Atomic Dynamics Research Articles

Development of attosecond pulses and their application to ultrafast dynamics of atoms and molecules

In the past two decades, the development of laser technology has made attosecond science become a cutting-edge research field, providing various novel perspectives for the study of quantum few-body ultrafast evolution. At present, the attosecond pulses prepared in laboratories are widely used in experimental research in the form of isolated pulses or pulse trains. The ultrafast changing light field allows one to control and track the motions of electrons on an atomic scale, and realize the real-time tracking of electron dynamics on a sub-femtosecond time scale. This review focuses on the research progress of ultrafast dynamics of atoms and molecules, which is an important part of attosecond science. Firstly, the generation and development of attosecond pulses are reviewed, mainly including the principle of high-order harmonic and the separation method of single-attosecond pulses. Then the applications of attosecond pulses are systematically introduced, including photo-ionization time delay, attosecond charge migration, and non-adiabatic molecular dynamics. Finally, the summary and outlook of the application of attosecond pulses are presented.

How structural and vibrational features affect optoelectronic properties of non-stoichiometric quantum dots: computational insights.

While stoichiometric quantum dots (QDs) have been well studied, a significant knowledge gap remains in the atomistic understanding of the non-stoichiometric ones, which are predominantly present during the experimental synthesis. Here, we investigate the effect of thermal fluctuations on structural and vibrational properties of non-stoichiometric cadmium selenide (CdSe) nanoclusters: anion-rich (Se-rich) and cation-rich (Cd-rich) using ab initio molecular dynamics (AIMD) simulations. While the excess atoms on the surface fluctuate more for a given QD type, the optical phonon modes are mostly composed of Se atoms dynamics, irrespective of the composition. Moreover, Se-rich QDs have higher bandgap fluctuations compared to Cd-rich QDs, suggesting poor optical properties of Se-rich QDs. Additionally, non-adiabatic molecular dynamics (NAMD) suggests faster non-radiative recombination for Cd-rich QDs. Altogether, this work provides insights into the dynamic electronic properties of non-stoichiometric QDs and proposes a rationale for the observed optical stability and superiority of cation-rich candidates for light emission applications.

Floquet Superradiance Lattices in Thermal Atoms.

Floquet modulation has been widely used in optical lattices for coherent control of quantum gases, in particular for synthesizing artificial gauge fields and simulating topological matters. However, such modulation induces heating which can overwhelm the signal of quantum dynamics in ultracold atoms. Here we report that the thermal motion, instead of being a noise source, provides a new control knob in Floquet-modulated superradiance lattices, which are momentum-space tight-binding lattices of collectively excited states of atoms. The Doppler shifts combined with Floquet modulation provide effective forces along arbitrary directions in a lattice in frequency and momentum dimensions. Dynamic localization, dynamic delocalization, and chiral edge currents can be simultaneously observed from a single transport spectrum of superradiance lattices in thermal atoms. Our Letter paves a way for simulating Floquet topological matters in room-temperature atoms and facilitates their applications in photonic devices.

A hydrogen atom in strong elliptically polarized laser fields within discrete variable representation

The nondirect product discrete variable representation (npDVR) is developed for the time-dependent Schrödinger equation with nonseparable angular variables and is applied to a hydrogen atom in elliptically polarized strong laser fields. The 2D npDVR is constructed on spherical harmonics orthogonalized on the 2D angular grids of the Popov and Lebedev 2D cubatures for the unit sphere. With this approach we have investigated the dynamics of a hydrogen atom initially in its ground state in elliptically polarized laser fields with the intensity up to W cm−2 and a wavelength of λ = 800 nm. For these parameters of the laser field and the entire range of ellipticity variation, we have calculated the total excitation and ionization yields of the atom. The performed analysis of the method convergence shows that the achieved accuracy of our calculations significantly exceeds the accuracy of recent works of other authors relevant to the problem, due to the high efficiency of the 2D npDVR in approximating the angular part of the 3D time-dependent Schrödinger equation. We also propose a new simple procedure for infinite summation of the transition probabilities to the bound states of the hydrogen atom when calculating the total excitation yield and prove its accuracy by comparison with conventional methods. The obtained results show the potential prospects of the 2D npDVR for investigating atomic dynamics in stronger laser fields.

