Two-temperature Molecular Dynamics Research Articles

A modified two temperature molecular dynamics (2T-MD) model for cascades

Two-Temperature molecular dynamics (2T-MD) is a common approach for describing how electrons contribute to the evolution of a damage cascade by addressing their role in the redistribution of energy in the system. However, inaccuracies in 2T-MD’s treatment of the high-energy particles have limited its utilisation. Here, we propose a reformulation of the traditional 2T-MD scheme to overcome this limitation by addressing the spurious double-interaction of high-energy atoms with electrons. We conduct a series of radiation damage cascades for 30, 50, and 100 keV primary knock-on atoms in increasingly large cubic W cells. In the simulations, we employ our modified 2T-MD scheme along with other treatments of electron–phonon coupling to explore their impact on the cascade evolution and the number of remnant defects. The results suggest that with the proposed modification, 2T-MD simulations account for the temperature time evolution during the ballistic phase and remove arbitrary choices, thus providing a better description of the underlying physics of the damage process.

Inverted nucleation for photoinduced nonequilibrium melting.

Ultrafast photoinduced melting provides an essential platform for studying nonequilibrium phase transitions by linking the kinetics of electron dynamics to ionic motions. Knowledge of dynamic balance in their energetics is essential to understanding how the ionic reaction is influenced by femtosecond photoexcited electrons with notable time lag depending on reaction mechanisms. Here, by directly imaging fluctuating density distributions and evaluating the ionic pressure and Gibbs free energy from two-temperature molecular dynamics that verified experimental results, we uncovered that transient ionic pressure, triggered by photoexcited electrons, controls the overall melting kinetics. In particular, ultrafast nonequilibrium melting can be described by the reverse nucleation process with voids as nucleation seeds. The strongly driven solid-to-liquid transition of metallic gold is successfully explained by void nucleation facilitated by photoexcited electron-initiated ionic pressure, establishing a solid knowledge base for understanding ultrafast nonequilibrium kinetics.

Latent ion tracks were finally observed in diamond

Injecting high-energy heavy ions in the electronic stopping regime into solids can create cylindrical damage zones called latent ion tracks. Although these tracks form in many materials, none have ever been observed in diamond, even when irradiated with high-energy GeV uranium ions. Here we report the first observation of ion track formation in diamond irradiated with 2–9 MeV C60 fullerene ions. Depending on the ion energy, the mean track length (diameter) changed from 17 (3.2) nm to 52 (7.1) nm. High resolution scanning transmission electron microscopy (HR-STEM) indicated the amorphization in the tracks, in which π-bonding signal from graphite was detected by the electron energy loss spectroscopy (EELS). Since the melting transition is not induced in diamond at atmospheric pressure, conventional inelastic thermal spike calculations cannot be applied. Two-temperature molecular dynamics simulations succeeded in the reproduction of both the track formation under MeV C60 irradiations and the no-track formation under GeV monoatomic ion irradiations.

Observing femtosecond orbital dynamics in ultrafast Ge melting with time-resolved resonant X-ray scattering.

Photoinduced nonequilibrium phase transitions have stimulated interest in the dynamic interactions between electrons and crystalline ions, which have long been overlooked within the Born-Oppenheimer approximation. Ultrafast melting before lattice thermalization prompted researchers to revisit this issue to understand ultrafast photoinduced weakening of the crystal bonding. However, the absence of direct evidence demonstrating the role of orbital dynamics in lattice disorder leaves it elusive. By performing time-resolved resonant X-ray scattering with an X-ray free-electron laser, we directly monitored the ultrafast dynamics of bonding orbitals of Ge to drive photoinduced melting. Increased photoexcitation of bonding electrons amplifies the orbital disturbance to expedite the lattice disorder approaching the sub-picosecond scale of the nonthermal regime. The lattice disorder time shows strong nonlinear dependence on the laser fluence with a crossover behavior from thermal-driven to nonthermal-dominant kinetics, which is also verified by ab initio and two-temperature molecular dynamics simulations. This study elucidates the impact of bonding orbitals on lattice stability with a unifying interpretation on photoinduced melting.

