Real-space Dynamics Research Articles

Sequential single-shot imaging of nanoscale dynamic interactions with a table-top soft x-ray laser

We demonstrate the first real-space recording of nanoscale dynamic interactions using single-shot soft x-ray (SXR) full-field laser microscopy. A sequence of real-space flash images acquired with a table-top SXR laser was used to capture the motion of a rapidly oscillating magnetic nanoprobe. Changes of 30 nm in the oscillation amplitude were detected when the nanoprobe was made to interact with stray fields from a magnetic sample. The table-top visualization of nanoscale dynamics in real space can significantly contribute to the understanding of nanoscale processes and can accelerate the development of new nanodevices.

Soliton magnetization dynamics in spin-orbit-coupled Bose-Einstein condensates

Ring-trapped Bose-Einstein condensates subject to spin-orbit coupling support localized dark-soliton excitations that show periodic density dynamics in real space. In addition to the density feature, the solitons also carry a localized pseudospin magnetization that exhibits a rich and tunable dynamics. Analytical results for Rashba-type spin-orbit coupling and spin-invariant interactions predict a conserved magnitude and precessional motion for the soliton magnetization that allows for the simulation of spin-related geometric phases recently seen in electronic transport measurements.

Two-Dimensional Crystallization of Hexagonal Bilayer with Moiré Patterns

Direct observation of crystallization dynamics in real space is of special interest to scientists in various disciplines. Although direct observation of transient structural transformation in a nanocrystalline system has been recently achieved using the state-of-the-art aberration-corrected transmission electron microscopy (AC-TEM), the small length scales of individual species in molecular systems still preclude routine observation of crystallization dynamics. Unidirectional packing of microbeads can serve as an experimental model system, as their dynamics can be observed and recorded readily in the laboratory due to their larger size and slower time scale. Herein, we present direct observation of a two-dimensional (2D) crystallization enabled by such a packing process. The direct imaging approach not only allows observation of the dynamics in a bead-by-bead fashion but also reveals intriguing phenomena, such as the formation of grain boundaries, disorder-order transitions, and the Moiré patterns which arise when two periodic monolayers are overlaid at certain angles. In addition, the imaging afforded by confocal microscopy facilitates a structural analysis of height-dependent polygonal tiling of the top monolayer, which has implication to the formation of 2D quasicrystals.

Accurate multi-boson long-time dynamics in triple-well periodic traps

In order to solve the many-boson Schr\"odinger equation we utilize the multiconfigurational time-dependent Hartree method for bosons (MCTDHB). To be able to attack larger systems and/or to propagate the solution for longer times, we implement a parallel version of the MCTDHB method, thereby realizing the recently proposed [Streltsov et al., Phys. Rev. A 81, 022124 (2010)] idea on how to construct efficiently the result of the action of the Hamiltonian on a bosonic state vector. As an illustrative example of its own interest, we study the real-space dynamics of repulsive bosonic systems made of $N=12$, 51, and 3003 bosons in triple-well periodic potentials. The ground state of this system is threefold fragmented. By suddenly strongly distorting the trap potential, the system performs complex many-body quantum dynamics. At long times it reveals a tendency to an oscillatory behavior around a threefold fragmented state. These oscillations are strongly suppressed and damped by quantum depletions. In spite of the richness of the observed dynamics, the three time-adaptive orbitals of MCTDHB($M=3$) are capable of describing the many-boson quantum dynamics of the system for short and intermediate times. For longer times, however, more self-consistent time-adaptive orbitals are needed to correctly describe the nonequilibrium many-body physics. The convergence of the MCTDHB($M$) method with the number $M$ of self-consistent time-dependent orbitals used is demonstrated.

Polarization and Propagation of Polariton Condensates

With the use of the generalized Gross-Pitaevskii equation it is shown that exciton polaritons in semiconductor microcavities form a linearly polarized condensate having two branches of the excitation spectrum. The splitting between these branches is strongly anisotropic. This anisotropy noticeably affects the real-space dynamics of polariton condensates.

Electron density changes and high harmonics generation in H2 molecule under intense laser fields

Under interaction with a high-intensity laser field, the real-time femtosecond dynamics of the electron density in the H 2 molecule has been studied quantum mechanically. For this purpose, a time-dependent generalized nonlinear Schrodinger equation of motion, developed earlier in our laboratory by combining density functional theory and quantum fluid dynamics in real space, is solved numerically at the equilibrium internuclear distance of the molecule. By employing various time-dependent calculated properties as probes, information and insight are obtained about the phenomena of excitation, ionization, bond-softening, dipole formation and high-harmonics generation. The present approach goes beyond the linear response formalism.

Equilibrium configurations and energetics of point defects in two-dimensional colloidal crystals.

We demonstrate a novel method of introducing point defects (mono- and divacancies) in a confined monolayer colloidal crystal by manipulating individual particles with optical tweezers. Digital video microscopy is used to study defect dynamics in real space and time. We verify the numerical predictions that the stable configurations of the defects have reduced symmetry compared to the triangular lattice and discover that in addition they are characterized by distinct topological arrangements of the particles in the defect core. Surprisingly, point defects are thermally excited into separated dislocations, from which we extract the dislocation pair potential.

