Atomic Velocity Research Articles

First and Second Sound in Two-Dimensional Bosonic and Fermionic Superfluids

We review our theoretical results of the sound propagation in two-dimensional (2D) systems of ultracold fermionic and bosonic atoms. In the superfluid phase, characterized by the spontaneous symmetry breaking of the U(1) symmetry, there is the coexistence of first and second sound. In the case of weakly-interacting repulsive bosons, we model the recent measurements of the sound velocities of 39K atoms in 2D obtained in the weakly-interacting regime and around the Berezinskii–Kosterlitz–Thouless (BKT) superfluid-to-normal transition temperature. In particular, we perform a quite accurate computation of the superfluid density and show that it is reasonably consistent with the experimental results. For superfluid attractive fermions, we calculate the first and second sound velocities across the whole BCS-BEC crossover. In the low-temperature regime, we reproduce the recent measurements of first-sound speed with 6Li atoms. We also predict that there is mixing between sound modes only in the finite-temperature BEC regime.

Dynamic strength, reinforcing mechanism and damage of ceramic metal composites

• We fully study the shock responses of ceramic metal composites via NEMD . • The interaction of shock-wave with the interface affects dislocation strengthening. • An abnormal softening effect is captured under shock loading . • The interface possesses negative effect on damage for composites. Shock tolerance is desirable for ceramic particles-reinforced metal matrix composites in many applications, where the dislocation dynamics evolution under the extreme load is the key but still elusive. Herein, we have investigated the dislocation motion and interaction under shock loading of SiC/Al nanocomposites using molecular dynamics simulations. We have demonstrated that the plastic deformation occurs at an impact velocity (0.5 km/s) lower than the Hugoniot elastic limit of aluminum due to the reflected shear wave effect. The Al/SiC interfaces act as a dislocation emitter to control dislocation multiplication density and slip direction, opening a new pathway to achieve ultrahigh-strength via shock loading. When the impact velocity (1.0 or 1.5 km/s) exceeds the Hugoniot elastic limit, the effect of nanoparticles on dislocation structure has changed from multiplying to retarding dislocations. The spall strength of composites improves due to dislocations pile-up at interface. Instead, the damage in the matrix is exacerbated, owing to the enhanced residual peak stress and interface reflection waves. In addition, the effect of abnormal shock softening determined by atomic velocity is revealed, which could be suppressed by increasing impact energy dissipation. Meanwhile, dynamic compressive strength depends on pressure and dislocation structures evolution. Our atomistic insights might be helpful in designing advanced shock-tolerant materials.

Efficient state-symmetric beamsplitters and mirrors for atom interferometers using optimized pulses

Atom interferometers that employ atoms in superpositions of different electronic states are sensitive to any noise that affects these superposed states differently. Resilience to such noise results from using superpositions where the atomic states differ in momentum only, but implementation of such ‘state-symmetric’ diffraction can lead to population loss into unwanted states and restricts the atomic velocity acceptance of the interferometer. In this paper, by varying the laser intensities and phases as functions of time, we present optimized pulses designed for use in state-symmetric interferometers that overcome these restrictions. We extend this optimization to multi-pulse sequences designed to increase the interferometer area and demonstrate significant improvements in the fringe visibility compared with sequences of π/2 and π pulses. We discuss the limits on the temperature of the atomic source required for efficient atomic diffraction and show how optimized pulse sequences enable efficient diffraction with considerably warmer clouds, hence reducing the need for velocity selection and increasing the measurement signal-to-noise ratio.

Effects of undercooling on atomic crystallization behaviors and growth mechanisms of pure metals

The atomic crystallization behaviors at the crystal–melt interfaces in a broad range of undercoolings are investigated by molecular dynamics simulations for two representative pure metals, FCC Cu and BCC Ta. Results show that the atomic transformation displacements against temperature for both metals have the same trend, i.e., increasing significantly as temperature goes up at small undercooling and keeping invariant at large undercooling. By classifying the interfacial atomic attachment behaviors into ballistic and diffusive motions based on the displacement analysis, it is found that the crystal growth of both metals involves many ballistic attachments, and a small increment of diffusive attachments at the Ta interface leads to a significant energy barrier for crystallization comparing to that of Cu. The temperature effects on the interfacial structures and atomic dynamics to attach onto the crystal are also studied in detail, and their correlations with the different growth mechanisms at low and deep undercoolings are disclosed. Finally, the crystallization rate is proved to be dominated by the atomic transformation displacement and interfacial atomic movement rate for either metal, rather than the atomic thermal velocity or liquid diffusion coefficient.

