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We use equilibrium molecular dynamics (MD) simulations to study heat transport in bulk singlelayer graphene. Through a modal analysis of the MD trajectories employing a time-domain formulation, we find that collective excitations involving flexural acoustic (ZA) phonons, which have been neglected in the previous MD studies, actually dominate the heat flow, generating as much as 78% of the flux. These collective excitations are, however, much less significant if the atomic displacements are constrained in the lattice plane. Although relaxation is slow, we find graphene to be a regular (non-anomalous) heat conductor for sample sizes of order 40 lm and more. V C 2015 AIP Publishing LLC. . [http://dx.Heat transport in graphene has raised considerable interest recently, for both practical and theoretical reasons. 1 Yet, some basic questions remain unanswered. Experimental values for the heat conductivity of bulk suspended single-layer graphene around room temperature, for example, have been reported in the wide range of 600-5300 W/m K (Refs. 2-6) with recent results even suggesting that it diverges logarithmically with the length of the samples. 7 Therefore, it is fair to say, that a definitive value is still to be found. To address this point, many theoretical investigations using various transport equations 8-12 and molecular dynamics (MD) schemes 7,13-20 have been carried out. In particular, much attention has focused on the relative conductivities of the various phonon branches, especially the flexural acoustic (ZA) branch. Early studies concluded that these were negligible in view of their low group velocity and high Gruneisen parameter. 21,22 More recently, and in contrast, full solutions of the linearized phonon Boltzmann equation suggest that the contribution of ZA modes is dominant and amounts to 88% 10 or 76% 11 of the total conductivity. However, these results rely on symmetrybased selection rules which reduce ZA phonon scattering but are likely broken in real materials. Modal analyses of MD simulations, finally, conclude that the contributions of the three acoustic branches are similar, about 30% each. 13,16,19 Below, we demonstrate that this last result is erroneous and due to a theoretical oversight and that ZA phonons actually dominate the heat flow.An interesting possibility recently raised by Fugallo et al. 12 is that collective phonon excitations in graphene lead to greatly enhanced thermal conductivity as opposed for considering single-mode excitations only. Such collective modes have already been observed in polyethylene chains, 23 in various one-dimensional models, 24 as well as in the twodimensional triangular Lennard-Jones lattice. 25 To investigate this possibility, we have performed modal analysis of equilibrium MD simulations, which naturally allows the treatment of scattering mechanisms to all orders. Modal analyses of MD trajectories in graphene have already been reported, 13,19,26 as noted above. However, these calculations used a frequency-domain formulation which, we demonstrate in this ...

We address the role of laser pulse fluence on expansion dynamics and size distribution of the nanoparticles produced by irradiating a metallic target with an ultrashort laser pulse in a vacuum, an issue for which contrasting indications are present in the literature. To this end, we have carried out a combined theoretical and experimental analysis of laser ablation of a bulk copper target with 50 fs, 800 nm pulses, in an interval of laser fluencies going from few to several times the ablation threshold. On one side, molecular dynamics simulations, with two-temperature model, describe the decomposition of the material through the analysis of the evolution of thermodynamic trajectories in the material phase diagram, and allow estimating the size distribution of the generated nanoaggregates. On the other side, atomic force microscopy of less than one layer nanoparticles deposits on witness plates, and fast imaging of the nanoparticles broadband optical emission provide the corresponding experimental characterization. Both experimental and numerical findings agree on a size distribution characterized by a significant fraction (90%) of small nanoparticles, and a residual part (10%) spanning over a rather large size interval, evidencing a weak dependence of the nanoparticles sizes on the laser pulse fluence. Numerical and experimental findings show a good degree of consistency, thus suggesting that modeling can realistically support the search for experimental methods leading to an improved control over the generation of nanoparticles by ultrashort laser ablation