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We present molecular dynamics (MD) simulations of radiation damage in pure Fe nanoparticles (NP) and bimetallic FeCu core-shell nanoparticles (CSNP). The CSNP includes a perfect body centered cubic (bcc) Fe core coated with a face-centered cubic (fcc) Cu shell. Irradiation with Fe Primary Knock-on Atoms (PKA) with energies between 1 and 7 keV leads to point defects, without clustering beyond divacancies and very few slightly larger vacancy clusters, and without interstitial clusters, unlike what happens in bulk at the same PKA energies. The Fe-Cu interface and shell can act as a defect sink, absorbing radiation-induced damage and, therefore, the final number of defects in the Fe core is significantly lower than in the Fe NP. In addition, the Cu shell substantially diminishes the number of sputtered Fe atoms, acting as a barrier for recoil ejection. Structurally, the Cu shell responds to the stress generated by the collision cascade by creating and destroying stacking faults across the shell width, which could also accommodate further irradiation defects. Sputtering yield (Y) is underestimated by BCA, which is also expected since the simulation is for a thin film at normal incidence. We also compare MD defect production to bulk predictions of the analytic Athermal Recombination Corrected Displacements Per Atom (arc-dpa) model. The number of vacancies in the Fe core is only slightly lower than arc-dpa predictions, but the number of interstitials is reduced by about one order of magnitude compared to vacancies, at 5 keV. According to the radiation resistance found for FeCu CSNP in our simulations, this class of nanomaterial could be suitable for developing new radiation resistant coatings, nanostructured components, and shields for use in extreme environments, for instance, in nuclear energy and astrophysical applications., Comment: In the SM we compare our MD results to Monte Carlo Binary Collision Approximation (BCA) simulations using the SRIM code, for the irradiation of an amorphous 3-layer thin film with thickness equal to the CSNP diameter. BCA does not include defect recombination, so the number of Frenkel pairs is significantly higher than in MD, as expected

We use spin-lattice dynamics simulations to study the possibility of modeling the magnetic hysteresis behavior of a ferromagnetic material. The temporal evolution of the magnetic and mechanical degrees of freedom is obtained through a set of two coupled Langevin equations. Hysteresis loops are calculated for different angles between the external field and the magnetocrystalline anisotropy axes. The influence of several relevant parameters is studied, including the field frequency, magnetic damping, magnetic anisotropy (magnitude and type), magnetic exchange, and system size. The role played by a moving lattice is also discussed. For a perfect bulk ferromagnetic system we find that, at low temperatures, the exchange and lattice dynamics barely affect the loops, while the field frequency and magnetic damping have a large effect on it. The influence of the anisotropy magnitude and symmetry are found to follow the expected behavior. We show that a careful choice of simulation parameters allows for an excellent agreement between the spin-lattice dynamics measurements and the paradigmatic Stoner-Wohlfarth model. Furthermore, we extend this analysis to intermediate and high temperatures for the perfect bulk system and for spherical nanoparticles, with and without defects, reaching values close to the Curie temperature. In this temperature range, we find that lattice dynamics has a greater role on the magnetic behavior, especially in the evolution of the defective samples. The present study opens the possibility for more accurate inclusion of lattice defects and thermal effects in hysteresis simulations

Armchair graphene nanoribbons, when forming a superlattice, can be classified in different topological phases, with or without edge states. By means of tight-binding and classical molecular dynamics (MD) simulations, we studied the electronic and mechanical properties of some of these superlattices. MD shows that fracture in modulated superlattices is brittle, as for unmodulated ribbons, and that occurs at the thinner regions, with staggered superlattices achieving a larger fracture strain. We found a general mechanism to induce a topological transition with strain, related to the electronic properties of each segment of the superlattice, and by studying the sublattice polarization we were able to characterize the transition and the response of these states to the strain. For the cases studied in detail here, the topological transition occurred at $\sim$3-5 \% strain, well below the fracture strain. The topological states of the superlattice -- if present -- are robust to strain even close to fracture. Unlike the zero-energy edge states found in the zig-zag edges of graphene nanoribbons, the superlattice states shows signatures of being particularly insensitive to disorder, even in real space.

The magnetic behavior of bcc iron nanoclusters, with diameters between 2 and 8 nm, is investigated by means of spin dynamics (SD) simulations coupled to molecular dynamics (MD-SD), using a distance-dependent exchange interaction. Finite-size effects in the total magnetization as well as the influence of the free surface and the surface/core proportion of the nanoclusters are analyzed in detail for a wide temperature range, going beyond the cluster and bulk Curie temperatures. Comparison is made with experimental data and with theoretical models based on the mean-field Ising model adapted to small clusters, and taking into account the influence of low coordinated spins at free surfaces. Our results for the temperature dependence of the average magnetization per atom M(T), including the thermalization of the transnational lattice degrees of freedom, are in very good agreement with available experimental measurements on small Fe nanoclusters. In contrast, significant discrepancies with experiment are observed if the translational degrees of freedom are artificially frozen. The finite-size effects on M(T) are found to be particularly important near the cluster Curie temperature. Simulated magnetization above the Curie temperature scales with cluster size as predicted by models assuming short-range magnetic ordering (SRMO). Analytical approximations to the magnetization as a function of temperature and size are proposed.

