Monatomic Liquids Research Articles

Relationship between the potential energy landscape and the dynamic crossover in a water-like monatomic liquid with a liquid-liquid phase transition

In the case of fragile liquids, dynamical properties such as the structural relaxation time evolve from Arrhenius at high-temperatures to non-Arrhenius at low temperatures. Computational studies show that (i) in the Arrhenius dynamic domain, the liquid samples regions of the potential energy landscape (PEL) that are insensitive to temperature (PEL-independent regime) and the relaxation is exponential, while (ii) in the non-Arrhenius dynamic domain, the topography of the PEL explored by the liquid varies with temperature (PEL-influenced regime) and the relaxation is non-exponential. In this work we explore whether the correlation between dynamics and PEL regimes, points (i) and (ii), holds for the Fermi-Jagla (FJ) liquid. This is a monatomic model liquid that exhibits many of the water anomalous properties, including maxima in density and diffusivity. The FJ model is a rather complex liquid that exhibits a liquid-liquid phase transition and a liquid-liquid critical point (LLCP), as hypothesized for the case of water. We find that, for the FJ liquid, the correlation between dynamics and the PEL regimes is not always present and depends on the density of the liquid. For example, at high density, the liquid exhibits Arrhenius/non-Arrhenius (AnA) dynamical crossover, exponential/non-exponential (EnE) relaxation crossover, and a PEL-independent/PEL-influenced regime crossover, consistent with points (i) and (ii). However, in the vicinity of the LLCP, the AnA crossover is absent but the liquid exhibits EnE relaxation and PEL regime crossovers. At very low density, crystallization intervenes and the PEL regime crossover is suppressed. Yet, the AnA dynamical crossover and the EnE relaxation crossover remain. It follows that the dynamics in liquids (AnA and EnE crossovers) are not necessarily correlated with the changes between the PEL regimes, as one could have expected. Interestingly, the AnA crossover in the FJ liquid is not related to the presence of the Widom line. This result may seem to be at odds with previous studies of polymorphic model liquids, and a simple explanation is provided.

Short-pulse laser formation of monatomic metallic glass in tantalum nanowire

The short-pulse laser heating of a tantalum nanowire is simulated by a hybrid method coupling the two-temperature model into the molecular dynamics. It is shown that the monatomic metallic glasses can be formed by short-pulse lasers. The critical cooling rate for vitrifying the pure metallic liquids in tantalum nanowire is estimated to be on the order of 1013 K s−1. Further simulations with different laser parameters and heated spot size are performed, demonstrating that the short-pulse laser quenching is a robust and promising alternative to the vitrification of monatomic metallic liquids into glassy state.

V-T theory for the self-intermediate scattering function in a monatomic liquid

In V-T theory the atomic motion is harmonic vibrations in a liquid-specific potential energy valley, plus transits, which move the system rapidly among the multitude of such valleys. In its first application to the self intermediate scattering function (SISF), V-T theory produced an accurate account of molecular dynamics (MD) data at all wave numbers q and time t. Recently, analysis of the mean square displacement (MSD) resolved a crossover behavior that was not observed in the SISF study. Our purpose here is to apply the more accurate MSD calibration to the SISF, and assess the results. We derive and discuss the theoretical equations for vibrational and transit contributions to the SISF. The time evolution is divided into three successive intervals: the vibrational interval when the vibrational contribution alone accurately accounts for the MD data; the crossover when the vibrational contribution saturates and the transit contribution becomes resolved; and the diffusive interval when the transit contribution alone accurately accounts for the MD data. The resulting theoretical error is extremely small at all q and t. V-T theory is compared to mode-coupling theories for the MSD and SISF, and to recent developments in Brownian motion experiments and theory.