Local intermolecular structure, hydrogen bonding and related dynamics in the liquid cis/trans N-methylformamide mixture: A density functional theory based Born-Oppenheimer molecular dynamics study

The local intermolecular structure and related dynamics in the liquid cis/trans N-methylformamide mixture at ambient temperature and density conditions have been systematically studied by employing Born-Oppenheimer molecular dynamics simulation techniques. Particular attention has been paid to the local structure around the cis- and trans- conformers and the formation and dynamics of hydrogen bonds with their closest neighbors. The calculated atom–atom radial distribution functions are in very good agreement with available experimental data and reveal the existence of different types of hydrogen bonding intermolecular interactions. The average number of hydrogen bonds formed by the cis- conformers is higher in comparison with the one corresponding to the trans- conformers. Moreover, the lifetimes of the hydrogen bonds formed in the liquid are longer when the cis- conformers participate in the bond formation, either as donors or acceptors. These findings clearly indicate that the local structural network around the cis- conformers in the liquid is more cohesive. The latter finding is also reflected in the slower reorientational dynamics of the cis- conformers and the low- and high-frequency region of the spectral densities of the atomic velocity time correlation functions. Finally, the calculated average dipole moments of the trans- and cis- conformers are significantly higher than their corresponding gas-phase values, signifying the importance of polarization effects in this particular polar liquid solvent.

Optical pumping and relaxation of atomic population in assorted conditions

Precise control and knowledge over atomic dynamics is central to the advancement of quantum technology. The different experimental conditions namely, atoms in a vacuum, antirelaxation coated and buffer gas filled atomic cells provide complementary platforms for such investigations. The extent of changes in optical pumping, velocity changing collision and hyperfine changing collision rates associated with these conditions are discussed. There is a phenomenal change in the optical density by a factor of >25 times in the presence of a control field in a buffer gas environment. We found confinement induced enhanced optical pumping as the mechanism behind the observed transparency in buffer gas cell. The diffusive velocity of atoms were measured to be ∼25 ± 12 m s−1 and ⩽8 ± 4 m s−1 for antirelaxation coated and buffer gas filled cells respectively. The measurements were carried out for 85Rb atoms in natural isotopic composition using pump–probe spectroscopy. The studies will have useful application in measurements of relaxation rates, quantum memory, quantum repeaters and atomic devices.

A cold cesium beam source based on a two-dimensional magneto-optical trap

A beam source is proposed for the production of an intense cold cesium atomic beam that can be used in cesium beam atomic clocks. The source is based on a two-dimensional magneto-optical trap (2D-MOT), but introduces hollow cooling and pushing lights in the axial direction to create a 2D+-MOT, which separates the cooling and pushing functions while the low-power pushing light pushes atoms out to form a cold atomic beam. This cold cesium atomic beam source reduces the light shift due to leakage light and retains longitudinal cooling to increase the flux of the cold atomic beam compared with that of the conventional 2D+-MOT scheme. The specifics of the design are investigated, the atomic velocity and beam flux are calculated, and the results are experimentally verified. The results demonstrate that when the power of the pushing light is 180 µW and when its frequency resonates with the 4 → 5′ transition of the Cs D2 line, the most probable longitudinal velocity of the outgoing cold atomic beam, the width of velocity distribution, and the atomic beam flux are 19.38 m/s, 8.1 m/s, and 1.7 × 1010 atoms/s, respectively.

Impact of complex boundary on the hydrodynamic properties of methane nanofluidic flow via non-equilibrium multiscale molecular dynamics simulation

Understanding the impact of complex boundary on the hydrodynamic properties of methane nanofluidic is significant for production optimization and design of energy-saving emission reduction devices. In the molecule scale, however, the microscopic mechanisms of the influence of the complex boundary on the hydrodynamic characteristics are still not well understood. In this study, a mixture boundary Poiseuille flow model is proposed to study the hydrodynamic properties and explore the molecular mechanisms of confined methane nanofluidic using the Non-equilibrium multiscale molecular dynamics simulation (NEMSMD). In order to investigate the influences of nonslip and rough boundary on hydrodynamic behavior of nanofluidic by the present model in one simulation, the coordinate transformation methods regarding the local symmetry is showed. Simulation results show that the atom number density, velocity and temperature profiles present significant differences near the nonslip boundary and rough wall surface. Moreover, the slip length of methane nanofluidic near the rough boundary decreases with the increasing of the temperature. Furthermore, the viscosity values are calculated by parabolic fit of the local velocity data based on the present model, which demonstrates that the impact of the nonslip boundary on the shear viscosity compared with the experiment result is less than one obtained using the rough boundary. In addition, the local contours of rotational and translational energy are plotted, which show that the rotational and translational energies of nonslip boundary are obvious higher than those of rough boundary. These numerical results are very significant in understanding the impact of complex boundary conditions on hydrodynamic properties in nanofluidic theory and the design of nano-devices.

