Standards for molecular dynamics modelling and simulation of relaxation

Cluster-Assisted Mesoplasma Chemical Vapor Deposition for Fast Epitaxial Growth of SiGe/Si Heterostructures: A Molecular Dynamics Simulation Study

Co-condensation of mixed SiGe nanoclusters and impingement of SiGe nanoclusters on a Si substrate were applied using molecular dynamics (MD) simulation in this study to mimic the fast epitaxial growth of SiGe/Si heterostructures under mesoplasma chemical vapor deposition (CVD) conditions. The condensation dynamics and properties of the SiGe nanoclusters during the simulations were investigated first, and then the impingement of transient SiGe nanoclusters on both Si smooth and trench substrate surfaces under varying conditions was studied theoretically. The results show that the mixed nanoclusters as precursors demonstrate potential for enhancing epitaxial SiGe film growth at a high growth rate, owing to their loosely bound atomic structures and high mobility on the substrate surface. By varying cluster sizes and substrate temperatures, this study also reveals that smaller clusters and higher substrate temperatures contribute to faster structural ordering and smoother surface morphologies. Furthermore, the formed layers display a consistent SiGe composition, closely aligning with nominal values, and the cluster-assisted deposition method achieves the epitaxial bridging of heterostructures during cluster impingement, highlighting its additional distinctive characteristics. The implications of this work make it clear that the mechanism of fast alloyed epitaxial film growth by cluster-assisted mesoplasma CVD is critical for extending it as a versatile platform for synthesizing various epitaxial films.

Freezing in two-length-scale systems: complexity, universality and prediction

Two-length-scale pair potentials arise ubiquitously in condensed matter theory as effective interparticle interactions in molecular, metallic and soft matter systems. The existence of two different bond lengths generated by the shape of potential causes complicated behavior in even one-component systems: polymorphism in solid and liquid states, water-like anomalies, the formation of quasicrystals and high stability against crystallization. Here we address general properties of freezing in one-component two-length-scale systems and argue that solidification of a liquid during cooling is essentially determined by the radial distribution function (RDF) of the liquid. We show that different two-length-scale systems having similar RDFs freeze into the same solid phases. In some cases, the similarity between RDFs can be expressed by the proximity of two dimensionless effective parameters: the ratio between effective bond lengths,λ, and the fraction of short-bonded particlesφ. We validate this idea by studying the formation of different solid phases in different two-length-scale systems. The method proposed allows predicting effectively the formation of solid phases in both numerical simulations and self-assembling experiments in soft matter systems with tunable interactions.

Effects of structural inhomogeneity on equilibration processes in Langevin dynamics

In recent decades, computer experiments have led to an accurate and fundamental understanding of atomic and molecular mechanisms in fluids, such as different kinds of relaxation processes toward steady physical states. In this paper, we investigate how exactly the configuration of initial states in a molecular-dynamics simulation can affect the rates of decay toward equilibrium for the widely known Langevin canonical ensemble. For this purpose, we derive an original expression relating the system relaxation time τ_{sys} and the radial distribution function g(r) in the near-zero and high-density limit. We found that, for an initial state which is slightly marginally inhomogeneous in the number density of atoms, the system relaxation time τ_{sys} is much longer than that for the homogeneous case and an increasing function of the Langevin coupling constant, γ. We also found, during structural equilibration, g(r) at large distances approaches 1 from above for the inhomogeneous case and from below for the macroscopically homogeneous one.

Morphological aspect of crystal nucleation in wall-confined supercooled metallic film

In this paper, we simulate the nucleation and growth of crystalline nuclei in a molybdenum film cooled at different rates confined between two amorphous walls. We also compare the results for the wall-confined and wall-free systems. We apply the same methodology as in the work (Kirova and Pisarev 2019J. Cryst. Growth528125266) which is based on reconstructing the probability density function for the largest crystalline nucleus in the system. The size of the nucleus and the asphericity parameter are considered as the reaction coordinates. We demonstrate that in both the free and confined systems there are two mechanisms of crystal growth: the attachment of atoms to the biggest crystal from the amorphous phase and the merging of the biggest crystal cluster with small ones (coalescence). We show that the attachment mechanism is dominant in the melt cooled down at a slower rate, and the mechanism gradually shifts to coalescence as cooling rate increases. We also observe the formation of long-lived crystal clusters and demonstrate that amorphous walls do not affect their geometric characteristics. However, system confined between walls demonstrates higher glass-forming ability.

Modeling of the phase transition inside graphene nanobubbles filled with ethane

A liquid–gas phase transition of ethane inside graphene nanobubbles below the critical temperature leads to a ‘forbidden range’ of radii, in which no stable bubbles exist.

Excitation spectra of liquid iron up to superhigh temperatures

Investigation of excitation spectra of liquids is one of the hot test topics nowadays. In particular, recent experimental works showed that liquid metals can demonstrate transverse excitations and positive sound dispersion. However, the theoretical description of these experimental observations is still missing. Here we report a molecular dynamics study of excitation spectra of liquid iron. We compare the results with available experimental data to justify the method. After that we perform calculations for high temperatures to find the location of the Frenkel line introduced in our previous works.