Glass formation and cluster evolution in the rapidly solidified monatomic metallic liquid Ta under high pressure

Abstract Molecular dynamics (MD) simulations have been performed to examine the glass formation and cluster evolution during the rapid solidification of monatomic metallic liquid Ta under high pressure. The atomic structures in the systems are characterized by the radical distribution function (RDF), Honeycutt–Anderson (H–A) bond-type index method and cluster-type index method (CTIM). It is observed that the defective icosahedra play the critical role in the formation of Ta monatomic metallic glasses (MGs) rather than (12 0 12 0) perfect icosahedra, which have been identified as the basic local atomic units in many multi-component MGs. With the increase of pressure P , the fraction of icosahedral type clusters decreases remarkably in Ta MGs, while the fraction of bcc type clusters rises evidently. The evolution of vitrification degree ( D SRO or D MRO ) of the rapidly cooled metal Ta system further reveals that a higher pressure P is disadvantageous to the formation of Ta monatomic MGs. The weaker glass forming ability (GFA) of liquid metal Ta obtained under higher pressure P can be contributed to the decrease of D SRO or D MRO which is induced by increasing high pressure P to some extent.

Atomic motion from the mean square displacement in a monatomic liquid

V-T theory is constructed in the many-body Hamiltonian formulation, and is being developed as a novel approach to liquid dynamics theory. In this theory the liquid atomic motion consists of two contributions, normal mode vibrations in a single representative potential energy valley, and transits, which carry the system across boundaries between valleys. The mean square displacement time correlation function (the MSD) is a direct measure of the atomic motion, and our goal is to determine if the V-T formalism can produce a physically sensible account of this motion. We employ molecular dynamics (MD) data for a system representing liquid Na, and find the motion evolves in three successive time intervals: on the first ‘vibrational’ interval, the vibrational motion alone gives a highly accurate account of the MD data; on the second ‘crossover’ interval, the vibrational MSD saturates to a constant while the transit motion builds up from zero; on the third ‘random walk’ interval, the transit motion produces a purely diffusive random walk of the vibrational equilibrium positions. This motional evolution agrees with, and adds refinement to, the MSD atomic motion as described by current liquid dynamics theories.

Confinement effects on the liquid-liquid phase transition and anomalous properties of a monatomic water-like liquid.

We use molecular dynamics simulations to study the effects of confinement on the phase behavior of a water-like monatomic liquid that exhibits a liquid-liquid phase transition (LLPT) and a liquid-liquid critical point (LLCP). The liquid is confined between parallel walls and we focus on the effects of wall separation and surface chemistry (solvophobicity/solvophilicity) on the location of the LLCP, temperature of maximum density (TMD) line, and loci of compressibility maxima (CM). It is found that, independently of the surface solvophobicity/solvophilicity, the LLCP, TMD, and CM lines shift rapidly towards higher pressures and lower temperatures as the wall separation is reduced. It follows that the effects of confinement on the TMD and CM lines are indicative of the confinement effects on the LLCP/LLPT. Confinement effects are observable already when the liquid particles form ≈15 layers between the walls. For the case of water, this corresponds to a separation of ≈4-5 nm between the surfaces, larger than the confining dimension of the nanopores commonly used to study the hypothesized LLPT in confined water. Hence, our results suggest that such experiments should not be interpreted in terms of the phase diagrams proposed for bulk water.

Excess entropy and crystallization in Stillinger-Weber and Lennard-Jones fluids.

Molecular dynamics simulations are used to contrast the supercooling and crystallization behaviour of monatomic liquids that exemplify the transition from simple to anomalous, tetrahedral liquids. As examples of simple fluids, we use the Lennard-Jones (LJ) liquid and a pair-dominated Stillinger-Weber liquid (SW16). As examples of tetrahedral, water-like fluids, we use the Stillinger-Weber model with variable tetrahedrality parameterized for germanium (SW20), silicon (SW21), and water (SW(23.15) or mW model). The thermodynamic response functions show clear qualitative differences between simple and water-like liquids. For simple liquids, the compressibility and the heat capacity remain small on isobaric cooling. The tetrahedral liquids in contrast show a very sharp rise in these two response functions as the lower limit of liquid-phase stability is reached. While the thermal expansivity decreases with temperature but never crosses zero in simple liquids, in all three tetrahedral liquids at the studied pressure, there is a temperature of maximum density below which thermal expansivity is negative. In contrast to the thermodynamic response functions, the excess entropy on isobaric cooling does not show qualitatively different features for simple and water-like liquids; however, the slope and curvature of the entropy-temperature plots reflect the heat capacity trends. Two trajectory-based computational estimation methods for the entropy and the heat capacity are compared for possible structural insights into supercooling, with the entropy obtained from thermodynamic integration. The two-phase thermodynamic estimator for the excess entropy proves to be fairly accurate in comparison to the excess entropy values obtained by thermodynamic integration, for all five Lennard-Jones and Stillinger-Weber liquids. The entropy estimator based on the multiparticle correlation expansion that accounts for both pair and triplet correlations, denoted by S(trip), is also studied. S(trip) is a good entropy estimator for liquids where pair and triplet correlations are important such as Ge and Si, but loses accuracy for purely pair-dominated liquids, like LJ fluid, or near the crystallization temperature (T(thr)). Since local tetrahedral order is compatible with both liquid and crystalline states, the reorganisation of tetrahedral liquids is accompanied by a clear rise in the pair, triplet, and thermodynamic contributions to the heat capacity, resulting in the heat capacity anomaly. In contrast, the pair-dominated liquids show increasing dominance of triplet correlations on approaching crystallization but no sharp rise in either the pair or thermodynamic heat capacities.

