Jürgen Klier

Triple-point wetting of molecular hydrogen isotopes

Triple-point wetting is a well-known phenomenon of simple adsorbates on solid substrates, which involves, in the liquid phase above the triple-point temperature, T3, complete wetting with the formation of arbitrary thick films being observed, whereas below T3 only a few monolayers of the solid phase are adsorbed at saturated vapour pressure. This effect is usually ascribed to the substrate-induced strain in the solid film, which occurs due to the lattice mismatch and the strong van der Waals pressure in the first few monolayers. Molecular hydrogen is a suitable system in which to investigate this phenomenon, in particular by tailoring the adsorbate–substrate interaction by means of thin preplating layers of other adsorbates, and by introducing disorder into the system by using not only the pure systems H2 and D2, but also mixtures thereof. The experiments show that triple-point wetting is a rather dominant effect which, in contrast to expectations, persists even if the system parameters are widely varied. This indicates that the present understanding of this effect is incomplete. We present an investigation of the influence of the roughness of the substrate which is expected to be responsible for the dewetting of the solid phase.

Characterization of Quench-Condensed Cesium on a Nanometer Scale at Low Temperatures

The alkali metals Cs and Rb are the only surfaces which are not wetted by liquid 4He below a certain temperature. Especially for the system 4He–Cs this has been observed many times. However, there exist discrepancies in the measured contact angles and accompanying hysteresis, and in the movement of helium contact lines. Surface roughness is expected to have a significant influence on the wetting properties, but the working mechanism is not clearly understood yet. For this reason we have developed a special low temperature setup, which allows simultaneously an in situ evaporation of the alkali metals and an investigation of the substrate surface using a scanning tunneling microscope (STM). A characterization of quench-condensed Cs substrates is given and a manipulation of their surfaces on the nanometer scale is demonstrated. These investigations contain the first STM images of such a Cs surface at low temperatures.PACS numbers: 67.70.+n, 68.08.Bc, 68.37.-d, 68.55.-a

The growth of the non-wetting liquid 4He on Cs

Abstract It was theoretically predicted that the heavy alkali metals provide the only surfaces non-wetted by superfluid 4 He below a certain temperature Tw. This was experimentally proven both for Cs and for Rb. However, investigations have shown that the non-wetting thin-film state for the He–Cs system is extremely dilute for T⪡Tw, yet close to Tw it can be much thicker and of the order of monolayers. Using the photoelectron tunneling method we have sensitively measured the growth of the non-wetting thin-film state of 4He on a quench-condensed Cs surface. It turns out that far from co-existence there is little adsorption of helium. In contrast, close to co-existence a rapid growth up to two to three monolayers of helium is observed, but the surface is still non-wet under the usual convention.

Cyclotron resonance for 2D electrons on helium films above rough substrates

Abstract An investigation of the microwave absorption for two-dimensional electron systems (2DES) on helium films and in the presence of a cyclotron resonance (CR) magnetic field are presented. Measured data are explained by a recently proposed two-fraction model of the 2DES, which makes the general structure of the microwave absorption understandable. The fraction of localized and free electrons can be precisely determined and its dependence on the thickness of the helium film above the roughness of the underlying solid substrate is understood.

Influence of the concentration of H2–D2 mixtures on their triple-point dewetting behavior

Triple-point dewetting of pure gases like hydrogen and deuterium on solid substrates is a well-known phenomenon. This property persists even for the mixed system of H2 and D2. There exists an effective triple-point temperature T3(mix), between the T3 of pure H2 and that of pure D2, which depends on the species concentrations. We present new investigations for a wide range of H2–D2 concentrations measured under different thermodynamic conditions. This allows us to map out T3(mix) as function of concentration, which can be different in the melting or solidifying direction. Furthermore, it turns out that the time the system needs to reach an equilibrium state can be very long and depends on concentration. This is not observed for the pure H2 and D2 systems. Sometimes the relaxation times are so extremely long that significant hysteresis occurs during ramping of the temperature, even if this is done very slowly on a scale of hours. This behavior can be understood on the basis of mixing and demixing processes....

Equilibrium helium film in the thick film limit

There are still some open questions about how the thickness of a liquid or solid quantum film, such as liquid helium or solid hydrogen, develops in certain limits. One of these is the thick-film limit, i.e., the crossover from the thick film to the bulk. We have performed measurements in this range using the surface plasmon resonance technique and an evaporated Ag film deposited on a glass substrate. The thickness of the adsorbed helium film is varied by changing the distance h of the bulk reservoir to the surface of the substrate. In the limiting case when h→0 the film thickness approaches about 100 nm, following the van der Waals law in the retarded regime. The film thickness and its dependence on h is determined precisely and modeled theoretically. The behavior of the equilibrium film thickness is discussed in detail. The agreement between theory and experiment is very good.

Electron tunnelling through a quantifiable barrier of variable width

This is the first study of electron tunnelling through a quantifiable barrier of adjustable width. We find quantitative agreement between the measured and calculated tunnelling probability with no adjustable constants. The tunnel barrier is a thin film of 3He on Cs1 which it wets. We excite photoelectrons which have to tunnel through the barrier to escape. The image potential must be included in calculating the barrier and hence the tunnelling current. This has been a debatable point until now. We confirm that an electron has a potential of 1.0 eV in liquid 3He for short times before a bubble forms. We show that the thickness of the 3He is given by thermodynamics for films of thickness at least down to 3 monolayers.