Uwe Albrecht

Detection of surface plasmons by scanning tunneling microscopy

The influence of surface plasmons excited in a polycrystalline silver film on the tunneling current of a scanning tunneling microscope (STM) has been analyzed. The plasmons cause an additional flow of electrons from the tungsten tip to the silver surface on the order of up to 50 pA. This process is independent of the polarity of the applied bias voltage, thereby excluding effects of thermal expansion. The different nature of the ordinary tunneling current and the surface plasmon induced current is clearly revealed by their different dependence on the gap distance. The local distribution of the intensity of the surface plasmon induced signal reveals structures on a nanometer scale. Some of them are correlated to the surface topography.

Multielectron bubbles in liquid helium

We have investigated multielectron bubbles in liquid 4He, which develop when the charged helium surface undergoes an electrohydrodynamic instability. Well-defined bubbles with charges ranging from 105 to 107 electrons are easily generated. Oscillations and fissioning of the bubbles are reported, and a new upper limit on their lifetime is set.

Surface-state electrons on a hydrogen film. 1. Annealing of the film

We have investigated the surface of thin films (thickness ∼2 µm) of solid H2 between 1.5 and 4.2 K by measuring the ac conductivity of surface-state electrons (SSE). The films were prepared on a glass substrate by quench condensation at 1.5 K and were therefore initially strongly disordered. In fact the surface of the virgin films before any heat treatment was so rough that no current due to SSE could be observed. Annealing the films decreased the surface roughness and gave rise to a thermal-activation-type temperature dependence of the SSE conductivity. By proper heat treatment up to 8 K the activation energy could be reduced to 10kB.

Annealing behavior of quench-condensed hydrogen and deuterium films

We have studied the annealing behavior of thin molecular hydrogen and deuterium films (typical thickness 10 nm) prepared by quench-condensation at 1.5 K on a silver substrate. Changes in the optical thickness and the heterogeneity of the films were monitored using optically excited surface plasmons. Upon annealing the films exhibit an irreversible change from a homogeneous to a strongly heterogeneous structure on a length scale of some hundred nm, accompanied by an apparent loss in optical thickness. This behavior is interpreted as being due to the formation of large bulk crystallites on the surface, resulting from the incomplete wetting of solid hydrogen on silver.

Investigation of quantum systems with surface plasmons and surface state electrons

We report on investigations of thin films of the quantum systems hydrogen and helium both in thermodynamic equilibrium and in metastable states. Information about the film thickness and surface roughness is obtained from the excitation of surface plasmons and mobility measurements of surface state electrons. The equilibrium studies show triple-point wetting for H2 on Au substrates, in agreement with earlier results for similar systems. Unexpectedly a distinct hysteretic behavior of the saturated film thickness is found near the triple-point. Superfluid4He films on as prepared Ag display complete wetting. Investigations of H2 films in metastable states prepared by quench-condensation at 1.5K reveal pronounced changes in the film structure upon annealing. These changes set in far below the desorption temperature and are ascribed to surface diffusion.

Unexpected Hydrodynamic Behavior of Multielectron Bubbles in Liquid Helium I

We have investigated multielectron bubbles (with charges Z ~ 106 electrons) in normal liquid 4He. The drift velocity in an electric field and the charge of the bubbles were measured simultaneously. We found the drag coefficient to be over an order of magnitude higher than expected from ordinary hydrodynamics. The velocities did not show a correlation with the charge and could vary up to a factor of 3 for equally charged bubbles.

Surface state electrons on a hydrogen film. 2. Influence of adsorbed helium films

We have investigated the conductivity of surface state electrons on liquid4He films supported by a quench-condensed thin solid hydrogen substrate. A pronounced dip in the ac-conductivity was observed as the He film thickness reached about 10 layers. This phenomenon was studied at different temperatures (range between 1.6 and 2 K), electron densities from 0.9 to 19×108 cm−2, frequencies between 1 and 100 kHz, and for various amplitudes of the ac driving voltage. Ripplon scattering and a ripplonic polaron are discussed as sources for the conductivity dip.

On the structure of thin4He films on solid hydrogen

We have investigated the adsorption of4He onto a quench-condensed H2 film by means of surface state electrons. Oscillations in the surface state electron conductivity clearly reveal the layerwise character of the adsorption. At temperatures below 2K we have observed an anomaly in the conductivity around the completion of the first monolayer. This new feature is interpreted as an indication of a liquid-to-solid phase transition in the first layer.

Diffusive two-dimensional spreading of a polymer

Abstract We have measured the thickness profiles of ultrasmall (picoliter volume) droplets of polydimethylsiloxane during their spreading on a silver surface at central thicknesses below one monolayer. The resulting profile shapes can be approximated well by shperical caps, and the diffusion dynamics are quite different from those expected for pointlike particles. The experimental data are in remarkable agreement with Monte Carlo simulations of a spreading two-dimensional polymer lattice-gas, which are performed using the bond fluctuation method.

Solid hydrogen films in a nonequilibrium state

Abstract Molecular hydrogen can be prepared as a metastable quantum solid by quench-condensation onto a substrate at sufficiently low temperature. Using surface plasmon resonance and light scattering techniques we have investigated such H2 films with thicknesses up to several 100A˚, condensed onto a silver surface at 1.5 K. Since solid H2 does not wet silver these films are in an extreme nonequilibrium state. At a temperature of 2.3 K we observe a dramatic rearrangement of the film overstructure on scales of ∼1 μm, which implies an extremely high mobility of the H2 molecules at a temperature far below the desorption threshold (3.8 K).