Laser-induced electron dynamics and surface modification in ruthenium thin films

We performed the experimental and theoretical study of the heating and damaging of ruthenium thin films induced by femtosecond laser irradiation. We present the results of an optical pump-probe thermoreflectance experiment with rotating sample allowing to significantly reduce heat accumulation in irradiated spot. We show the evolution of surface morphology from growth of a heat-induced oxide layer at low and intermediate laser fluences to cracking and grooving at high fluences. Theoretical analysis of thermoreflectance in our pump-probe experiment allows us to relate behavior of hot electrons in ruthenium to the Fermi smearing mechanism. This conclusion invites more research on Fermi smearing of transition metals. The analysis of heating is performed with the two-temperature modeling and molecular dynamics simulation, results of which demonstrate that the calculated single-shot melting threshold is higher than experimental damage threshold. We suggest that the onset of Ru film damage is caused by the heat-induced stresses that lead to cracking of the Ru film. Such damage accumulates during repetitive exposure to light.

Two-temperature molecular dynamics simulation study of copper thin film irradiation with femtosecond and picosecond laser pulses

Metal targets irradiated with laser pulses have a wide range of applications in thin film preparation, nanomaterial synthesis, bio-medical imaging, and metal ablation. Here, using two-temperature model based molecular dynamics simulation, we investigate laser mediated ablation in copper. Ablation of the film starts with the formation of voids within it. This void forming mechanism at low laser fluences (≤100 mJ/cm2) is studied using both picosecond and femtosecond pulses. At the same fluence, shorter laser pulse transfers more energy to the atoms generating temperatures greater than the melting temperature of the crystal. This increases the kinetic energy of the atoms and they start vibrating with different velocities. If these vibrations cross a threshold of 5 Å per picosecond (500 m/s), voids and faults start appearing in the system. At the same fluence, higher concentration of voids are also created at a faster rate with the femtosecond pulse.

Electronic effects on the radiation damage in high-entropy alloys

High-entropy alloys (HEAs) are exceptional candidates for radiation-resistant materials due to their complex local chemical environment and slow defect migration. Despite commonly overlooked, electronic effects on defects evolution in radiation environments also play a crucial role by dissipating excess energy through electron-phonon coupling and electronic heat conduction during cascade events. We present a systematic study on electronic properties in random-solid solutions (RSS) in four and five principal elements HEAs and their effect on defect formation, clustering, and recombination. Electronic properties, including electron-phonon coupling factor (Ge−ph), the electronic specific heat (Ce), and the electronic thermal conductivity (κe), are computed within first-principles calculations. Using the two-temperature molecular dynamics simulations, we show that the electron-phonon coupling factor and electronic specific heat play a critical role in Frenkel pairs formation. Specifically, the electron-phonon coupling factor quickly dissipates the kinetic energy during primary knock-on atom events via plasmon excitations and is subsequently dissipated via the free-electrons conduction. We show that these effects are more critical than the elastic distortion effects produced by the atomic mismatch. Of tremendous interest, we show that including lighter elements helps to increase Ge−ph suggesting the possibility to improve radiation resistance in HEA through optimal composition.

Two-temperature molecular dynamics simulations of crystal growth in a tungsten supercooled melt

In this work we use the two-temperature model (TTM) coupled to molecular dynamics (MD) with sinks at the boundaries of the electronic subsystem to study crystal-growth rate in a quasi-one-dimensional tungsten crystal into a supercooled melt. The possibility of varying the extension of the electronic grid along with the sinks allows a more realistic description of the electronic thermal transport away from the system, providing a considerable heat dissipation from the crystallization front. Based on this approach, our results regarding crystal-growth rates are not affected even if the size of the system is changed. Moreover, comparisons are established with respect to MD and standard TTM simulations. For these comparisons between models, something remarkable is found, and it is that the temperature and the value of the maximum growth rate are the same. In contrast, the inclusion of sinks has a great impact with respect to the standard approaches specially reflected at low temperatures, where a frustration of the liquid–crystal interface dynamics is seen until a state of zero crystal growth is reached, which is not possible to characterize quantitatively since a kind of stochastic behavior is present.

