Dynamics of the Vortex-Particle Complexes Bound to the Free Surface of Superfluid Helium.

We present an experimental and theoretical study of the 2D dynamics of electrically charged nanoparticles trapped under a free surface of superfluid helium in a static vertical electric field. We focus on the dynamics of particles driven by the interaction with quantized vortices terminating at the free surface. We identify two types of particle trajectories and the associated vortex structures: vertical linear vortices pinned at the bottom of the container and half-ring vortices traveling along the free surface of the liquid.

Metallic nanowires and mesoscopic networks on a free surface of superfluid helium and charge-shuttling across the liquid-gas interface.

We investigate the motion of electrically charged metallic nano- and microparticles produced by laser ablation in He gas and injected into superfluid helium. In the presence of a vertical static electric field, the particles either perform a repetitive shuttle-like motion transporting the charge across the liquid-gas interface or become trapped under the free surface of liquid He and coalesce into long filaments and complex two-dimensional mesoscopic networks. A classical electrohydrodynamic model is used to describe the motion of charged microparticles in superfluid He. The resulting filaments and networks are analyzed using electron microscopy. It is demonstrated that each filament is in fact composed of a large number of nanowires with a characteristic diameter of order of 10 nm and extremely large aspect ratios.

Transient increase of the energy gap of superconducting NbN thin films excited by resonant narrow-band terahertz pulses.

Observations of radiation-enhanced superconductivity have thus far been limited to a few type-I superconductors (Al, Sn) excited at frequencies between the inelastic scattering rate and the superconducting gap frequency 2Δ/h. Utilizing intense, narrow-band, picosecond, terahertz pulses, tuned to just below and above 2Δ/h of a BCS superconductor NbN, we demonstrate that the superconducting gap can be transiently increased also in a type-II dirty-limit superconductor. The effect is particularly pronounced at higher temperatures and is attributed to radiation induced nonthermal electron distribution persisting on a 100 ps time scale.

Enhanced reactivity and related optical changes of Ag nanoparticles on amorphous Al2O3 supports.

Pairs of samples containing Ag nanoparticles (NPs) of different dimensions have been produced under the same conditions but on different substrates, namely standard glass slides and a thin layer of amorphous aluminum oxide (a-Al2O3) on-glass. Upon storage in ambient conditions (air and room temperature) the color of samples changed and a blue-shift and damping of the surface plasmon resonance was observed. The changes are weaker for the samples on-glass and tend to saturate after 12 months. In contrast, the changes for the samples on a-Al2O3 appear to be still progressing after 25 months. While x-ray photoelectron spectroscopy shows a slight sulfurization and negligible oxidation of the Ag for the on-glass samples upon 25 months aging, it shows that Ag is strongly oxidized for the on a-Al2O3 samples and sulfurization is negligible. Both optical and chemical results are consistent with the production of a shell at the expense of a reduction of the metal core dimensions, the latter being responsible for the blue-shift and related to the small (<10 nm initial diameter) of the NPs. The enhanced reactivity of the Ag NPs on the a-Al2O3 supports goes along with specific morphological changes of the Ag NPs and the observation of nitrogen.

Point-contact transport properties of strongly correlated electrons on liquid helium.

We present transport measurements of a nondegenerate two-dimensional electron system on the surface of liquid helium at a point constriction. The constriction is formed in a microchannel by a split gate beneath the helium surface. The electrostatic energy of the electron system, which depends in part on the electron density, determines the split-gate voltage threshold of current flow through the constriction. Steplike increases in conductance are observed as the confinement strength is reduced. As the Coulomb interaction between electrons is strong, we attribute this effect to the increase in the number of electrons that can pass simultaneously through the constriction. Close to the threshold, single-electron transport is observed.

Energy-gap dynamics of superconducting NbN thin films studied by time-resolved terahertz spectroscopy.

Using time-domain terahertz spectroscopy we performed direct studies of the photoinduced suppression and recovery of the superconducting gap in a conventional BCS superconductor NbN. Both processes are found to be strongly temperature and excitation density dependent. The analysis of the data with the established phenomenological Rothwarf-Taylor model enabled us to determine the bare quasiparticle recombination rate, the Cooper pair-breaking rate and the electron-phonon coupling constant, λ=1.1±0.1, which is in excellent agreement with theoretical estimates.