A Kinetic Monte Carlo Approach to Model Barite Dissolution: The Role of Reactive Site Geometry

Barite (Ba[SO4]) is one of the promising candidates for sequestration of radioactive waste. Barite can incorporate radium (Ra) and form ideal solid solutions, i.e., (Ba,Ra)[SO4]. Together with isostructural celestite (Sr[SO4]), ternary solid solutions, (Ba,Sr,Ra)[SO4], may exist in natural conditions. Our fundamental understanding of the dissolution kinetics of isostructural sulfates is critically important for a better risk assessment of nuclear waste repositories utilizing this mineral for sequestration. So far, the barite-water interface has been studied with experimental methods and atomistic computer simulations. The direct connection between the molecular scale details of the interface structure and experimental observations at the microscopic scale is not yet well understood. Here, we began to investigate this connection by using a kinetic Monte Carlo approach to simulate the barite dissolution process. We constructed a microkinetic model for the dissolution process and identified the reactive sites. Identification of these sites is important for an improved understanding of the dissolution, adsorption, and crystal growth mechanisms at the barite–water interface. We parameterized the molecular detachment rates by using the experimentally observed etch pit morphologies and atomic step velocities. Our parameterization attempts demonstrated that local lattice coordination is not sufficient to differentiate between the kinetically important sites and estimate their detachment rates. We suggest that the water structure and dynamics at identified sites should substantially influence the detachment rates. However, it will require more work to improve the parameterization of the model by means of Molecular Dynamics and ab initio calculations.

Investigation of energy accommodation coefficient at gas-solid interface of a hypersonic flying vehicle

An effective thermal protection system is crucial for hypersonic flying vehicles performing multiple cycles of flight under high aerodynamic heating loads. Accurate calculation of heat transfer between a flying vehicle and its front shock waves is not possible due to complex gas-solid interactions. In this study, it was observed that the energy accommodation coefficient (EAC) at the interface between the shock wave formed by monoatomic gas and the flying vehicle exhibited a decrease-increase trend, which is attributed to that the averaged lasting time for each gas atom interacting with the solid decreases exponentially while the interacting depth increases logarithmically with the increase of the flight velocity. A one-dimensional collision model disclosed that the minimum EACs at different spring-mass systems occurred at the same Deborah number, indicating that the incident velocity of gas atoms corresponding to the minimum EAC is proportional to the natural frequency of the spring-mass system. Our research results suggest that the EAC strongly depends on the vehicle flying velocity and provide a rule for the optimal design of an efficient thermal protection system.

Generalized Theory for DISes in a Large Deformed Solid

Analysis of diffusion-induced stress in the electrode materials of metal-ion batteries, especially in silicon-anode lithium-ion battery with large deformation, involves an in-depth understanding of the interaction between atomic migration and matrix volume expansion. Under large migration velocity of solute atoms in a host matrix, such chemo-mechanical coupling can result in the damage or structural degradation of the matrix (electrode), leading to capacity fading. Incorporating the effects of concentration changing rate on strain energy in Helmholtz free energy and the finite deformation framework, a generalized theory has been established to solve stresses in an elasto-plastic electrode induced by lithiation. Numerical calculations of lithiation-induced stresses are performed in an amorphous nano-spherical silicon electrode by using the free-volume-based constitutive relation of amorphous materials and the generalized theory. We analyze the effects of the volumetric expansion coefficient associate with solute atoms and the volumetric expansion coefficient associate with the concentration changing rate of solute atoms on the stress evolution. The results reveal that the concentration changing rate of solute atoms likely accelerates surface cracking of electrode materials during lithiation.

Focusing of an Atomic Beam for the Efficient Loading of an Atom Chip

A method has been proposed to increase the rate of loading of atoms in a U-magneto-optical trap near an atom chip. The method is based on the focusing of a slow atomic beam into the localization region of the atom chip. The overdamped focusing regime has been considered. In this case, the focal length is independent of the initial transverse velocity of atoms. It has been shown that the focusing of the atomic beam makes it possible to increase the loading rate in the localization region 250 μm in diameter by a factor of 160.

