We report about our investigations on two-dimensional electron ensembles floating above thin helium films supported by metallic substrates. We found a surprisingly high stability of these electrons, although the substrates were conducting which allows breakthrough at weak spots of the helium film due to roughness peaks. We have measured the electron densities and corresponding relaxation times. By pulling the electrons from the area above the loading electrode to the area above a separate electrode one can enhance the lifetime of these surface state electrons.
Triple-point dewetting is a well-known behaviour of molecular hydrogen and other van der Waals systems like noble gases on a solid substrate. Recent theoretical and experimental investigations (Phys. Rev. Lett. 88 (2002) 55702) suggest that it is caused primarily by the roughness of the substrate. Strain induced due to the mismatch of the lattice constant of the substrate and the growing layers of the adsorbed materials is increased by the micro-roughness of the substrate which eventually leads to the growth of only a thin solid film of the adsorbate. The dominating role of the substrate roughness is demonstrated, e.g., by ellipsometric measurements on smooth Si surfaces (rms 0.15 nm), where a thicker solid hydrogen film than the 3 monolayers on "usual" substrates is observed. We present a way to modify and improve the surface quality of snbstrates for such wetting studies of solid van der Waals films. substrate-adsorbate interaction. Recently, a more realistic calculation has been done by Esztemann et al. [4], in which the roughness profile of the substrate is taken into account. The computed results OF that investigation predict a significant variation of the film thickness on substrates with different roughnessas, i.e., the adsorbed film will be thicker on the smoother substrate, and for a certain range of substrate strength ( 1 . 5~ R 6 3 ) it will diverge as the roughness goes to zero.In this work we present results of adsorbed HI on a Si-wafer with an average roughness (ms) of 0.15 nm. Its surface is distinctly smoother than the previously used evaporated gold substrates on glass [3,4], which had an rms roughness of 1.3 nrn and more. We use an elIipsometry setup, as shown in Fig. 1, to measure the Hz film thickness on the Si substrate. The sample is mounted vertically in a Cu cell with windows for optical access to the sample surface. The whole assembly is placed inside an optical flow cryostat. A laser diode provides a very stable laser beam which passes through a polarizer and compensator before entering the cryostat. After reflection on the sampIe the beam leaves the cryostat, passes through an analyser and is detected by a photodiode.0921-4526/03/% -see front matter 0
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