Laurent Beland

Oak Ridge National Laboratory, Materials Science and Technology, Post-Doc

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Papers by Laurent Beland

The Activation-Relaxation Technique: ART Nouveau and Kinetic ART

Journal of Atomic, Molecular, and Optical Physics, 2012

ABSTRACT The evolution of many systems is dominated by rare activated events that occur on timesc... more ABSTRACT The evolution of many systems is dominated by rare activated events that occur on timescale ranging from nanoseconds to the hour or more. For such systems, simulations must leave aside the full thermal description to focus specifically on mechanisms that generate a configurational change. We present here the activation relaxation technique (ART), an open-ended saddle point search algorithm, and a series of recent improvements to ART nouveau and kinetic ART, an ART-based on-the-fly off-lattice self-learning kinetic Monte Carlo method.

Numerical study of the thermal ablation of wet solids by ultrashort laser pulses

Physical Review B, 2008

The ablation by ultrashort laser pulses at relatively low fluences ͑i.e., in the thermal regime͒ ... more The ablation by ultrashort laser pulses at relatively low fluences ͑i.e., in the thermal regime͒ of solids wetted by a thin liquid film is studied using a generic numerical model. In comparison with dry targets, the liquid is found to significantly affect ablation by confining the solid and slowing down the expansion of the laser-heated material. These factors affect the relative efficiency of the various ablation mechanisms, leading, in particular, to the complete inhibition of phase explosion at lower fluences, a reduced ablation yield, and significant changes in the composition of the plume. As a consequence, at fluences above the ablation threshold, the size of the ejected nanoclusters is lower in presence of the liquid. Our results provide a qualitative understanding of the effect of wetting layers on the ablation process.

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Investigation of defect clusters in ion-irradiated Ni and NiCo using diffuse X-ray scattering and electron microscopy

Journal of Nuclear Materials, 2015

Influence of chemical disorder on energy dissipation and defect evolution in concentrated solid solution alloys

Nature communications, 2015

A grand challenge in materials research is to understand complex electronic correlation and non-e... more A grand challenge in materials research is to understand complex electronic correlation and non-equilibrium atomic interactions, and how such intrinsic properties and dynamic processes affect energy transfer and defect evolution in irradiated materials. Here we report that chemical disorder, with an increasing number of principal elements and/or altered concentrations of specific elements, in single-phase concentrated solid solution alloys can lead to substantial reduction in electron mean free path and orders of magnitude decrease in electrical and thermal conductivity. The subsequently slow energy dissipation affects defect dynamics at the early stages, and consequentially may result in less deleterious defects. Suppressed damage accumulation with increasing chemical disorder from pure nickel to binary and to more complex quaternary solid solutions is observed. Understanding and controlling energy dissipation and defect dynamics by altering alloy complexity may pave the way for ne...

A computational atomistic study of the relaxation of ion-bombarded c-Si on experimental time-scales: an application of the kinetic Activation Relaxation Technique

The kinetic activation relaxation technique (kinetic ART) method, an off-lattice, self-learning k... more The kinetic activation relaxation technique (kinetic ART) method, an off-lattice, self-learning kinetic Monte Carlo (KMC) algorithm with on-the-fly event search,ootnotetextL. K. B'eland, P. Brommer, F. El-Mellouhi, J.-F. Joly and N. Mousseau, Phys. Rev. E 84, 046704 (2011). is used to study the relaxation of c-Si after Si^- bombardment at 3 keV. We describe the evolution of the damaged areas at room-temperature and above for periods of the order of seconds, treating long-range elastic deformations exactly. We assess the stability of the nanoscale structures formed by the damage cascade and the mechanisms that govern post-implantation annealing.