Metastable states as a key to the dynamics of supercooled liquids

Computer simulations of a model glass-forming system are presented, which study the correlation between the dynamics in real space and the topography of the potential energy landscape. This analysis clearly reveals that in the supercooled regime the dynamics is strongly influenced by the presence of deep valleys in the energy landscape, corresponding to long-lived metastable amorphous states. We explicitly relate nonexponential relaxation effects and dynamic heterogeneities to these metastable states and thus to the specific topography of the energy landscape.

Direct observation of dynamical heterogeneities in colloidal hard-sphere suspensions

The real-space dynamics in a model system of colloidal hard spheres was studied by means of time-resolved fluorescence confocal scanning microscopy. Direct experimental evidence for the presence of dynamical heterogeneities in a dense liquid was obtained from an analysis of particle trajectories in two-dimensional slices of the bulk sample. These heterogeneities manifest themselves as a non-Gaussian probability distribution of particle displacements and also affect the onset of long-time diffusive behavior.

Quantum tunneling cyclic loop in high temperature superconductor probably is the microscopic origin of high temperature superconductivity

4-particle (electrons or electrons with hole) or higher order quantum tunneling cyclic loops (QTCLs) may play a key role in high Tc superconductivity. Because of the existence of mixed valences on the CuO 2 plane, the electron/holes in high Tc superconductor are weekly localized and strongly correlated thus able to form QTCL. The QTCLs have k=0 in the center of the mass frame, and a non-zero angular momentum. Each QTCL has total spin S=0. Each QTCL can contain 2 or more Cooper pairs in real space. The QTCL may explain superfluid 3He phenomenon, in which angular momentum is not equal to zero case. As a result of the strong correlations between those tunneling loops, high Tc superconductivity will occur if the connectivity of tunneling loops is large enough to form an infinite cluster, under these conditions: T<Tc and H<Hc. This high Tc superconducting transition is a special kind of percolation transition which is consistent with the experimental results of uniaxial stress experiments and of thermal expansion measurements. This microscopic dynamics process on CuO2 planes is consistent with the experimental results of uniaxial stress experiments and of thermal expansion measurements and many other experiments. Because of the existence of QTCL, half intrinsic flux quanta h/4e and higher order fractional flux quanta such as h/20e and other fractional flux quanta should be observed in all kind of high temperature superconductors. Some recent observations have indicated the above predictions. This model is consistent with the fact that the coherence length of high temperature superconductor is in the order of 5–20 angstroms. Therefore, the consideration of the microscopic dynamics in real space (not only in k-space) is necessary for a reasonable model. From quantum mechanics view, the coherent position exchange in a QTCL only changes the phase of a wavefunction of a QTCL and the energy of a QTCL is conserved, this is consistent with gauge invariance. This microscopic scale energy conservation and the phase relation is the foundation of high temperature superconductivity. This microscopic scale energy conservation and the phase of fractional flux quanta will be observed in all different high temperature superconductors as well as in those doped carbon 60 superconductors in the near future.

Monocrystalline gold-aqueous interfaces as a model experimental link between macroscopic electrochemistry and in-situ electrochemical surface science

Comparisons are undertaken between some voltammetric and ac impedance responses of ordered monocrystalline gold electrodes in the presence of adsorbed anions, and the electrode potential-dependent atomic structures and surface phase transitions sensed by in situ scanning tunneling microscopy (STM). Emphasis is placed on facilitating such links between macroscopic and microscopic double-layer phenomena by acquiring individual and/or temporal sequences of so-called “potentiodynamic” STM images (PDSTM) during potential sweeps or steps. Two types of double-layer phenomena are considered. The first involves the formation and removal of reconstruction on low-index gold surfaces, chiefly in iodide electrolytes. The marked influence of anion adsorption on the real-space dynamics as well as the thermodynamics of potential-induced reconstruction are discussed in terms of surface bonding and electronic factors. The detailed information on local variations in reconstructed atomic patterns discernable by STM is also briefly described, along with the virtues of PDSTM for unraveling the atomic-level transport mechanisms associated with the potential-induced formation and removal of reconstruction. The other type of phenomenon considered is the formation of ordered anionic adlayers at higher potentials, and the nature of phase transitions that are thereby involved, specifically for iodide, bromide, and sulfate adsorption. The overall synergetic value of coupled electrochemical and probe microscopic measurements for enhancing our atomic-level understanding of double-layer phenomena is pointed out, along with the unique value of ordered gold surfaces for this endeavor.

The unified approach to density functional and molecular dynamics in real space

The lagrangean introduced in ref.[1] is modified with the introduction of an auxiliary field that represents the Hartree potential. While leading to the same results, the present version of the theory offers considerable practical and theoretical advantages. In particular, it provides the possibility of working entirely in real space, reducing the number of operations required. An interesting and potentially useful isomorphism of local density theory to a continuous spin lattice model is pointed out.