Atomistic investigation of cavitation and ablation in tantalum foils under irradiation with x-rays approaching 5 keV

The rapid irradiation and heating of matter can lead to material removal via a process known as ablation. While previous investigations have focused on ablation with optical and soft x-ray pulses, the process is not well understood for the high-energy x-rays delivered at current x-ray free electron laser facilities. In this paper, we use hybrid two-temperature model molecular dynamics simulations to determine the damage threshold and dynamics for tantalum foils under irradiation with x-rays in the range 1--5 keV. We report that damage occurs for foils with thickness $\ensuremath{\ge}300$ nm when heated to around 1.25 eV/atom. This damage results from the combined processes of melting and cavitation, finally resulting in the removal of material layers. The predictions of this study, in terms of the cavitation threshold and underlying dynamics, could guide interpretation of experiments as well as applications including development of beamline optics for free-electron lasers. We report consistency between cavitation and ablation behavior in isochoric heating experiments and spall processes in hydrodynamic compression and release experiments, confirming the primary modes of damage are mechanical in nature for the x-ray energies investigated.

Development of a Hybrid Intelligent Process Model for Micro-Electro Discharge Machining Using the TTM-MDS and Gaussian Process Regression.

This paper proposed a hybrid intelligent process model, based on a hybrid model combining the two-temperature model (TTM) and molecular dynamics simulation (MDS) (TTM-MDS). Combined atomistic-continuum modeling of short-pulse laser melting and disintegration of metal films [Physical Review B, 68, (064114):1–22.], and Gaussian process regression (GPR), for micro-electrical discharge machining (micro-EDM) were also used. A model of single-spark micro-EDM process has been constructed based on TTM-MDS model to predict the removed depth (RD) and material removal rate (MRR). Then, a GPR model was proposed to establish the relationship between input process parameters (energy area density and pulse-on duration) and the process responses (RD and MRR) for micro-EDM machining. The GPR model was trained, tested, and tuned using the data generated from the numerical simulations. Through the GPR model, it was found that micro-EDM process responses can be accurately predicted for the chosen process conditions. Therefore, the hybrid intelligent model proposed in this paper can be used for a micro-EDM process to predict the performance.

A molecular dynamics study of laser-excited gold

The structural evolution of laser-excited systems of gold has previously been measured through ultrafast MeV electron diffraction. However, there has been a long-standing inability of atomistic simulations to provide a consistent picture of the melting process, leading to large discrepancies between the predicted threshold energy density for complete melting, as well as the transition between heterogeneous and homogeneous melting. We make use of two-temperature classical molecular dynamics simulations utilizing three highly successful interatomic potentials and reproduce electron diffraction data presented by Mo et al. [Science 360, 1451–1455 (2018)]. We recreate the experimental electron diffraction data, employing both a constant and temperature-dependent electron–ion equilibration rate. In all cases, we are able to match time-resolved electron diffraction data, and find consistency between atomistic simulations and experiments, only by allowing laser energy to be transported away from the interaction region. This additional energy-loss pathway, which scales strongly with laser fluence, we attribute to hot electrons leaving the target on a timescale commensurate with melting.