Coupling of Hubbard fermions with phonons in La2 CuO4: A combined study using density-functional theory and the generalized tight-binding method

Journal of Alloys and Compounds, 2015

Preparation, morpho-structural and photo-characterization of Ag/TiO 2 eSiO 2 (0.5e1wt%) composite... more Preparation, morpho-structural and photo-characterization of Ag/TiO 2 eSiO 2 (0.5e1wt%) composites were reported. The silver ions were reduced on the titaniaesilica surface in two different routes. The best Ag/TiO 2 eSiO 2 photocatalysts must have high specific surface area and low Ag content, when the Ag nanoparticles homogeneous dispersed onto the mixed oxide surface generates an oxidation state of titanium (Ti Ag 4þ ), that improves the charge carriers transfer, thus increasing their photoactivity. A negative value of the activation energy suggested that the temperature does not influence the photoactivity of the composites.

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Self-Evolving Atomistic Kinetic Monte Carlo simulations of defects in materials

Computational Materials Science, 2015

The recent development of on-the-fly atomistic kinetic Monte Carlo methods has led to an increase... more The recent development of on-the-fly atomistic kinetic Monte Carlo methods has led to an increased amount attention on the methods and their corresponding capabilities and applications. In this review, the framework and current status of Self-Evolving Atomistic Kinetic Monte Carlo (SEAKMC) are discussed. SEAKMC particularly focuses on defect interaction and evolution with atomistic details without assuming potential defect migration/interaction mechanisms and energies. The strength and limitation of using an active volume, the key concept introduced in SEAKMC, are discussed. Potential criteria for characterizing an active volume are discussed and the influence of active volume size on saddle point energies is illustrated. A procedure starting with a small active volume followed by larger active volumes was found to possess higher efficiency. Applications of SEAKMC, ranging from point defect diffusion, to complex interstitial cluster evolution, to helium interaction with tungsten surfaces, are summarized. A comparison of SEAKMC with molecular dynamics and conventional object kinetic Monte Carlo is demonstrated. Overall, SEAKMC is found to be complimentary to conventional molecular dynamics, especially when the harmonic approximation of transition state theory is accurate. However it is capable of reaching longer time scales than molecular dynamics and it can be used to systematically increase the accuracy of other methods such as object kinetic Monte Carlo. The challenges and potential development directions are also outlined.

format_quoteVariation in initial displacement magnitude affects saddle point detection efficiency in SEAKMC, with Gaussian distribution proving optimal.format_quote

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Interstitial loop transformations in FeCr

Journal of Alloys and Compounds, 2015

ABSTRACT We improve the Self-Evolving Atomistic Kinetic Monte Carlo (SEAKMC) algorithm by integra... more ABSTRACT We improve the Self-Evolving Atomistic Kinetic Monte Carlo (SEAKMC) algorithm by integrating the Activation Relaxation Technique nouveau (ARTn), a powerful open-ended saddle-point search method, into the algorithm. We use it to investigate the reaction of 37-interstitial 1/2[1 1 1] and 1/2[ ] loops in FeCr at 10 at.% Cr. They transform into 1/2[1 1 1], 1/2[ ], [1 0 0] and [0 1 0] 74-interstitial clusters with an overall barrier of 0.85 eV. We find that Cr decoration locally inhibits the rotation of crowdions, which dictates the final loop orientation. The final loop orientation depends on the details of the Cr decoration. Generally, a region of a given orientation is favored if Cr near its interface with a region of another orientation is able to inhibit reorientation at this interface more than the Cr present at the other interfaces. We also find that substitutional Cr atoms can diffuse from energetically unfavorable to energetically favorable sites within the interlocked 37-interstitial loops conformation with barriers of less than 0.35 eV.