Universal self-assembly of one-component three-dimensional dodecagonal quasicrystals

Using molecular dynamics simulations, we study computational self-assembly of one-component three-dimensional dodecagonal (12-fold) quasicrystals in systems with two-length-scale potentials. Existing criteria for three-dimensional quasicrystal formation are quite complicated and rather inconvenient for particle simulations. So to localize numerically the quasicrystal phase, one should usually simulate over a wide range of system parameters. We show how to universally localize the parameter values at which dodecagonal quasicrystal order may appear for a given particle system. For that purpose, we use a criterion recently proposed for predicting decagonal quasicrystal formation in one-component two-length-scale systems. The criterion is based on two dimensionless effective parameters describing the fluid structure which are extracted from the radial distribution function. The proposed method allows reduction of the time spent for searching the parameters favoring a certain solid structure for a given system. We show that the method works well for dodecagonal quasicrystals; this result is verified on four systems with different potentials: the Dzugutov potential, the oscillating potential which mimics metal interactions, the repulsive shoulder potential describing effective interactions for the core/shell model of colloids and the embedded-atom model potential for aluminum. Our results suggest that the mechanism of dodecagonal quasicrystal formation is universal for both metallic and soft-matter systems and it is based on competition between interparticle scales.

Crossover of collective modes and positive sound dispersion in supercritical state

Supercritical state has been viewed as an intermediate state between gases and liquids with largely unknown physical properties. Here, we address the important ability of supercritical fluids to sustain collective excitations. We directly study propagating modes on the basis of correlation functions calculated in molecular dynamics simulations and find that the supercritical system sustains propagating solid-like transverse modes below the Frenkel line but not above where there is one longitudinal mode only. Important thermodynamic implications of this finding are discussed. We directly detect positive sound dispersion (PSD) below the Frenkel line where transverse modes are operative and quantitatively explain its magnitude on the basis of transverse and longitudinal velocities. PSD disappears above the Frenkel line which therefore demarcates the supercritical phase diagram into two areas where PSD does and does not operate.

The behavior of cyclohexane confined in slit carbon nanopore

It is well known that confining a liquid into a pore strongly alters the liquid behavior. Investigations of the effect of confinement are of great importance for many scientific and technological applications. Here we present a molecular dynamics study of the behavior of cyclohexane confined in carbon slit pores. The local structure and orientational ordering of cyclohexane molecules are investigated. It is shown that the system freezes with decreasing the pore width, and the freezing temperature of nanoconfined cyclohexane is higher than the bulk one.

Direct simulations of homogeneous bubble nucleation: Agreement with classical nucleation theory and no local hot spots

We present results from direct, large-scale molecular dynamics simulations of homogeneous bubble (liquid-to-vapor) nucleation. The simulations contain half a billion Lennard-Jones atoms and cover up to 56 million time steps. The unprecedented size of the simulated volumes allows us to resolve the nucleation and growth of many bubbles per run in simple direct micro-canonical simulations while the ambient pressure and temperature remain almost perfectly constant. We find bubble nucleation rates which are lower than in most of the previous, smaller simulations. It is widely believed that classical nucleation theory (CNT) generally underestimates bubble nucleation rates by very large factors. However, our measured rates are within two orders of magnitude of CNT predictions; only at very low temperatures does CNT underestimate the nucleation rate significantly. Introducing a small, positive Tolman length leads to very good agreement at all temperatures, as found in our recent vapor-to-liquid nucleation simulations. The critical bubbles sizes derived with the nucleation theorem agree well with the CNT predictions at all temperatures. Local hot spots reported in the literature are not seen: Regions where a bubble nucleation event will occur are not above the average temperature, and no correlation of temperature fluctuations with subsequent bubble formation is seen.

Solid-density plasma nanochannel generated by a fast single ion in condensed matter

A plasma model of relaxation of a medium in heavy-ion tracks in condensed matter is proposed. The model is based on three assumptions: the Maxwell distribution of plasma electrons, localization of plasma inside the track nanochannel, and constant values of the plasma electron density and temperature during the x-ray irradiation. The model of multiple ionization of target atoms by a fast projectile ion is used to determine the initial conditions. An analysis of the results of the calculations performed makes it possible to define when the atomic relaxation model is a very rough approximation and the plasma relaxation model must be used. It is demonstrated that the plasma relaxation model adequately describes the x-ray spectra observed upon interaction of a fast ion with condensed target. The comparison with the experimental data justifies the reliability of the plasma relaxation model. Preassumptions of plasma relaxation model are validated by the molecular-dynamics simulation. An x-ray spectral method based on the plasma relaxation model is proposed for diagnostics of the plasma in fast ion tracks. The results obtained can be useful in examining the initial stage of defect formation in solids under irradiation with single fast heavy ions.

Cavitation in liquid metals under negative pressures. Molecular dynamics modeling and simulation

An approach to study cavitation in stretched liquids via molecular dynamics (MD) simulation is presented. It is based on the stochastic properties of MD and allows one to study cavitation as a stochastic phenomenon. The approach is used to study equation of state and stability limits of the metastable liquid phase, cavitation kinetics and dynamics properties for different temperatures. Particular examples of metals under consideration include Pb, Li and Pb(83)Li(17). Quantitative and qualitative disagreements between the classic nucleation theory estimates and the MD results are found. The Kolmogorov-Johnson-Mehl-Avrami equation is used as an alternative way to estimate cavitation rate. The two methods show good mutual agreement. Decay at a constant stretching rate is also considered.