Exploring Femtosecond Laser Ablation by Snapshot Ultrafast Imaging and Molecular Dynamics Simulation

Femtosecond laser ablation (FLA) has been playing a prominent role in precision fabrication of material because of its circumvention of thermal effect and extremely high spatial resolution. Molecular dynamics modeling, as a powerful tool to study the mechanism of femtosecond laser ablation, still lacks the connection between its simulation results and experimental observations at present. Here we combine a single-shot chirped spectral mapping ultrafast photography (CSMUP) technique in experiment and a three-dimensional two-temperature model-based molecular dynamics (3D TTM-MD) method in theory to jointly investigate the FLA process of bulky gold. Our experimental and simulated results show quite high consistency in time-resolved morphologic dynamics. According to the highly accurate simulations, the FLA process of gold at the high laser fluence is dominated by the phase explosion, which shows drastic vaporized cluster eruption and pressure dynamics, while the FLA process at the low laser fluence mainly results from the photomechanical spallation, which shows moderate temperature and pressure dynamics. This study reveals the ultrafast dynamics of gold with different ablation schemes, which has a guiding significance for the applications of FLA on various kinds of materials.

Elastoplastic and polymorphic transformations of iron at ultra-high strain rates in laser-driven shock waves

Elastoplastic and polymorphic α–ε transformations in iron films induced by ultra-short laser-driven shock waves are studied. Interpretation of time-resolved interferometric measurements is performed using an inverse analysis technique of experimental rear-side velocity profiles. The lasts are obtained by numerical differentiation of free surface displacements detected by probe laser pulses. The inverse analysis techniques are validated in consistent two-temperature hydrodynamics and molecular dynamics simulations of laser energy deposition and diffusion, generation, and propagation of shock waves in a polycrystalline iron sample. The stress–strain diagrams containing information about elastoplastic deformation and phase transformation are reconstructed by the inverse analysis. We found that the polymorphic transformation in iron under picosecond duration of loading requires much higher stress in contrast to that in microsecond-scale plate-impact experiments. Moreover, such transition may be accomplished partially even at very high stresses if an unloading tail after the shock front is too short.

Erratum: “Simulation of femtosecond laser ablation and spallation of titanium film based on two-temperature model and molecular dynamics” [J. Laser Appl. 33, 012047 (2021)

Erratum: “Simulation of femtosecond laser ablation and spallation of titanium film based on two-temperature model and molecular dynamics” [J. Laser Appl. 33, 012047 (2021)

Melting characteristics and dynamic evolution of microstructure of single-crystal gold film irradiated by laser under the non-Fourier effect

The process of ultra-thin single crystal gold film irradiated by femtosecond laser is simulated by the combination of improved two-temperature model (TTM) and molecular dynamics (MD). The results indicate that under the non-Fourier effect, the gold film shows special melting characteristics and spalling mechanism. The phenomena of homogeneous melting and pore spalling of gold film are successfully distinguished by curing rate method and atomic density method respectively. Due to the tiny thickness of the gold film studied, the damage mechanism of the gold film irradiated by laser has also changed.

Simulation of femtosecond laser ablation and spallation of titanium film based on two-temperature model and molecular dynamics

In order to study the interaction between a femtosecond laser and a titanium film, the method of combining the two-temperature model and molecular dynamics was used to simulate the ablation and spallation of the titanium film by the femtosecond laser with a wavelength of 800 nm. The spallation of the titanium film was explained from the perspective of the temperature and the stress wave. It is found that the femtosecond laser with a pulse width of 300 fs loads the titanium film, and the ablation threshold is around 0.05 J/cm2. The effects of different pulse widths in the range of 100–500 fs and different energy densities in the range of 0.10–0.20 J/cm2 on the spallation results of titanium films were also studied. We found that the femtosecond laser with a pulse width of 100 fs promotes the spallation phenomenon to occur more quickly. The surface spallation products of the titanium film were large clusters of atoms when the energy density is 0.10 J/cm2, but most of the surface spallation products of the titanium film were single atoms when the energy density is 0.20 J/cm2. These indicated that the energy density is an important factor affecting the spallation products of the titanium film. The evolution of maximum electron temperature, electron-lattice equilibrium temperature, and coupling time with pulse width and energy density are also given and analyzed. These results have important reference significance for studying the fracture and damage of titanium, and they help us to understand the mechanism of femtosecond laser processing materials.