Salt-concentration-dependent nucleation rates in low-metastability colloidal charged sphere melts containing small amounts of doublets.

We determined bulk crystal nucleation rates in aqueous suspensions of charged spheres at low metastability. Experiments were performed in dependence on electrolyte concentration and for two different particle number densities. The time-dependent nucleation rate shows a pronounced initial peak, while postsolidification crystal size distributions are skewed towards larger crystallite sizes. At each concentration, the nucleation rate density initially drops exponentially with increasing salt concentration. The full data set, however, shows an unexpected scaling of the nucleation rate densities with metastability times the number density of particles. Parameterization of our results in terms of classical nucleation theory reveals unusually low interfacial free energies of the nucleus surfaces and nucleation barriers well below the thermal energy. We tentatively attribute our observations to the presence of doublets introduced by the employed conditioning technique. We discuss the conditions under which such small seeds may induce nucleation.

Influence of chopped laser light onto the electronic transport through atomic-sized contacts.

This paper reports on the influence of laser irradiation onto the electrical conductance of gold nanocontacts established with the mechanically controllable breakjunction technique. We concentrate here on the study of reversible conductance changes which can be as high as 200%. We investigate the dependence on the initial conductance of the contacts, the wavelength, the intensity and position of the laser spot with respect to the sample. Under most conditions an enhancement of the conductance is observed. We discuss several physical mechanisms which might contribute to the observed effect including thermal expansion, rectification and photon-assisted transport. We conclude that thermal expansion is not the dominating one.

Taylor cone and electrospraying at a free surface of superfluid helium charged from below.

Electrically charged metallic micro- and nanoparticles are trapped under a free surface of superfluid He in a vertical static electric field. We observe a static deformation of the charged liquid surface in the form of a Taylor cone and the emission of a charged liquid helium jet (electrospray). Our numeric calculations reproduce the static shape of the cone.

Direct observation of the dynamics of electronic excitations in molecules and small clusters

Femtosecond time-resolved photoelectron spectroscopy is applied to study relaxation paths of excited states of mass-selected negatively charged clusters. As a first example, the lifetime of an excited state of the carbon trimer anion is measured directly. In addition, the mechanism of the decay, i.e., the configurations of the participating electronic states, is determined from the photoelectron spectra. In general, this method can be used to study all kinds of electronic excitation and relaxation processes in mass-selected nanoparticles.

One millisecond of light suffices to suppress nighttime pineal melatonin synthesis in rats.

The effect of a single high-intensity light pulse with a duration of 1 ms on nighttime pineal activity of male Sprague-Dawley rats was investigated. 10 minutes after light exposure pineal N-actyltransferase activity and melatonin content were significantly reduced. These results show that the rat pineal is capable of responding to very short light flashes of high intensity.

Non-monotonic crossover from single-file to regular diffusion in micro-channels.

The diffusion behavior of interacting particles determines the behavior of a large number of systems ranging from pedestrians crossing a road to ions passing through channels in living cells. Here we present a system in which the nature of the diffusion process varies with changes in the external conditions. We find this special behavior in a colloidal model system, consisting of micron sized particles which are confined to narrow channels and interact via induced magnetic dipoles. When the density of these particles is changed, diffusion alternates between normal Fickian behavior and single-file diffusion. This anomalous behavior is induced by the order of the particles in the restricted geometry and does not depend on the exact nature of the inter-particle interactions.

Density reduction and diffusion in driven two-dimensional colloidal systems through microchannels.

The behavior of particles driven through a narrow constriction is investigated in experiment and simulation. The system of particles adapts to the confining potentials and the interaction energies by a self-consistent arrangement of the particles. It results in the formation of layers throughout the channel and of a density gradient along the channel. The particles accommodate to the density gradient by reducing the number of layers one by one when it is energetically favorable. The position of the layer reduction zone fluctuates with time while the particles continuously pass this zone. The flow behavior of the particles is studied in detail. The velocities of the particles and their diffusion behavior reflect the influence of the self-organized order of the system.