Spatial phase effect on quantum resonance ratchet transport of cold atoms

We investigate the transport dynamics of a cold atom exposed to a sawthooth-like optical lattice potential subject to periodic δ-kicks. By means of the computed transport current, at Quantum Resonance regimes, interesting phenomenon induced by the potential spatial phase θ have been resolved. From a zero-phase High Order Quantum Resonance case, where current reversal is possible, we show by turning on the spatial phase θ that current enhancement, current rectification and zero current are achieved. Specifically, θ can be used to turn a negative current into a positive one and vice-versa, to enhance current, or to set current to zero. With the help of current landscapes displayed as function of θ and the potential P, optimal currents are generated. Such a picture clearly shows regions of optimal currents, at critical values of θ and P, where prospective experiments can be done with present-day optical lattice set-up.

Deciphering a structural signature of glass dynamics by machine learning

The dynamics of atoms plays a key role in governing various dynamical and transport properties of glasses. However, it remains elusive which structural features (if any) control atom dynamics in glasses. Here, based on million-atom molecular dynamics simulations and classification-based machine learning, we extract a needle in a haystack by identifying a local, nonintuitive structural signature (a revised version of the recently developed softness metric) that governs glass dynamics. We do so by investigating the ion mobility in sodium silicate glasses---a realistic, archetypal glass---finding that the sodium ion mobility is largely encoded in its initial softness, wherein softer Na atoms exhibit higher mobility. Importantly, our approach allows us to interpret the machine-learned softness metric and thus elucidate the atomistic origin of the ion mobility. Namely, we find that Na mobility is anticorrelated with the local density of defect oxygen neighbors that are located between the nearest two coordination shells. This local packing order offers a potential path to develop glass formulations with tailored dynamical properties. Finally, we demonstrate that the softness is strongly anticorrelated with the activation energy for Na atom reorganization.

Intelligent dissipative particle dynamics: Bridging mesoscopic models from microscopic simulations via deep neural networks

We develop a bottom-up coarse graining framework to construct mesoscopic force fields directly from microscopic dynamics by a machine learning technique. This scheme, called the intelligent dissipative particle dynamics (IDPD), holds various consistencies, including configuration, phase, static and dynamic consistencies that are incorporated into a deep neural network. The network is trained with full atomic data in a special way for preserving the natural symmetries of the system, and generates both the conservative and non-conservative force fields in the mesoscopic system. Subsequently, the force fields are used in the dissipative particle dynamics (DPD) that is considered as the effective mesoscopic dynamics resulting from the Mori–Zwanzig projection of the underlying atomic dynamics. To show the validity and applicability of IDPD, we coarse grain star polymer and methane fluids respectively, from full molecular dynamics (MD) simulations. Quantitative comparisons between the MD and IDPD indicate that the IDPD is able to reproduce both the static and dynamic properties of underlying MD system, with the aids of a pressure correction and a diffusion rescaling, even when coarse graining multiple molecules into a particle. It follows that the IDPD is competent for preserving the dynamic properties and coarse graining non-bonded molecules, which are common challenges for most of coarse graining methods.

Disruption of electrostatic contacts in the HNH nuclease from a thermophilic Cas9 rewires allosteric motions and enhances high-temperature DNA cleavage.

Allosteric signaling within multidomain proteins is a driver of communication between spatially distant functional sites. Understanding the mechanism of allosteric coupling in large multidomain proteins is the most promising route to achieving spatial and temporal control of the system. The recent explosion of CRISPR-Cas9 applications in molecular biology and medicine has created a need to understand how the atomic level protein dynamics of Cas9, which are the driving force of its allosteric crosstalk, influence its biophysical characteristics. In this study, we used a synergistic approach of nuclear magnetic resonance (NMR) and computation to pinpoint an allosteric hotspot in the HNH domain of the thermostable GeoCas9. We show that mutation of K597 to alanine disrupts a salt-bridge network, which in turn alters the structure, the timescale of allosteric motions, and the thermostability of the GeoHNH domain. This homologous lysine-to-alanine mutation in the extensively studied mesophilic S. pyogenes Cas9 similarly alters the dynamics of the SpHNH domain. We have previously demonstrated that the alteration of allostery via mutations is a source for the specificity enhancement of SpCas9 (eSpCas9). Hence, this may also be true in GeoCas9.

Temperature-dependent effect of cooling rate on the melt-quenching process of metallic glasses

The cooling rate during the melt-quenching process is a crucial factor in the formation of metallic glasses. This study reveals the temperature- and thermal-history-dependent effects of cooling rate on the relaxation state of metallic glasses prepared by the rapid quenching of a liquid alloy. Using molecular dynamics techniques, we conducted melt-quenching simulations in which the cooling rates were varied during the quenching process. Maps were constructed to demonstrate the effect of the cooling rate on the quenching process as a function of temperature. The critical temperature range varied depending on the thermal history of the cooling process. Analyses of the local atomic order and atomic dynamics revealed the atomistic origin of the temperature- and thermal-history-dependent effects of the cooling rate. This study also discusses the physical background of the upper and lower ends of the critical temperature range based on the potential energy landscape and atomic dynamics.