Probing Potential Energy Surface Exploration Strategies for Complex Systems

by Laurent Beland and Gawonou Ntsouaglo

Journal of Chemical Theory and Computation, 2015

The efficiency of minimum-energy configuration searching algorithms is closely linked to the ener... more The efficiency of minimum-energy configuration searching algorithms is closely linked to the energy landscape structure of complex systems. Here we characterize this structure by following the time evolution of two systems, vacancy aggregation in Fe and energy relaxation in ion-bombarded c-Si, using the kinetic Activation-Relaxation Technique (k-ART), an off-lattice kinetic Monte Carlo (KMC) method, and the well-known Bell-Evans-Polanyi (BEP) principle. We also compare the efficiency of two methods for handling non-diffusive flickering states -an exact solution and a Tabu-like approach that blocks already visited states. Comparing these various simulations allow us to confirm that the BEP principle does not hold for complex system since forward and reverse energy barriers are completely uncorrelated. This means that following the lowest available energy barrier, even after removing the flickering states, leads to rapid trapping: relaxing complex systems requires crossing high-energy barriers in order to access new energy basins, in agreement with the recently proposed replenish-and-relax model [Béland et al., PRL 111, 105502 (2013)] This can be done by forcing the system through these barriers with Tabu-like methods. Interestingly, we find that following the fundamental kinetics of a system, though standard KMC approach, is at least as efficient as these brute-force methods while providing the correct kinetics information. arXiv:1407.7371v1 [cond-mat.mtrl-sci]

$FIG. 5. Comparison of k-ART vs. BEP with structural evo- lution for the 50-vacancy FE system. Top: Evolution of the average cluster size as function of logarithmic of simulated time. Bottom: Evolution of the fraction of mono-vacancies. Blue line: KMC; green line: BEP. FIG. 4. Comparison of k-ART vs. ABC relaxation of ar 2000-atom iron system with 50 vacancies initially distributec at random. Red line: total energy evolution as function o: logarithmic of simulated time; green line: evolution of cluste1 size; blue line: evolution of the fraction of mono-vacancies. The horizontal dashed black line corresponds to the energy level reached after 20 000 s with ABC.$

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Strain effects and intermixing at the Si surface: Importance of long-range elastic corrections in first-principles calculations

Physical Review B, 2014

We investigate Ge mixing at the Si(001) surface and characterize the 2 × N Si(001) reconstruction... more We investigate Ge mixing at the Si(001) surface and characterize the 2 × N Si(001) reconstruction by means of hybrid quantum and molecular mechanics calculations (QM/MM). Avoiding fake elastic dampening, this scheme allows to correctly take into account long range deformation induced by reconstruted and defective surfaces. We focus in particular on the dimer vacancy line (DVL) and its interaction with Ge adatoms. We first show that calculated formation energies for these defects are highly dependent on the choice of chemical potential and that the latter must be chosen carefully. Characterizing the effect of the DVL on the deformation field, we also find that the DVL favors Ge segregation in the fourth layer close to the DVL. Using the activation-relaxation technique (ART nouveau) and QM/MM, we show that a complex diffusion path permits the substitution of the Ge atom in the fourth layer, with barriers compatible with mixing observed at intermediate temperature.

format_quoteEstablishes that Ge intermixing occurs at room temperature with deeper intermixing possible at higher temperatures, affecting third and fourth atomic layers.format_quote

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Slow relaxation of cascade-induced defects in Fe

Physical Review B, 2015

On-the-fly kinetic Monte Carlo (KMC) simulations are performed to investigate slow relaxation of ... more On-the-fly kinetic Monte Carlo (KMC) simulations are performed to investigate slow relaxation of non-equilibrium systems. Point defects induced by 25 keV cascades in α-Fe are shown to lead to a characteristic time-evolution, described by the replenish and relax mechanism. Then, we produce an atomistically-based assessment of models proposed to explain the slow structural relaxation by focusing on the aggregation of 50 vacancies and 25 self-interstital atoms (SIA) in 10-latticeparameter α-Fe boxes, two processes that are closely related to cascade annealing and exhibit similar time signature. Four atomistic effects explain the timescales involved in the evolution: defect concentration heterogeneities, concentration-enhanced mobility, cluster-size dependent bond energies and defect-induced pressure. These findings suggest that the two main classes of models to explain slow structural relaxation, the Eyring model and the Gibbs model, both play a role to limit the rate of relaxation of these simple point-defect systems. arXiv:1409.0529v2 [cond-mat.mtrl-sci]

format_quote25 keV cascade annealing in α-Fe shows logarithmic relaxation over seven orders of magnitude due to point defects interaction.format_quote

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Kinetic Activation–Relaxation Technique and Self-Evolving Atomistic Kinetic Monte Carlo: Comparison of on-the-fly Kinetic Monte Carlo algorithms

