Revisiting the role of magnetic field fluctuations in nonadiabatic acceleration of ions during dipolarization

Using energetic (9–212 keV/e) ion flux data obtained by the Geotail spacecraft, Ono et al. (2009) statistically examined changes in the energy density of H+ and O+ ions in the near-Earth plasma sheet during substorm-associated dipolarization. They found that ions are nonadiabatically accelerated by the electric field induced by the magnetic field fluctuations whose frequencies are close to their gyrofrequencies. The present paper revisits this result and finds it still holds

Divergent Thermal Conductivity in Three-dimensional Nonlinear lattices

Heat conduction in three-dimensional nonlinear lattices is investigated using a particle dynamics simulation. The system is a simple three-dimensional extension of the Fermi-Pasta-Ulam $\beta$ (FPU-$\beta$) nonlinear lattices, in which the interparticle potential has a biquadratic term together with a harmonic term. The system size is $L\times L\times 2L$, and the heat is made to flow in the $2L$ direction the Nose-Hoover method. Although a linear temperature profile is realized, the ratio of enerfy flux to temperature gradient shows logarithmic divergence with $L$. The autocorrelation function of energy flux $C(t)$ is observed to show power-law decay as $t^{-0.98\pm 0,25}$, which is slower than the decay in conventional momentum-cnserving three-dimensional systems ($t^{-3/2}$). Similar behavior is also observed in the four dimensional system.Comment: 4 pages, 5 figures. Accepted for publication in J. Phys. Soc. Japan Letter

Thermal Conductivity and Thermal Rectification in Graphene Nanoribbons: a Molecular Dynamics Study

We have used molecular dynamics to calculate the thermal conductivity of symmetric and asymmetric graphene nanoribbons (GNRs) of several nanometers in size (up to ~4 nm wide and ~10 nm long). For symmetric nanoribbons, the calculated thermal conductivity (e.g. ~2000 W/m-K @400K for a 1.5 nm {\times} 5.7 nm zigzag GNR) is on the similar order of magnitude of the experimentally measured value for graphene. We have investigated the effects of edge chirality and found that nanoribbons with zigzag edges have appreciably larger thermal conductivity than nanoribbons with armchair edges. For asymmetric nanoribbons, we have found significant thermal rectification. Among various triangularly-shaped GNRs we investigated, the GNR with armchair bottom edge and a vertex angle of 30{\deg} gives the maximal thermal rectification. We also studied the effect of defects and found that vacancies and edge roughness in the nanoribbons can significantly decrease the thermal conductivity. However, substantial thermal rectification is observed even in the presence of edge roughness.Comment: 13 pages, 5 figures, slightly expanded from the published version on Nano Lett. with some additional note

Finite thermal conductivity in 1d lattices

We discuss the thermal conductivity of a chain of coupled rotators, showing that it is the first example of a 1d nonlinear lattice exhibiting normal transport properties in the absence of an on-site potential. Numerical estimates obtained by simulating a chain in contact with two thermal baths at different temperatures are found to be consistent with those ones based on linear response theory. The dynamics of the Fourier modes provides direct evidence of energy diffusion. The finiteness of the conductivity is traced back to the occurrence of phase-jumps. Our conclusions are confirmed by the analysis of two variants of this model.Comment: 4 pages, 3 postscript figure

Carbon superatom thin films

Assembling clusters on surfaces has emerged as a novel way to grow thin films with targeted properties. In particular, it has been proposed from experimental findings that fullerenes deposited on surfaces could give rise to thin films retaining the bonding properties of the incident clusters. However the microscopic structure of such films is still unclear. By performing quantum molecular dynamics simulations, we show that C_28 fullerenes can be deposited on a surface to form a thin film of nearly defect free molecules, which act as carbon superatoms. Our findings help clarify the structure of disordered small fullerene films and also support the recently proposed hyperdiamond model for solid C_28.Comment: 13 pages, RevTeX, 2 figures available as black and white PostScript files; color PostScript and/or gif files available upon reques

Stability of gold nanowires at large Au-Au separations

The unusual structural stability of gold nanowires at large separations of gold atoms is explained from first-principles quantum mechanical calculations. We show that undetected light atoms, in particular hydrogen, stabilize the experimentally observed structures, which would be unstable in pure gold wires. The enhanced cohesion is due to the partial charge transfer from gold to the light atoms. This finding should resolve a long-standing controversy between theoretical predictions and experimental observations.Comment: 7 pages, 3 figure

Network structure and dynamics of hydrogenated amorphous silicon

In this paper we discuss the application of current it ab initio computer simulation techniques to hydrogenated amorphous silicon (a-Si:H). We begin by discussing thermal fluctuation in the number of coordination defects in the material, and its temperature dependence. We connect this to the ``fluctuating bond center detachment" mechanism for liberating H bonded to Si atoms. Next, from extended thermal MD simulation, we illustrate various mechanisms of H motion. The dynamics of the lattice is then linked to the electrons, and we point out that the squared electron-lattice coupling (and the thermally-induced mean square variation in electron energy eigenvalues) is robustly proportional to the localization of the conjugate state, if localization is measured with inverse participation ratio. Finally we discuss the Staebler-Wronski effect using these methods, and argue that a sophisticated local heating picture (based upon reasonable calculations of the electron-lattice coupling and molecular dynamic simulation) explains significant aspects of the phenomenon.Comment: 10 pages, 5 figures, accepted in J. Non. Cryst. So