Active Cancellation of Servo-Induced Noise on Stabilized Lasers via Feedforward

Many precision laser applications require active frequency stabilization. However, such stabilization loops operate by pushing noise to frequencies outside their bandwidth, leading to large ``servo bumps'' that can have deleterious effects for certain applications. The prevailing approach to filtering this noise is to pass the laser through a high-finesse optical cavity, which places constraints on the system design. Here, we propose and demonstrate a different approach where a frequency error signal is derived from a beat note between the laser and the light that passes through the reference cavity. The phase noise derived from this beat note is fed forward to an electro-optic modulator after the laser, carefully accounting for relative delay, for real-time frequency correction. With a hertz-line-width laser, we show $\ensuremath{\gtrsim}20\phantom{\rule{0.2em}{0ex}}\mathrm{dB}$ noise suppression at the peak of the servo bump (approximately $250\phantom{\rule{0.2em}{0ex}}\mathrm{kHz}$) and a noise-suppression bandwidth of approximately $5\phantom{\rule{0.2em}{0ex}}\mathrm{MHz}$---well beyond the servo bump. By simulating the Rabi dynamics of a two-level atom with our measured data, we demonstrate substantial improvements to the pulse fidelity over a wide range of Rabi frequencies. Our approach offers a simple and versatile method for obtaining a clean spectrum of a narrow-line-width laser, as required in many emerging applications of cold atoms, and is readily compatible with commercial systems that may even include wavelength conversion.

Microscopic structure and dynamics of glass forming Zr2Co melts and the impact of different late transition metals on the melt properties

We studied the short-range order and the atomic dynamics of stable and undercooled binary Zr 2 Co alloy melts as well as their density and viscosity. The containerless processing technique of electrostatic levitation was used to achieve deep undercooling and to avoid contaminations. Static structure factors are determined by combining this technique with neutron and high energy X-ray diffraction. Co self-diffusion coefficients are measured by quasielastic neutron scattering. Our results reveal that the short-range order of the Zr 2 Co melts closely resembles that previously observed for Zr 64 Ni 36 . We consider this as the origin of the very similar melt dynamics of these two alloys at same temperatures. On the other hand, the difference in the structure and dynamics when compared with those of Zr 2 Cu and Zr 2 Pd shows clearly that not only the atomic sizes, but also electronic properties or chemical bonding have an important influence on the melt properties of Zr-based glass forming melts. PACS number(s): 61.20.−p, 61.25.Mv, 66.30.Fq, 61.05.F-, 61.05.cp

Velocity Dependence of Moiré Friction.

Friction force microscopy experiments on moiré superstructures of graphene-coated platinum surfaces demonstrate that in addition to atomic stick-slip dynamics, a new dominant energy dissipation route emerges. The underlying mechanism, revealed by atomistic molecular dynamics simulations, is related to moiré ridge elastic deformations and subsequent relaxation due to the action of the pushing tip. The measured frictional velocity dependence displays two distinct regimes: (i) at low velocities, the friction force is small and nearly constant; and (ii) above some threshold, friction increases logarithmically with velocity. The threshold velocity, separating the two frictional regimes, decreases with increasing normal load and moiré superstructure period. Based on the measurements and simulation results, a phenomenological model is derived, allowing us to calculate friction under a wide range of room temperature experimental conditions (sliding velocities of 1-104 nm/s and a broad range of normal loads) and providing excellent agreement with experimental observations.

Highly tunable β-relaxation enables the tailoring of crystallization in phase-change materials

In glasses, secondary (β-) relaxations are the predominant source of atomic dynamics. Recently, they have been discovered in covalently bonded glasses, i.e., amorphous phase-change materials (PCMs). However, it is unclear what the mechanism of β-relaxations is in covalent systems and how they are related to crystallization behaviors of PCMs that are crucial properties for non-volatile memories and neuromorphic applications. Here we show direct evidence that crystallization is strongly linked to β-relaxations. We find that the β-relaxation in Ge15Sb85 possesses a high tunability, which enables a manipulation of crystallization kinetics by an order of magnitude. In-situ synchrotron X-ray scattering, dielectric functions, and ab-initio calculations indicate that the weakened β-relaxation intensity stems from a local reinforcement of Peierls-like distortions, which increases the rigidity of the bonding network and decreases the dynamic heterogeneity. Our findings offer a conceptually new approach to tuning the crystallization of PCMs based on manipulating the β-relaxations.