On the mechanism of the shape elongation of embedded nanoparticles

The mechanism of the shape elongation of metal nanoparticles (NPs) in silica, which is induced under swift heavy ion irradiation, is discussed with comparing the two candidates: (i) the synergy between the ion hammering and the transient melting of NPs by the inelastic thermal spike and (ii) the thermal pressure and flow model. We show that three experimental results are inconsistent with (i). The latter is supported by two-temperature molecular dynamics simulations, which simulate not only the atomic motions but also the local electron temperatures. A remarkable correlation was observed between the temporal evolution of the silica density around the ion trajectory and that of the aspect ratio of the NP later than ~ 1 ps after the ion impact, while no correlation was observed earlier than ~ 1 ps, even under the assumption of the instantaneous energy deposition.

Structural transition dynamics of the formation of warm dense gold: From an atomic scale view

With the highly optimized embedded-atom-method (EAM) potential and electron-phonon coupling factor obtained from experimental data, the dynamics of the formation of warm dense gold and the nuclear response of gold foils upon intense laser excitation were investigated using two-temperature molecular dynamics simulations. Considering laser energy densities ranging from 0.18 to 1.17 MJ/kg, we provide a microscopic picture of the formation of warm dense gold. A threshold (0.19 MJ/kg) for the laser energy density was determined, identifying two different melting mechanisms. For an energy density below 0.19 MJ/kg, the melting of the foil is controlled by the propagation of melt fronts from external surfaces, which results in heterogeneous melting on the time scale of hundreds of picoseconds. For an energy density above 0.19 MJ/kg, homogeneous nucleation and growth of liquid regions inside the foil play the leading role, and homogeneous melting occurs with several picoseconds. Compared with previous simulations and experimental measurements, the evaluated different threshold value indicates that the improvement in the electron heat capacity for the two-temperature model by including the kinetic information of electrons may predict better laser-matter interactions under such extreme non-equilibrium conditions.

Electron-phonon coupling induced defect recovery and strain relaxation in Ni and equiatomic NiFe alloy

Abstract We study the effects of local heating due to electron-phonon coupling in cumulative 10 MeV Ni ion irradiation on pre-damaged pure Ni and equiatomic NiFe alloy. In particular, we estimate the defect recovery in NiFe compared to pure Ni due to the chemical disorder. The simulations are done using the two-temperature molecular dynamics approach that includes the electronic stopping and electron-phonon interactions. A total of 50% of the damage that consists of isolated point defects and small defect clusters is recovered in NiFe, whereas no noticeable recovery is observed in elemental Ni. We have also demonstrated strain relaxation associated with defect recombination based on the results from both simulation and X-ray diffraction measurements in NiFe. The damage recovery primarily depends on pre-existing damage level, defect configuration, ion energy (stopping powers) and electron-phonon coupling parameter of the materials of interest.

Thermal transport in model copper-polyethylene interfaces.

Thermal transport through model copper-polyethylene interfaces is studied using two-temperature nonequilibrium molecular dynamics. This approach treats electronic and phonon contributions to the thermal transport in the metallic region, but only phonon mediated transport is assumed in the polymer. Results are compared with nonequilibrium molecular dynamics simulations of heat transport in which only phonon contributions are incorporated. The influence of the phase of the polymer component (crystalline, amorphous, and lamella) and, where relevant, its orientation relative to the metallic interface structure is explored. These computational studies suggest that the thermal conductivity of the metal-polymer interface can be more than 40 times greater when the polymer chains of the lamella are oriented perpendicular to the interface than the situation when the interface is formed by an amorphous polymer or a crystalline polymer phase in which the chains orient parallel to the interface. The simulations suggest that the phonon contribution to the thermal conductivity of the copper region can be increased by as much as a factor of three when coupling between the electrons and phonons in the metal region is incorporated. This, combined with the explicit inclusion of the purely electronic component of the thermal transport in the metal region, can lead to a substantial increase in the heat flux promoted by the interface while maintaining a constant temperature drop. These simulation results have important implications for designing materials that have excellent electrical insulation properties but can also be highly effective in heat conduction.