Ab initio molecular dynamics study of the local atomic structures in monatomic metallic liquid and glass

Abstract Monatomic metallic glasses (MGs), being the simplest glass formers, have been obtained successfully in recent experiments, offering unique possibilities for studying the local atomic structures in glasses. In this study, the atomic-level structures in tantalum (Ta) liquid and Ta monatomic MG have been analyzed using ab initio molecular dynamics simulations. A comparison of atomic structures in monatomic MGs and in multicomponent MGs has been made. It is found that the dominant Voronoi polyhedra in Ta monatomic MG is the distorted icosahedra 〈0, 1, 10, 2〉 rather than the full icosahedra 〈0, 0, 12, 0〉, which has been identified as the basic local structure in many multicomponent MGs. Our results reveal that the fundamental process of Ta MG formation is the formation of icosahedral type clusters, including the distorted and full icosahedra. The global atomic packing in Ta MG can be characterized by the spherical-periodic order combined with the local translational symmetry of atomic arrangements in MGs, in good agreement with previous theoretical and experimental studies on multicomponent MGs. Our findings have implications in understanding the formation of MGs, and provide a check for the existing atomic structural models proposed over the years for MGs.

Synergy and pinning effects in a monatomic liquid film in confined conditions.

The freezing behavior of a monatomic liquid film in confined conditions during rapid cooling was studied by molecular dynamics simulations. We illustrated the synergy and pinning effects of the local icosahedral order during freezing. Our results show that the icosahedron contributes to nucleation through the synergy with other short-range ordered structures and participates in crystal growth via assimilation, but the pinning effect should be overcome when crystals grow. Furthermore, a semi-ordered morphology with maze-like nano-patterns emerged due to the cooperation between the synergy effect and the pinning effect. Our findings shed light on the correlation between the local icosahedral order and the crystalline medium-range order, providing a better understanding of the rapid solidification.

Substrate effects on glass formation in simple monatomic supercooled liquids

Glass formation in simple monatomic supercooled liquids on substrate is studied via molecular dynamics simulations. We find that glass formation in thin films on substrate exhibits a ‘heterogeneous’ behavior, i.e. solidlike atoms initiate/enhance simultaneously in the near substrate region and in the interior of the system, then solidlike domain grows outward to the free surface. Substrate causes strong layering of the near substrate region of liquid and glass. Layering is enhanced with decreasing temperature and layering region exhibits a glassy behavior instead of crystalline one. We find that structure and dynamics of liquid and glass in the near substrate region is different from those of the remaining part of thin films. This means that thin films formed on substrate can be divided into three distinct parts: near substrate region, interior and free surface region. We find a significant amount of liquidlike atoms even at T<Tg which may perform local motion like Johari–Goldstein process.

Cooling rate effects on structure of amorphous graphene

Simple monatomic amorphous 2D models with Honeycomb structure are obtained from 2D simple monatomic liquids with Honeycomb interaction potential (Rechtsman et al., Phys. Rev. Lett. 95, 228301 (2005)) via molecular dynamics (MD) simulations. Models are observed by cooling from the melt at various cooling rates. Temperature dependence of thermodynamic and structural properties including total energy, mean ring size, mean coordination number is studied in order to show evolution of structure and thermodynamics upon cooling from the melt. Structural properties of the amorphous Honeycomb structures are studied via radial distribution function (RDF), coordination number and ring distributions together with 2D visualization of the atomic configurations. Amorphous Honeycomb structures contain a large amount of structural defects including new ones which have not been previously reported yet. Cooling rate dependence of structural properties of the obtained amorphous Honeycomb structures is analyzed. Although amorphous graphene has been proposed theoretically and/or recently obtained by the experiments, our understanding of structural properties of the system is still poor. Therefore, our simulations highlight the situation and give deeper understanding of structure and thermodynamics of the glassy state of this novel 2D material.