The sputtering yield, Y, from a cylindrical thermal spike is calculated using a two dimensional fluid dynamics model which includes the transport of energy, momentum and mass. The results show that the high pressure built-up within the spike causes the hot core to perform a rapid expansion both laterally and upwards. This expansion appears to play a significant role in the sputtering process. It is responsible for the ejection of mass from the surface and causes fast cooling of the cascade. The competition between these effects accounts for the nearly linear dependence of $Y$ with the deposited energy per unit depth that was observed in recent Molecular Dynamics simulations. Based on this we describe the conditions for attaining a linear yield at high excitation densities and give a simple model for this yield., Comment: 10 pages, 9 pages (including 9 figures), submitted to PRB

Using classical molecular dynamics simulations we examine the formation of craters during 0.4 - 100 keV Xe bombardment of Au. Our simulation results, and comparison with experiments and simulations of other groups, are used to examine to what extent analytical models can be used to predict the size and properties of craters. We do not obtain a fully predictive analytical model (with no fitting parameters) for the cratering probability, because of the difficulty in predicting the probability of cascades splitting into subcascades, and the relation of the heat spike lifetime and energy density. We do, however, demonstrate that the dependence of the crater size on the incident ion energy can be well understood qualitatively in terms of the lifetime of the heat spike and the cohesive energy of the material. We also show that a simple energy density criterion can not be used to predict cratering in a wide ion energy range because of the important role of the heat spike lifetime in high-energy cascades. The cohesive energy dependence differs from that obtained for macroscopic cratering (observed e.g. in astrophysics) because of the crucial role of melting in the development of heat spikes., Comment: 12 pages, 8 figures

An incident fast ion in the electronic stopping regime produces a track of excitations which can lead to particle ejection and cratering. Molecular Dynamics simulations of the evolution of the deposited energy were used to study the resulting crater morphology as a function of the excitation density in a cylindrical track for large angle of incidence with respect to the surface normal. Surprisingly, the overall behavior is shown to be similar to that seen in the experimental data for crater formation in polymers. However, the simulations give greater insight into the cratering process. The threshold for crater formation occurs when the excitation density approaches the cohesive energy density, and a crater rim is formed at about six times that energy density. The crater length scales roughly as the square root of the electronic stopping power, and the crater width and depth seem to saturate for the largest energy densities considered here. The number of ejected particles, the sputtering yield, is shown to be much smaller than simple estimates based on crater size unless the full crater morphology is considered. Therefore, crater size can not easily be used to estimate the sputtering yield., Comment: LaTeX, 7 pages, 5 EPS figures. For related figures/movies, see: http://dirac.ms.virginia.edu/~emb3t/craters/craters.html New version uploaded 5/16/01, with minor text changes + new figure 2

A fast ion penetrating a solid creates a track of excitations. This can produce displacements seen as an etched track, a process initially used to detect energetic particles but now used to alter materials. From the seminal papers by Fleischer et al. [Phys. Rev. 156, 353 (1967)] to the present [C. Trautmann, S. Klaumunzer and H. Trinkaus, Phys. Rev. Lett. 85, 3648 (2000)], `Coulomb explosion' and thermal spike models are treated as conflicting models for describing ion track effects. Here molecular dynamics simulations of electronic-sputtering, a surface manifestation of ion track formation, show that `Coulomb explosion' produces a `heat' spike so that these are early and late aspects of the same process. Therefore, differences in scaling are due to the use of incomplete spike models., Comment: Submitted to PRL. 4 pages, 3 figures. For related movies see: http://dirac.ms.virginia.edu/~emb3t/coulomb/coulomb.html PACS added in new version

Porous materials are ubiquitous in the universe and weathering of porous surfaces plays an important role in the evolution of planetary and interstellar materials. Sputtering of porous solids in particular can influence atmosphere formation, surface reflectivity, and the production of the ambient gas around materials in space, Several previous studies and models have shown a large reduction in the sputtering of a porous solid compared to the sputtering of the non-porous solid. Using molecular dynamics simulations we study the sputtering of a nanoporous solid with 55% of the solid density. We calculate the electronic sputtering induced by a fast, penetrating ion, using a thermal spike representation of the deposited energy. We find that sputtering for this porous solid is, surprisingly, the same as that for a full-density solid, even though the sticking coefficient is high.