Session X26: Focus Session: Non-Adiabatic Dynamics in Irradiated Materials

First-principles calculation of electronic stopping power in metals and insulators via time-dependent DFT simulations

Projectiles interacting with solid or liquid targets are subject to two main inelastic collision channels: electronic and nuclear. The end result is a slowing down -- or stopping -- of the projectile due to energy deposition onto electronic excitations or motion of the target nuclei. At high projectile velocities, cross sections for nuclear stopping are exceedingly small, thus leaving electronic stopping as the only relevant channel. At low velocities nuclear stopping becomes predominant. In metals, it coexists with electronic stopping, but in insulators the existence of an energy gap for electronic excitations translates into a velocity threshold for electronic stopping. In this presentation I will introduce a recently developed computational methodology designed to study electronic stopping at the first-principles level, by means of time-dependent density-functional (TDDFT) simulations. I will then discuss the results for a variety of systems, starting from the first application to the ionic crystal LiF, moving to metallic systems like Al and Au, and then to the insulating water ice. In all cases I will compare to available experimental data to emphasize the astonishing accuracy of TDDFT results. I will also discuss how these simulations provide further insight into open questions such as the difference in stopping between H and He projectiles, and I will focus on the interplay between electronic excitations and non-adiabatic forces on the nuclei. Finally, I will touch on computational methods to simulate the combined dynamics of nuclei and electrons.   

Nonadiabatic Molecular Dynamics with Trajectories

In the mixed quantum-classical description of molecular systems, only the quantum character of the electronic degrees of freedom is considered while the nuclear motion is treated at a classical level. In the adiabatic case, this picture corresponds to the Born-Oppenheimer limit where the nuclei move as point charges on the potential energy surface (PES) associated with a given electronic state. Despite the success of this approximation, many physical and chemical processes do not fall in the regime where nuclei and electrons can be considered decoupled. In particular, most photoreactions pass through regions of the PES in which electron-nuclear quantum interference effects are sizeable and often crucial for a correct description of the phenomena. Recently, we have developed a trajectory-based nonadiabatic molecular dynamics scheme that describes the nuclear wavepacket as an ensemble of particles following classical trajectories on PESs derived from time-dependent density functional theory (TDDFT) [1]. The method is based on Tully's fewest switches trajectories surface hopping (TSH) where the nonadiabatic coupling elements between the different potential energy surfaces are computed \textit{on-the-fly} as functionals of the ground state electron density or, equivalently, of the corresponding Kohn-Sham orbitals [2]. Here, we present the theoretical fundamentals of our approach together with an extension that allows for the direct coupling of the dynamics to an external electromagnetic field [3] as well as to the external potential generated by the environment (solvent effects) [4]. The method is applied to the study of the photodissociation dynamics of simple molecules in gas phase and to the description of the fast excited state dynamics of molecules in solution (in particular Ruthenium (II) tris(bipyridine) in water). \\[4pt] [1] E. Tapavicza, I. Tavernelli, U. Rothlisberger, \textit{Phys. Rev. Lett.,} \textbf{98}, (2007) 023001. \\[0pt] [2] Tavernelli I.; Tapavicza E.; Rothlisberger U., \textit{J. Chem. Phys}., \textbf{130}, (2009) 124107; Tavernelli I., Curchod B.F.E., Rothlisberger U., \textit{J. Chem. Phys}., \textbf{131}, (2009) 196101; Tavernelli I., Curchod B.F.E., Laktionov A., Rothlisberger U., \textit{J. Chem. Phys.}, \textbf{133}, (2010) 194104. \\[0pt] [3] Tavernelli I., Curchod B.F.E., Rothlisberger U., \textit{Phys}. \textit{Rev. A}, \textbf{81}, (2010) 052508. \\[0pt] [4] Tavernelli I., Curchod B.F.E., Rothlisberger U., \textit{Phys. Chem.,} accepted 2011.   

Advances on Time-dependent DFT Simulations of Electronic Stopping

Radiation damage of reactor materials is a topic of interest and continuous research in the nuclear industry. A single nuclear decay event (e.g. alpha ) produces a cascade of collisions involving the displacement of thousands of atoms in a crystalline material. While atomistic-scale simulations would be the ideal tool to understand these processes, the fact that they currently work on the assumption that electrons respond adiabatically to the atomic motion does not provide valid answers, for example to the stopping power problem. An alternative approach to attacking this problem is a method which explicitly takes into account electron non-adiabatic dynamics. We will present results obtained by time-depending DFT on the electronic stopping power of channeling protons in prototypical metals and insulators obtained by recent implementations of non-adiabatic electron dynamics methods.   