Molecular dynamics simulation analysis of the stress around an up-migrating helium bubble under tungsten surface and its bursting

Tungsten has been considered as the most qualified candidate for the plasma-facing material of modern fusion devices up to date. However, in the future tokamaks such as ITER and CFETR, the helium species will be trapped inevitably in tungsten, and these alien species gradually agglomerate and coalesce into bubbles under appropriate conditions. The building-up of internal pressure in a bubble near the tungsten surface finally engenders the bubble bursting. The bubble rupture not only produces helium and tungsten impurities but also changes the topography of the tungsten surface; the latter further affects the sputtering. This paper uses molecular dynamics simulations to show how a helium bubble behaves in the course of its migration in tungsten towards the surface at different temperatures, and correspondingly, estimates the critical bursting pressure of bubbles when they come close to the surface. In the meantime, the paper also studies how the helium bubble at different depths affects its surrounding stress in tungsten at the atomic level. The study indicates that the helium bubble makes the stress of its close neighboring part of tungsten increase and that the internal pressure of the bubble is higher when it is deeper below the surface. The study also simulates how the surface morphology evolves after the bubble bursts. It is found that the bubble can burst easily when the temperature is over 1200 K and a considerably deep, cone-shaped pit forms after the bursting. During the bubble bursting, tungsten atoms are ejected out of the bulk. The velocity of these emitted atoms increases with temperature. The emitted atoms fall around the pit more locally during the bursting at 1200 K than at 2300 K. Further discussion is also made over the implications towards the formation of fuzz.

Accelerated causal Green's function molecular dynamics

A Green's function formalism has been applied to solve the equations of motion in classical molecular dynamics simulations. This formalism enables larger time scales to be probed for vibration processes in carbon nanomaterials. In causal Green's function molecular dynamics (CGFMD), the total interaction potential is expanded up to the quadratic terms, which enables an exact solution of the equations of motion to be obtained for problems within the harmonic approximation, reasonable energy conservation, and fast temporal convergence. Differently from conventional integration algorithms in molecular dynamics, CGFMD performs matrix multiplications and diagonalizations within its main loop, which make its computational cost high and, therefore, has limited its use. In this work, we propose a method to accelerate CGFMD simulations by treating the full system of N atoms as a collection of N smaller systems of size n. Diagonalization is performed for smaller nd×nd dynamical matrices rather than the full Nd×Nd matrix (d=1,2, or 3). The eigenvalues and eigenvectors are then used in the CGFMD equations to update the atomic positions and velocities. We applied the method for one-dimensional lattices of oscillators and have found that the method rapidly converges to the exact solution as n increases. The computational time of the proposed method scales linearly with N, providing a considerable gain with respect to the O(N3) full diagonalization. The method also exhibits better accuracy and energy conservation than the velocity-Verlet algorithm. An OpenMP parallel version has been implemented and tests indicate a speedup of 14× for N= 50000 in affordable computers. Our findings indicate that CGFMD can be an alternative, competitive integration technique for molecular dynamics simulations.

Energy shift of a uniformly moving two-level atom through a thermal reservoir

We investigate the implications of an atomic constant velocity in the energy shift of a two-level atom inside the thermal bath of a quantum scalar field, which is described by the Bose-Einstein distribution. The use of DDC formalism shows that the contribution of thermal fluctuations on the atomic level shifts depends on the atomic velocity and the temperature of the heat reservoir but the contribution of radiation reaction is totally insusceptible. The resulting energy shifts are analyzed and examined in detail under different circumstances. The atomic uniform linear motion always broadens the atomic level spacing in the limit of low temperature but narrows down it in the limit of high temperature. Our work clearly indicates that the moving heat reservoir shifts the atomic levels in a way quite different from that of the static one.

Revealing Intra- and Intermolecular Interactions Determining Physico-Chemical Features of Selected Quinolone Carboxylic Acid Derivatives.