Formation of monatomic metallic glasses through ultrafast liquid quenching

It has long been conjectured that any metallic liquid can be vitrified into a glassy state provided that the cooling rate is sufficiently high. Experimentally, however, vitrification of single-element metallic liquids is notoriously difficult. True laboratory demonstration of the formation of monatomic metallic glass has been lacking. Here we report an experimental approach to the vitrification of monatomic metallic liquids by achieving an unprecedentedly high liquid-quenching rate of 10(14)Ks(-1). Under such a high cooling rate, melts of pure refractory body-centred cubic (bcc) metals, such as liquid tantalum and vanadium, are successfully vitrified to form metallic glasses suitable for property interrogations. Combining in situ transmission electron microscopy observation and atoms-to-continuum modelling, we investigated the formation condition and thermal stability of the monatomic metallic glasses as obtained. The availability of monatomic metallic glasses, being the simplest glass formers, offers unique possibilities for studying the structure and property relationships of glasses. Our technique also shows great control over the reversible vitrification-crystallization processes, suggesting its potential in micro-electromechanical applications. The ultrahigh cooling rate, approaching the highest liquid-quenching rate attainable in the experiment, makes it possible to explore the fast kinetics and structural behaviour of supercooled metallic liquids within the nanosecond to picosecond regimes.

‘Graphenization’ of 2D simple monatomic liquids

We present molecular dynamics (MD) simulations of the formation of 2D materials with a honeycomb structure from 2D simple monatomic liquids with honeycomb interaction potential (Rechtsman et al 2005 Phys. Rev. Lett. 95 228301). Models are observed by cooling from the melt at various cooling rates. Thermodynamics of the phase transitions is analyzed in detail. Depending on the cooling rate, amorphous or crystalline honeycomb structures have been found. Structural properties of the crystalline honeycomb structure are studied via radial distribution function (RDF), coordination number and ring distribution, including 2D visualization of the atomic configurations. We find evidence for the existence of polycrystalline honeycomb structures and new structural defects, not previously reported. The atomic mechanism that forms the solid phase of a honeycomb structure from the liquid state has been analyzed by monitoring the spatio-temporal arrangement of atoms in 6-fold rings and/or atoms with the coordination number , occurring upon cooling from the melt. Since knowledge of how real 2D solids with honeycomb structures form from the vapor or liquid phase is still completely lacking, our simulations highlight the situation and give a deeper understanding of the structure and thermodynamics of real 2D materials such as graphene, silicene, germanene, etc.

A diagrammatic kinetic theory of density fluctuations in simple liquids in the overdamped limit. I. A long time scale theory for high density

Starting with a formally exact diagrammatic kinetic theory for the equilibrium correlation functions of particle density and current fluctuations for a monatomic liquid, we develop a theory for high density liquids whose interatomic potential is continuous and has a strongly repulsive short ranged part. We assume that interparticle collisions via this short ranged part of the potential are sufficient to randomize the velocities of the particles on a very small time scale compared with the fundamental time scale defined as the particle diameter divided by the mean thermal velocity. When this is the case, the graphical theory suggests that both the particle current correlation functions and the memory function of the particle density correlation function evolve on two distinct time scales, the very short time scale just mentioned and another that is much longer than the fundamental time scale. The diagrams that describe the motion on each of these time scales are identified. When the two time scales are very different, a dramatic simplification of the diagrammatic theory at long times takes place. We identify an irreducible memory function and a more basic function, which we call the irreducible memory kernel. This latter function evolves on the longer time scale only and determines the time dependence of the density and current correlation functions of interest at long times. In Paper II, a simple one-loop approximation for the irreducible memory kernel is used to calculate correlation functions for a Lennard-Jones fluid at high density and a variety of temperatures.

Glass transitions in a monatomic liquid with two glassy states.