A giant atom with modulated transition frequency

Giant atoms are known for the frequency-dependent spontaneous emission and associated interference effects. In this paper, we study the spontaneous emission dynamics of a two-level giant atom with dynamically modulated transition frequency. It is shown that the retarded feedback effect of the giant-atom system is greatly modified by a dynamical phase arising from the frequency modulation and the retardation effect itself. Interestingly, such a modification can in turn suppress the retarded feedback such that the giant atom behaves like a small one. By introducing an additional phase difference between the two atom-waveguide coupling paths, we also demonstrate the possibility of realizing chiral and tunable temporal profiles of the output fields. The results in this paper have potential applications in quantum information processing and quantum network engineering.

Interactions between CTAB and montmorillonite by atomic force microscopy and molecular dynamics simulation

The tailing dewatering process meets significant challenges due to the contained clay minerals. The clays often have great hydrophilicity and high electronegativity, which improve slurry stability and prevent particle aggregation. Hydrophobic aggregation of chemical additives such as surfactants is an efficient way to enhance dewatering of clay-rich tailings. This work aims to investigate the interactions between CTAB, a cationic surfactant, and montmorillonite by experiments and molecular simulation. In macroscopic dewatering experiments, the results show that, after the treatment with 0.3 mol/L CTAB, the contact angle of montmorillonite cake and the zeta potential increased from 13.0° to 34.1° and from –21.5 mV to 0.22 mV, respectively. The addition of CTAB resulted in improved clay-rich tailings dewatering due to reduced surface tension of the filtrate, increased particle hydrophobicity, and charge neutralization. Then, the interaction forces between montmorillonite particles at various CTAB concentrations were directly measured. AFM results indicated that the interaction forces between montmorillonite particles in water were always repulsive during the approaching process. As the concentration of CTAB increased, the interaction force changed from repulsive to attractive. The adhesion force between montmorillonites increased to approximately 6.46 nN at 0.3 mmol/L CTAB. These adhesion forces help the montmorillonites readily adhere to each other. Finally, molecular simulation results indicated that the interactions between montmorillonite and water molecules were weakened by CTAB. The thickness of the water film was dramatically reduced by CTAB adsorption, enhancing the hydrophobicity of the montmorillonite surface.

Model for Nanopore Formation in Two-Dimensional Materials by Impact of Highly Charged Ions.

We present a first qualitative description of the atomic dynamics in two-dimensional (2D) materials induced by the impact of slow, highly charged ions. We employ a classical molecular dynamics simulation for the motion of the target atoms combined with a Monte Carlo model for the diffusive charge transport within the layer. Depending on the velocity of charge transfer (hopping time or hole mobility) and the number of extracted electrons which, in turn, depends on the charge state of the impinging ions, we find regions of stability of the 2D structure as well as parameter combinations for which nanopore formation due to Coulomb repulsion is predicted.

Density matrix formulation of dynamical systems.

Physical systems that are dissipating, mixing, and developing turbulence also irreversibly transport statistical density. However, predicting the evolution of density from atomic and molecular scale dynamics is challenging for nonsteady, open, and driven nonequilibrium processes. Here, we establish a theory to address this challenge for classical dynamical systems that is analogous to the density matrix formulation of quantum mechanics. We show that a classical density matrix is similar to the phase-space metric and evolves in time according to generalizations of Liouville's theorem and Liouville's equationfor non-Hamiltonian systems. The traditional Liouvillian forms are recovered in the absence of dissipation or driving by imposing trace preservation or by considering Hamiltonian dynamics. Local measures of dynamical instability and chaos are embedded in classical commutators and anticommutators and directly related to Poisson brackets when the dynamics are Hamiltonian. Because the classical density matrix is built from the Lyapunov vectors that underlie classical chaos, it offers an alternative computationally tractable basis for the statistical mechanics of nonequilibrium processes that applies to systems that are driven, transient, dissipative, regular, and chaotic.

