronic states in fluid methane are extended throughout the whole sample, in amorphous PE there is a transition between localized and delocalized states slightly above the vacuum level (∼+0.06 eV). The localized states in our amorphous PE model extend to −0.33 eV below this level. Using the Kubo–Greenwood equation we compute the zero-field electron mobility in pure amorphous PE to be μ≈2×10−3 cm2/V s. Our results highlight the importance of electron transport through extended states in amorphous regi...

Self-diffusion in freely evolving granular gases

A self-diffusion equation for a freely evolving gas of inelastic hard disks or spheres is derived starting from the Boltzmann–Lorentz equation, by means of a Chapman–Enskog expansion in the density gradient of the tagged particles. The self-diffusion coefficient depends on the restitution coefficient explicitly, and also implicitly through the temperature of the system. This latter introduces also a time dependence of the coefficient. As in the elastic case, the results are trivially extended to the Enskog equation. The theoretical predictions are compared with numerical solutions of the kinetic equation obtained by the direct simulation Monte Carlo method, and also with molecular dynamics simulations. An excellent agreement is found, providing mutual support to the different approaches.

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

Abstract We 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 C 27 H 56 . 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.

Single electron states in polyethylene

We report computer simulations of an excess electron in various structural motifs of polyethylene at room temperature, including lamellar and interfacial regions between amorphous and lamellae, as well as nanometre-sized voids. Electronic properties such as density of states, mobility edges, and mobilities are computed on the different phases using a block Lanczos algorithm. Our results suggest that the electronic density of states for a heterogeneous material can be approximated by summing the single phase density of states weighted by their corresponding volume fractions. Additionally, a quantitative connection between the localized states of the excess electron and the local atomic structure is presented.

Hydrodynamic Transport Coefficients of Granular Gases

Some transport properties of granular gases are investigated. Starting from a kinetic theory level of description, the hydrodynamic transport equations to Navier-Stokes order are presented. The equations are derived by means of the Chapman-Enskog procedure. To test the existence of a normal solution and the possibility of a hydrodynamic description, the theoretical predictions are compared with numerical simulations of the underlying kinetic equation for small deviations around the reference homogeneous state. An excellent agreement is found for all the range of dissipation in collisions considered. Similar analysis is presented for self-diffusion and Brownian motion. In the former case, also Molecular Dynamics results are shown to agree with the theoretical predictions. Quantitative and also qualitative differences with the elastic limit are discussed.

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 A. 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.

Current reversals in a rocking ratchet: dynamical versus symmetry-breaking mechanisms.

Directed transport in ratchets is determined by symmetry breaking in a system out of equilibrium. A hallmark of rocking ratchets is current reversals: an increase in the rocking force changes the direction of the current. In this work for a biharmonically driven spatially symmetric rocking ratchet we show that a class of current reversal is precisely determined by symmetry breaking, thus creating a link between dynamical and symmetry-breaking mechanisms.

High-frequency effects in the Fitzhugh-Nagumo neuron model

The effect of a high-frequency signal on the FitzHugh-Nagumo excitable model is analyzed. We show that the firing rate is diminished as the ratio of the high-frequency amplitude to its frequency is increased. Moreover, it is demonstrated that the excitable character of the system, and consequently the firing activity, is suppressed for ratios above a given threshold value. In addition, we show that the vibrational resonance phenomenon turns up for sufficiently large noise strength values.

Vibrational Mechanics in an Optical Lattice: Controlling Transport via Potential Renormalization

We demonstrate theoretically and experimentally the phenomenon of vibrational resonance in a periodic potential, using cold atoms in an optical lattice as a model system. A high-frequency (HF) drive, with a frequency much larger than any characteristic frequency of the system, is applied by phase modulating one of the lattice beams. We show that the HF drive leads to the renormalization of the potential. We used transport measurements as a probe of the potential renormalization. The very same experiments also demonstrate that transport can be controlled by the HF drive via potential renormalization.

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 theorem a 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.