The intra- and intermolecular interactions of selected quinolone carboxylic acid derivatives were studied in monomers, dimers and crystals. The investigated compounds are well-recognized as medicines or as bases for further studies in drug design. We employed density functional theory (DFT) in its classical formulation to develop gas-phase and solvent reaction field (PCM) models describing geometric, energetic and electronic structure parameters for monomers and dimers. The electronic structure was investigated based on the atoms in molecules (AIM) and natural bond orbital (NBO) theories. Special attention was devoted to the intramolecular hydrogen bonds (HB) present in the investigated compounds. The characterization of energy components was performed using symmetry-adapted perturbation theory (SAPT). Finally, the time-evolution methods of Car–Parrinello molecular dynamics (CPMD) and path integral molecular dynamics (PIMD) were employed to describe the hydrogen bond dynamics as well as the spectroscopic signatures. The vibrational features of the O-H stretching were studied using Fourier transformation of the autocorrelation function of atomic velocity. The inclusion of quantum nuclear effects provided an accurate depiction of the bridged proton delocalization. The CPMD and PIMD simulations were carried out in the gas and crystalline phases. It was found that the polar environment enhances the strength of the intramolecular hydrogen bonds. The SAPT analysis revealed that the dispersive forces are decisive factors in the intermolecular interactions. In the electronic ground state, the proton-transfer phenomena are not favourable. The CPMD results showed generally that the bridged proton is localized at the donor side, with possible proton-sharing events in the solid-phase simulation of stronger hydrogen bridges. However, the PIMD enabled the quantitative estimation of the quantum effects inclusion—the proton position was moved towards the bridge midpoint, but no qualitative changes were detected. It was found that the interatomic distance between the donor and acceptor atoms was shortened and that the bridged proton was strongly delocalized.

Spectroscopic effects of velocity-dependent Casimir-Polder interactions induced by parallel plates

Casimir-Polder interactions cause energy and momentum exchange between microscopic and macroscopic bodies, a process mediated by quantum fluctuations in the coupled matter-electromagnetic field system. The dynamics of such effects are yet to be experimentally investigated due to the dominance of static effects at currently attainable atomic velocities. However, Y. Guo and Z. Jacob [\textit{Opt. Express}, 22:26193-26202, 2014] have proposed a non-static two-plate set-up where quantum fluctuation mediated effects have a strong velocity-dependent resonance, leading to a giant friction force on the plates. Here a more easily realisable set-up, a moving atom between two stationary plates, is analysed within a QED framework to establish the spectroscopic Casimir-Polder effects on the atom, and their velocity dependence. While no large velocity-dependent enhancement is found, expressions for the plate-induced spectroscopic effects on the atom were found, and further shown to be equivalent to the Doppler-shifted static result within certain velocity constraints. A numerical analysis investigates the behaviour of this system for the well studied case of the $6D_{3/2}\rightarrow 7P_{1/2}$ transition in $^{133}$Cs interacting with sapphire plates.

Single- and two-photon absorption laser-induced fluorescence spectroscopy in rare gases for gridded ion thruster diagnostics

Methods based on laser-induced fluorescence spectroscopy are widely used for spatially resolved non-intrusive diagnostics of atomic or molecular densities and velocity distributions in plasma applications. With regard to electric space propulsion, one focus is on the investigation of rare gases such as xenon or krypton, which are currently the favored propellants in gridded ion- and Hall-effect thrusters. For gridded ion engines, diagnostics of neutral atoms is of interest since charge-exchange processes between neutrals and ions are the main driver of accelerator grid erosion, which limits the lifetime of a gridded ion thruster. Extending the capabilities of the advanced electric propulsion diagnostics platform which has been developed by the IOM and partners, single- and two-photon absorption laser-induced fluorescence diagnostics have been set-up recently at our institute. Both experimental set-ups, and as a series of first applications, measurements of krypton neutrals in the plume of the radiofrequency ion thruster RIT-10 (ArianeGroup GmbH), and xenon neutrals within the discharge chamber of a gridded radiofrequency ion source developed at IOM, are presented.

Generalized virial equationfor nonlinear multiplicative Langevin dynamics: Application to laser-cooled atoms.

The virial theorem, and the equipartition theorem in the case of quadratic degrees of freedom, are handy constraints on the statistics of equilibrium systems. Their violation is instrumental in determining how far from equilibrium a driven system might be. We extend the virial theorem to nonequilibrium conditions for Langevin dynamics with nonlinear friction and multiplicative noise. In particular, we generalize it for confined laser-cooled atoms in the semiclassical regime. The resulting relation between the lowest moments of the atom position and velocity allows to measure in experiments how dissipative the cooling mechanism is. Moreover, its violation can reveal the departure from a strictly harmonic confinement or from the semiclassical regime.

Мезоатомын гадаад Оже эффектийг бодох нэгэн арга

A method for calculating mesic-atom external Auger transition is proposed. The transition proceed in two stages. In the first stage, the quasi-molecular complex, consisting of the mesic atom and the atom is formed in their collisions and on the second stage, the decay of this complex leads to external Auger effects. At given distance between two atoms of the quasi-molecular system, the partial and total width of external Auger transition are calculated. The average Auger transition rates of pionic helium (Helium gass pressure 59.44 atm, velocity of the pionic atom is v=105 cm/s) are calculated and compared the rates of other competitive processes (Internal Auger and Radiation transitions).