Computational Materials Science, 2015

We present a comparison of the kinetic Activation-Relaxation Technique (k-ART) and the Self-Evolv... more We present a comparison of the kinetic Activation-Relaxation Technique (k-ART) and the Self-Evolving Atomistic Kinetic Monte Carlo (SEAKMC), two off-lattice, on-the-fly kinetic Monte Carlo (KMC) techniques that were recently used to solve several materials science problems. We show that if the initial displacements are localized the dimer method and the Activation-Relaxation Technique nouveau provide similar performance. We also show that k-ART and SEAKMC, although based on different approximations, are in agreement with each other, as demonstrated by the examples of 50 vacancies in a 1950-atom Fe box and of interstitial loops in 16000-atom boxes. Generally speaking, k-ART's treatment of geometry and flickers is more flexible, e.g. it can handle amorphous systems, and rigorous than SEAKMC's, while the later's concept of active volumes permits a significant speedup of simulations for the systems under consideration and therefore allows investigations of processes requiring large systems that are not accessible if not localizing calculations.

format_quoteThe accuracy of saddle searches was achieved with a precision of 0.0285 eV/Å, enabling effective energy landscape exploration.format_quote

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The Activation-Relaxation Technique: ART Nouveau and Kinetic ART

by Eduardo Machado-charry and Laurent Beland

Journal of Atomic, Molecular, and Optical Physics, 2012

Following atomistic kinetics on experimental timescales with the kinetic Activation–Relaxation Technique

by Laurent Beland and Gawonou Ntsouaglo

Computational Materials Science, 2014

The properties of materials, even at the atomic level, evolve on macroscopic time scales. Followi... more The properties of materials, even at the atomic level, evolve on macroscopic time scales. Following this evolution through simulation has been a challenge for many years. For lattice-based activated diffusion, kinetic Monte Carlo has turned out to be an almost perfect solution. Various accelerated molecular dynamical schemes, for their part, have allowed the study on long time scale of relatively simple systems. There is still a need, however, for methods able to handle complex materials such as alloys and disordered systems. Here, we review the kinetic Activation-Relaxation Technique (k-ART), one of a handful of off-lattice kinetic Monte Carlo methods, with on-the-fly cataloging, that have been proposed in the last few years.

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Replenish and Relax: Explaining Logarithmic Annealing in Ion-Implanted c-Si

Physical Review Letters, 2013

Fatigue and aging of materials are, in large part, determined by the evolution of the atomic-scal... more Fatigue and aging of materials are, in large part, determined by the evolution of the atomic-scale structure in response to strains and perturbations. This coupling between microscopic structure and long time scales remains one of the main challenges in materials study. Focusing on a model system, ion-damaged crystalline silicon, we combine nanocalorimetric experiments with an off-lattice kinetic Monte Carlo simulation to identify the atomistic mechanisms responsible for the structural relaxation over long time scales. We relate the logarithmic relaxation, observed in a number of systems, with heat-release measurements. The microscopic mechanism associated with logarithmic relaxation can be described as a two-step replenish and relax process. As the system relaxes, it reaches deeper energy states with logarithmically growing barriers that need to be unlocked to replenish the heat-releasing events leading to lower energy configurations.

format_quoteCoupling NC measurements with K-ART simulates low-energy disorder annealing, enhancing temporal resolution in material studies.format_quote

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Kinetic activation-relaxation technique

Physical Review E, 2011

We present a detailed description of the kinetic activation-relaxation technique (k-ART), an off-... more We present a detailed description of the kinetic activation-relaxation technique (k-ART), an off-lattice, self-learning kinetic Monte Carlo (KMC) algorithm with on-the-fly event search. Combining a topological classification for local environments and event generation with ART nouveau, an efficient unbiased sampling method for finding transition states, k-ART can be applied to complex materials with atoms in off-lattice positions or with elastic deformations that cannot be handled with standard KMC approaches. In addition to presenting the various elements of the algorithm, we demonstrate the general character of k-ART by applying the algorithm to three challenging systems: self-defect annihilation in c-Si (crystalline silicon), self-interstitial diffusion in Fe, and structural relaxation in a-Si (amorphous silicon).