We perform out-of-equilibrium molecular dynamics simulations of a monatomic liquid that exhibits liquid and glass polymorphism, with two distinct glasses, low- (LDA) and high-density (HDA) amorphous solids. By performing isobaric heating simulations of LDA and HDA at different pressures, we determine (a) the glass transition temperature of LDA and HDA, TgLDA(P) and TgHDA(P), as well as (b) the corresponding glass-glass transformation temperatures, TLDA-HDA(P) and THDA-LDA(P). It is found that TgLDA(P) is anomalous; i.e., it decreases with increasing pressure, while TgHDA(P) increases with increasing pressure. Interestingly, the TgLDA(P) and TLDA-HDA(P) loci, as well as the TgHDA(P) and THDA-LDA(P) loci, constitute smooth single lines in the P-T plane, suggesting that heating-induced glass-glass and glass transitions are related. We discuss the present results in the context of water experiments and simulations.

Estimating the density-scaling exponent of a monatomic liquid from its pair potential

This paper investigates two conjectures for calculating the density dependence of the density-scaling exponent γ of a single-component, pair-potential liquid with strong virial potential-energy correlations. The first conjecture gives an analytical expression for γ directly in terms of the pair potential. The second conjecture is a refined version of this involving the most likely nearest-neighbor distance determined from the pair-correlation function. The conjectures are tested by simulations of three systems, one of which is the standard Lennard-Jones liquid. While both expressions give qualitatively correct results, the second is more accurate.

分子動力学計算系におけるバルク液体の自己拡散係数に対する計算系形状の影響

In the present study, molecular dynamics (MD) simulations on simple liquids like monatomic Lennard-Jones and water bulk liquids in periodic boundary systems were performed to clarify the effect of the system size and shape on self-diffusion coefficient. So far, in the previous publications, the system size effect in cubic computational domains has been reported, and these studies showed that the diffusion coefficient becomes smaller as the system size decreases. Here we examined the system size effect not only in the cubic cell systems, but also in the rectangular cell systems which were created by changing one side length of the cubic cell with the system density kept constant. It is found that the diffusion coefficient in the direction perpendicular to the long side of the rectangular cell significantly increases more or less linearly as the side length increases for both liquids. On the other hand, the diffusion coefficient in the direction along the long side is almost constant. In consequence, anisotropy of the diffusivity appears in the rectangular cell with a periodic boundary condition even though the diffusion property of bulk liquids is examined.

Identification of structure in condensed matter with the topological cluster classification

We describe the topological cluster classification (TCC) algorithm. The TCC detects local structures with bond topologies similar to isolated clusters which minimise the potential energy for a number of monatomic and binary simple liquids with m ≤ 13 particles. We detail a modified Voronoi bond detection method that optimizes the cluster detection. The method to identify each cluster is outlined, and a test example of Lennard-Jones liquid and crystal phases is considered and critically examined.

Formation of the smectic-Bcrystal from a simple monatomic liquid

We report a molecular dynamics simulation demonstrating that the smectic-B crystalline phase (Cry-B), commonly observed in mesogenic systems of anisotropic molecules, can be formed by a system of identical particles interacting via a spherically symmetric potential. The Cry-B phase forms as a result of a first-order transition from an isotropic liquid phase upon isochoric cooling at appropriate number density. Its structure, determined by the design of the pair potential, corresponds to the Cry-B structure formed by elongated particles with the aspect ratio 1.8. The diffraction pattern and the real-space structure inspection demonstrate dominance of the ABC-type of axial layer stacking. This result opens a general possibility of producing smectic phases using isotropic interparticle interaction both in simulations and in colloidal systems.

Isomorphs, hidden scale invariance, and quasiuniversality

This paper first establishes an approximate scaling property of the potential-energy function of a classical liquid with good isomorphs (a Roskilde-simple liquid). This "pseudohomogeneous" property makes explicit that-and in which sense-such a system has a hidden scale invariance. The second part of the paper gives a potential-energy formulation of the quasiuniversality of monatomic Roskilde-simple liquids, which was recently rationalized in terms of the existence of a quasiuniversal single-parameter family of reduced-coordinate constant-potential-energy hypersurfaces [J. C. Dyre, Phys. Rev. E 87, 022106 (2013)]. The new formulation involves a quasiuniversal reduced-coordinate potential-energy function. A few consequences of this are discussed.