Complex dynamics in low electron density lithium ammonia solutions. Interacting modes revealed by comparing neutron and x-ray inelastic scattering

We present an experimental study of the collective dynamics of lithium-ammonia solutions in order to investigate the role played by the metallic character of the liquid on the atomic dynamics and the relevance of almost free ammonia. To understand the intrinsically complex dynamical behavior due to the presence of clusters composed of at least four ammonia molecules coordinated by a Li atom, we measured the neutron dynamic structure factor in an extended region of wave-vector transfer $Q$, i.e., from a value as low as $0.2\phantom{\rule{4pt}{0ex}}{\AA{}}^{\ensuremath{-}1}$ up to $2.4\phantom{\rule{4pt}{0ex}}{\AA{}}^{\ensuremath{-}1}$, going beyond the limit of twice the Fermi wave vector $2{k}_{F}\ensuremath{\simeq}0.98\phantom{\rule{4pt}{0ex}}{\AA{}}^{\ensuremath{-}1}$. This allowed us to probe the system with wavelengths between $\ensuremath{\approx}3\phantom{\rule{4pt}{0ex}}\AA{}$ (comparable with the ammonia size) and $\ensuremath{\approx}30\phantom{\rule{4pt}{0ex}}\AA{}$, a range ensuring a good sensitivity to the collective atomic motions, where the metallic character is known to play a major role. Three different Li concentrations (0.10, 0.17, and 0.20 molar fraction) have been analyzed at 220 K and, in the case of the intermediate concentration, the behavior at 155 K (still in the liquid phase) has also been explored. The effect of the metallic nature, dominated by the electron response when $Q<2{k}_{F}$, is confirmed down to 0.1 molar fraction but, thanks to the availability of better data and the synergy enabled by the simultaneous analysis of neutron and previous x-ray data, we were able to detect the presence of a second excitation affecting the global dynamics of the system. This second mode shows, with respect to the acoustic collective mode, an increasing energy-integrated intensity as $Q$ grows, i.e., in a range where the data obviously better reveal the local (short length scale) atomic behavior. Finally, by comparing the results of the mentioned analysis with those obtained recently by techniques specially focused on the intrinsic metallic properties, like photoemission spectroscopy, it has been possible to correlate the dynamics with the presence of metallic clusters, at almost constant electron density, embedded into free fluid ammonia. The present results also allow for a coherent interpretation of the elastic behavior of lithium ammonia solutions, as determined by ultrasound velocity data in a rather wide concentration range, and the identification of the different concentration regions where agreement with the interacting electron gas paramagnetic response and velocity of collective mode is obtained. Whether the dispersion curve anomaly at $\ensuremath{\approx}2{k}_{F}$ (observed down to 0.1 molar fraction also for the second mode) is simply related to the dielectric response or some other mechanism may also contribute to this phenomenon, remains an open question.

The Application of Deep Learning on Room Temperature Conductivity of Li10GeP2S12 Type Solid State Electrolyte

Room temperature ionic conductivity of the solid-state electrolyte is the essencial element of the commercialization of solid-state lithium-ion batteries. First-principles method is a good tool as it simulates the atomic level dynamics, and provides accurate insides of the ion moving velocity, path, and the microscopic environment. However, the first-principles method is time and computational resources-consuming. It is usually applied to systems with hundreds of atoms and the simulation time is less than 100 picoseconds. The classical molecular dynamics method applies to systems containing thousands of atoms and the simulation time can reach the scale of nanoseconds. However, it is based on the experimental force-field parameters and can only be applied to limited systems with experimentally-proofed good parameters. The method used in this paper is a combination of the first-principles simulation and classical dynamics. Deep-learning tool Deepmd-kit is used to train the first-principles simulation data of the targeted system into a tailor-made force-field model. The verified model for the specific system will then be used in the classical molecular dynamics simulation. With the first-principles precise and classical efficiency, the molecular dynamics simulation of ion movement at room temperature is achieved. The results of Li10GeP2S12 and Li10SiP2S12 showed good agreement with the experiments.

Control of coherent extreme-ultraviolet emission around atomic potential through laser chirp

Substantial neutral atoms can tunnel to excited states in an intense laser field and subsequently generate coherent emission through free induction decay. We experimentally observe an enhanced coherent emission in the harmonic slightly below the threshold, which is consistent with the free induction decay of Rydberg states produced by the frustrated tunnelling ionization (FTI) process. We further find that the intensity of the coherent emission significantly depends on the chirp of laser pulses. The simulations based on the strong field approximation model show that laser chirp affects the probability that the returned electrons recombine to the Rydberg states. Our result shows that coherent emission can be controlled by laser chirp, which facilitates understanding the dynamics of the Rydberg atom and coupling mechanism between the below-threshold harmonics and atomic energy level. In addition, the coherent below-threshold FTI emission we observed has small divergence which is good for EUV light source applications.