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First stages of silicon oxidation with the activation relaxation technique

Physical Review B, 2012

ABSTRACT Using the art nouveau method, we study the initial stages of silicon oxide formation. Af... more ABSTRACT Using the art nouveau method, we study the initial stages of silicon oxide formation. After validating the method&#39;s parameters with the characterization of point defects diffusion mechanisms in pure Stillinger-Weber silicon, which allows us to recover some known results and to detail vacancy and self-interstitial diffusion paths, the method is applied onto a system composed of an oxygen layer deposited on a silicon substrate. We observe the oxygen atoms as they move rapidly into the substrate. From these art nouveau simulations, we extract the energy barriers of elementary mechanisms involving oxygen atoms and leading to the formation of an amorphouslike silicon oxide. We show that the kinetics of formation can be understood in terms of the energy barriers between various coordination environments.

Long-time relaxation of ion-bombarded silicon studied with the kinetic activation-relaxation technique: Microscopic description of slow aging in a disordered system

Physical Review B, 2013

ABSTRACT Diffusion and relaxation of defects in bulk systems is a complex process that can only b... more ABSTRACT Diffusion and relaxation of defects in bulk systems is a complex process that can only be accessed directly through simulations. We characterize the mechanisms of low-temperature aging in self-implanted crystalline silicon, a model system used extensively to characterize both amorphization and return to equilibrium processes, over 11 orders of magnitudes in time, from 10 ps to 1 s, using a combination of molecular dynamics and kinetic activation-relaxation technique simulations. These simulations allow us to reassess the atomistic mechanisms responsible for structural relaxations and for the overall logarithmic relaxation, a process observed in a large number of disordered systems and observed here over the whole simulation range. This allows us to identify three microscopic regimes, annihilation, aggregation, and reconstruction, in the evolution of defects and to propose atomistic justification for an analytical model of logarithmic relaxation. Furthermore, we show that growing activation barriers and configurational space exploration are kinetically limiting the system to a logarithmic relaxation. Overall, our long-time simulations do not support the amorphous cluster model but point rather to a relaxation driven by elastic interactions between defect complexes of all sizes.

Contribution of vacancies to relaxation in amorphous materials: A kinetic activation-relaxation technique study

Physical Review B, 2013

The nature of structural relaxation in disordered systems such as amorphous silicon (a-Si) remain... more The nature of structural relaxation in disordered systems such as amorphous silicon (a-Si) remains a fundamental issue in our attempts at understanding these materials. While a number of experiments suggest that mechanisms similar to those observed in crystals, such as vacancies, could dominate the relaxation, theoretical arguments point rather to the possibility of more diverse pathways. Using the kinetic activation-relaxation technique, an off-lattice kinetic Monte Carlo method with on-the-fly catalog construction, we resolve this question by following 1000 independent vacancies in a well-relaxed a-Si model at 300 K over a timescale of up to one second. Less than one percent of these survive over this period of time and none diffuse more than once, showing that relaxation and diffusion mechanisms in disordered systems are fundamentally different from those in the crystal.

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Optimization of the Kinetic Activation-Relaxation Technique, an off-lattice and self-learning kinetic Monte-Carlo method

Journal of Physics: Conference Series, 2012

We present two major optimizations for the kinetic Activation-Relaxation Technique (k-ART), an of... more We present two major optimizations for the kinetic Activation-Relaxation Technique (k-ART), an off-lattice self-learning kinetic Monte Carlo (KMC) algorithm with onthe-fly event search THAT has been successfully applied to study a number of semiconducting and metallic systems. K-ART is parallelized in a non-trivial way: A master process uses several worker processes to perform independent event searches for possible events, while all bookkeeping and the actual simulation is performed by the master process. Depending on the complexity of the system studied, the parallelization scales well for tens to more than one hundred processes. For dealing with large systems, we present a near order 1 implementation. Techniques such as Verlet lists, cell decomposition and partial force calculations are implemented, and the CPU time per time step scales sublinearly with the number of particles, providing an efficient use of computational resources.

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