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Caro, A.

Publications and source records attributed to Caro, A..

10 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Role of electrons in collision cascades in solids. I. Dissipative model

In this article, we present a detailed model for the nonadiabatic coupling between ions and electrons in energetic ion-solid interactions over a wide range of energies in concentrated solid-solution fcc alloys of the 3d transition metals Ni, Co, Fe, and Cr. The model is based on general statistical mechanical principles and results in a stochastic modification of the classical nuclei motion which is parameterized by the first-principles calculation of a dissipation function produced by explicit time-dependent electronic evolution. This model provides a full picture of an entire collision process, from the ballistic to the thermal phases of a cascade, giving a detailed description of the energy exchange between ions and electrons till their final thermalization, removing in this way some ad hoc assumptions used in the state-of-the-art atomistic two-temperature models. Here, this work is separated in two papers; in the present Part I, we report on the ab initio methodology used to translate stopping power and electron-phonon interaction into a parameterized dissipation function; Part II, to be published, addresses the nonadiabatic ion dynamics using our modified Langevin dynamics [Tamm et al. Phys. Rev. Lett. 120, 185501 (2018)] applying the dissipation functions developed here to specific collision cascade events.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Role of electrons in collision cascades in solids. II. Molecular dynamics

In this work, we present a model for the role of electrons in collision cascades in solids in which the coupling between ions and electrons is calculated using first-principles models and introduced into the classical ion equations of motion using our modified Langevin dynamics [A. Tamm et al., Phys. Rev. Lett. 120, 185501 (2018)]. This model gives a full picture of the entire collision process, from the ballistic to the thermal phases of a cascade, giving a detailed representation of the energy exchange between ions and electrons until their final thermalization, removing in this way some ad hoc assumptions used in the state-of-the-art two-temperature model. This work is separated into two papers: Part I [M. Caro et al., Phys. Rev. B 99, 174301 (2019)] reports on the ab initio methodology used to translate stopping power into the parametrized dissipation function. Finally, part II applies the nonadiabatic ion dynamics using the dissipation functions developed in Part I to specific collision cascade events.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

On the local density dependence of electronic stopping of ions in solids

We use here time-dependent density functional theory to calculate the electronic stopping S e in binary Ni-Ni atomic collisions and for a Ni projectile along channeling directions in a Ni crystal. Our results show that when S e is reported as a function of the ground state target electronic density ρ 0 the stopping is not a single-valued function of the local density, as assumed in formalisms that date back to the origins of quantum mechanics, but shows loops, suggesting that it is inaccurate to model stopping as a dissipative force of the type F = β ( ρ 0 ) v , as it is customarily done in non-adiabatic molecular dynamics simulations of ion-solid interactions. We compare our results with S e in a uniform electron gas where the above definition for the force holds, and conclude on the validity of using jellium as a crude approximation for more realistic inhomogeneous electron gases.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

Stopping power beyond the adiabatic approximation

Energetic ions traveling in solids deposit energy in a variety of ways, being nuclear and electronic stopping the two avenues in which dissipation is usually treated. This separation between electrons and ions relies on the adiabatic approximation in which ions interact via forces derived from the instantaneous electronic ground state. In a more detailed view, in which non-adiabatic effects are explicitly considered, electronic excitations alter the atomic bonding, which translates into changes in the interatomic forces. In this work, we use time dependent density functional theory and forces derived from the equations of Ehrenfest dynamics that depend instantaneously on the time-dependent electronic density. With them we analyze how the inter-ionic forces are affected by electronic excitations in a model of a Ni projectile interacting with a Ni target, a metallic system with strong electronic stopping and shallow core level states. We find that the electronic excitations induce substantial modifications to the inter-ionic forces, which translate into nuclear stopping power well above the adiabatic prediction. Particularly, we observe that most of the alteration of the adiabatic potential in early times comes from the ionization of the core levels of the target ions, not readily screened by the valence electrons.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Electron-phonon interaction within classical molecular dynamics

Here, we present a model for nonadiabatic classical molecular dynamics simulations that captures with high accuracy the wave-vector q dependence of the phonon lifetimes, in agreement with quantum mechanics calculations. It is based on a local view of the e-ph interaction where individual atom dynamics couples to electrons via a damping term that is obtained as the low-velocity limit of the stopping power of a moving ion in a host. The model is parameter free, as its components are derived from ab initio -type calculations, is readily extended to the case of alloys, and is adequate for large-scale molecular dynamics computer simulations. We also show how this model removes some oversimplifications of the traditional ionic damped dynamics commonly used to describe situations beyond the Born-Oppenheimer approximation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Impact of Short-Range Forces on Defect Production from High-Energy Collisions

Primary radiation damage formation in solid materials typically involves collisions between atoms that have up to a few hundred keV of kinetic energy. The distance between two colliding atoms can approach 0.05 nm during these collisions. At such small atomic separations, force fields fitted to equilibrium properties tend to significantly underestimate the potential energy of the colliding dimer. To enable molecular dynamics simulations of high-energy collisions, it is common practice to use a screened Coulomb force field to describe the interactions and to smoothly join this to the equilibrium force field at a suitable interatomic spacing. But, there is no accepted standard method for choosing the parameters used in the joining process, and our results prove that defect production is sensitive to how the force fields are linked. A new procedure is presented that involves the use of ab initio calculations to determine the magnitude and spatial dependence of the pair interactions at intermediate distances, along with systematic criteria for choosing the joining parameters. Results are presented for the case of nickel, which demonstrate the use and validity of the procedure.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Analytical model of the effect of misfit dislocation character on the bubble-to-void transition in metals

Here, this paper addresses the role of misfit dislocations in the nucleation and growth of nanoscale He bubbles at interfaces. In a recent work, we studied the nanoscale effects on the capillarity equation and on equilibrium conditions. We proposed an expression for surface energy and for the equation of state, EOS, for He in bubbles, which have a size dependence that captures the role of the interface forces, which become relevant at the nanoscale. Here we determine the EOS for several twist grain boundaries in Fe and Cu and incorporate these results into the rate equation that determines the bubble-to-void transition, focusing on the influence of interface dislocations on the evaporation rate of vacancies. We find a significant effect of the magnitude of the Burgers vector of the dislocations on the critical radius for the transition. In conclusion, these results give a quantitative way to characterize grain boundaries in their ability to capture He and alter the onset of swelling.

36 MATERIALS SCIENCE↗
NASA NTRS2012

Sputtering from a Porous Material by Penetrating Ions

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.

Rodriguez-Nieva, J. F.↗