Electronic stopping of slow H and He atoms in gold from first principles

In spite of a long history, the quantitative understanding of non-adiabatic processes in condensed matter and our ability to perform predictive theoretical simulations of processes coupling many adiabatic energy surfaces is very much behind what accomplished for adiabatic situations, for which first-principles calculations provide predictions of varied properties within a few percent accuracy. We will present here high-accuracy results for the electronic stopping power of H and He moving through gold, using time-evolving density-functional theory, thereby conveying usual first-principles accuracies to strongly coupled, continuum non-adiabatic processes in condensed matter. The two key unexplained features of what observed experimentally have been reproduced and understood: ($i$) The non-linear behavior of stopping power versus velocity is a gradual crossover as excitations tail into the $d$-electron spectrum; and ($ii$) the higher stopping for He than for H at low velocities is explained by the substantial involvement of the $d$ electrons in the screening of the projectile even at the lowest velocities where the energy loss is generated by $s$-like electron-hole pair formation only.   

Golden Rule of Radiation Hardness: a Study of Strain Effect on Controlled Radiation Damage

Stain is widely presented in microstructures. Strain effect to radiation hardness is critical in understanding and engineering nano-materials. Here we studied the strain effect on the controlled radiation damage in monolayer hexagonal boron nitride (h-BN) through {\em ab initio} density functional theory calculations. We observed a general behavior of reduction in the radiation hardness by the strain, for both B-vacancy and N-vacancy configurations, in both compressive and tensive strain states, at the directions of zigzag, armchair and bi-axial. We proposed a golden rule of the radiation hardness states that any effort adding energy to the system will reduce the radiation hardness. Such golden rule of radiation hardness could be widely applied to material design and engineering for those devices working in irradiation-enrich environments, for example, electronic and optoelectronic devices in outer space.   

First-principles simulation of laser irradiation of graphene and graphane

In the framework of real-time real-space time-dependent density functional theory complemented with classical molecular dynamics for ions, we have studied the behavior of graphene and graphane fragments irradiated with strong laser pulses. In particular, we have investigated how the response of graphene and graphane changes when laser pulses of different frequency (near IR, visible, and UV) are shot. Damage thresholds have been established and compared with existing experimental data.   

Electron-Phonon Coupling in a CdSe Nanowire

It is important to calculate the coupling between phonons and electrons in realistic nanostructures, e.g. to understand carrier cooling and dynamics in a nanowire. In this talk, we will present results of phonon spectrum calculations using a customized valence force field (VFF) method. This customized VFF method is developed to be fittable to the results of any ab-initio calculations, with density functional theory (DFT) results being used in this work. By fitting many different DFT calculations on different motifs and their perturbations, we have obtained in the custom VFF a very efficient method that closely reproduces DFT phonons for CdSe nanowires with (10-10) surfaces having Cd-Se dimerization. We have also combined the results of these phonon spectrum calculations with electronic structure calculations to obtain the electron-phonon coupling. We will present this result and and show how the electron-phonon coupling affects the carrier dynamics in the nanowire.   

Deviational formulations for efficient simulation of multiscale phonon transport

We show that by simulating only the deviation from equilibrium, considerable computational savings can be realized in Monte Carlo solutions of the Boltzmann equation describing phonon transport at small scales. The computational savings manifest themselves in the form of significantly smaller statistical uncertainty compared to standard Monte Carlo solution methods in the limit of small deviation from equilibrium (e.g. small temperature differences). Additional computational savings are realized in multiscale problems where the degree of deviation from equilibrium varies considerably over the simulation domain. By developing rigorous evolution equations for the deviation from equilibrium from the governing kinetic equation, the resulting algorithm seamlessly bridges the near- and far-equilibrium regions without introducing any approximation. We also show that considering an energy-based Boltzmann equation lends itself naturally to algorithms that conserve energy exactly, thus improving the simulation fidelity. Application of the proposed methods to practical problems of current interest, such as a transient thermoreflectance experiment used to extract information about the mean free path of heat carriers in various materials, will be presented and discussed.